In chemistry, an equivalent -- or 1,000 milliequivalents -- is defined as the number of moles of a substance that will combine with one mole of hydrogen ions. Hydrogen ions have a +1 charge, so one mole of hydrogen ions will combine with one mole of an ion with a -1 charge, a half a mole of an ion with a -2 charge and a third of a mole of an ion with a -3 charge. The rules for calculating equivalents, or milliequivalents, require an understanding of how these ions combine.
The rule for an ion with a charge of +1 or -1 -- a monovalent ion -- is: 1 milliequivalent of a monovalent ion is equal to 1 millimole of that ion. A chlorine ion has a charge of -1. If you have 20 mmol of chlorine ion, use the conversion factor for a monovalent ion to set up the following calculation, as follows:
20 mmol chlorine ion x (1 meq chlorine ion / 1 mmol chlorine ion) = 20 meq chlorine ion
The rule for an ion with a charge of +2 or -2 -- a divalent ion -- is: 1 milliequivalent of a divalent ion is equal to 0.5 millimole of that ion. A sulfate ion has a charge of -2. If you have 20 mmol of sulfate ion, use the conversion factor divalent ion to set up the following calculation:
20 mmol sulfate ion x (1 meq sulfate ion / 0.5 mmol sulfate ion) = 40 meq sulfate ion
The rule for an ion with a charge of +3 or -3 -- a trivalent ion -- is: 1 milliequivalent of a trivalent ion is equal to 0.33 millimole of that ion. A phosphate ion has a charge of -3. If you have 20 mmol of phosphate ion, use the conversion factor for a trivalent ion to set up the following calculation:
20 mmol phosphate ion x (1 meq phosphate ion / 0.33 mmol sulfate ion) = 61 meq phosphate ion
