An isotonic solution has the same osmotic pressure as body fluids, preventing cell damage by maintaining balance between fluids.
Isotonicity and Freezing Point Depression
Isotonic solutions match the freezing point depression of body fluids, around -0.52°C.
Example: 0.9% NaCl solution is isotonic with blood (normal saline).
Isotonicity and Molecular Weight
The osmotic pressure depends on the number of particles, not their mass.
Formula for freezing point depression:
ΔTf=i×Kf×m
Where:
i= Van't Hoff factor (dissociation particles).
Kf = Freezing point depression constant.
m = Molality.
E-value (NaCl Equivalent)
The E-value helps compare other solutes to NaCl for isotonic solutions:
E=17/(i×MW)
Where MW = Molecular weight and i = Van’t Hoff factor.
Example: To make an isotonic solution with dextrose (C₆H₁₂O₆), calculate based on molecular weight and freezing point depression.
Key Points:
0.9% NaCl has a freezing point depression of 0.52°C.
Use molecular weight, Van’t Hoff factor, and E-value to calculate the isotonic concentration of other solutes.