A note on the preparation of Dakin's hypochlorite solution

Abstract

In the preparation of Dakin's hypochlorite solution it has been customary to adjust the reaction by the use of solid phenolphthalein. In the preparation from sodium carbonate and bleaching powder Dakin added boric acid to the strongly alkaline hypochlorite solution until it no longer colored solid phenolphthalein. Daufresne modified this technique by using a mixture of sodium carbonate and sodium bicarbonate for decomposing the bleach and also used solid phenolphthalein as the test for the reaction of the solution. Investigations in this laboratory indicate that the reaction at which solid phenolphthalein turns red with a hypochlorite solution is at a hydrogen ion concentration of about 1 × 10−10 (pH = 10), i. e., an alkalinity in terms of hydroxyl ions about 1,000 times that of water. Alkaline phenolphthalein solutions show a flash color in hypochlorite solutions at a hydrogen ion concentration of about 1 × 10−8 (pH = 8) or in the same unit an alkalinity 10 times that of water or one one-hundredth that of the turning point of solid phenolphthalein in hypochlorite solution. Hypochlorite solutions at a reaction corresponding to the color change of phenolphthalein solution, i. e., pH = 8, are not stable. It is desirable therefore to find the minimum alkalinity at which a hypochlorite solution can maintain its concentration for a convenient period. We have found that this point is at a hydrogenion concentration of about 10-8.5(pH = 8.5). At this reaction a solution will maintain its hypochlorite content within the desired range for about two days. Hypochlorite solutions prepared by the use of phenolphthalein may vary as much as one hundred times in alkalinity, depending upon the bleach and conditions. This explains the variations in irritation that different observers have reported.