Provisional Method for Carbonate, Dissolved; Bicarbonate, Dissolved; and Carbonate Alkalinity, Dissolved; Electrometric Tritration, Incremental, Field In Reply Refer To: December 11, 1981 EGS-Mail Stop 412 QUALITY OF WATER BRANCH TECHNICAL MEMORANDUM 82.05 Subject: WATER QUALITY--Provisional Method for Carbonate, Dissolved; Bicarbonate, Dissolved; and Carbonate Alkalinity, Dissolved; Electrometric Tritration, Incremental, Field Attached is a description of the incremental field tritration for carbonate, bicarbonate, and carbonate alkalinity, which is similar to the method described by Barnes in U.S. Geological Survey Water-Supply Paper 1535-H (1964). The method of Barnes was referred to in Quality of Water Branch Technical Memorandums 80.27 and 81.04 as the only acceptable method for determination of the carbonate species. This method is provisional and will remain so until the precisions of the determinations in the field have been established. We would urge the users of this method to assist in the determination of the field precision in the following manner. At an appropriate sampling site when the streamflow is stable, collect six replicate samples and determine the carbonate, bicarbonate, and carbonate alkalinity in each by this method. Report the original data plus any statistical analyses you may choose to make to this office. We would also appreciate any comments, discussions of difficulties, or suggestions for any improvements that might be made to the method description. After further review and incorportation of any comments, the method will be published in the forthcoming Field Methods TWRI. R. J. Pickering Chief, Quality of Water Branch Attachment This memorandum supplements Quality of Water Branch Technical Memorandums 80.27 and 81.04 Key words: water quality, analytical methods, carbonate, bicarbonate, carbonate alkalinity WRD Distribution: A, B, S, FO, PO PROVISIONAL METHOD Carbonate, Dissolved; Bicarbonate Dissolved; Carbonate Alkalinity, Dissolved; Electrometric Titration, Incremental, Field Parameters and Codes: Carbonate, dissolved (mg/L as C03): 99445 Bicarbonate, dissolved (mg/L as HC03): 99440 Carbonate Alkalinity, dissolved (mg/L as CaC03): 99430 1. Application 1.1 This procedure is applicable to surface and ground waters, either filtered or unfiltered, although an unfiltered sample is preferred (see Sections 3.4 and 3.5). 1.2 Accurate values for pH, and carbonate and bicarbonate ion concentrations in an aqueous solution are essential in studies involving carbonate chemistry and equilibrium calculations. These parameters are especially subject to rapid changes in water samples owing to loss of dissolved gases. Determinations by this procedure are to be made in the field to minimize exchange of carbon dioxide gas between the sample and its surroundings. 2. Summary of Method Although method I-1030-78 (Skougstad and others, 1979), a fixed-pH endpoint (pH 4.5) titration, is commonly used to determine alkalinity, accurate determination of alkalinity due to carbonate species, as well as determination of the concentrations of those species, can be made by constructing a titration curve from measurements of pH versus volume of strong acid titrant added to the sample in small increments. The endpoints for titration of successive proton absorbing species are taken as the inflection points of the titration curve, or as the maximum rates of change of pH per volume of titrant added (see fig. 1). A detailed description of the theory of potentiometric titration of carbonate and bicarbonate, sampling procedures, and the occurrence of alkalinity, are given in Barnes (1964), Wood (1976), and Hem (1970). 3. Interferences 3.1 Any ionized substance that reacts with a strong acid can contribute to alkalinity if the reaction occurs at a pH above that of the specified endpoint; examples are salts of weak organic and inorganic acids. The weak inorganic bases SiO(OH)3, H2B04, NH3 and Al(OH)2(H20)4+ will accept a hydrogen ion in the titration to the first endpoint near pH 8.3. The weak inorganic bases H2P04 and Al(OH)(H20)5 as well as most of the weak organic bases found in natural waters will accept a hydrogen ion in the titration between the first endpoint and the second endpoint near pH 4.5. Corrections for these interferences may be required. 