9045D - 1 Revision 4

November 2004

METHOD 9045D

SOIL AND WASTE pH

1.0 SCOPE AND APPLICATION

1.1 This method is an electrometric procedure for measuring pH in soils and waste

samples. Wastes may be solids, sludges, or non-aqueous liquids. If water is present, it must

constitute less than 20% of the total volume of the sample.

2.0 SUMMARY OF METHOD

2.1 The sample is mixed with reagent water, and the pH of the resulting aqueous

solution is measured.

3.0 INTERFERENCES

3.1 Samples with very low or very high pH may give incorrect readings on the meter. 

For samples with a true pH of >10, the measured pH may be incorrectly low. This error can be

minimized by using a low-sodium-error electrode. Strong acid solutions, with a true pH of <1,

may give incorrectly high pH measurements.

3.2 Temperature fluctuations will cause measurement errors.

3.3 Errors will occur when the electrodes become coated. If an electrode becomes

coated with an oily material that will not rinse free, the electrode can (1) be cleaned with an

ultrasonic bath, or (2) be washed with detergent, rinsed several times with water, placed in 1:10

HCl so that the lower third of the electrode is submerged, and then thoroughly rinsed with water,

or (3) be cleaned per the manufacturer's instructions.

4.0 APPARATUS AND MATERIALS

4.1 pH meter with means for temperature compensation.

4.2 Glass electrode.

4.3 Reference electrode -- A silver-silver chloride or other reference electrode of

constant potential may be used.

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 maintenance.

4.4 Beaker -- 50-mL.

4.5 Thermometer and/or temperature sensor for automatic compensation.

4.6 Analytical balance -- capable of weighing 0.1 g.
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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 American Chemical Society, where such specifications are available. Other

grades may be used, provided it is first ascertained that the reagent is of sufficiently high purity

to permit its use without lessening the accuracy of the determination.

5.2 Reagent water. All references to water in this method refer to reagent water, as

defined in Chapter One.

5.3 Primary standard buffer salts are available from the National Institute of Standards

and Technology (NIST) 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 solutions should be replaced at least once each month.

5.4 Secondary standard buffers may be prepared from NIST salts or purchased as

solutions from commercial vendors. These commercially available solutions, which have been

validated by comparison with NIST standards, are recommended for routine use.

6.0 SAMPLE PRESERVATION AND HANDLING

Samples should be analyzed as soon as possible.

7.0 PROCEDURE

7.1 Calibration

7.1.1 Because of the wide variety of pH meters and accessories, detailed

operating procedures 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 readings are within 0.05 pH units of the buffer solution value. If an

accurate pH reading based on the conventional pH scale [0 to 14 at 25 EC] is required,

the analyst should control sample temperature at 25 ± 1 EC when sample pH approaches

the alkaline end of the scale (e.g., a pH of 11 or above).

7.2 Sample preparation 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 soils and salts or other problematic matrices. 

7.2.2 Let the soil suspension stand for about 1 hr to allow most of the

suspended clay to settle out from the suspension or filter or centrifuge off the aqueous

phase for pH measurement.
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7.2.3 Adjust the electrodes in the clamps of the electrode holder so that, upon

lowering the electrodes into the beaker, the glass electrode will be immersed just deep

enough into the clear supernatant solution to establish a good electrical contact through

the ground-glass joint or the fiber-capillary hole. Insert the electrodes into the sample

solution in this manner. For combination electrodes, immerse just below the suspension.

7.2.4 If the sample temperature differs by more than 2 EC from the buffer

solution, the measured pH values must be corrected.

7.2.5 Report the results as "soil pH measured in water at EC" where " EC" is

the temperature at which the test was conducted.

7.3 Sample preparation and pH measurement of waste materials

7.3.1 To 20 g of waste sample 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 wastes and salts or other problematic matrices. 

7.3.2 Let the waste suspension stand for about 15 min to allow most of the

suspended waste to settle out from the suspension or filter or centrifuge off aqueous

phase for pH measurement.

NOTE: If the waste is hygroscopic and absorbs all the reagent water, begin the

experiment again using 20 g of waste and 40 mL of reagent water.

NOTE: If the supernatant is multiphasic, decant the oily phase and measure the pH of

the aqueous phase. The electrode may need to be cleaned (Step 3.3) if it

becomes coated with an oily material.

7.3.3 Adjust the electrodes in the clamps of the electrode holder so that, upon

lowering the electrodes into the beaker, the glass electrode will be immersed just deep

enough into the clear supernatant to establish good electrical contact through the groundglass joint or the fiber-capillary hole. Insert the electrode into the sample solution in this

manner. For combination electrodes, immerse just below the suspension.

7.3.4 If the sample temperature differs by more than 2 EC from the buffer

solution, the measured pH values must be corrected.

7.3.5 Report the results as "waste pH measured in water at EC" where " EC"

is the temperature at which the test was conducted.

8.0 QUALITY CONTROL

8.1 Refer to Chapter One for the appropriate QC protocols.

8.2 Electrodes must be thoroughly rinsed between samples.

9.0 METHOD PERFORMANCE

9.1 No data provided.
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10.0 REFERENCES

1. Black, Charles Allen; Methods of Soil Analysis; American Society of Agronomy: 

Madison, WI, 1973.

2. National Bureau of Standards, Standard Reference Material Catalog, 1986-87, Special

Publication 260.
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November 2004

METHOD 9045D

SOIL AND WASTE pH