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.
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.
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.
When using an electrical conductivity meter that measures over 100mS/cm2, always aim to have your diluted sample reading in the 100-150 mS/cm2 range. Because the relationship curve between dissolved salts and electrical conductivity flattens out in the upper ranges, errors are larger in this area. If you are using a diluted sample, you will multiply the size of any error.