Methods Manual for Salt Lake Studies/Salinity/measuring electrical conductivity

Authors: PSJ Coleman,

Electrical conductivity overview

Electrical conductivity is the most commonly used measure of salinity, however its relationship to the mass of dissolved salts is not constant. This becomes most noticeable as the salinity increases. In addition, the various ions that may be present in the brine of salt lakes may be more or less conductive than a simple sodium chloride salt solution. The electrical conductivity relationship to TDS (g/L) given on the salinity table was derived using a range of sodium chloride concentrations, not from concentrated seawater. The differening ionic compositions of tidally connected lagoons and inland salt lakes make the development of any such relationship fairly site specific, so it is rarely attempted.

Four pole conductivity meters are available that are capable of measuring from freshwater to nearly four times the salinity of seawater (from 0-200 mS/cm). Prices for conductivity meters that can measure this wide range are considerably more expensive than the two pole 'pocket testers' commonly available. The necessity to calibrate the meter regularly adds further to the cost of ownership.

Electrical conductivity method for estimating salinity

Calibrate the meter following the directions in the EC meter manufacturer's manual. Ensure your calibration solution is fresh. If your conductivity meter measures a wide range of conductivity, make sure you use a calibration solution with a similar conductivity range to the brine you plan to measure.

To measure your brine sample, insert the EC probe and temperature probe (if separate) into the brine sample and stir gently. Wait until the reading stabilises and record reading in the laboratory daybook or field book.

Report the EC as mS/cm². Be aware that EC does not directly correlate to total soluble salts.

An approximation useful for salinities up to seawater strength is to multiply the EC by 670, to obtain Total Soluble Salts in mg/L. Alternatively, use the salinity conversion table.

Williams (1984) developed a relationship between hypersaline lake waters in Australia and TDS, that may be used for brines up to 100 mS/cm²

TDS g/L = 0.4665*EC^1.0878

Overcoming the limitations of EC in hypersaline situations

Prof Brian Timms (Australia) recommends carrying volumetric flasks in the field (polypropylene ones do not break as readily as glass ones) to allow you to dilute your sample with distilled water. This allows you to measure the EC of very concentrated brines approaching the crystallisation point of common salt. Convert the EC to TDS mg/L (or g/L) and then correct the result for the dilution. Brine stronger than 100 mS/cm² should not be converted using Williams (1984) formula. Dilute all samples to well below that.

When using an electrical conductivity meter that measures over 100mS/cm², aim to have your diluted sample reading in the 80mS/cm² range. When using a conductivity meter that only reads up to 100mS/cm², aim to have your diluted sample in the mid range for your meter. The EC reading that results from the diluted sample is converted using the Williams 1984 formula (or other site specific relationship if one exists) and the resulting TDS g/L result is then multiplied by the dilution factor.

Quality assurance and reporting of salinity measures derived from electrical conductivity

When salinity has been obtained by conversion of electrical conductivity readings, the salinity should not be simply reported as TDS g/L. That may give the erroneous impression that a direct measure of dissolved salts (via density or gravimetry) was used. If salinity is reported as TDS g/L, It should be specified that EC was used to determine the salinity and the conversion formula used should also be provided. Alternatively, report the salinity as EC in either mS/cm or uS/cm.