Hydrology Field Methods

Specific Conductance - Calibration Solution

Purpose:

This solution is used to calibrate specific conductance water quality sampling probes.
The objective is to generate a solution that is approximately equal to the specific conductance expected under field conditions.

Supplies and Equipment:

Drying plate
Weighing paper
KCl, reagent grade
1-L beaker
Deionized water
Label and pen
Precision balance (±0.001 g precision)
General balance (±0.1 g precision)

Procedure:

Oven-dry a small quantity of KCl, approximately 0.5 g, on a drying plate
Place a label on the beaker.
Record the Sample ID on the label along with the date.
Record the Sample ID in Column (1) and the date in Column (2) of the reporting form.
Record the mass of a weighing paper in Column (3) using the precision balance.
Weigh out a small quantity of KCl (between 0.05 and 0.10 g) onto the weighing paper. The less KCl added, the lower the conductance.
Record the mass of the weighing paper plus KCl in Column (4).
Transfer the KCl from the weighing paper to the beaker.
Record the final mass of the weighing paper in Column (5).
Record the mass of added KCl in Column (6) by subtracting Column (5) from Column (4).
Record the mass of the beaker plus KCl in Column (7) using the general balance.
Add approximately 500 to 1,000 g of deionized water to the beaker. The more water added, the lower the specific conductance.
Record the mass of the beaker, KCl, and water in Column (8).
Record the mass of added water in Column (9) by subtracting Column (7) from Column (8).
Record the KCl concentration in Column (10) by dividing Column (6) by Column (9) and multiplying by 1,000,000, or C[ppm] = 1,000,000 × C[g-KCl / g-water].
Record the specific conductance in Column (11) by multiplying Column (10) by 1.8885, or SC[µS/cm] = 1.8885 C[ppm]
Record the specific conductance on the beaker label.
Allow the KCl to dissolve completely before using.

Worksheet used to derive relationship between KCl concentration and Specific Conductance

KCl Concentration			Specific Conductance at 25°C
M	g/kg	mg/kg	S/m	µS/cm
0.01	0.74551	745.51	0.14079	1,407.9

Source: Wu, Y.C., K.W. Pratt, and W.F. Koch, 1989, Determination of the Absolute Specific Conductance of Primary Standard KCl Solutions, J Solution Chem., 18(6):515-528. Cited in CRC, 1990, Handbook of Chemistry and Physics, 71st Edition, p. 5-94

Specific Conductance Form

Sample ID	Date	KCl Measurements (precision balance, g)				Water Measurements (general balance, g)			KCl Concentration (ppm, mg/kg)	Specific Conductance (µS/cm)
Sample ID	Date	Initial Tare Mass	KCl + Tare Mass	Final Tare Mass	KCl Mass	Initial Tare Mass	Water + Tare Mass	Water Mass	KCl Concentration (ppm, mg/kg)	Specific Conductance (µS/cm)
(1)	(2)	(3)	(4)	(5)	(6)= (4) - (5)	(7)	(8)	(9)= (8) - (7)	(10) = 10⁶ × (6) / (9)	(11) = 1.8885 × (10)