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, inorganic, 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 reverse osmosis, distillation, and deionization can produce reagent water, as can ultrafiltration and/or ultraviolet irradiation. 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 remove. For details on preparing water for microbiological tests, see Section 9020B.4d. * Reviewed by Standard Methods Committee, 2011. 1080 B. Methods for Preparing Reagent-Grade Water 1. Distillation acid solution. Remove CO2 by boiling the water for 15 min and cooling rapidly to room temperature; exclude atmospheric CO2 by using a tube containing soda lime or a commercially available CO2-removing agent.* 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 * 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 Deionization Reverse osmosis Carbon adsorption Filtration Ultrafiltration Ultraviolet oxidation G–E† E G‡ P P P P P E P P§ P P P G P G G–E㛳 P G# G–E** E P E P E E P E P E P E E G†† E P E P P E 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 CO2 and sometimes H2S, NH3, 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. 1 REAGENT WATER (1080)/Reagent Water Quality Impurities may be added to the water during boiling if they leach from the container. Also, freshly replaced filters, cartridges, and resins initially can release impurities. Pretreat feedwater and maintain still periodically to minimize scale formation. Pretreatment may be required if the feedwater contains significant concentrations of calcium, magnesium, and bicarbonate ions; it may involve demineralization via reverse osmosis or ion exchange. 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 regeneration 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, remove the organics first to minimize the potential for resin fouling. Organics can be removed via prefiltration, distillation, reverse osmosis, or adsorption. If using commercially prepared resin columns, follow supplier’s recommendations for monitoring QC of reagent water from specific equipment. 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 spiralwound and hollow-fiber configurations; the choice depends on the feedwater’s characteristics and fouling potential. Obtain rejection data for feedwater contaminants (levels of salt and impurities 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 permeate (reagent water) quality. Pretreatment steps (e.g., filtration) may be needed to minimize membrane fouling (due to colloids or particulates) and/or degradation (due to chlorine, iron, and other oxidizing compounds). Also, the membrane modules need to be backwashed periodically to clean the surface of the membranes. If using a commercially available reverse osmosis system, follow manufacturer’s instructions for quality control (QC) and maintenance. 4. Adsorption In adsorption, water is fed into a reactor filled with an adsorbent material (typically, granular activated carbon, although 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 efficiency is inversely proportional to the solubility of the organics in water and the adsorption process may be inadequate for removing low-molecular-weight, polar compounds. Performance differences among activated carbons are attributable 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 recommended flow and QC steps. Using activated carbon may adversely affect the reagent water’s resistivity. This effect may be controlled via reverse osmosis, 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). 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 substituted with specific ions on the resins (ones acceptable in treated water systems), thereby purifying the water. To prepare deionized water, direct feedwater through a mixed-bed ion exchanger, which contains both strong anion and strong cation resins. Proper 1080 C. Reagent Water Quality 1. Quality Guidelines deionization, or reverse osmosis of feedwater followed by mixed-bed deionization and membrane filtration (0.2-m pore). Guidelines for reagent water vary with the intended use.1 Table 1080:II lists some characteristics of various qualities of reagent water. In general, low-quality reagent water has a minimum 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 distillation 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, TABLE 1080:II. REAGENT WATER SPECIFICATIONS Quality Parameter Resistivity, megohm-cm at 25°C Conductivity, mho/cm at 25°C SiO2, mg/L 2 High ⬎10 Medium ⬎1 ⬍0.1 ⬍1 ⬍0.05 ⬍0.1 Low ⬎0.1 ⬍10 ⬍1 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 resistivity (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 contaminants 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 significantly. 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. 3