eli5: Why are electrolytes measured in mEq/L and not just mM?

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I don’t get it. Why are electrolytes measured in mEq/L and not mM?

In: Biology

Electrolytes are dissolved charged particles in water, generally salts. The thing about them is that, for the purposes of biology, you don’t really care how much of the electrolyte itself exists, but how much its presence will affect other things in the water.

The most important difference is the charge of the electrolyte. mEq/L is a count of how many hydrogen ions will react with your dissolved electrolyte, so having a charge of -2 will make the ion react with twice the number of hydrogen ions that an ion of charge -1 would. In addition, some ions don’t fully dissociate in water. There will always be some particles of the full salt compound floating around, and those particles won’t react with hydrogen ions at all. The mEq/L of these weak electrolytes will be lower per mole of salt added to the water than you would get for a salt which completely dissociates as a result. Furthermore, adding two different salts to the solution will produce the same electrolytic properties as adding twice as much of just one of them (assuming the ions are things like sodium and potassium, rather than something inherently dangerous like cyanide). If you added 1 mole of sodium chloride and one mole of potassium chloride to one liter of water, the solution would have the same mEq/L as if you added two moles of sodium chloride. The measure only cares about the negative ions floating around, not exactly what you added to make that happen.