CONVERSIONIt is often necessary in the practice of phar-macy to convert from one system to another inorder to dispense the substances that have beenordered in their proper amounts. Although thedenominations of the metric system are not com-mensurate with those of the common systems, theBureau of International Standards has establishedconversion standards that will satisfy the degreeof accuracy required in almost any practical situa-tion. Ordinary pharmaceutical proceduresgenerally require something between two- andthree-figure accuracy, and the following tables ofconversion are more than sufficient for practicaluse. Naturally, if potent agents are involved, youmust use a more precise conversion factor for pur-poses of calculation.Conversion Table for Weights and LiquidMeasuresTo convert from:REDUCING AND ENLARGINGFORMULAS AND DOSESREDUCING FORMULASIn compounding, you will often find itnecessary to reduce or enlarge the original recipeor formula. Most of the formulas in the USP-NFare given in quantities of 100 g or 1000 g of weightor milliliters of volume total.There are many ways of reducing and enlarg-ing formulas. The methods most commonly usedare:1. Ratio and proportion.Example: Reduce the following formula forpotassium arsenite solution to make 120 ml.Arsenic trioxide . . . . . . . . . . . . . . . . . . . . . 10gPotassium bicarbonate . . . . . . . . . . . . . . .7.6 gAlcohol . . . . . . . . . . . . . . . . . . . . . . . . . . . . .30mlDistilled water, a sufficientquantity to make, . . . . . . . . . . . . . . . 1000mlBy using ratio and proportion, the amount ofarsenic trioxide to be used:of arsenic trioxide needed.For potassium bicarbonate:of potassium bicarbonateneeded.For alcohol:x = 3.6ml of alcohol needed.The new formula is written as follows:Arsenic trioxide . . . . . . . . . . . . . . . . . . . . .1.2 gPotassium bicarbonate . . . . . . . . . . . . .0.912 gAlcohol . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.6 mlDistilled water, q.s. . . . . . . . . . . . . . . . .120.0 ml2. Fractional method.The numerator will be the amount of the newformula, and the denominator will be the amountof the original formula. Example: Reduce the for-mula for the potassium arsenite solution in thepreceding item to make 120 ml.For arsenic trioxide:potassium bicarbonate:alcohol:water:q.s. to 120 ml8-4

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