If one measures cell constant using 0.01N KCl and 0.001N KCl the values differ. When cell constant is fully governed by the geometry of the electrode, why does such a difference arise?
According to the basic law of electrochemistry:
Specific conductance (K, kappa) =C. l/a.
Where l/a is cell constant.
i.e., Specific conductance (K, kappa) is equal to the product of the conductance of the electrolyte and the cell constant.
Or cell constant= Specific conductance/ Conductance.
(Cell constant can be determined on dividing Specific conductance by Conductance)
NOW CONDUCTANCE CAN BE EXPRESSED EITHER AS MOLAR CONDUCTANCE OR EQUIVALENT CONDUCTANCE.
The molar conductance of the standard KCl solution is inversely proportional to its molarity while its equivalent conductance is inversely proportional to its normality for a given volume Vcm^3.So cell constant will also change with the change with molarity or normality of KCl solution.
The explanation given by Dr. Sehgal is not correct. As per the equation we cannot give this conclusion because cell constant is governed by two parameters conductance and specific conductance. With change in molarity of KCl both conductance and specific conductance will change and ultimately the ratio will be constant. Probably small contribution due to polarisation with change in concentration of test solution may attribute the shift in cell constant.
If I surveyed the literature rightly, following are given values of specific conductances of N/50 and N/35 KCl solutions (298K; ohm-1 cm-1) for your kind perusal. It is upon these values that tried to answer your question.
For N/50=0.002765 and for N/35=0.002768.
Anyway, I will humbly accept your BIG NO if the literature survey failed me.
Thank God, you were nice to have agreed with my answer to your second question
If you use an electrode of a particular geometry with cell constant ~1 cm-1 and using this you are the cell constant with 0.1 N, 0.01N and 0.001N KCl the cell constant value differs from one particular concentration to the other
If you use an electrode of a particular geometry with cell constant ~1 cm-1 and using this you are verifying the cell constant with 0.1 N, 0.01N and 0.001N KCl the cell constant value differs from one particular concentration to the other. Moreovedr in ASTM they have strictly recommended which electrode of particular cell constant can be used for particular ionic solution with a fixed range of conductivity.
Maybe too long time ago, but I still would like to try to answer your question. Yes, theoretically the cell constant should not change with the electrolyte concentration. But due to the following effects it can change:
• Polarisation
• Contamination
• Geometry (field effects)
• Cable resistance
• Cable capacitance
• Frequency change
• Temperature
Measuring the cell constant in different concentrations of KCl is used to control your measurement cell. Important is to keep the temperature constant. Other options to optimise your cell are:
- use platinised electrodes
- use 4-pole cell
- use clean electrodes
- adjust frequency to the conductivity of your sample
- cables as short as possible
- use alternate current
Kind attention to Dr. Reich. Thank you for your answer. You have mentioned very general information available in text book. But we have to look in different way to find the actual phenomenon occuring in the cond cell system.
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