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DETERMINATION OF CONDUCTIVITY

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DETERMINATION OF CONDUCTIVITY Powered By Docstoc
					DETERMINATION OF CONDUCTIVITY (Mainly from EMEP manual for sampling and chemical analysis. Integration from ISO and ASTM standard methods) 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.
Principle

Conductance is the reciprocal 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

Rρ

l A

where l is distance (in metres) between the electrodes and A their area (m2).  is the specific resistivity. The specific conductance, or conductivity  is

κ

l 1 l or  ρ R A

where

l is the cell constant. A

Temperature coefficient of electrical conductivity

The temperature coefficient of conductivity is given by the equation 1     25   ,25   100  25    25   

where 25 and °C are the temperatures at which the electrical conductivities 25 and  respectively were measured. In order to make comparisons, it is essential that measurements are corrected to a chosen reference temperature, usually 25,0 °C, even if the temperature of the water sample differs only slightly from that temperature. Conversions to the electrical conductivity at 25 °C, 25, can be made using the equation   25  1   100   25 
Temperature correction

If measurement at 25,0 + 0,l OC is not possible, for example in field or plant work, measure the electrical conductivity of the sample at a known temperature, 19 °C. Many instruments are fitted with temperature compensation devices, and with reference to the temperature coefficient of samples, may automatically correct measurements obtained over a range of temperatures to electrical conductivity at 25,0 °C. Such instruments shall be calibrated strictly in accordance with the manufacturer’s instructions. If the temperature coefficient of the sample is not known, it may be derived by substituting electrical conductivity values experimentally determined at 25.0  0.l °C and temperatures   0.l °C. Where instruments do not incorporate a temperature compensation device, the electrical conductivity measured at  °C shall be corrected to 25.0 °C using the appropriate correction factor taken from table A. Whichever form of temperature compensation is applied to the measurement of electrical conductivity at  °C, the result will be less accurate than that actually measured at the reference temperature of 25.0 °C. In some aspects of routine fieldwork, it may not be necessary to transpose values measured at  °C to 25.0 °C. However, such measurements should be interpreted with great care and comparison with other values may be difficult or even meaningless.
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.

Chemicals

 Deionized water, conductivity < 0.5 µS/cm  Potassium chloride p.a. quality
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 1. The solutions A trough D should be kept well closed in plastic flasks at room temperature. The stability is 6 months at most.

Immediately before preparing solution E trough G the water shall be freed from carbon dioxide by purging with pure nitrogen or by boiling. During work with these solutions any contact with the atmosphere shall be minimized. Prepare these solutions shortly before use,
Table 1: Calibration standards for conductivity at 25C. Solution A B C D E F G Concentration M KCl 0.0500 0.0200 0.0100 0.0050 0.0010 0.0005 0.0001 Conductivity µS/cm 6668 2767 1413 717.8 147.0 73.9 14.94 Upper limit µS/cm 6801 2822 1441 735 149 77.8 16.5 Lower limit µS/cm 6535 2711 1395 700 145 70.2 13.5

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 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 1 the instrument must be checked as specified in the manufacturers instrument manual followed by measurements with all calibration solutions in Table 1. Reference temperature should be 25C. Results obtained at other temperatures can be corrected to 25C as seen in the next Section.
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, 25, corrected to 25C when the measurement , is carried out at a temperature . Alternatively used the enclosed Table A.

κ 25 

κ 1.0  0.02   25

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.
Examples of reported results

Example 1 25 = 2.52 mS/m. Measuring temperature 25,0 °C Example 2 25 = 25.8 mS/m. Measuring temperature 11.5°C. Mathematical correction Example 3

25= 48 mS/m. Measuring temperature 12.l °C. Correction by means of a temperature compensation device

APPENDIX Table A. Temperature correction factor, f25, for the conversion of conductivity values of natural waters from  °C to 25°C.