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Archive-name: sci/chem-faq/part6
Posting-Frequency: monthly
Last-modified: 22 October 1999
Version: 1.17
Subject: 26. Electrochemical Techniques
26.1 What is pH?
The pH scale determines the degree of acidity or alkalinity of a solution,
but as it involves a single ion activity it can not be measured directly.
pH = - log10 ( gamma H x m H )
where gamma H = hydrogen ion single ion activity coefficient
m H = molality of the hydrogen ion.
As pH can not be directly measured, it is defined operationally according to
the method used to determine it. IUPAC recommend several standardised methods
for the determination of pH in solution in aqueous solutions. There are
seven primary reference standards that can be used, including 0.05 mol/kg
potassium hydrogen phthalate as the Reference Value Standard. There is an
ongoing debate concerning the relative merits of having a multiple primary
standard scale ( that defines pH using several primary standards, and their
values are determined using a cell without a liquid junction ) or a single
primary standard ( that requires a cell with a liquid junction ). Interested
readers can obtain further information on the debate in [1]. Bates [2], is a
popular text covering both theory and practise of pH measurement.
26.2 How do pH electrodes work?
Contributed by Paul Willems <Paul.Willems@rug.ac.be>, and slightly modified
by Bruce Hamilton.
The most common type of pH electrodes are the "glass" electrodes. They
consist of a special glass membrane that is sensitive to variations in pH,
as pH variation also changes the electrical potential across the glass. In
order to be able to measure this potential, a second electrode, the
"reference" electrode, is required. Both electrodes can be present in a
"combined" pH electrode, or two physically-separate electrodes can be used.
The glass electrode consists of a glass shaft on which a bulb of a special
glass is mounted. The inner is usually filled with 3 Mol/Litre aqueous KCl
and sealed. Electrical contact is provided by a silver wire immersed in the
KCl.
For "combined" electrodes, the glass electrode is surrounded by a concentric
reference electrode. The reference electrode consists of a silver wire in
contact with the almost-insoluble AgCl. The electrical contact with the meter
is through the silver wire. Contact with the solution being measured is via
a KCl filling solution. To minimise mixing of the solution to be measured and
the filling solution, a porous seal, the diaphragm, is used. This is usually
a small glass sinter, however other methods which allow a slow mixing contact
can also be used, especially for samples with low ionic strength. Besides the
"normal" KCl solutions, often solutions with an increased viscosity, and
hence lower mixing rate are used. A gel filling can also be used, which
eliminates the necessity for slow mixing devices.
In contact with different pH solutions a typical glass electrode gives, when
compared to the reference electrode, a voltage of about 0 mV at pH 7,
increasing by 59 mV per pH unit above 7, or decreasing by 59 mV per pH unit
below 7. Both the slope, and the intercept of the curve between pH and
generated potential, are temperature dependent. The potential of the
electrode is approximated by the Nernst equation :
E = E0 - RT log [H+] = E0 + RT pH
Where E is the generated potential, E0 is a constant, R is universal
gas constant and T is the temperature in degrees Kelvin.
All pH-sensitive glasses are also susceptible to other ions, such as Na or K.
This requires a correction in the above equation, so the relationship between
pH and generated voltage becomes nonlinear at high pH values. The slope tends
to diminish both as the electrode ages, and at high pH. As the electrode has
a very high impedance, typically 250 Megohms to 1 Gigohm, it is necessary to
use a very high impedance measuring instrument.
The reference electrode has a fairly constant potential, but it is
temperature dependent, and also varies with activity of the silver ions in
the reference electrode. This occurs if a contaminant enters the reference
electrode.
Calibration
From the preceding, it is obvious that frequent calibration and adjustment of
pH meters are necessary. To check the pH meter, at least two standard buffer
solutions are used to cover the range of interest. The pH meter should be
on for at least 30 minutes prior to calibration to ensure that all components
are at thermal equilibrium, and calibration solutions should be immersed for
at least a minute to ensure equilibrium.
First use the buffer at pH 7, and adjust the zero (or the intercept).
Then, after thorough rinsing with water, use the other buffer to adjust the
slope. This cycle in repeated at least once, or until no further adjustments
are necessary. Many modern pH meters have an automatic calibration feature,
which requires each buffer only once.
Errors
People assume pH measurements are accurate, however many potential errors
exist. There can be errors caused by the pH-sensitive glass, reference
electrode, electrical components, as well as externally generated errors.
Glass Electrode Errors
The pH-sensitive glass can be damaged. Major cracks are obvious, but minor
damage can be difficult to detect. If the internal liquid of the pH-measuring
electrode and the external environment are connected, a pH value close to 7
will be obtained. It will not change when the electrode is immersed in a
known solution of different pH. The electrical resistance of the glass
membrane will also be low, often below 1 megohm, and it must be replaced.
Similar results occur if the glass wall between the inner and outer part of
a combined electrode breaks. This may occur if the outer part is plastic.
The inner part can crack without any external signs. The electrical
resistivity over the glass electrode is intact, but actual measuring between
both electrodes reveals a low resistivity. The electrode must be replaced.
The glass can wear out. This gives slow response times, as well as a lower
slope for the mV versus pH curve. To rejuvenate, immerse the electrode in a
3 Molar KCl solution at 55 degrees Celsius for 5 hours. If this does not
solve the problem, try removing a thin layer of the glass by immersion for
two minutes in a mixture of HCl and KF (be careful, do not breathe the fumes,
and wear gloves). The electrode is then immersed for two more minutes in HCl,
and rinsed thoroughly. As an outer layer of glass has been removed, the new
surface will be like a new electrode, however the thinner glass will result
in a shorter electrode life. Frequent recalibration will be required for
several days.
The glass can be dirty. A deposit on the glass will slow the response time,
make the response sensitive to agitation and ionic strength, and also give
the pH of the film, not the sample solution. If the deposit is known, use a
appropriate solvent to remove it, and rehydrate the electrode in 3M KCl.
If the deposit is not known, first immerse the electrode for a few minutes
in a strongly alkaline solution, rinse thoroughly, and immerse it in a
strong acid (HCl) solution for several minutes. If this does not help, try
using pepsin in HCl. If still unsuccessful, use the above HCl/KF method.
Reference Electrode Errors
The diaphragm of the reference can become blocked. This is seen as unstable
or wrong pH measurements. If the electrical resistivity of the diaphragm is
measured, high values are reported (Most multimeters will give an over-range
error). The most common reason is that AgS formed a precipitate in the
diaphragm. The diaphragm will be black in this case. The electrode should be
immersed in a solution of acidic thiourea until the diaphragm is white, and
then replace the internal filling liquid of the reference electrode
There is no contact across the diaphragm, due to air bubbles. This appears
as if the diaphragm were blocked, except that the diaphragm is white. Ensure
that the filling solution level in the reference electrode is always well
above the sample, so that liquid is always slowly flowing from the reference
electrode towards the sample.
The electrode filling solution is contaminated. This appears as unstable or
wrong pH measurements. Often the 0mV pH differs considerably from pH 7. The
diaphragm has its normal colour and the electrical resistivity is normal.
