Testing changing sheet resistance with impurities
Assuming the chemical film is mainly an insulator, trying to measure purity with an ohmmeter method may imply high voltage or extremely low currents.

I have found the best dielectric impurity testing measures the discharge of contaminants as they are excited with sufficient electric field either AC (preferred) or DC hitting a conductor surface. 

This is often calibrated and measured in picocoulombs , but I have done it successfully in volts from a simple current loop around the source wire connected to coax terminated by 50 Ohms into a high speed scope. The scope can be replaced with a high speed charge amp or peak detector to detect the concentration of contaminants.  There will be a threshold for 1 pulse per minute which then rapidly increases in rate above this threshold.

The effect of HV excitation is a migration of the charged particle which has a different property than the dielectric medium and the mobility in the fluid and its velocity under the E field determines the discharge rate like a geiger counter or a Unijunction relaxation oscillator.

In my case I was using transformer oil with a 50kV/mm ideal threshold and and a supplied rating of 25kV/mm with an actual case of these Partial Discharge (PD) events starting around 16kV/mm which were caused by silicate iron contaminants that were invisible to the naked eye.

A laser particle reflector may be useful to identify in a glass container, contaminant particles and there are Omega ion insulation ohmmeters but I found my test method to be most effective for empirical measurements.

What chemicals are involved?