It is common practice to reduce a recipe for dispensing or enlarge the recipe as required for the purpose of patient. This is required when a manufacturing formula is to be taken from the laboratory scale to a manufacturing floor and a formula for the large scale manufacturing is to be adopted for prescribing to a patient. Such calculations may also involve conversions from one system of weights and measures to another.
The following rules are generally useful in such system
Rule 1:
To convert a recipe in imperial system to metric system
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“Write grams in place of grains, divide the minims by 1.1 and write the result as milliliters. For large batch, read avoirdupois ounce as grams and fluid ounces as milliliters”
Rule 2:
To convert a recipe in metric system to imperial system
“For a small batch write grains in place of grams and multiply the milliliters by 1.1 and write the result as minims. For large batch, write ounce in place of grams and fluid ounce in place of minims”
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Example
Rx
Sodium hypophosphite gr. Xx
Sulphuric acid fl dr. I
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Cinnamon water q.s. ad fl.oz. ii
Dispense 50 ml
Calculation:
Convert to metric system
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Sodium hypophosphite 20 g. = 20 x 50/872.7 = 1.15 g
Sulphuric acid 60/1.1 = 55.5 ml = 55.45 x 50 / 872.7 = 3.2 ml
Cinnamon water q.s. ad 2 x 480/1.1 = 872.5 ml = 50 ml
Dispense 50 ml
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However, the simpler solution of the problem would be to retain the original recipe and prepare 2 fl. oz. of the mixture, which corresponds to about 60 ml. Out of 2fl.oz, 60 ml, could be dispensed and the rest rejected.
Example
Rx
Pt. Nitrate
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Sod. Chloride
Camphor of each 3 g
Tincture opium 15 ml
Alcohol to make 200 ml
Dispense 4 fl.oz.
Solution
4 fl oz of the prescription is to be dispensed hence quantities can be converted to Imperial system.
R
Pt. Nitrate
Sod. Chloride
Camphor each 3 grains
Tincture opium = 15 x 1.1 = 16.5 minims
Alcohol = 200 x l.l = 220 minims
4 x 480= 1920 minims
Quantity of Pt. Nitrate, Sod. Chloride and Camphor = 3 x 1920/220 =26.2 grains
So to avoid this we can prepare 9 times the formulal980 minims and dispense 4 fl.oz and reject 60 minims
Example
Calculate the amounts of ingredients required to make 50 ml of calamine lotion.
Rx
Calamine 150 g
Zinc oxide 50 g
Bentonite 30 g
Sodium Citrate 5 g
Liquefied Phenol 5 ml
Purified water, q.s. 1000 ml
Dispense 50 ml.