However, the solution often becomes contaminated due to low filling solution
levels, and air bubbles may also appear in the diaphragm, which obviously
affects electrical resistivity. Replacing the reference filling solution
several times should solve the problem, but the electrode may have been
permanently damaged. The problem can be avoided by choosing gel-filled
reference electrodes, double-junction electrodes, or ensuring there is an
outflow of reference filling solution towards the sample.
The electrode was filled with the wrong reference solution. This appears as
as displaced pH measurements. Flush and replace the reference liquid.
Electrical errors
Condensation or sample contamination of the electrode connecting cable. This
appears as an almost-constant measurement of about pH 7, even when the pH
electrode is disconnected from the cable, or as a pH which changes less than
it should, when tested with two standard solutions. If the cable is
disconnected from the meter, the pH will start to drift.
There is a short circuit in the cable. The symptoms are similar to the above
case, except that bending the cable may create sharp, spurious readings. In
most pH cables, between the two copper conductors there are two layers which
appear to be insulators. The inner layer is an insulator, whereas the outer
layer is a conductor to avoid trace electrical effects. If this outer layer
does contact the inner conductor, there will be a short circuit. Replace
suspect cables.
The input stage of the meter is contaminated with conducting liquid. The
symptoms are the same as above, except that removing the cable has no
effect. Closely examine the input stage of the meter for liquid or deposits.
If present, rinse with distilled water, then ethanol, and dry thoroughly.
The input stage of the meter is faulty. This gives random measurements.
Shorting both input wires does not make any difference. Repair the meter.
The input stage appears faulty. Shorting both input wires gives a stable
pH measurement of about 7. The meter may be faulty, but probably the problem
is elsewhere in the electrical circuit.
Externally-generated Errors
If a significant flow of liquid passes the electrode, then there can
be a minor electrical effect. This generates a potential on the glass
membrane, which is superimposed on the actual pH measurement. This effect
becomes negligible for highly-conducting liquids. It is seldom observed.
If the trace electric effect does influence pH measurements, the addition
of a little salt to increase the conductivity, or changing the flux of
liquid around the electrode, should solve the problem.
Ground loops and spurious electrical currents may generate unexpected
electrical signals. Such signals can strongly influence pH measurements.
A pH reading in the range of -15 to +20 is possible, even if the pH is 7.
Ground loops can be eliminated by grounding the system according to the
manufacturer's instructions, and ensuring insulation is in good condition.
Often these problems can be extremely difficult to detect and remedy.
Low ionic strength samples can be affected by electrolyte from the electrode,
and special electrodes are available.
26.3 What are ion-selective electrodes?
Ion selective electrodes are electrochemical sensors whose potential varies
with the logarithm of the activity of an ion in solution. Available types:
1. The membrane is a single compound, or a homogeneous mixture of compounds.
2. The membrane is a thin glass whose chemical composition determines the
response to specific ions.
3. The support, containing an ionic species, or uncharged species, forms the
membrane. The support can be solid or porous.
Popular texts on applications of ion-selective electrodes include
"Ion-Selective Electrodes in Analytical Chemistry" [3], and "Ion-selective
Electrode Methodology" [4].
26.4 Who supplies pH and ion-selective electrodes?
The best known manufacturer of ion-selective electrodes is Orion Research.
There are several pH electrode manufacturers, including Radiometer and
Metrohm.
Subject: 27. Fuel Chemistry
27.1 Where does crude oil come from?.
The generally-accepted origin of crude oil is from plant life up to 3
billion years ago, but predominantly from 100 to 600 million years ago [1].
"Dead vegetarian dino dinner" is more correct than "dead dinos".
The molecular structure of the hydrocarbons and other compounds present
in fossil fuels can be linked to the leaf waxes and other plant molecules of
marine and terrestrial plants believed to exist during that era. There are
various biogenic marker chemicals such as isoprenoids from terpenes,
porphyrins and aromatics from natural pigments, pristane and phytane from
the hydrolysis of chlorophyll, and normal alkanes from waxes, whose size
and shape can not be explained by known geological processes [2]. The
presence of optical activity and the carbon isotopic ratios also indicate a
biological origin [3]. There is another hypothesis that suggests crude oil
is derived from methane from the earth's interior. The current main
proponent of this abiotic theory is Thomas Gold, however abiotic and
extraterrestrial origins for fossil fuels were also considered at the turn
of the century, and were discarded then. A large amount of additional
evidence for the biological origin of crude oil has accumulated, however
Professor Gold still actively promotes his theory worldwide, even though
it does not account for the location and composition of all crude oils.
27.2 What are CNG/LPG/gasoline/kerosine/diesel?.
Crude oil consists mainly of hydrocarbons with carbon numbers between one and
forty. The petroleum refinery takes this product and refines it into several
fuel fractions that are optimised for their intended application. For spark
ignition engines, the very volatile and branched chain alkane hydrocarbons
have desirable combustion properties, and several fractions are produced.
CNG ( Compressed Natural Gas ) is usually around 70-90% methane with 10-20%
ethane, 2-8% propanes, and decreasing quantities of the higher HCs up to
pentane. The major disadvantage of compressed gaseous fuels is the reduced
range. Vehicles may have between one to three cylinders ( 25 MPa, 90-120
litre capacity), and they usually provide about 50% of the gasoline range.
LPG ( Liquefied Petroleum Gas ) is predominantly propane with iso-butane
and n-butane. It has one major advantage over CNG, the tanks do not have
to be high pressure, and the fuel is stored as a liquid. The fuel offers
most of the environmental benefits of CNG, including high octane - which
means higher compression, more efficient, engines can be used. Approximately
20-25% more fuel than gasoline is required, unless the engine is optimised
( CR 12:1 ) for LPG, in which case there is no decrease in power or any
significant increase in fuel consumption [4,5].
Gasoline contains over 500 hydrocarbons that may have between 3 to 12
carbons, and gasoline used to have a boiling range from 30C to 220C at
atmospheric pressure. The boiling range is narrowing as the initial boiling
point is increasing, and the final boiling point is decreasing, both
changes are for environmental reasons. A detailed description of the
composition of gasoline, along with the properties and compositions of CNG,
LPG, and oxygenates can be found in the Gasoline FAQ, which is posted monthly
to rec.autos.tech.
Kerosine is a hydrocarbon fraction that typically distils between 170-270C
(narrow cut kerosine, or Jet A1) or 100-250C ( wide cut kerosine, or JP-4 ).
It contains around 20% of aromatics, however the aromatic content will be
reduced for high quality lighting kerosines, as the aromatics reduce the
smoke point. The major use for kerosines today is as aviation turbine (jet)
fuels. Special properties are required for that application, including high
flash point for safe refuelling ( 38C for Jet A1 ), low freezing point for
high altitude flying ( -47C for Jet A1 ), and good water separation
characteristics. Details can be found in any petroleum refining text and
Kirk Othmer.
