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NOTE: Combination electrodes incorporating both measuring and referenced functions are convenient to use and are available with solid, gel-type filling materials that require minimal malintenance. 4.4 Magnetic stirrer and Teflon-coated stirring bar. 4.5 Thermometer and/or temperature sensor for automatic compensation. 5.0 REAGENTS 5.1 Reagent grade chemicals shall be used in all tests. Unless otherwise indicated, it is intended that all reagents shall conform to the specifications of the Committee on Analytical Reagents of the Am rican Chemical Society, where such specifications are available. Other grades may be use , provided it is first ascertained that the reagent is of sufficiently high purity to permit its use wit out lessening the accuracy of the determination. 5.2 Primary standard buffer salts are available from the National Institute of Standards and Technology (NI T) and should be used in situations where extreme accuracy is necessary. Preparation of refer nce solutions from these salts requires some special precautions and handling, such as to -conductivity dilution water, drying ovens, and carbon-dioxide-free purge gas. These solution should be replaced at least once each month. 5.3 Second ry standard buffers may be prepared from NIST salts or purchased as solutions from co m ercial vendors. These commercially available solutions have been validated by compar son with NIST standards and are recommended for routine use. 6.0 SAMPLE COLLECTION, PRESERVATION, AND HANDLING Samples shoul be analyzed as soon as possible. 7.0 PROCEDURE 7.1 Calibrati n 7.1.1 Because of the wide variety of pH meters and accessories, detailed operating proc dures cannot be incorporated into this method. Each analyst must be acquainted wit the operation of each system and familiar with all instrument functions. Special attenti n to care of the electrodes is recommended. 7.1.2 Each instrument/electrode system must be calibrated at a minimum of two points that racket the expected pH of the samples and are approximately three pH units or more apart. (For corrosivity characterization, the calibration of the pH meter should include 6 buffer of pH 2 for acidic wastes and a pH 12 buffer for caustic wastes; also, for corrosivity characterization, the sample must be measured at 25 ± 1 •C if the pH of the waste is Bove 12.0.) Various instrument designs may involve use of a dial (to "balance" or "st ndardize") or a slope adjustment, as outlined in the manufacturer's instructions. R peat adjustments on successive portions of the two buffer solutions until readings are wi I hin 0.05 pH units of the buffer solution value. 7.2 Place thsample or buffer solution in a clean glass beaker using a sufficient volume to cover the sensinglements of the electrodes and to give adequate clearance for the 9040C - 2 Revision 3 November 2004