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31.10 What are the different grades of laboratory water?


This article is from the Chemistry FAQ, by Bruce Hamilton B.Hamilton@irl.cri.nz with numerous contributions by others.

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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