Diesel is used in compression ignition engines, and is a hydrocarbon fraction
that typically distils between 250-380C. Diesel engines use the Cetane
(n-hexadecane) rating to assess ignition delay. Normal alkanes have a high
cetane rating, ( nC16=100 ) whereas aromatics ( alpha methylnaphthalene = 0 )
and iso-alkanes ( 2,2,4,4,6,8,8-hexamethylnonane = 15 ) have low ratings,
which represent long ignition delays. Because of the size of the hydrocarbons,
the low temperature flow properties control the composition of diesel, and
additives are used to prevent filter blocking in cooler temperatures. There
are usually summer and winter grades. Environmental legislation is reducing
the amount of aromatics and sulfur permitted in diesel, and the emission of
small particulates ( diameters of <10um ) that are considered possibly
carcinogenic, and are known to cause other adverse health effects. Details
can be found in any petroleum refining text and Kirk Othmer.
27.3 What are oxygenates?.
Oxygenates are just pre-used hydrocarbons :-). They contain oxygen, which can
not provide energy, but their structure provides a reasonable anti-knock
value, thus they are good substitutes for aromatics, and they may also reduce
the smog-forming tendencies of the exhaust gases [6]. Most oxygenates used
in gasolines are either alcohols ( Cx-O-H ) or ethers (Cx-O-Cy), and contain
1 to 6 carbons. Alcohols have been used in gasolines since the 1930s, and
MTBE was first used in commercial gasolines in Italy in 1973, and was first
used in the US by ARCO in 1979. The relative advantages of aromatics and
oxygenates as environmentally-friendly and low toxicity octane-enhancers are
still being researched.
Ethanol C-C-O-H C2H5OH
C
|
Methyl tertiary butyl ether C-C-O-C C4H9OCH3
(aka tertiary butyl methyl ether ) |
C
They can be produced from fossil fuels eg methanol (MeOH), methyl tertiary
butyl ether (MTBE), tertiary amyl methyl ether (TAME), or from biomass, eg
ethanol(EtOH), ethyl tertiary butyl ether (ETBE)). MTBE is produced by
reacting methanol ( from natural gas ) with isobutylene in the liquid phase
over an acidic ion-exchange resin catalyst at 100C. The isobutylene was
initially from refinery catalytic crackers or petrochemical olefin plants,
but these days larger plants produce it from butanes.
Oxygenates have significantly different physical properties to hydrocarbons,
and the levels that can be added to gasolines are controlled by the EPA in
the US, with waivers being granted for some combinations. Initially the
oxygenates were added to hydrocarbon fractions that were slightly-modified
unleaded gasoline fractions, and these were commonly known as "oxygenated"
gasolines. In 1995, the hydrocarbon fraction was significantly modified, and
these gasolines are called "reformulated gasolines" ( RFGs ). The change to
reformulated gasoline requires oxygenates to provide octane, but also that
the hydrocarbon composition of RFG must be significantly more modified than
the existing oxygenated gasolines to reduce unsaturates, volatility, benzene,
and the reactivity of emissions.
Oxygenates that are added to gasoline function in two ways. Firstly they
have high blending octane, and so can replace high octane aromatics
in the fuel. These aromatics are responsible for disproportionate amounts
of CO and HC exhaust emissions. This is called the "aromatic substitution
effect". Oxygenates also cause engines without sophisticated engine
management systems to move to the lean side of stoichiometry, thus reducing
emissions of CO ( 2% oxygen can reduce CO by 16% ) and HC ( 2% oxygen can
reduce HC by 10%)[7]. However, on vehicles with engine management systems,
the fuel volume will be increased to bring the stoichiometry back to
the preferred optimum setting. Oxygen in the fuel can not contribute
energy, consequently the fuel has less energy content. For the same
efficiency and power output, more fuel has to be burnt, and the slight
improvements in combustion efficiency that oxygenates provide on some
engines usually do not completely compensate for the oxygen.
There are huge number of chemical mechanisms involved in the pre-flame
reactions of gasoline combustion. Although both alkyl leads and oxygenates
are effective at suppressing knock, the chemical modes through which they
act are entirely different. MTBE works by retarding the progress of the low
temperature or cool-flame reactions, consuming radical species, particularly
OH radicals and producing isobutene. The isobutene in turn consumes
additional OH radicals and produces unreactive, resonantly stabilised
radicals such as allyl and methyl allyl, as well as stable species such as
allene, which resist further oxidation [8,9].
The major concern with oxygenates is no longer that they may not be
effective at reducing atmospheric pollution, but that their greater water
solubility, and very slow biodegradability, can result in groundwater
pollution that may be difficult to remove. Their toxicological and
environmental effects are also still being researched.
27.4 What is petroleum ether?.
Petroleum ether ( aka petroleum spirits ) is a narrow alkane hydrocarbon
distillate fraction from crude oil. The names "ether" and "spirit" refer
to the very volatile nature of the solvent, and petroleum ether does not
have the ether ( Cx-O-Cy ) linkage, but solely consists of hydrocarbons.
Petroleum ethers are defined by their boiling range, and that is typically
20C. Typical fractions are 20-40C, 40-60C, 60-80C, 80-100C, 100-120C etc.
up to 200C. There are specially refined grades that have any aromatic
hydrocarbons removed, and there are specially named grades, eg pentane
fraction (30-40C), hexane fraction (60-80C, 67-70C). It is important to
note that most "hexane" fractions are mixtures of hydrocarbons, and pure
normal hexane is usually described as "n-hexane".
27.5 What is naphtha?.
Naphtha is a refined light distillate fraction, usually boiling below 250C,
but often with a fairly wide boiling range. Gasoline and kerosine are the
most well-known, but there are a whole range of special-purpose hydrocarbon
fractions that can be described as naphtha. The petroleum refining industry
calls the 0-100C fraction from the distillation of crude oil "light virgin
naphtha" and the 100-200C fraction " heavy virgin naphtha". The product
stream from the fluid catalytic cracker is often split into three fractions,
<105C = "light FCC naphtha", 105-160C = "intermediate FCC naphtha" and
160-200C "heavy FCC naphtha".
27.6 What are white spirits?.
White spirits are petroleum fractions that boil between 150-220C. They can
have aromatics contents between 0-100%, and Shell lists eight grades with
aromatics contents below 50%, and six grades with aromatics contents above
50%. The two common "white spirits" are defined by British Standard 245,
which states Type A should have aromatics content of less that 25% v/v and
Type B should have an aromatics content of 25-50% v/v. The most common
" white spirit" is type A, and it typically has an aromatics content of
20%, boils between 150-200C, and has an aniline point of 58C, and is
sometimes known as Low Aromatic White Spirits. The next most common is
Mineral Turpentine (aka High Aromatic White Spirits ), which typically has
an aromatics content of 50%, boils between 150-200C and has an aniline
point of 25C. For safety reasons, most White Spirits have Flash Points
above ambient, and usually above 35C. Note that "white gas" is not white
spirits, but is a volatile gasoline fraction that has a flash point below
0C, which is also known by several other names. Do not confuse the two
when purchasing fuel for camping stoves and lamps, ensure you purchase the
correct fuel.
