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Positively charged particles are called cations, and negatively charged particles are called anions. When cations and anions combine, they do so according to their ionic charge (or valence) and not according to their weight. Electrochemical equivalence refers to the combining power of an ion.
One equivalent (Eq) is defined as the weight in grams of an element that combines with or replaces 1 g of hydrogen ion (H+). Since 1 g of H+ is equal to 1 mol of H+ (containing approximately 6.02 x 1023 particles), 1 mol of any univalent anion (charge equals 1-) will combine with this H+ and is equal to one equivalent (Eq). For example,
1 mol H+ + 1 mol Cl- —> 1 mol HCl
(1 g) (35.5 g) (36.5 g)
By similar reasoning, 1 mol of a univalent cation (charge equals 1+) also is equal to 1 Eq, since it can replace H+ and combine with 1 Eq of Cl-. For example,
1 mol Na+ + 1 mol Cl- —> 1 mol NaCl
(23 g) (35.5 g) (58.5 g)
By contrast, ionized calcium (Ca2+) is a divalent cation (charge equals 2+). Consequently, 1 mol of Ca2+ will combine with 2 mol of Cl- and is equal to 2 Eq:
1 mol Ca2+ + 2 mol Cl- —> 1 mol CaCl2
(40 g) (71 g) (111 g)
The body fluids are relatively dilute, and most ions are present in milliequivalent quantities (one-thousandth of 1 Eq equals 1 mEq). To convert from units of millimols per liter to milliequivalents per liter, the following formulae can be used:
mEq/l = mmol/l x valence
mEq/l = (mg/dl x 10 x valence) ÷ mol wt
The measurement of ionic concentrations in milliequivalents per liter emphasizes the principle that ions combine milliequivalent for milliequivalent, not millimol for millimol or milligram for milligram. It also highlights electroneutrality. There is an equal number of milliequivalents of cations and anions in the body fluids. The need to preserve electroneutrality is an important determinant of ion transport in the kidney and in ion movement between the cells and the extracellular fluid. This obligatory relationship could not be appreciated if the ionic concentrations were measured in millimols per liter or in milligrams per deciliter.
It should be noted that not all ions can be easily measured in milliequivalents per liter. The total calcium (Ca2+) concentration in the blood is approximately 10 mg/dl.
mEq/l of Ca2+ = (10 x 10 x 2) ÷ 40 = 5 mEq/L
However, approximately 50 to 55 percent of plasma Ca2+ is bound by albumin and, to a much lesser degree, citrate so that the physiologically important ionized (or unbound) Ca2+ concentration is only 2.0 to 2.5 mEq/L.
There is a different problem with phosphate since it can exist in different ionic forms, as H2PO4-or as HPO4(2-) or as PO4(3-), and an exact valence cannot be given. We can estimate an approximate valence of minus 1.8 because roughly 80 percent of extracellular phosphate exists as HPO4(2-) and 20 percent as H2PO4-. If the normal serum phosphorus concentration is 3.5 mg/dl (phosphate in the blood is measured as inorganic phosphorus), then,
mEq/l of phosphate = (3.5 x 10 x 1.8) ÷ 31 = 2 mEq/L
Similar problems apply to other elements which occur in more than one valence state in physiological fluids.
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