Environmental Engineering Reference
In-Depth Information
problems to fix, but its quality may be also one of the least understood factors in
many analytical laboratories. There are many ways to prepare pure water to meet
quality requirements for the analysis of organic, inorganic, or biological parameters.
Reagent water has been defined at three or four distinct purity levels or types by
various professional organisations including National Committee for Clinical
Laboratory Standards (NCCLS), the College of American Pathologists (CAP),
and the American Society for Testing and Materials (ASTM). Table 6.1 is a set of
water purity specifications established by ASTM. For example Type I water has
the highest quality and can be used for test methods requiring minimum interference
and bias. It is prepared by distillation, deionization, or reverse osmosis followed
by polishing with a mixed bed deionizer and passing through a 0.2-
m pore size
membrane filter. The polishing steps remove trace organics, particulate matter, and
bacteria. Type II water is prepared by distillation or deionization and can be used
if bacterial presence can be tolerated. Type III water can be used for glassware
washing, preliminary rinsing of glassware and a feed water for the production of
higher purity waters.
m
Table 6.1 ASTM reagent-grade water specification
Type I
Type II
Type III
Type IV
Total Matter, max. mg/L
0.1
0.1
1.0
2.0
Electrical conductivity, max.
0.06
1.0
1.0
5.0
mmho/cm at 25
C
Electrical Resistivity, min.
16.67
1.0
1.0
0.2
M/cm at 25
C
pH at 25
C
NS
NS
6.2-7.5
5.0-8.0
Min. color retention time
60
60
10
10
of KMNO
4
, minutes
Max. soluble silica
ND
ND
10 mg/L
No limit
*NS¼not specified. The measurements of pH in Type I and Type II reagent waters are meaningless,
since electrodes used in this test contaminate the water. ND¼not detected.
Distilled water is excellent with regard to particulates and bacteria, good in
dissolved organics, but may be poor in dissolved ionized gases. DI water is excellent
in terms of ionized species, but poor in dissolved organics, particulate and bacteria.
The highest purity reagent-grade water is equivalent to the Type I water. The reagent
water is the water with no detectable concentration of compound or element to be
analyzed at the detection limit of the analytical method (APHA, 1998). However,
this definition of reagent water has limited use.
In practice, a commonly used indicator for high quality water is the high
resistivity (
10 megaohm-cm at 25
C) or low conductivity (
mho/cm at
25
C). These parameters, however, do not tell the amount of organics or non-
ionized contaminants, nor will they provide an accurate assessment of ionic
contaminants at the ppb level. Fortunately, with most commercially available water
purification systems, obtaining a desired quality of water is not as problematic as
>
<
0.1
m
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