27.7 What are biofuels?.
Biofuels are produced from biomass ( land and aquatic vegetation, animal
wastes, and photosynthetic organisms ), and are thus considered renewable
within relatively short time-frames. Examples of biofuels include wood,
dried animal dung, methyl esters from triglyceride oils, and methane from
land-fills. The renewable aspect of most biofuels is essentially the use
of solar energy to grow crops that can be converted to energy. There is
a large monograph "Fuels from Biomass" in Kirk Othmer, and the subject
is frequently discussed in alt.energy.renewable, sci.energy, and
sci.energy.hydrogen.
27.8 How can I convert cooking oil into diesel fuel?.
Diesel engines can run on plant and animal triglycerides such as tallow
and seed oils, however most trials have resulted in reduced engine life, or
increased service costs. The solution is to transesterify the triglycerides
into esters, taking care to avoid the formation of monoacylglycerides
that will precipitate out at low temperatures or when diesel is encountered.
There are several plants in Austria that produce Rapeseed Oil Methyl Esters
as fuels for diesel engines. The economics of the process are very
dependant on the price of diesel and the market for the glycerol byproduct.
The common catalysts used to transesterify triglycerides are sodium
hydroxide, sodium methoxide and potassium carbonate. If the esters are to
be blended with diesel fuel, then a two stage reaction is usually required
to ensure that monoacylglycerides are kept below 0.05%. Usually this
involves using 22g of methanol ( containing 0.6g of sodium hydroxide ) and
100g of tallow refluxed for 30 minutes. The mixture is cooled, the glycerol
layer removed, and a further 0.2g of sodium hydroxide is reacted for 5
minutes at 35C in a stirred reactor. The glycerol phase is allowed to
separate, and the ester phase is washed with water to remove residual
catalyst, glycerol and methanol. Note that sodium hydroxide is the most
cost-effective catalyst, but also has the worst tendency to form soaps.
The catalyst and methanol can be of industrial grade without further
purification required, however care should be taken to prevent additional
water entering the reaction [10].
The fuel can be converted into other esters, such as ethyl and butyl, but
it really depends on the availability of cheap alcohol along with the
desired properties of the fuels. The effect of catalysts, reagent ratio,
temperature, and moisture on the production of methyl, ethyl, and butyl
esters from some common oils has been investigated [11]. The New Zealand
government investigated a wide range of techniques for turning various
vegetable and animal triglycerides into esters for diesel, and the reports
cover many aspects of the kinetics and efficiencies [12]. There is a general
overview of the current processes and technology available in Inform [13].
A specific technique for analysing the monoglycerides has been published
[14], however I have found that acetylation followed by narrow bore
( 0.1mm ID ) capillary chromatography is faster and cheaper.
Subject: 28. Pharmaceutical Chemistry
28.1 Does Thalidomide racemise in humans?.
Thalidomide ( N-phthaloyl-alpha-aminoglutarimide ) is well known as an
enantiomeric sedative-hypnotic drug that caused tragic birth defects in
the early 1960s. It has often been claimed that the defects were caused by
the presence of the other isomer in the production batches, and if the pure
enantiomer had been sold, then the tragic defects would have been avoided.
Unfortunately, thalidomide is optically unstable in solution; the pure
isomers of thalidomide racemise by the opening of the phthalimide ring, with
half-lives of 4-5 hours in buffer at pH 7.4, and less than 10 minutes in the
blood. Thus shortly after administration of either enantiomer, the other
enantiomer will be present in significant quantities [1].
Some recent work has revealed that thalidomide inhibits the production of
tumour necrosis factor alpha. Elevated levels of TNF-alpha are associated
with several inflammatory conditions. This has led to the development of
analogues that are chirally stable in reconstituted human plasma, and which
are undergoing development as anti-inflammatory drugs [2].
Subject: 29. Adhesive Chemistry
Subject: 30. Polymer Chemistry
30.1 How can I simply identify common plastics?.
Read the recycle code :-). Alternatively, give it to the nearest IR
spectroscopist who has a polymer library. But if you want some fun, try the
following.
There are several simple tests that can be performed in the home that can
assist in separating common plastics, however it is important to realise that
formulated products contain large quantities of pigments, plasticisers, and
fillers that can dramatically alter the following properties. If possible
repeat the tests on a reference sample of the plastic.
a. Visually examine the sample, looking for recycle codes :-)
While you are at it, you can check for indications of how the plastic
was made - moulded, injected, rolled, machined etc.
b. Try assessing the flexibility by bending, and look at the bending zone
- does the material stretch or is it brittle?
c. Test the hardness, try scratching it with pencils of differing hardness
( B,HB,1-6H ) to ascertain which causes a scratch in the plastic.
Alternatively, attempt to scuff the sample with a fingernail.
d. Cut a small slither with a sharp knife. Does the sample cut cleanly
( thermoplastic )?, or does it crumble ( thermosetting )?.
e. Hold sample in small flame, note whether it burns, self-extinguishes on
removal from the flame, colour of the flame, and smell/acrid nature of
fumes when flame is blown out ( Caution - the fumes are likely to be
toxic ). Also attempt to press melted sample against a cold surface, and
pull away - does sample easily form long threads.
f. Drop onto a hard surface, does the sample "ring" or "thud"?
g. Place it in water. Does it float, sink slowly, or sink rapidly?
If it sinks rapidly, it is likely to be halogenated ( PVC, Viton, PTFE )
If it sinks slowly, possibly nylon
If it floats possibly polyethylene or polypropylene.
- you can ascertain the actual density by adding measured volumes of a
low density solvent like methanol until the sample neither rises nor
sinks.
Cutting thin slivers results in powdery chips ( thermosetting )
- carbolic smell in flame, self extinguishing = phenol formaldehyde
- self extinguishing, black smoke, acrid = epoxide
- fishy smell = urea formaldehyde, or melamine formaldehyde
cutting thin slivers results in smooth sliver ( thermoplastic )
- metallic "ring", burns (styrene smell) = polystyrene
(note that high impact polystyrene may not give "ring" )
- "thud", floats, hard, glossy surface, burns (paraffin wax smell) =
polypropylene
- "thud", floats, medium-hard surface, burns (sealing wax smell) =
high density polyethylene
- "thud", floats, soft surface, burns (paraffin wax smell) =
low density polyethylene
- "thud", sinks, burns ( fruity smell ) = acrylic
- "thud", sinks, burns ( burning paper smell ) = cellulose acetate or
propionate.
- "thud", sinks, burns ( rancid butter smell ) = cellulose acetate butyrate
- "thud", sinks, difficult to ignite ( greenish tinge ) = PVC
- "thud", sinks, difficult to ignite ( yellow colour, formaldehyde smell )
= polyacetal
- "thud", sinks, difficult to ignite ( yellow colour, weak smell ), draws
into long threads = Nylon
- "thud", sinks, difficult to ignite ( minimal flame, decomposition but no
charring, cellular structure forms = polycarbonate.
30.2 What do the plastics recycling codes mean?.
The recycle codes for plastics are currently being reviewed, and new codes
( probably inside a totally different symbol ) will soon be introduced.
