To measure the purity of water you need to measure the conductivity (usually measured in micro-siemens) have a look at this article it may give you some ideas
New to this forum but it appears Fig. 1 has the circuit. You just have to do the scaling. I'm sure there is a cheaper way but I would buy with a cheap digital meter and scale the voltage. You will need physical constants to create the scaling. (Note you can't do standards below 100 uS without being in a nitrogen blanket....and still they are difficult.