If the equivalent conductance at infinite dilution of pairs of salts having an anion in common are examined certain regularities become evident. For example, the difference in conductance of sodium and potassium salts of the same anion is independent of the nature of the anion. It follows that each ion contributes a definite portion to the total conductance

 

                 Thus,                                                                   (8)

 

Now, the quantity of electricity that can pass through an electrolyte (and, thus, it’s conductance) will depend on the product of the number of ions, the charge on each ion, and the velocity of the ions. Since the total charge is constant (it is the equivalent conductance) then the equivalent conductance at infinite dilution can only depend on the speeds of the ions. If (u) and (v) are the velocities of the cation and anion respectively under a potential gradient of 1 volt per cm, then,

 

                                                           (9)

 

where  (s) is the same constant for all electrolytes.

 

Consequently,                                (10)

 

It is clear that by measuring the electrical conductivity of a solution the data can be used to determine the amount of dissolved solids (assuming they are ionisable) and in particular the salinity of aqueous samples. In the analytical laboratory, measurement of electrical conductivity of a solution is often employed in measuring the total dissolved solids (TDS) in salt water, wastewater, cooling tower water and boiler condensate. It has also been used to verify reverse osmosis systems, the quality of drinking water and to monitor the concentration of the nutrient solutions employed in hydroponics. An example of a commercially available conductivity meter manufactured by Orion Inc is shown in figure 3.

 

 

Figure 3.  The Orion 145A+ Conductivity Meter Manufactured by Orion Inc.

The types of measurement that are selectable can be TDS, salinity or the direct measurement of conductivity. The device is supplied with a graphite cell containing an integral temperature sensor for automatic temperature compensation. Stainless steel flow cells are also available. There is a single step calibration in which a calibration solution is measured and the meter output adjusted to the correct conductivity. The cell and instrument are then fully calibrated. 

 

Conductivity electrodes are also generally available and are manufactured for specific types of sample. The vast majority of them contain integral temperature compensation sensors. The electrode series can extend over the range 1 to 10,000 mS. An example of a set of conductivity electrodes manufactured by Pulse Instruments is shown in figure 4. These electrodes are complete in themselves and, after connecting to the appropriate meter (for which the electrodes have been specifically designed), need only be calibrated using the appropriate calibration solution and then merely dipped into the sample under examination.

Figure 4. Conductivity Electrodes Manufactured by Pulse Instruments for Measuring of Solutions Having Different Conductivity Ranges