1 = PET
2 = High density polyethylene
3 = Vinyl
4 = Low density polyethylene
5 = Polypropylene
6 = Polystyrene
7 = Others, including multi-layer
Subject: 31. Others
31.1 How does remote sensing of chemical pollutants work?.
The are several techniques, but the one of most interest to the public is
the system being used to identify grossly polluting vehicles. The system
consists of an infra-red source on one side of the road, and a detector
system on the other. The collimated beam of IR is directed at a gas filter
radiometer equipped with two liquid-nitrogen-cooled indium antimide
photovoltaic detectors. The beam is split, and passes through a 4.3um
bandpass filter to isolate the CO2 spectral region, a 4.6um filter to
isolates the CO region, and a third filter to isolate the HC region.
A non-absorbing region is also used to compensate for signal strength.
There are various specific enhancements, such as the spinning gas-filter
correlation cell in the University of Denver FEAT ( Fuel Efficiency
Automobile Test ) system used to cost-effectively identify grossly-polluting
vehicles [1]. "Optical remote sensing for air pollutants - review " by
M.Simonds et al [2], provides a good introduction to the diverse range of
instruments used for remote sensing of pollutants.
31.2 How does a Lava Lamp work?.
Contributed by: Jim Webb <jnw4347@email.unc.edu>
A container filled with clear or dyed liquid contains a non-water-soluble
substance (the "lava") that's just a little bit denser (heavier), and has
a greater thermal coefficient of expansion, than the liquid around it.
Thus, it settles to the bottom of the container. A heat source at the
bottom of the container warms the substance, making it expand and become
less dense than the liquid around it. Thus, it rises. As it moves away
from the heat source, it cools, contracts a bit, and becomes (once again)
heavier than the medium. Thus, it falls. Heavy, light, heavy, light.
Sounds like a Milan Kundera novel.
(Actually, to be more precise: dense, less dense, dense, less dense.)
31.3 How do I make a Lava Lamp?.
Contributed by: Jim Webb <jnw4347@email.unc.edu>
Method 1. A new, easy, simple, cheap lava lamp recipe
Use mineral oil as the lava. Use 90% isopropyl alcohol (which most
drugstores can easily order) and 70% isopropyl alcohol (grocery-store
rubbing alcohol) for the other ingredient. In 90% alcohol the mineral oil
will sink to the bottom; slowly add the 70% alcohol (gently mixing all
the while; take your time) until the oil seems lighter and is about to
"jump" off the bottom. Use the two alcohols to adjust the responsiveness
of the "lava."
This mixture is placed in a closed container (the "lava lamp shape" is
not required, although something fairly tall is good) and situated over a
40-watt bulb. If the "lava" tends to collect at the top, try putting a
dimmer on the bulb, or a fan at the top of the container.
To dye the lava, use an oil-based dye like artists' oil paints or a
chopped-up sharpie marker. To dye the liquid around it, use food
coloring.
Two suggestions for better performance: 1) Agitation will tend to make
the mineral oil form small bubbles unlike the large blobs we're all used
to. The addition of a hydrophobic solvent to the mixture will help the
lava coalesce. Turpentine and other paint solvents work well. To make
sure what you use is hydrophobic, put some on your hand (if it's so toxic
you can't put it on your hand, do you want to put it in a container that
could break all over your room/desk/office?) and run a little water on
it. If the water beads, it should work fine. 2) For faster warm-up time,
add some antifreeze or (I've not tried it) liquid soap. Too much will
cloud the alcohol. Keep in mind that the addition of these chemicals may
necessitate your readjusting the 90% to 70% alcohol mixture.
Method 2. The "official" way - from a patent [3].
The patent itself is not very specific as to proportions of ingredients.
The solid component (i.e., the waxy-looking stuff that bubbles) is said
to consist of "a mineral oil such as Ondina 17 (R.T.M.) with a light
paraffin, carbon tetrachloride, a dye and paraffin wax."
The medium this waxy stuff moves in is roughly 70/30% (by volume) water
and a liquid which will raise the coefficient of cubic thermal expansion,
and generally make the whole thing work better. The patent recommends
propylene glycol for this; however, glycerol, ethylene glycol, and
polyethylene glycol (aka PEG) are also mentioned as being sufficient.
This mixture is placed in a closed container (the "lava lamp shape" is
not required, although something fairly tall is good) and situated over a
40-watt bulb. If the "lava" tends to collect at the top, try putting a
dimmer on the bulb, or a fan at the top of the container.
Method 3. The "less official" way - from Popular Electronics [4].
Several non-water-soluble chemicals fall under the category of being
"just a little bit heavier" than water, and are still viscous enough to
form bubbles, not be terribly poisonous, and have a great enough
coefficient of expansion. Among them: Benzyl alcohol (Specific Gravity
1.043 g/cm3), Cinnamyl Alcohol (SG 1.04), Diethyl phthalate (SG 1.121)
and Ethyl Salicylate (SG 1.13). [The specific gravity of distilled water
is 1.000.]
Hubscher recommends using Benzyl Alcohol, which is used in the
manufacture of perfume and (in one of its forms) as a food additive. It can
be obtained from chemical or laboratory supply houses (check your yellow
pages); the cheapest I could find it for was $25 for 500 ml (probably 2,
maybe 3 regular-sized lava lamps' worth). An oil-soluble dye is nice to
color the "lava"; Hubscher soaked the benzyl in a chopped up red felt-tip
pen and said it worked great. [Benzyl alcohol is "relatively harmless",
but don't drink it, and avoid touching & breathing it.]
Hubscher found that the benzyl and the water alone didn't do much, so he
raised the specific gravity of the water a little bit by adding table
salt. A 4.8% salt solution (put 48 grams of salt in a container and fill
it up to one liter with water) has a specific gravity of about 1.032,
closer to benzyl's 1.043. I find that the salt tends to cloud the water a
bit.. you might want to experiment with other additives. (Antifreeze?
Vinegar?)
This is put into a closed container and placed above a 40-watt bulb, as
above. Either way, I would suggest using distilled water and consider
sterilising the container by immersing it in boiling water for a few
minutes.. algae growing in lava lamps is not very hip.
Caveat: Some of these chemicals are not good for you. Caveat 2: Some of
these companies are not good for you if they find you've been infringing
on their patent rights and trying to sell your new line of "magma
lights." Be careful.
31.4 What is Goretex?.
Goretex is a dispersion-polymerised PTFE that is patented by W.L.Gore and
Associates [5]. It is classed as a stretched semi-crystalline film, and is
produced by extrusion under stress ( faster take-up rate than extrusion
rate ). The extrudate is stretched below the melting temperature, often
in the presence of an aromatic hydrocarbon that swells the amorphous region,
creating porosity. The hydrophobic nature of the PTFE means that liquid
water is repelled from the pores, whereas water vapour can pass through.
It is important to realise that once the PTFE pores are filled with liquid
water, the fabric can allow liquid water to pass though until it is dry
again. Thus Goretex-containing fabrics ( such as Nomex/Goretex - which
consists of an outer aramid fabric, a central Goretex layer, and a cotton
backing ) should never be used as protection from chemicals as many will
pass straight through. Any water-miscible solvent ( eg alcohol ) can fill
the pores, and then liquid water can displace it and continue to rapidly
pass through until the fabric is fully dried out.
31.5 What causes an automobile airbag to inflate?.
The final cause is the production of nitrogen from 10s of grams of sodium
azide, but there are some extra chemicals involved along the way.
