Conductivity, the measurement of how well electrical current passes through, is in direct relation to the ion concentration of most solutions. The higher the ion concentration in your solution, the better it will conduct electricity. Although some highly concentrated solutions do not follow the linear relationship of conductivity-to-concentration and conductivity can be affected by temperature, a standard conversion factor can be used to make a best estimate of concentration if conductivity is known.
Measure the conductivity of your solution. Different conductivity meters will vary in their operation, but the basic principle is to place the probe in the solution and wait until the reading stabilizes on the display. The current will typically be in microohms or microsiemens (these units are equal to each other), although some older meters may only read resistivity.
Convert the current reading to ohms. If your meter does not convert to microohms or microsiemens for you, write down the resistivity reading and use Ohms Law to find conductivity. For the following formulas, G is conductivity in ohms, R is resistivity, V is voltage and I is amps:
R= I/V G= 1/R.
G then will have to be divided by 1 million to get microohms or microsiemens.
Calculate PPM (parts per million--the concentration) from microohms (the measure of conductivity). Multiply microohms or microsiemens by 0.64 to obtain the ppm. So concentration in ppm = conductivity in microohms x 0.64.
Change ppm to molarity. In most cases, you will want to know molarity rather than ppm for your solution. Use the following principles to calculate molarity:
ppm = 0.001 g of solute in 1 liter of solution (a solute is the substance that is dissolved into the solvent to make up the solution)
Molarity = moles/liter, so by taking the atomic weight (grams/moles) of the solute (found either in the periodic table or on the solute bottle's label) you can calculate molarity.
ppm (grams/liter) divided by atomic weight (grams/mole) equals molarity (moles/liter).
TL;DR (Too Long; Didn't Read)
Temperature can affect conductivity. For the best readings, either measure your solution in an environment that is 25 degrees Celsius or use a conductivity meter that adjusts its readings based on ambient temperature.
If your solution has numerous solutes in it, you will not be able to calculate molarity from conductivity. The conductivity-to-concentration conversion works best in a solution with only one solute.