The milliequivalent is related to the total number of ionic charges in solution and it takes note of the valency of the ions. In other words, it is a unit of measurement of the amount of chemical activity of an electrolyte. The total concentration of cations always equals the total concentration of anions.
A milliequivalent (mEq) weight is 1/1000 of the equivalent weight. An equivalent weight of an element is the atomic weight expressed in grams, divided by its valency.
1mEq =
atomic weight in mg valency
e.g. 1 mEq Na+ = | 23 | = 23 mg |
e.g. 1 mEq Na+ = | = 23 mg | |
1
1 mEq Cl- = | 35.5 | = 35.5 mg |
1 mEq Cl- = | = 35.5 mg | |
1
Thus, a solution containing 1 mEq of Sodium per litre contains
23 mg Sodium. A solution containing 1 mEq of Sodium Chlo-
ride contains 23 mg Na+ and 35.5 mg Cl-, i.e. 58.5 mg Sodium
Chloride per litre.
In a salt containing ions of different valencies,
Weight of a salt containing 1 mEq = of specified ion
Sum of the atomic weights (valency of the specified ion) x no. of speci- fied ions in molecule
e.g. Weight of magnesium chloride ( mgCl2.6H2O) required to prepare a solution containing 1 mEq of magnesium per litre.
24.3 + (2 x 35.5) + 6[(2 x 1) + 16]
2 x 1
= 101.7 mg mgCl2.6H2O
For the conversion of grams per 100 ml (percentage) of a solution to mEq/litre, the following formula may be used.
percentage strength x 10000
mEq per litre =
mg of salt containing 1 mEq of specified ions
e.g. Number of mEq of Na+ per litre contained in Sodium Chlo- ride injection 0.9% (1mEq Sodium is contained in 58.5 mg Sodium Chloride)
Appendix 2: Calculation of Electrolytes
mEq per litre of Na+ in Sodium Chloride 0.9% | 0.9 x 10,000 | = 154 approximately |
mEq per litre of Na+ in Sodium Chloride 0.9% | = 154 approximately | |
solution =
58.5
mEq =
Wt (g)
mEq weight (g)
e.g. mEq in 5g of Potassium Chloride (atomic weight of Potas- sium Chloride = 74.6, valency = 1)
ly
0.0746
To convert mg per 100 ml ( mg%) to mEq per litre, the following formula is used :
mg. per 100 ml. x 10 x valency atomic weight
= mEq. per litre
In the case of gases (i.e. carbon dioxide) the volume per cent of the gas is multiplied by 10 and divided by 22.4, the later figure being the volume in litres occupied by a molecular weight of a gas. The results are expressed in millimoles per litre.
vol. percent x 10
22.4
= mM. per litre
To convert mg per 100 ml ( mg%) to milliosmoles:
mg. per 100 ml x 10
molecular weight
= mOsm per litre
Normal Ranges, Plasma or Serum
Electrolyte | Atomic Weight | Valency | mg. per 100 ml | mEq. per litre |
Na | 23 | 1 | 310-34 | 135-147 |
NaCl | 58.5 | 1 | 570-620 | 98-106 |
K | 39 | 1 | 16-22 | 4.1-5.7 |
Ca (total) | 40 | 1 | 9.1-11.5 | 2.1-2.6 |
Ca2+ (ionized) | 40 | 1 | 4.25-5.25 | 2.1-2.6 |
Appendix 2: Calculation of Electrolytes
(HCO3)
SO4
Ion | weight of mEq in mg | Salt | mg of salt containing 1 mEq of the specified ion |
Ca++ | 20.0 | Calcium Chloride, CaCl2,2H2O Calcium Gluconate, C12H22CaO14,H2O Calcium Lactate, C6H10 CaO6,5H2O | 147 224 154 |
K+ | 39.1 | Potassium Chloride, KCI Potassium Citrate, C6H5K3O7,H2O | 74.5 108 |
Mg++ | 12.5 | Magnesium Sulphate, mgSO4,7H2O | 123 |
Na+ | 23.0 | Sodium Acetate, C2H3NaO2,3H2O Sodium Acid Citrate, C6H6Na2O7,1½H2O Sodium Bicarbonate, NaHCO3 Sodium Chloride, NaCI Sodium Citrate, C6H5Na3O7,2H2O Sodium Lactate* | 136 131 84 58.5 98 |
CI | 35.5 | Calcium Chloride, CaCI2,2H2O Potassium Chloride, KCI Sodium Chloride, NaCI | 73.5 74.5 58.5 |
C2H3O2 (Acetate) | 59.0 | Sodium Acetate, C2H3NaO2,3H2O | 136 |
HCO3
*Prepared in solution by neutralising lactic acid with sodium hydroxide: 1.0 ml of 1 M sodium contains the equivalent of 112 mg.