Here are some suggestions. The measurement will be affected by the size of the sample compared to the electrode spacing. This is no problem in a big vat of liquid or if your test chamber is filled to the same point each time.
You should calibrate your meter with a know solution.
Use AC so that there is no plating on the electrodes to cause measurement errors.
I have seen a system 25 years ago where the liquid was in a pipe which formed a loop and there were toroid cores around the pipe at opposite sides of the loop. There were windings on the cores. The circuit was formed by transformer coupling between the cores by the liquid which was the dominant resistance in the system.