pH and ELECTRICAL CONDUCTIVITY

Reagents
Calcium chloride (CaCl2.2H2O)
Potassium chloride (KCl)

Preparation of Reagents
0.02M CaCl2:  Dissolve 2.94g CaCl2.2H2O per liter in deionized water (or 2.22g CaCl anhydrous) using a volumetric flask and make to volume.
1.0N KCl:  Dissolve 74.56g KCl per liter in deionized water using a volumetric flask and make to volume.

Sample Preparation and Procedures
Weigh, in duplicate, 25g soil (10g for organic) into plastic cups. Add 25mL deionized water to one set and 25mL 1.0N KCl to the duplicates. Let stand for one hour, stirring at regular intervals.

While samples are standing, turn on conductivity meter to allow for warm up. Calibrate pH meter.

Measure conductivity of water samples after calibrating meter. Pour supernatant into 25mL beaker or plastic vial to obtain reading to avoid error due to soil contacting the sensor electrodes. Reading is in microhms per centimeter.

Measure pH of soil-water supernatant after reading the conductivity. Return supernatant to cup. Add 25mL of 0.02M CaCl2 to water samples. Let stand for 15 minutes stirring twice. Read pH in plastic cup since there is sufficient supernatant to immerse electrodes without contacting the soil.

Measure pH of KCl samples as in water pH.

This determination should proceed in the below order:

KCl pH
Electrical conductivity
Water pH
CaCl2 pH


Sample Calculations
(microhms reading)(0.01)(dilution factor) = mmho/cm
0.001 is a factor based on the cell constant of the electrode used.

The dilution factor is for other than 1:1 soil ratio.

Additional Notes
For most soils the pH values will be: H2O>CaCl2>KCl
Ratios other than 1:1 do not significantly affect the pH values.

References
Soil Survey Laboratory Methods Manual. August 1992.  p.388.  U.S. Department of Agriculture.