4.9  Determination of conductivity

The conductivity of precipitation samples depends on the concentrations of the various ion species and their different ability to transport electric charges in a solution, i.e. the ion species equivalent conductivity. This conductivity is temperature dependent and increases approximately 2% per degree in aqueous solutions for most ion species. Conductivity measurements can through comparison with estimated conductivity and in combination with ion balance calculations and records of old data help identify ion concentrations which are wrong or inaccurate.

4.9.1  Principle

Conductance is the inverse of resistance in a solution and the conductivity the inverse of specific resistance. Conductivity is measured with a bridge and a measuring cell, and it is dependent upon distance between the electrodes and their area, in the measurement cell. This is expressed by the cell constant which is a characteristic of the measurement cell. The resistance, R, can be expressed as

         

where l is distance between the electrodes and A their area. r is the specific resistivity. The specific conductance, or conductivity k is

         

where  is the cell constant.

4.9.2  Instrumentation

The conductivity meter applied should have a measurement range 1–1000 µS/cm, a precision within this range of 0.5% and an accuracy within 1%. Conductivity meters may be able to give the result at a pre-selected reference temperature while the actual measurement is carried out at room temperature. Other meters need a waterbath for the measurement cell in order to give a result at 25 °C, which is the temperature used for EMEP’s and WMO GAW’s conductivity measurements. Besides the conductivity meter itself, a platinum conductivity cell is needed, and possibly a water bath and a thermometer.

4.9.3  Chemicals

4.9.4  Calibration solutions

0.1M KCl stock solution
Transfer 7.4560 g KCl, dried at least 2 hours at 110°C, to a volumetric flask and dilute to 1000.0 ml with deionized water. The solution should be transferred to a plastic flask. The stability of the solution is one year at most.

A series of calibration solutions based on the 0.1 M KCl stock solution is used for the calibration procedure, as seen from Table 4.9.1. The solutions should be kept well closed in plastic flasks at room temperature. The stability is 6 months at most.

Table 4.9.1: Calibration standards for conductivity at 25°C.

Concentration
M KCl

Conductivity
µS/cm

Upper limit
µS/cm

Lower limit
µS/cm

0.0500

6668

6801

6535

0.0200

2767

2822

2711

0.0100

1413

1441

1395

0.0050

717.8

735

700

0.0010

147.0

149

145

0.0005

73.9

77.8

70.2

0.0001

14.94

16.5

13.5

 

4.9.5  Calibration of the instrument

Calibration of the cell constant
The cell constant should be calibrated whenever the conductivity of the 0.0010 M KCl calibration solution is outside the upper and lower limits given in Table 4.9.1. The age of the calibration solution must be checked before the calibration. Enter the new constant after having followed the cell constant calibration procedure given in the instrument manual. Reference temperature (or measurement temperature) should be 25°C.

Calibration with calibration solutions
Before running a series of precipitation samples, measurements should be carried out with the 0.0001, 0.001 and 0.0100 M KCl calibration solutions. Check the age of the calibration solutions. If the measurements are outside the limits given in Table 4.9.1 the instrument must be checked as specified in the manufacturers instrument manual followed by measurements with all calibration solutions in Table 4.9.1. Reference temperature should be 25°C. Results obtained at other temperatures can be corrected to 25°C as seen in the next Section.

4.9.6  Measurement procedure

The procedure given in the instrument manual must be followed. In general the measurement cell has to be rinsed well with deionized water, dried with a Kleenex, and rinsed again with the measurement solution a few times before a correct reading can be made. The display will also need some time to stabilize before the reading. The reference temperature should be 25°C, and the result expressed in µS/cm. If the measurement is carried out at a different temperature, the result should be corrected to 25°C. The temperature coefficient for aqueous solutions is approximately 2% pr. degree. The formula below will give the conductivity, k25, corrected to 25°C when the measurement lt, is carried out at a temperature t.

         

4.9.7  Maintenance and storage of measurement cell

It is essential that the manufacturers instructions are followed. Cleaning of the measurement cell is needed if a contamination is discovered.

 


Last revision: November 2001