1080

1080 REAGENT WATER*
1080 A. Introduction
One of the most important aspects of analysis is preparing the
reagent water used to prepare and dilute reagents and prepare
blanks. Reagent water is water with no detectable concentration
of the compound or element to be analyzed (i.e., it is below the
analytical method’s detection level). Reagent water should also
be free of substances that interfere with analytical methods.
However, its overall quality (concentrations of organic, inor-
ganic, and biological constituents) will depend on the water’s
intended use(s).
Use any method to prepare reagent water that can meet the
applicable quality requirements. Various combinations of re-
verse osmosis, distillation, and deionization can produce re-
agent water, as can ultrafiltration and/or ultraviolet irradia-
tion. Keep in mind, however, that improperly operated or
maintained water purification systems may add rather than
remove contaminants.
This section provides general guidelines for preparing reagent
water. Table 1080:I lists commonly available water purification
processes and the major classes of contaminants that they re-
move. For details on preparing water for microbiological tests,
see Section 9020B.4d.
1080 B. Methods for Preparing Reagent-Grade Water
1. Distillation
Distillation is the process of heating a liquid until it boils,
capturing and cooling the resultant hot vapors, and collecting the
condensed vapors. Laboratory-grade distilled water should be
generated in a still made of all-borosilicate glass, fused
quartz, tin, or titanium. To remove ammonia, distill from an
acid solution. Remove CO
2
by boiling the water for 15 min
and cooling rapidly to room temperature; exclude atmospheric
CO
2
by using a tube containing soda lime or a commercially
available CO
2
-removing agent.*
* Reviewed by Standard Methods Committee, 2011.
* Ascarite II, Fisher Scientific Co., or equivalent.
TABLE 1080:I. WATER PURIFICATION PROCESSES
Major Classes of Contaminants*
Process
Dissolved
Ionized Salts
Dissolved
Ionized Gases
Dissolved
Organics Particulates Bacteria
Pyrogens/
Endotoxins
Distillation G–E† P G E E E
Deionization E E P P P P
Reverse osmosis G‡ P G E E E
Carbon adsorption P P§ G–E㛳PPP
Filtration P P P E E P
Ultrafiltration P P G# E E E
Ultraviolet oxidation P P G–E** P G†† P
Permission to use this table from C3-A2, Vol. 11, No. 13, Aug. 1991, “Preparation and Testing of Reagent Water in the Clinical
Laboratory — Second Edition” has been granted by the National Committee for Clinical Laboratory Standards. The complete current standard may be obtained from
National Committee for Clinical Laboratory Standards, Lancaster Ave.,771 E. Villanova, PA 19085.
*E⫽Excellent (capable of complete or near total removal), G ⫽Good (capable of removing large percentages), P ⫽Poor (little or no removal).
† The resistivity of water purified via distillation is an order of magnitude less than that in water produced via deionization, mainly due to the presence of CO
2
and sometimes
H
2
S, NH
3
, and other ionized gases (if present in feedwater).
‡ The resistivity of dissolved ionized solids in product water depends on original feedwater resistivity.
§ Activated carbon removes chlorine via adsorption.
㛳When used with other purification processes, special grades of activated carbon and other synthetic adsorbents are excellent at removing organic contaminants. Their use,
however, is targeted toward specific compounds and applications.
# Ultrafilters reduce specific feedwater organic contaminants based on the membrane’s rated molecular weight cut-off.
** 185-nm UV oxidation (batch process) removes trace organic contaminants effectively when used as post-treatment. Feedwater makeup plays a critical role in their
performance.
†† While 254-nm UV sterilizers do not physically remove bacteria, they may have bactericidal or bacteriostatic capabilities limited by intensity, contact time, and flow rate.
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Impurities may be added to the water during boiling if they
leach from the container. Also, freshly replaced filters, car-
tridges, and resins initially can release impurities. Pretreat feed-
water and maintain still periodically to minimize scale forma-
tion. Pretreatment may be required if the feedwater contains
significant concentrations of calcium, magnesium, and bicarbon-
ate ions; it may involve demineralization via reverse osmosis or
ion exchange.
2. Reverse Osmosis
In reverse osmosis, water is forced under pressure through a
semipermeable membrane, thereby removing some dissolved
constituents and suspended impurities. The reagent water quality
will depend on both feedwater quality and the type and condition
of membranes used.
Reverse osmosis membranes are available in both spiral-
wound and hollow-fiber configurations; the choice depends on
the feedwater’s characteristics and fouling potential. Obtain re-
jection data for feedwater contaminants (levels of salt and im-
purities that will pass through the membranes compared to
feedwater levels) at the operating pressure that will be used to
prepare reagent water. Set the water-production rate to make the
most economical use of feedwater without compromising per-
meate (reagent water) quality.
Pretreatment steps (e.g., filtration) may be needed to minimize
membrane fouling (due to colloids or particulates) and/or deg-
radation (due to chlorine, iron, and other oxidizing compounds).