3.2 The hydroxide ion (OH-) will contribute significant alkalinity when the initial sample pH exceeds 11.0 units. In this case, a correction for the OH- ion should be applied to the calculation of carbonate. 3.3 All surface waters carry particulate matter ranging in concentrations from several lO's of milligrams per liter to several lOO's of grams per liter. Compostion varies from mainly organic to mainly inorganic rock fragments and is largely controlled by the geologic terrain traversed by the stream. Particulates can take up some strong acid by dissolution, adsorption, or ion exchange and, thereby, cause anomalously high measurements. For this reason, filtration through a 0.45 micron pressure-type stainless steel barrel filter using an inert gas such as nitrogen or argon for pressure may be needed for some samples. Experience indicates that small amounts of particulate matter do not interfere appreciably. The effect of particulate matter should be determined by comparing shapes of and analytical values obtained from titration curves determined on filtered versus unfiltered samples. 3.4 Ground water obtained from wells usually has much less particulate matter than surface water but may contain sufficient particulate matter to justify filtration. For discussion of ground-water sample filtration see Wood (1976). An in-line filter that permits exclusion of the atmosphere during filtration is recommended. 3.5 Wind-borne dust can be a source of contamination of the sample and direct sunlight can warm the sample appreciably, causing loss of carbon dioxide. Every precaution should be taken to minimize these effects. The field procedure is best performed in an enclosed van-type vehicle containing all the reagents and equipment required for on-site determination of unstable constituents. 4. Apparatus 4.1 Beakers, 150 mL capacity, glass or disposable plastic. 4.2 Bottle, 500 mL capacity, plastic squeeze, for rinsing with distilled or deionized water. 4.3 Buret, 25-mL capacity with 0.1 mL graduations. A lO-mL semi-micro buret with .02 graduations may be used for samples containing less than 200 mg/L of carbonate and bicarbonate. 4.4 Buret stand and holder. 4.5 pH electrode, combination; Orion 91-62 or equivalent and filling solution. 4.6 pH meter, battery operated, with expanded scale or scale length of at least 15 cm for detailed work of +0.02 pH units. 4.7 Pipets, class A volumetric, 50 mL, or other appropriate volume. 4.8 Stirrer, portable magnetic with small teflon coated stirring bar. If this is not available, hand stirring with glass stirring rod or with hand-held battery-powered cocktail stirrer with glass stirring rod will suffice. 4.9 Thermometer, 0! to 50!C, graduated in 0.1!C. 5. Reagents 5.1 pH buffer solutions, pH 4, 7 and 10 accurate to +.02 units and changes with temperature specified. 5.2 Sodium carbonate standard solution, 0.01639N: 1.00 mL = 1.00 mg HC03: Dry 1.0 g primary standard Na2C03 at 150!C to 160!C for 2 h. Cool in a desiccator. Dissolve 0.8685 g in carbon dioxide-free water prepared by boiling for 15 minutes and cooling without agitation; dilute to 1,000 mL in a volumetric flask. 5.3 Sulfuric acid standard solution, 0.01639N: 1.00 mL = 1.00 mg HC03: Cautiously add 0.5 mL concentrated H2S04 (sp gr 1.84) to 950 mL water. After the solution has been thoroughly mixed, standardize by titrating 25.00 mL Na2C03 standard solution to pH 4.5. Adjust the concentration of the sulfuric acid standard solution to exactly 0.01639N by dilution with water or by addition of dilute acid as indicated by the first titration. Confirm the exact normality by restandardization. Keep the solution in a sealed glass bottle or volumetric flask until used. Although the sulfuric acid standard solution is reasonably stable if protected from ammonia fumes, the normality should be verified at least monthly (NOTE 1). NOTE 1. It may be found more convenient to prepare standard sulfuric acid that is not exactly 0.01639N but the exact normality of which is known. Such standards may be used if the appropriate factor is applied in the calculations. 