milliequivalents

Milliequivalents (mEq) are units used to express the amount of a substance in terms of its reactive capacity, or charge, in a given chemical reaction. They are commonly used in medicine, clinical laboratories, and nutrition to quantify electrolytes and drug doses.

Calculation is based on charge and amount. mEq = mmol × valence. For a monovalent ion (valence 1), mEq equals mmol; for a divalent ion (valence 2), mEq equals twice the mmol. In terms of mass, if a substance X has mass m (in mg), molar mass M (in g/mol), and valence z, then mEq = (mass mg × z) / M. Concentrations are often reported as mEq/L, indicating milliequivalents per liter.

Examples help illustrate the concept. Sodium (Na+) has valence 1, so 1 mmol Na+ equals 1 mEq (1 mEq/L roughly equals 1 mmol/L for Na+ in solution). Calcium (Ca2+) has valence 2, so 1 mmol Ca2+ equals 2 mEq; correspondingly, 0.5 mmol Ca2+ equals 1 mEq. Clinically, electrolyte measurements are frequently expressed as mEq/L to reflect charge balance in the body.

Limitations and context are important. The concept of equivalents depends on the chemical reaction or biological context, so mEq is not an SI base unit. In many settings, mmol/L or mg/dL may be preferred, but mEq/L remains useful for assessing charge balance and dosing in electrolyte therapies.