The conductivity of a solution is defined as the capacity to carry an electric current through it. It varies with the number and type of ions the solution contains. A conductivity cell connected to a wheat stone bridge circuit measures it.
It gives information about the dissolved solids in the sample.If conductivity and dissolved solids concentration of the same water sample is found out separately and a correlation factor is found out previously, dissolved solids need not be determined in future as it can be computed by multiplying conductivity with correlation factor.
mg/L dissolved ionic matter in a solution = conductivity x 0.55 to 0.9
Similarly while finding out the corrections or validity of chemical analysis of water sample conductivity data becomes an important parameter. Ionic balance computed on the basis of analysis report must show cation = anions. If these values do not equal each other due to some reasons, product of conductivity x 0.01 is found out and the ion (may be cation or anion) coming nearer to this product is taken as correct.
Reagents
a) Conductivity water: pass distilled water thought a mixed bed deionizer and discard first liter conductivity should be less than 1mhos/cm.
b) Standard potassium chloride solution, KCl, 0.01m: Dissolve745.6 mg anhydrous KCl in conductivity water and dilute to ml at 25oC. This is the standard reference solution, which at 25oC has conductivity of 14 µmhos/cm. It is satisfactory for most samples when the cell has a constant between 1 & 2. For other cell constant use stronger or weaker KCl solution. Store in a glass stoppered borosilicate glass bottle.
Procedure
Conductivity measurement: Rinse cell with one or more portions of sample. Adjust temperature of a final portion to 25 ± 0.1oC Measure sample resistance or conductivity and note the temperature.
Calculation
The temperature coefficient of most of the water is only approximately the same as that of standard KCl solution, the more the temperature of measurement deviates from 25oC, the greater the uncertainty in applying the temperature correction. Report all conductivities at 25oC.
(a) When sample resistance is measured conductivity at 25oC is -
K =(1000000 )(C) / Rm(1 0.0191)(t 25)
Where,
K = Conductivity mhos / cm
C = cell constant, cm-1
Rm = measured resistance of sample, ohms and
t = temperature of measurement
(b) When sample conductivity is measured, conductivity at 25°C
K = (1000000)(C) / 1 0.0191 (t 25)
Where
Km = measured conductivity mhos at toC and other units are defined as above
Note: If the conductive readout is micromhos per centimeter delete the factor 1000000 in the numerator.
(c) For instruments giving values in SI units 1 ms/m = 10 mhos/cm, or conversely 1µ mhos/cm = 0.1 ms/m