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How many milliliters of a 0.9% solution of sodium chloride should be used in compounding the following 0/1
prescription: Phenylephrine 1%, Chlorobutanol 0.5%, Sodium Chloride q.s., Purified water 15ml, Make isotonic.

There are steps to remember when determining isotonicity. Here are the steps when using the NaCl method

    a. Determine the amount of each ingredient in the formula in grams

         PE 0.01 x 15 mL = 0.15 g     CH 0.005 x 15 mL = 0.075 g



    b. Calculate the amount of NaCl represented by each ingredient by multiplying each amount by its E value; E for PE=0.32, CH = 0.24

        PE 0.15 g x 0.32 = 0.048 g    CH 0.075 g x 0.24 = 0.018g



    c. Calculate the amount of NaCl that would be needed to make the quantity of 15 mL isotonic if nothing else was added. Remember, the concentration for an isotonic soln of NaCl is 0.9%

               0.009 x 15 = 0.135 g NaCl needed



    d. Subtract the amount of NaCl represented by each ingredient from the amount to make 15 mL isotonic

       0.135 g - (0.048 g + 0.018 g) = 0.069 g NaCl to be added



    e. Determine the volume of a 0.9% soln that would contain 0.069 g NaCl

           0.9 g/100 mL = 0.069 g/x, x = 7.67 mL