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SI Units

SI (le Systéme International d’Unités) units are used in many countries to express clinical laboratory and serum drug concentration data.* Instead of using units of mass (eg, micrograms), the SI system uses moles (mol) to represent the amount of a substance. A molar solution contains 1 mole (the molecular weight of the substance in grams) of the solute in 1 L of solution. The following formula is used to convert units of mass to moles (mcg/mL to μmol/L or, by substitution of terms, mg/mL to mmol/L or ng/mL to nmol/L).

Micromoles per Liter

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An equivalent weight of a substance is the weight that will combine with or replace 1 g of hydrogen; a milliequivalent is 1/1000 of an equivalent weight.

Milliequivalents per Liter

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Approximate Milliequivalents: Weight Conversions for Selected Ions
Salt mEq/gSalt mg Salt/mEq
Calcium carbonate (CaCO3) 20.0 50.0
Calcium chloride (CaCl2 · 2H2O) 13.6 73.5
Calcium gluceptate (Ca[C7H13O8]2) 4.1 245.2
Calcium gluconate (Ca[C6H11O7]2 · H2O) 4.5 224.1
Calcium lactate (Ca[C3H5O3]2 · 5H2O) 6.5 154.1
Magnesium gluconate (Mg[C6H11O7]2 · H2O) 4.6 216.3
Magnesium oxide (MgO) 49.6 20.2
Magnesium sulfate (MgSO4) 16.6 60.2
Magnesium sulfate (MgSO4 · 7H2O) 8.1 123.2
Potassium acetate (K[C2H3O2]) 10.2 98.1
Potassium chloride (KCl) 13.4 74.6
Potassium citrate (K3[C6H5O7]· H2O) 9.2 108.1
Potassium iodide (KI) 6.0 166.0
Sodium acetate (Na[C2H3O2]) 12.2 82.0
Sodium acetate (Na[C2H3O2] · 3H2O) 7.3 136.1
Sodium bicarbonate (NaHCO3) 11.9 84.0
Sodium chloride (NaCl) 17.1 58.4
Sodium citrate (Na3[C6H5O7] · 2H2O) 10.2 98.0
Sodium iodide (NaI) 6.7 149.9
Sodium lactate (Na[C3H5O3]) 8.9 112.1
Zinc sulfate (ZnSO4 · 7H2O) 7.0 143.8

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Valences and Atomic/Molecular Weights of Selected Ions
Substance Electrolyte Valence Atomic/Molecular Weight
Calcium Ca2+ 2 40.1
Chloride Cl 1 35.5
Magnesium Mg2+ 2 24.3
Phosphate HPO42– (80%) 1.8 96.0a
(pH = 7.4) H2PO4 (20%)   97.0
Potassium K+ 1 39.1

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