Whenever I use the ÄKTA Pure Protein Purification System to separate proteins on an ion-exchange column, the y-axis that indicates salt concentration is labeled in mS/cm rather than molarity.
But what is mS/cm? Wikipedia says it is the unit of conductivity. So if the salt is NaCl, is it possible to convert mS/cm to molarity for convenience?
A Google search turned up a discussion about the reverse conversion, from molarity to electrical conductivity. One answer said:
Some data on the electrical conductivity of pure solutions relevant to saline soils or to seawater . As 10 mM NaCl has an EC of almost exactly 1.0 dS/m, and as it is the most common salt in solution in ground or river water, a ‘rule of thumb’ is to muliply the EC by ten to equate it to salt molarity.
So I get an equation: 10 mM -> 1 mS/cm, 1 M -> 100 mS/cm.
In my experience, this equation may not be accurate (there is a good chance my own error is much larger). One reason may be that my buffer is never a pure salt solution. But for my workflow, using conductivity to get a rough estimate is good enough.
I hope this little discovery helps you.
Reference:
Bouras, Hamza. (2022). Re: How is it possible to convert (NaCl concentration, in mM) to electrical conductivity (dS/m)or % of salts. ?. Retrieved from: https://www.researchgate.net/post/How_is_it_possible_to_convert_NaCl_concentration_in_mM_to_electrical_conductivity_dS_mor_of_salts/61f107189b9a8d052e6fb0cb/citation/download.