Also, the membrane modules need to be backwashed periodi-
cally to clean the surface of the membranes. If using a commer-
cially available reverse osmosis system, follow manufacturer’s
instructions for quality control (QC) and maintenance.
3. Ion Exchange
In an ion exchange process, water passes through a reactor
containing negatively charged (anionic) and/or positively
charged (cationic) resins. Targeted ions in the water are substi-
tuted with specific ions on the resins (ones acceptable in treated
water systems), thereby purifying the water. To prepare deion-
ized water, direct feedwater through a mixed-bed ion exchanger,
which contains both strong anion and strong cation resins. Proper
bed sizing is critical to resin performance. Be sure the bed’s
length-to-diameter ratio is in accordance with the maximum
process flow rate to ensure that optimal face velocities are not
exceeded and that residence time is sufficient.
If the system does not generate reagent water continuously,
recirculate the water through the ion exchanger. If resin regen-
eration is economically attractive, use separate anion and cation
resin beds, and position the anion exchanger downstream of the
cation exchanger to remove leachates from the cation resin. If the
feedwater contains significant quantities of organic matter, re-
move the organics first to minimize the potential for resin foul-
ing. Organics can be removed via prefiltration, distillation, re-
verse osmosis, or adsorption. If using commercially prepared
resin columns, follow supplier’s recommendations for monitor-
ing QC of reagent water from specific equipment.
4. Adsorption
In adsorption, water is fed into a reactor filled with an
adsorbent material (typically, granular activated carbon, al-
though some resins and other manmade adsorbents are used in
specific applications). Chlorine and other organic impurities
are drawn from the water to the surface of the adsorbent. How
well the process works depends on the organic contaminants
involved, the activated carbon’s physical characteristics, and
the operating conditions. In general, organics-adsorption ef-
ficiency is inversely proportional to the solubility of the
organics in water and the adsorption process may be inade-
quate for removing low-molecular-weight, polar compounds.
Performance differences among activated carbons are attrib-
utable to the raw materials and activation procedures. Even
with an optimal activated carbon, proper performance will not
be attained unless the column is sized to provide required face
velocity and residence time at the maximum process flow rate.
If using commercial sorbent systems, follow supplier’s rec-
ommended flow and QC steps.
Using activated carbon may adversely affect the reagent wa-
ter’s resistivity. This effect may be controlled via reverse osmo-
sis, mixed resins, or special adsorbents. To minimize organic
contamination, use mixtures of polishing resins with special
carbons and additional treatment steps (e.g., reverse osmosis,
natural carbons, ultraviolet oxidation, or ultrafiltration).
1080 C. Reagent Water Quality
1. Quality Guidelines
Guidelines for reagent water vary with the intended use.
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Table 1080:II lists some characteristics of various qualities of
reagent water. In general, low-quality reagent water has a min-
imum resistivity of 0.1 megohm-cm at 25°C. It may be used to
wash glassware, rinse glassware (as a preliminary step), and as a
source to produce higher-grade waters.
Medium-quality reagent water typically is produced via distilla-
tion or deionization. Resistivity should be ⬎1 megohm-cm at 25°C.
High-quality reagent water has a minimum resistivity of 10
megohms-cm at 25°C. It typically is prepared via distillation,
deionization, or reverse osmosis of feedwater followed by
mixed-bed deionization and membrane filtration (0.2-
␮ m pore).
TABLE 1080:II. REAGENT WATER SPECIFICATIONS
Quality Parameter High Medium Low
Resistivity, megohm-cm
at 25°C
⬎10 ⬎1⬎0.1
Conductivity,
␮ mho/cm
at 25°C
⬍0.1 ⬍1⬍10
SiO
2
, mg/L ⬍0.05 ⬍0.1 ⬍1
REAGENT WATER (1080)/Reagent Water Quality
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REAGENT WATER (1080)/Reagent Water Quality

It also could be prepared via reverse osmosis followed by carbon
adsorption and deionization.
Mixed-bed deionizers typically add small amounts of organic
matter to water, especially if the beds are fresh, so determine
reagent water quality immediately after preparation. Its resistiv-
ity (measured in-line) should be ⬎10 megohm-cm at 25°C.
However, resistivity measurements do not detect organics or
nonionized contaminants, nor accurately assess ionic contam-
inants at the microgram-per-liter level.
The pH of high- or medium-quality water cannot be measured
accurately without contaminating the water, so measure other
constituents required for individual tests.
High-quality water cannot be stored without degrading signif-
icantly. Medium-quality water may be stored, but keep storage
time to a minimum and make sure quality remains consistent
with the intended use. Only store it in materials that protect the
water from contamination (e.g., TFE and glass for organics
analysis or plastics for metals).
2. Reference
1. AMERICAN SOCIETY FOR TESTING AND MATERIALS. 2006. Annual Book
of ASTM Standards, Vol 11.01, D 1193-06. American Soc. Testing
& Materials, W. Conshohocken, Pa.
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REAGENT WATER (1080)/Reagent Water Quality