You would like to prepare 30 ml of an eye drop containing 1% of pilocarpine and 2% procaine. How many mg of sodium chloride should be used to make the above eye drops isotonic? (Freezing points depression of 1% solution of pilocarpine and 1% solution of procaine are 0.14 and 0.11, respectively.) 

(4 marks) 

You need to know:

• Freezing point depression (fdp) of a substance is proportional to its concentration.
• A 0.9% sodium chloride solution is isotonic
• fdp of 1% NaCl is 0.576 therefore fdp of 0.9% is 0.518
• The fdp of the final solution is equal to the sum of the ingredients fdps

NB Examiner has used 2 decimal places for the fpd values

Fdp 1% pilocarpine - 0.14

Fdp 2% procaine - 0.11*2 = 0.22

Fdp total = 0.14 + 0.22 = 0.36

Fdp of isotonic solution is 0.518

0.518-0.36 = 0.158

What percent NaCl has fdp of 0.158:

1% - 0.576

1*(0.158/0.576) = 0.275%

0.275% = 0.275 g/100ml

= 0.00275 g/ml

= 0.0825 g/30ml

82.5 mg NaCl