b) Isotonicity calculation using freezing point depression method
You would like to prepare 30 mL of eye drops containing 1% (w/v) of pilocarpine and 2% (w/v) procaine. How many mL of 0.9% (w/v) sodium chloride should be used to make the above eye drops isotonic?
(freezing point of depression of 1% solution of pilocarpine = 0.14, procaine = 0.11, sodium chloride = 0.576)
4 marks
Freezing point depression units are all in degrees Celsius
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)
fpd
Pilocarpine 1% 0.14
Procaine 2% 0.22 (1 % is 0.11)
Total fpd here is 0.14 + 0.22 = 0.36
You want the fpd to be 0.52 in order to be isotonic.
0.52 - 0.36 = 0.16 further depression needed.
What concentration of NaCl will produce 0.16 fdp:
x = 0.28 % NaCl will produce 0.16 fpd
FInal concentrations:
Final volume of eye drops required: 30 ml
Amount of g NaCl in 30 ml of 0.28% solution:
0.28% = 0.28 g/100ml
0.28 g x 30/100 = 0.084g
Amount of ml of 0.9% NaCl which contains 0.084 g:
0.9 % = 0.9 g / 100 ml
0.084/0.9 * 100 ml = 9.33 ml of 0.9% NaCl contains 0.084 g NaCl