Document Control No.: QM-AD002 SOP Code: AP09 Issue Date: Revision No.: 04 Page 1 of 6 Title: ph MEASUREMENT

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1 SOP Code: AP09 Issue Date: Revision No.: 04 Page 1 of 6 Crop and Environmental Sciences Division Standard Operating Procedure ph MEASUREMENT -AP09 This document is issued under the authority of MS. LILIA R. MOLINA Assistant Manager II THIS IS A CONTROLLED DOCUMENT

2 SOP Code: AP09 Issue Date: Revision No.: 04 Page 2 of 6 SIGNATURE PAGE The signatures below signify approval and concurrence of the signatories with the contents of this SOP coupled with the commitment to provide the resources necessary to ensure proper quality operation of the ASL laboratory. Issuing Authority: Name & Title Signature Date (yyyy-mm-dd) Lilia R. Molina Manager/QM Christy S. Daniel Deputy Quality Manager

3 SOP Code: AP09 Issue Date: Revision No.: 04 Page 3 of 6 -AP09 ph MEASUREMENT BY THE ELECTROMETRIC METHOD 1.0 Scope and application This SOP is designed primarily for the determination of soil ph, although the procedure applies equally well for the ph determination of any aqueous solutions or waters. ASL s default method is to use deionized water as the suspending medium in a 1:1 (soil:suspending medium) ratio unless specifically requested otherwise by the customer. Soil ph is measured to assess soil chemical properties, crop suitability, lime needs and relative nutrient availability. 2.0 Summary. Soil ph is measured electrometrically in a saturated paste or a supernatant liquid that is in equilibrium with a soil suspension of a 1:1 soil-to-liquid mixture. The most popular ph method is to use water or 0.01 M calcium chloride and a soil to liquid ratio of 1:2 w/v. ASL s default method is to use a 1:1 ratio and deionized water as the suspending liquid without stirring, unless requested otherwise to use an electrolyte (1 M KCl or 0.01 M CaCl 2 ) by the customer. Soils containing greater than 15% organic matter may require a 1:5 or 1:10 soil:water ratio for the determination of ph. The method is generally reproducible within ± 0.07 ph units (Kalra, 1995). 3.0 Apparatus and materials 3.1 ph meter preferably with automatic temperature compensation (Beckman Φ 310 ph meter or equivalent) 3.2 Numbered 125 ml polyethylene bottles, with a good fitting top 3.3 Dispenser bottle 3.4 Reciprocating shaker (Everbach Model Cat. No or equivalent) 3.5 Analytical balance, with a readability of ±0.01 g

4 SOP Code: AP09 Issue Date: Revision No.: 04 Page 4 of Reagents and standards 4.1 Standard buffer solutions (ph 4.0 ± 0.01, ph 7.00 ± 0.01 and ph ± 0.01) 4.2 Verification standard buffer of known ph obtained from a different manufacturer than M Potassium chloride solution Dissolve g potassium chloride (AR Grade) in ~200 ml deionized water in a 1000 ml volumetric flask. When dissolved, make to volume with deionized water and shake well to mix M Calcium chloride solution Dissolve 1.11 g of calcium chloride (AR Grade) in about 400 ml deionized water in a 1000 ml volumetric flask. When dissolved, make to volume with deionized water and shake well to mix M KCl solution (storage and fill solution for the electrode). 5.0 Sample preparation, preservation and storage 5.1 ASL does not prepare samples. 5.2 The customers are the one responsible for preparation of their samples and may use either the procedures prescribed by ASL in QM-AD036-MP24, or their own procedures. But the customer is required to indicate the method of sample preparation when they submit the sample charge slip to ASL. The soil sample must be air dried and sieved to 2 mm mesh size. 5.3 Samples submitted by customers in brown envelope are kept in the soils storage room until taken for analysis (no preservative is added). 6.0 Procedure 6.1 Weigh out 10 g of sample (air dried, passed in a 2 mm mesh sieve) and place in a clean 125 ml polyethylene bottle. 6.2 Add 10 g or 10 ml of the suspending liquid as specified by the customer (whether deionized water, 1M KCl or 0.01 M CaCl 2 ).

5 SOP Code: AP09 Issue Date: Revision No.: 04 Page 5 of Place all of the bottles in a reciprocating shaker and shake for 30 min. at high speed ( excursions per minute). 6.4 Remove from the shaker and allow soil particles to settle for 5 minutes but not more than 30 minutes before ph measurement. 6.5 Measure the ph of the sample in a previously calibrated ph meter (calibrate ph meter using standard buffers of ph 4, 7 and 10). Result is expressed as ph up to the first decimal point. 6.6 Rinse the electrodes thoroughly in-between measurements with deionized water making sure there are no adhering particles on the electrode bulb before making further measurements. 6.7 When all measurements are done, rinse the electrodes thoroughly before storing them in 3M KCl solution. 7.0 Calculations and reporting 7.1 The ph reading is reported in not more than one decimal point. 7.2 The extracting liquid is also indicated in the analysis report. 8.0 Quality control, acceptance criteria Each analytical batch of samples should include measurement of an independent buffer solution of known ph, in-house reference material, sample repeats and a buffer for continuing calibration verification. 9.0 References 9.1 Caroll, S.D., Herron, C.G., Lafex, D.C. Determination of Soil ph. University of Arkansas Soil Testing and Research. 9.2 FAO and ISRIC Guidelines for quality management in soil and plant laboratories. FAO Soils Bulleting Jackson, M.L. Soil Chemical Analysis Prentice Hall, Inc. Inglewood Cliffs. N.J. p 46.

6 SOP Code: AP09 Issue Date: Revision No.: 04 Page 6 of Kalra, Y.P.; Maynard, D.G Methods manual for forest soil and plant analysis. For. Can., Northwest Reg., North For. Cent., Edmonton, Alberta, Canada. Inf. Rep. NOR-X Thomas GW Soil ph and soil acidity. In: Sparks DL et al, editors. Methods of soil analysis: Part 3 Chemical methods. SSSA Book Series No. 5. Madison, Wisconsin (USA): Soil Science Society of America. pp (END OF DOCUMENT)