Re: How to determine salt content in water using electricity?

Hello Micheline.  You seem to have a good understanding of how salt 
solutions can conduct electricity.  Conductivity is an excellent way to 
measure the concentration of a salt in solution.

Conduction of electricity through a solution requires the movement of 
positively charged species (cations) towards the negative electrode or 
cathode and negatively charged species (anions) towards the positive 
electrode or anode.  The conductance of the solution is a measure of the 
current that flows under a given applied voltage and is proportional to 
the number of charged particles in solution.  SO, with the correct 
calibration, the conductivity could be used to determine the salt 
concentration down to very low levels (parts per million).  

If you want to measure the individual ions present in a mixture of 
different salts, you would have to do other experiments.  For example, you 
could separate the ions from each other by doing Ion Chromatography and 
then detecting the individual ions by conductivity.

Measurement of conductance can also be used as a way of finding the end 
point in titrations.  In addition to conductivity, other electrochemical 
techniques include potentiometry, amperometry and voltametry.   
ElectroAnalytical Chemistry is a massive area, indeed, many undergraduate 
courses will only scratch the surface.  I have searched for good 
introductory web sites but have not found anything suitable, so I refer 
you to any good book on Analytical Chemistry for a full explanation of 
electrochemistry and the various techniques and applications.

I hope this helps,

Steve.