Sodium azide is toxic, The airbag inflators are aluminium-encased units
that contain an igniter (squib), gas generating pellets ( or wafers of
sodium azide propellant ), and filters to screen out combustion products.
The electrical signal ignites a few milligrams of initiator pyrotechnic
material. The pyrotechnic material then ignites several grams of booster
material, which ignites the tens of grams of sodium azide, and the azide
burns very rapidly to produce nitrogen gas and sodium.
The sodium azide is pelletised to control the rate of gas generation by
controlling its surface area. The free sodium would form sodium
hydroxide when it contacts the water in people's noses, mouths, and
eyes so, to prevent this, the manufacturers mix in chemicals that will
produce sodium salts ( silicates, aluminates, borates ) on combustion.
Inflator units also often have a layer of matted material of alumina and
silica called Fiberfrax in the particulate filter. The Fiberfrax mat reacts
with most of the remaining free sodium in the generated gas. A typical
reaction pathway is as follows [6];-
300C
2 NaN3 ------> 2 Na + 3 N2
10 Na + 2 KNO3 ------> K2O + 5 Na20 + N2
K2O + Na2O + SiO2 ------> alkaline silicate glass.
There are apparently also corn starch and talcum powder used as lubricants
in the bag, and if the bag explodes these are the powders that contaminate
people - not the toxic chemicals in the inflator.
One article quotes 160 grams of propellant for a drivers-side bag
( 60 litres of gas) and 450 grams for a passengers-side bags
( which are 3-5 times larger) . I suspect that may include all of the
above ingredients in the igniter, but not the bag lubricants.
The bag fills until it reaches slightly above atmospheric pressure, and
the manufacturers now control the bag inflation speed to 90-200mph, which
is less than the early models - because they were too violent and could
harm occupants. The actual sequence goes something like:-
0 - Impact
15 - 20 milliseconds - sensors signal severe frontal collision.
18 - 23 milliseconds - pyrotechnic squib fired
21 - 27 milliseconds - nylon bag inflates
45 - 50 milliseconds - the driver ( who has moved forward 5 inches)
slams into the fully inflated bag
85 -100 milliseconds - the driver "rides the bag down" as the air
cushion deflates.
Recently, there have been calls to change the crash testing procedures to
allow the test dummy to be belted in, as seat belt usage is now about 67%.
Having a belted dummy would permit the use of slower inflating airbags, as
the deaths of 30 children ( up to Dec. 1996 ) have been attributed to the
speed of inflation of the larger passenger-side bag. Early in 1997, the
US NHTSA finally permitted depowering and/or disabling of passenger-side
airbags. A major airbag supplier is Breed Automotive, Boonton Township, N.J.
More details can be found in specialist articles [7-9], and research is
continuing into alternative inflation mechanisms, such as compressed gases.
There has been extensive work over the last decade on "hybrid" airbag
systems. These two-stage systems often use cylinders of compressed gas,
which can be released at ambient temperatures for situations where low-speed
deployment is appropriate, or the gas can be rapidly heated for high-speed
deployment.
31.6 How hazardous is spilt mercury?.
First step - ensure any broken thermometer actually contained mercury, as
many only contain alcohol. Mercury has an appreciable vapour pressure at
ambient temperatures, thus if the mercury has split somewhere warm and with
limited air circulation, then vapour concentrations can accumulate. When
mercury drops any distance onto a surface, it splatters into hundreds of
minute globules, resulting in a large surface area. The major hazard is
the mercury vapour produced from the spill. Mercury usually ends up in carpet
or cracks in the surface, and so really is only a significant hazard to
children crawling around the floor. Do not over-react. If the location is
relatively cool and well-ventilated, there is little danger to adults. Remove
as much mercury as conveniently possible, and just remember when toddlers
come visiting that there is a slight potential hazard if the area is not
well-ventilated and is warm. Obviously, if you increase the ventilation, the
concentrations will decrease faster. The USA ACGIH TLV for mercury vapour is
0.05mg/m3, whilst the DFG ( Germany ) limit is 0.01mg/m3, and the vapour
pressure of mercury at 25C is 0.0018mm. At 25C, the equilibrium concentration
would be about 20mg/m3, which is 400 times the permitted TLV. It is unlikely
that this equilibrium would be reached in areas where there are significant
airflows, unless the mercury had been finely dispersed ( as in a blown
manometer, or dropped onto a very rough surface ).
Mercury vapour is rapidly oxidised to divalent ionic mercury by the tissues
of the body. Human volunteers exposed to tracer doses of elemental Hg
demonstrated first order kinetics for excretion with a half life of 60 days.
The lethal concentration for humans is apparently not known, but acute
mercurialism has resulted from exposures to concentrations within the range
1.2 - 8.5mg/m3. The human organism is able to absorb and excrete substantial
amounts of mercury, in some cases as high as 2 mg/day without exhibiting
any abnormal symptoms or physical signs [10].
The Dietary uptake for mercury was estimated to be :-
3 micrograms/day Adults
1 " " young children
1 " " infants.
and the adult uptake was estimated to comprise of
0.3 air via Hg(0),
0.1 water via Hg(2+),
3 food via Hg(CH3Hg+).
( EPA Mercury Criteria Document 1979 )
The CRC Handbook of Laboratory Safety [11] has a chapter on mercury hazards.
A good discussion of mercury ( and other metals ) is found in "Metals and
their Compounds in the Environment: Occurrence, Analysis and Biological
Relevance" [12].
The best method of removing spilt mercury is to use a vacuum with a flask
and pasteur pipette and chase the little globules around the floor while not
breathing :-). Seriously, a simple vacuum system, or even a pasteur pipette,
can remove most of the large globules. There are special commercial vacuum
cleaners, but never use a household one - as the expelled air will contain
mercury vapour, and the fine metal globules will contaminate the cleaner.
For nooks, crannies, and cracks - where the mercury is likely to remain
undisturbed, you can either apply flowers of sulfur ( fine elemental sulfur )
or zinc dust, with vigorous brushing to facilitate contact, and sweep up the
excess. If the mercury is going to be re-exposed ( by cleaning, foot traffic
etc., ), then the zinc dust may be preferred because of an apparently faster
reaction rate. However, if you have a light-coloured carpet, pouring yellow
or grey powder is not usually an option, and if the location is warm and not
well-ventilated near ground level, ensure that toddlers do not spend hours
every day playing there.
There have been several studies on the best methods to clean up spills,
including "Vaporisation of Mercury spillage" [13]. The abstract reports " A
report on an investigation of the problem in laboratories and industries of
mercury (Hg) vaporisation from small droplets in cracks and floors. The
efficacy of other fixing agents besides flowers of sulfur was metered.
The results show that the use of a sulfur, calcium oxide and water mixture
was the most successful mixture for fixing mercury droplets. A second
convenient technique is the use of an aerosol hair spray. A chelating soap
is available in some countries, and this would presumably be the method of
choice in dealing with spillages."
