Question

Tetracaine Hydrochloride 0.5% (0.18 E Value)
Sol. Epinephrine Bitartrate 1:1000 10 (0.18 E Value)
Boric Acid q.s.
Purified Water ad 30.0
Make isoton. sol.
Sig. Eye drops.

The solution epinephrine bitartrate (1:1000) is already isotonic. How many grams of boric acid should be used in compounding the prescription?


Answer: 0.294g

Answer 1

Solution:

30 mL (total volume prescribed) − 10 mL (already isotonic) = 20 mL (to be made isotonic)

(Since given that 10 mL solution epinephrine bitartrate (1:1000) is already isotonic and hence sodium chloride contribution is not taken for calculation)

STEP 1
0.5% = 0.5g/100ml * 30ml = 0.15g Tetracaine HCl
0.18 x 0.15g = 0.027g of Sodium Chloride represented by Tetracaine HCl

(0.5% means 100 mL contain 0.5 g, E value = 0.18)

STEP 2
20 ml x 0.009g/1mL (or 0.9%) = 0.18 g of Sodium Chloride (total needed)

(0.9% solution of NaCl is isotonic )

Step 3

Sodium chloride to be used:

= 0.18g – 0.027g = 0.153g

Therefore, convert to Boric acid = 0.153g / 0.52 (sodium chloride equivalent of boric acid) = 0.294 g

In compounding the prescription, 0.294 g of boric acid should be used.

Note: Isotonic Solution: is a solution having the same osmotic pressure as a body fluid. Ophthalmic, nasal, and parenteral solutions should be isotonic.