6. Procedure 6.1 Standardize the pH meter as described in method I-1586-78 (Skougstad and others, 1979). 6.2 Rinse electrode thoroughly (at least 3 times) with sample water, and adjust temperature of titrant to +2!C of the sample temperature. 6.3 Measure and record pH value of a freshly-collected representative sample. 6.4 Pipet an appropriate volume of sample (usually not more than 50 mL) into a clean dry 150 mL beaker and insert pH probe and teflon stirring bar. (See section 3.4 and 3.5 for guidelines on need for prior filtration). 6.5 If pH is greater than 8.3, add sulfuric acid standard solution dropwise and carefully record the volume delivered in 0.02 mL increments (0.05 increments with a 25-mL burette) and record the pH after each addition until pH is below 8.0. Stir gently with magnetic stirrer or other appropriate stirring device while adding titrant and making readings. Allow 15-20 seconds for equilibration after each acid addition. 6.6 If initial pH is less than 8.3, skip step 6.5 and go directly to step 6.7 6.7 Titrate rapidly to pH 5.0 and record the volume of titrant at pH 5.0 to the nearest 0.02 mL (0.05 mL on 25-mL buret). Allow 15-20 seconds for equilibration. 6.8 From pH 5.0 to 4.0, add acid dropwise in 0.02 mL increments and record the pH after each addition, allowing 15-20 seconds for pH equilibration after each. The most sensitive part of the titration curve is usually between pH 4.8 and 4.3. 7. Calculations 7.1 The true endpoints are determined by either 7.lA or 7.lB. 7.lA Construct a titration curve plotting the volume of the titrant (sulfuric acid standard solution) added against pH. The endpoints are the inflection points of the curve, the points at which the pH changes are greatest for a given amount of titrant (see fig. 1). 7.lB Plot the rate of change of pH with change in titrant volume _ pH _ mL of titrant against the total volume of titrant. The endpoints are the volumes of titrant delivered at which there occur maximum rates of change of pH per volume of titrant added (see fig. 1). 7.2 Calculate carbonate C03 (mg/L) = 1000 x [mLa at ep near pH 8.3] x .9835 mLs (See NOTE 2). 7.3 Calculate bicarbonate HC03 (mg/L) = 1000 x [(mLa at ep near pH 4.5) mLs -2(mLa at ep near pH 8.3)] x 1.0 (See NOTE 2). where mLs = volume of sample in mL mLa = volume of titrant (surfuric acid standard solution) added in mL ep = endpoint determined by 7.lA or 7.lB NOTE 2: If the sulfuric acid standard solution has normality different from 0.01639N, compute new multiplying factors as follows: For C03 Factor = 0.9835 x Acid Normality .01639N For HC03 Factor = 1.00 x Acid Normality .01639N 7.4 Calculate carbonate alkalinity Carbonate Alkalinity (mg/L as CaC03) = 8. Report Report carbonate (99445), bicarbonate (99440), and carbonate alkalinity (99430) as follows: less than 1000 mg/L, whole numbers: 1000 mg/L and above, three significant figures. 9. Precision The precision of this method in the field has not been established. References Barnes, Ivan. 1964. Field measurement of alkalinity and pH. U.S. Geological Survey Water-Supply Paper 1535-H, p. 17. Hem, John D. 1970. Study and interpretation of the chemical characteristics of natural water. U.S. Geological Survey Water-Supply Paper 1473, second edition, p. 363. Skougstad, M., Fishman M., Friedman, L.C., Erdman, D.E. and Durran, S.S. (editors) 1979. Methods for determination of inorganic substances in water and fluvial sediments. Techniques of Water-Resources Investigations of the United States Geological Survey, book 5, chapter Al, p. 626. Wood, Warren W. 1976. Guidelines for collection and field analysis of groundwater samples for selected unstable constituents. Techniques of WaterResources Investigations of the United States Geological Survey, Book 1, chapter D2, p. 24.