Another article includes methods based on amalgamating with zinc impregnated
in a metal sponge or scrubbing pad for picking up mercury [14], and another
investigates substances that can be used to remove spilled mercury - such as
iodised activated carbon, copper or zinc powders, molecular sieves of copper
or silver ions, and silica gel [15].
Dental amalgam is apparently a finely divided powder of a silver, tin,
and copper alloy that is mixed with the mercury. The setting time probably
is a function of the slow dissolution of the alloy in the mercury due to
the particle size of the powder used. The mass % of each individual metal
amalgam when mercury is saturated at 20C is Ag = 0.04, Cu = 0.0032, and
Sn = 0.62, but I've no idea if that is the ratio actually used. I presume
the ratio may be varied to obtain the desired physical properties, and that
there would be a theoretical excess of the alloy to ensure minimal free
mercury. The actual amount of mercury vapour from dental amalgam is low, but
directly measurable by sensitive mercury vapour analysers. The significance
of mercury vapour from dental amalgam to health has been very controversial,
however there are now practical alternatives in widespread use.
31.7 Did molasses really kill 21 people in Boston?.
From: mica@world.std.com (mitchell swartz) Date: Sun, 4 Jul 1993
Subject: Molasses Accident
[excerpt from the Book of Lists #3 (Wallace et alia)]
THE GREAT BOSTON MOLASSES FLOOD
"On Jan. 15, 1919, the workers and residents of Boston's North End, mostly
Irish and Italian, were out enjoying the noontime sun of an unseasonably
warm day. Suddenly, with only a low rumble of warning, the huge cast-iron
tank of the Purity Distilling Company burst open and a great wave of raw
black molasses, two stories high, poured down Commercial Street and oozed
into the adjacent waterfront area. Neither pedestrians nor horse-drawn
wagons could outrun it. Two million gallons of molasses, originally
destined for rum, engulfed scores of persons - 21 men, women, and children
died of drowning or suffocation, while another 150 were injured. Buildings
crumbled, and an elevated train track collapsed. Those horses not
completely swallowed up were so trapped in the goo they had to be shot by
the police. Sightseers who came to see the chaos couldn't help but walk in
the molasses. On their way home they spread the sticky substance throughout
the city. Boston smelled of molasses for a week, and the harbor ran brown
until summer."
From this we see 21 people were killed, the half life was fairly short for
the contaminants. Long term effects were probably negligible.
31.8 What is the active ingredient in mothballs?.
Mothballs were originally made from camphor ( C10H16O, [76-22-2], MP 176C,
BP 204C ), or naphthalene ( C10H8, [91-20-3], MP 82C, BP 218C ),
but para-dichlorobenzene ( C6H4Cl2, [106-46-7], MP 55C, BP 173C ), became
cheaply available as an unwanted by-product of ortho-dichlorobenzene
production, and thus became the most common active ingredient. However
para-dichlorobenzene is also a suspected carcinogen, and naphthalene
has again become a common active ingredient. Consequently, the best
method of finding the active ingredient is to read the label on the packet,
Note that adding mothballs to modern gasolines will not increase the octane
rating of the fuel - refer to the Gasoline FAQ posted in rec.autos.tech for
more details.
31.9 Is vinegar just acetic acid?.
Most countries have food regulations that permit the use of acetic acid as
clearly-labelled "synthetic white vinegar". Most vinegars are actually malt
vinegars ( fermented ), and synthetic acetic acid is not allowed to be sold
as Malt Vinegar. Most natural, unfortified, malt vinegars are appropriately
labelled. The classification can get rather messy when bulk suppliers dilute
malt vinegar concentrates with acetic acid, which itself could either be
synthetic, or from another fermentation process. Regulations usually require
any addition of acetic acid to be clearly marked on the label, and the
product is not normally legally sold as pure "malt vinegar". The amount of
acetic acid in "natural" malt, cider, or wine vinegars usually ranges from
4% - 6%, but some examples can have up to approximately 20%. Vinegar is
produced by the exothermic aerobic bacterial oxidation of ethanol to acetic
acid via acetaldehyde.
31.10 What are the different grades of laboratory water?.
There are several techniques used in chemical laboratories to obtain the
required purity of water. There are several grading systems for water, but
the most well-known is the ASTM system, although certain applications (HPLC)
often require purer water than ASTM Type I, consequently additional
treatments such as ultrafiltration and UV oxidation may also be used to
reduce concentrations of uncontrolled impurities, such as organics.
ASTM Type I II III
Specific Conductance (max. uMhos/cm.) <0.06 <1.0 <1.0
Specific Resistance (min. Mohms/cm.) >16.67 >1.0 >1.0
Total Matter ( max. mg/l ) <0.1 <0.1 <1.0
Silicate ( max. mg/l ) N/D N/D 0.01
KMnO4 Reduction ( min. mins ) >60.0 >60.0 >10.0
Type A B C
Colony Count (Colony forming units/ml) 0 Bacteria <10 <100
pH NA NA 6.2-7.5
The techniques to purify natural waters - which may be almost saturated
with some contaminants - are frequently used in combination to obtain high
purity laboratory water. Some purification techniques use less energy than
distilling the water, and may be used in combination where large volumes of
"pure" water are required. The design of purified water systems, and the
materials used for construction, are selected according to the important
contaminants of the water. For some applications, 316L stainless steel may
be required, whereas other applications may require polyvinylidene difluoride
and polytetrafluoroethylene materials. Systems are carefully designed to
minimise the volume of water remaining static and in "dead ends" - where
microbes could grow.
The first treatment is usually a coarse physical filtration using a depth
filter that can remove undissolved large particles and other insoluble
material in the feed water.
For smaller volumes, distillation is the pretreatment method of choice.
Distilled water is water that has been boiled in a still and the vapour
condensed to obtained distilled water. While many impurities are removed
( especially dissolved and undissolved inorganics that make water "hard",
most organisms, etc. ), some impurities do remain ( volatile and some
non-volatile organics, dissolved gases, and trace quantities of fine
particulates ). Distilled water has lost many of the ionic species that
provided a pH buffer effect so, as it dissolves some CO2 from the air
during condensation and storage, the pH moves to around 5.5 ( usually from
close to the neutral pH of 7.0 ). Distilled water has the vast majority of
impurities removed, but often those residual compounds still make it
unsuitable for demanding applications, so there are alternative methods of
purifying water to remove specific undesirable species.
The next common treatment is ion-exchange, which involves using a bed of
resin that exchanges with unwanted dissolved species, such as those that
cause "hardness" ( calcium, magnesium ) in water. Two resins are used, one
that exchanges anions ( usually a strong anion exchanger such as Amberlite
IRA-400 - a quaternary ammonium compound on polystyrene ), and one that
exchanges cations ( usually a strong cation exchanger such as Amberlite
IR-120 - a sulfonic acid compound on polystyrene ). These resins can also
be combined in "mixed bed" resins, such as Amberlite MB-1A, which is a
mixture of IRA-400 [OH- form] and IR-120 [H+ form]. The porosity of the
polystyrene-based resin is dependant on the amount of cross-linking, which
is, in turn, dependant on the proportion of divinyl benzene used in the
process. Unfortunately, selectivity of a highly porous resin is inferior
to that of a less porous, more cross-linked, resin, so a balance between
the rate of exchange and the selectivity is sought. Agarose, cellulose,
or dextran can be used in place of the polystyrene base. Sophisticated
systems can have many beds in sequence, using both stronger and weaker
ion exchange resins.
