Milliequivalent (mEq) is a unit of measure often used for electrolytes. It indicates the chemical activity, or combining power, of an element relative to the activity of 1 mg of hydrogen. Thus, 1 mEq is represented by 1 mg of hydrogen (1 mole) or 23 mg of Na+, 39 mg of K+, etc. . An equivalent is the amount of a substance that will react with a certain number of hydrogen ions. A milliequivalent is one-thousandth of an equivalent. Milliequivalent dosages are written using Arabic numbers followed by mEq. For example, sodium ion has an atomic weight of 23 g/mol and a valence of 1 equivalent/mol. Therefore, one milliequivalent weight of any electrolyte is 1/1000 its equivalent weight. Concentrations of electrolytes are often expressed as milliequivalents per liter, which is an expression of the chemical combining power of the electrolyte in a fluid.