a) Isotonicity freezing point depression

An intravenous solution of sodium chloride 0.2% requires to be made isotonic by the addition of anhydrous glucose. What weight of anhydrous glucose is required in the preparation of 1L of the intravenous solution? (A 1% solution of sodium chloride depresses the freezing point of water by 0.576°C. A 1% solution of anhydrous glucose depresses the freezing point of water by 0.1037°C.) (4 marks)

You need to know:

  • The freezing point depression (fpd) of a solution is proportional to its concentration.
  • Reference values of freezing point depression are for 1% solutions (unless stated otherwise)
  • Total freezing point depression of an isotonic solution (NaCl 0.9 %) is 0.52C (NaCl 1 % is 0.58)

Freezing point depression for 1% NaCl is 0.576 °C

Freezing point depression for 0.2% NaCl -> 0.576*0.2 = 0.1152 °C

Freezing point depression for an isotonic NaCl solution (0.9%) is 0.576*0.9 = 0.5184 °C

0.5184 - 0.1152 = 0.4032

Freezing point depression of 1% glucose is 0.1037°C

freezing point depression required is 0.4032

0.4032/0.1037 = 3.9

Concentration of glucose required in the 1L solution is 3.9 %

3.9% = 3.9g in 100ml -> 39 g in 1 L

Weight of glucose required is 39g

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