The exchange potential for ions depends on a number of factors, including
molecular size, valency and concentration. In dilute solutions, exchange
potentials increase with increasing valency, but in concentrated solutions
the effect of valency is reversed, favouring the absorption of univalent
ions rather than polyvalent ions. This explains why calcium and magnesium
can be strongly absorbed from feedwater in softening processes, but then are
easily removed from the ion exchange resin when concentrated sodium chloride
is used as regenerant. In dilute solutions, the order of common anion
exchange potentials on strong anion exchangers is sulfate > chromate >
citrate > nitrate > phosphate > iodide > chloride. In dilute solutions, the
order of common cation exchange potentials on strong cation exchangers is
Fe3+ > Al2+ > Ba2+ > Pb2+ > Ca2+ > Cu2+ > Zn2+ = Mg2+ > NH4+ = K+ > Na+ >
H+ > Hg2+.
There are two forms of ion exchange for water purification. To "deionise"
feed water, the resins are in the OH- ( anion exchanger ) and H+ ( cation
exchanger ) forms. If sodium chloride was present in the feed water, the
sodium ion would displace the hydrogen ion from the cation resin, while
the chloride would displace the hydroxyl ion from the anion resin. The
displaced ions can combine to form water. Separate beds of resins can be
regenerated using 1 Normal acid ( HCl or H2SO4 ) for strongly-acid cation
resins, or 1 Normal sodium hydroxide for strongly-basic anion resins.
The amount of regenerant is approximately 150 - 500% of the theoretical
exchange capacity of the bed.
If the intention is to merely "soften" the feed water to reduce deposits,
the beds can be in the Cl- ( anion exchanger ) and Na+ ( cation exchanger )
forms. These are replaced by the dilute polyvalent species in the water that
rapidly form undesirable insoluble deposits as process water evaporates,
like calcium, magnesium and sulfate. The beds can be regenerated by passing
highly concentrated salt ( sodium chloride ) solutions through them until
all the polyvalent ions on the resins have been replaced. This technique
produces "soft" process water that used in industry.
When a dilute feedwater solution containing salt passes through a cation
exchange resin bed in the hydrogen form, the reaction that occurs is:-
Na+ + Cl + R.SO3H <=> H+ + Cl- + R.SO3Na
Obviously, the acidity of the water strongly increases as it moves down the
bed, which inhibits the exchange process. If a mixed bed is used, the
products soon encounter the anion exchange resin and are also removed:-
H+ + Cl- + R.NH2 <=> R.NH3 + Cl-
H+ + Cl- + R.NH3OH <=> R.NH3 + Cl- + H2O
Mixed bed resins are usually more efficient than equivalent single beds.
If the water feeding the resin beds has already been distilled ( very common
in laboratories - the resin beds then last much, much longer, and the
distillation has also removed other impurities ), then the water is called
"distilled and deionised". Laboratory water that has had most of the ionic
impurities removed will have a high electrical resistance, and is often known
as "18.3 megohm" water because the electrical resistance is >18,300,000
ohm/cm, but note that non-ionic impurities may still be present.
An alternative process that has increasingly replaced ion-exchange is
reverse-osmosis, which uses osmotic pressure across special membranes to
remove most of the impurities. It is called reverse-osmosis because the feed
side is pressurised to drive the purified water through the membrane in the
opposite direction than would occur if both sides were the same pressure.
The two common membrane materials are cellulose acetate or polysulfone
coated with polyamine, and typical rejection characteristics are:-
Monovalent Divalent Pyrogens, Bacteria
Ions Ions Organics > 200 MW
Cellulose Acetate >88% >94% >99%
Polyamine >90% >95% >99%
The huge advantage of RO is that membranes can easily be maintained
( occasional chemical sterilisations ), are largely self-cleaning, and can
produce large amounts of water with no chemical regeneration and minimal
energy requirements - just the pressure ( 200 psi ) required to push the
water along the membrane surfaces and improve the osmotic yield. RO is
commonly used as a pretreatment stage when very pure water is required, and
for situations where large volumes of reasonably pure water are required.
Organic species and free chlorine are usually removed from water by passing
the water through a bed of activated carbon where they form a low energy
chemical link with the carbon. These filters are often installed upstream
of the ion-exchange and reverse osmosis stages to protect them from chlorine
and organics in the feed water. Polyamine RO membranes require feedwater
containing <0.1ppm free chlorine, however cellulose acetate membranes can
tolerate up to 1.5ppm free chlorine.
The final stage of producing "pure" laboratory water usually involves
passing the deionised water through a 0.22um filter, which is sufficiently
small to remove the vast majority of organisms ( the smallest known
bacterium is around 0.3um ), thus sterilising the water.
Recently, ultrafiltration has become popular as a means of reducing pyrogens
( they are usually lipopolysaccharides from the degradation of gram negative
bacteria ). They are measured by either injecting a sample into test rabbits
and measuring body temperature increase or by the more sensitive Limulus
Amebocyte Lysate (LAL) test. The internal membrane of an ultrafiltration
system has a pore size of <0.005um. This will remove most particles,
colloidal silica, and high MW organics such as pyrogens, down to about
10,000MW. These are usually for cell-culture and DNA research, and are
located at the point of use, however the ultrafiltration unit has to be
regularly sanitized to prevent microbial growth.
Ultraviolet irradiation can be used as a bactericide (254nm) or to destroy
organics by photo-oxidation (185nm). The water is exposed to UV for periods
up to 30 minutes, and the UV interacts with dissolved oxygen to produce
ozone. The ozone promotes hydroxyl radical formation, which result in the
destruction of organic material. Usually a high intensity, quartz mercury
vapour lamp is used, and is followed by an ion exchange and organic scavenger
cartridge to collect decomposition products. The product water is very low in
total organic carbon.
Dissolved gases can be removed by passing the water through a vacuum
degassing module that utilises an inert, gas-permeable membrane surrounded
by a vacuum to remove dissolved gases from the water.
The purest laboratory water is usually obtained after passing through a
system that can include reverse osmosis or distillation of the feed water,
followed by activated carbon to remove chlorine and organics. The water is
passed through ion exchange resins to remove inorganic ions, through a
UV oxidation stage, followed by a combined ion exchange and organic scavenger
cartridge, and finally through a 0.22um filter. An additional stage of vacuum
degassing to remove dissolved gases may be added for some applications - such
as for semiconductor production.
These pure water systems are regarded as " point-of-use ", because it is
extremely difficult to prevent the reintroduction of contamination during
storage and distribution. The water is commonly known as " 18.3 Megohm "
water, because it has a specific resistance greater than 18.3 Megohm-cm
at 25C. It also contains < 5 ppb of total organic carbon, < 10 ppb of total
dissolved solids, and < 1 colony forming unit / mL of micro-organisms.
Details of laboratory and industrial water-purification processes are
available in the catalogues of equipment suppliers such as Barnstead [16]
and Millipore [17].
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