Electrolyte Solution Calculation in Pharmacy

Electrolyte solution calculation is necessary to measure the electrolytes used to replace fluids and minerals (such as sodium, potassium) lost due to diarrhea and vomiting. It helps to prevent or treat the dehydration and important for the normal functioning of the body.


- Substances that dissociate partly or completely in water yielding ions

- A solution that has ions can conduct electricity

Non Electrolytes

- Substances that dissolve in water without dissociating, rather they remain as molecules (not dissociated) 

- No ions, no conductivity 

Unit of Measurement

Electrolyte concentration can be expressed as:

  • Percent solutions (w/v)
  • Millimoles (mmol) and Micromoles (Β΅mol)
  • Milliequivalents (mEq)
  • Milliosmoles (mOsmol)

Percent solutions

  • Weight in gm/ 100 ml volume of a solution (w/v)


  • 5% Dextrose = 5 g dextrose in 100 ml solution

Millimoles and Micromoles

  • Mole= π‘€π‘’π‘–π‘”β„Žπ‘‘ (𝑔)/π‘€π‘Š
  • Millimoles = 10˄-3 of a mole
  • Micromoles= 10˄-6 of a mole

Milliequivalent (mEq)

A chemical unit, the milliequivalent (mEq), is now used in USA by clinicians, physicians, pharmacists, and manufacturers to express the concentration of electrolytes in solution. It measures the total no. of ionic charges in solution, and it takes note of the valence of the ions. 

  • mEq= mg/equivalent weight


Equivalent weight = the amount of a substance that will either supply or react with one mole of hydrogen ions (H+) in an acid-base reaction; or supply or react with one mole of electrons in a redox reaction.

  • Equivalent weight= atomic, molecular or formula weight/valence


  • mEq =  mg × valence/atomic, molecular or formula weight

Common Values for Some Important Ions


  • Takes into account the number of particles of solute in the solution.
  • The unit of osmotic concentration is the milliosmole (mOsmol).

For example,

  • 1 mmole of NaCl = 2 mOsmol (Na+ + Cl-)
  • 1 mmole of CaCl2 = 3 mOsmol (Ca++ + 2Cl-)

mOsmol = π‘€π‘’π‘–π‘”β„Žπ‘‘ (𝑔)/π‘€π‘Š × number of species × 1000

= moles × number of species × 1000

Electrolyte Solution Calculation Example

1. What is the concentration in (mg/ml) of a solution containing 3 mEq of potassium chloride per milliliter? M.W. of KCl = 74.5

mg/ml = mEq/π‘šπ‘™ π‘₯ π‘Žπ‘‘π‘œπ‘šπ‘–π‘,π‘šπ‘œπ‘™π‘’π‘π‘’π‘™π‘Žπ‘Ÿ,π‘œπ‘Ÿ π‘“π‘œπ‘Ÿπ‘šπ‘’π‘™π‘Ž π‘€π‘’π‘–π‘”β„Žπ‘‘/π‘£π‘Žπ‘™π‘’π‘›π‘π‘’

= 3 × 74.5/1

= 223.5 mg/ml

2. What is the percent (w/v) of a solution containing 100 mEq of Ammonium chloride per Liter? M.W. of NH4Cl = 53.5

mg/L = mEq/π‘šπ‘™ π‘₯ π‘Žπ‘‘π‘œπ‘šπ‘–π‘,π‘šπ‘œπ‘™π‘’π‘π‘’π‘™π‘Žπ‘Ÿ, π‘œπ‘Ÿ π‘“π‘œπ‘Ÿπ‘šπ‘’π‘™π‘Ž π‘€π‘’π‘–π‘”β„Žπ‘‘/π‘£π‘Žπ‘™π‘’π‘›π‘π‘’

= 100 π‘šπΈπ‘ž/𝐿 π‘₯ 53.5/1

= 5350 mg/L

= 5.35 g/L

= 0.535 g/100 ml or 0.535 g%

3. A solution containing 10 mg/100mL of Ca++. Express the solution in terms of milliequivalent per liter (mEq/L). Atomic weight of Ca++ = 40

mEq/ml= (π‘šπ‘”/π‘šπ‘™ π‘₯ π‘£π‘Žπ‘™π‘’π‘›π‘π‘’)/π‘Žπ‘‘π‘œπ‘šπ‘–π‘,π‘šπ‘œπ‘™π‘’π‘π‘’π‘™π‘Žπ‘Ÿ,π‘œπ‘Ÿ π‘“π‘œπ‘Ÿπ‘šπ‘’π‘Žπ‘™ π‘€π‘’π‘–π‘”β„Žπ‘‘

= (10 π‘šπ‘”/100π‘šπ‘™ × 2)/40

= 0.5 mEq/100 ml

= 5 mEq/L

4. A patient is given 125 mg of phenytoin sodium (m.w. 274) three times a day. How many milliequivalents of sodium are represented in daily dose?

mEq = π‘šπ‘” π‘₯ π‘£π‘Žπ‘™π‘’π‘›π‘π‘’/π‘Žπ‘‘π‘œπ‘šπ‘–π‘,π‘šπ‘œπ‘™π‘’π‘π‘’π‘™π‘Žπ‘Ÿ,π‘œπ‘Ÿ π‘“π‘œπ‘Ÿπ‘šπ‘’π‘Žπ‘™ π‘€π‘’π‘–π‘”β„Žπ‘‘

125 mg three times a day = 375 mg

mEq = 375 π‘₯ 1/274

= 1.37 mEq

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