Compounding Hotline

Author: Martin A. Erickson III, RPh

Your Compounding Questions Answered

Q: I was wondering if there is a recipe for compounding “alcohol 20%,” with the active ingredient being isopropyl alcohol.

A: This kind of question is very often confusing. Pharmaceutically speaking, “alcohol” (alcohol USP) is ethanol, or ethyl alcohol, 200 proof (100%). Isopropyl alcohol contains 99.0% to 100.5% by weight of C3H8O and is a transparent, colorless, mobile, volatile liquid with a characteristic odor and a slightly bitter taste. It has a specific gravity of 0.783 to 0.787 and is miscible with water, alcohol (ethanol), ether, and chloroform. It is used for disinfection of medical and surgical equipment and supplies and as a skin antiseptic. As a skin antiseptic in concentrations greater than 70%, it has effectiveness similar to ethanol.

Ethanolic solutions for skin antisepsis are becoming displaced by isopropanolic solutions, especially in community practice, chiefly due to potential for ethanol abuse (even where a denturant is added to ethanol). We will assume that you have clarified the order is for isopropyl alcohol.

Isopropyl Alcohol 20%
In this preparation, liquid is mixed with liquid (isopropyl alcohol with water), and it is assumed that no shrinkage occurs. The calculation for 1 liter of isopropyl alcohol 20% is 20%/100%=x/1000, and therefore 1000 mL of isopropyl alcohol 20% contains 200 mL isopropyl alcohol 100%.

The formulation may be expressed as:
Isopropyl alcohol 100%            200 mL
Purified water USP qs ad         1000 mL

Mr. Erickson is director of professional affairs at Gallipot Inc.