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5.0 REAGENTS 5.1 Reage t grade chemicals shall be used in all tests. Unless otherwise indicated, it is intended that all r agents 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 Reage t water. All references to water in this method refer to reagent water, as defined in Chapter in e. 5.3 Prima 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 reference solutions from these salts requires some special precautions and handling, such as low-conductivity dilution water, drying ovens, and carbon-dioxide-free purge gas. These solution should be replaced at least once each month. 5.4 Second ry standard buffers may be prepared from NIST salts or purchased as solutions from com ercial vendors. These commercially available solutions, which have been validated by compari on with NIST standards, are recommended for routine use. 6.0 SAMPLE PREISERVATION AND HANDLING Samples shou�d be analyzed as soon as possible. 7.0 PROCEDUREI 7.1 Calibrat on 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 with the operation of each system and familiar with all instrument functions. Special attention to care of the electrodes is recommended. 7.1.2 Each instrument/electrode system must be calibrated at a minimum of two points that bracket the expected pH of the samples and are approximately three pH units or more apart. Repeat adjustments on successive portions of the two buffer solutions until eadings are within 0.05 pH units of the buffer solution value. If an accurate pH re ding based on the conventional pH scale [0 to 14 at 25 •C] is required, the analyst sh uld control sample temperature at 25 ± 1 •E when sample pH approaches the alkaline en of the scale (e.g., a pH of 11 or above). 7.2 Sample reparation and pH measurement of soils: 7.2.1 To 20 g of soil in a 50-mL beaker, add 20 mL of reagent water, cover, and continuously stir the suspension for 5 min. Additional dilutions are allowed if working with hygroscopic sof Is and salts or other problematic matrices. 7.2.2 I Let the soil suspension stand for about 1 hr to allow most of the suspended cla to settle out from the suspension or filter or centrifuge off the aqueous phase for pH m asurement. 9045D - 2 Revision 4 November 2004