Abstract
The successful application of light filters in the Folin-Malmros 1 method for sugar suggested the possibility of compensating, in a similar way, the acid color of the bicolor indicators. Employing light filters consisting of the appropriate indicators in an acid solution, a procedure was worked out for determining pH with phenol red, brom cresol purple and brom cresol green.
The acid filter solutions for phenol red and brom cresol purple consists of 0.01% solutions of each indicator made up in acetate buffers having pH values of 5 and 3 respectively. In the case of brom cresol green ethyl alcohol must be added to the acid solution to prevent precipitation of the indicator. A 0.02% solution of the indicator is made up in hydrochloric acid and alcohol so that the final solution is equivalent to 0.1 N hydrochloric acid and contains 35% alcohol.
Battery jars 10 × 10 × 10 cm. or museum jars 16 × 10 × 8 cm. of clear, white glass make satisfactory containers for the filter solutions. The sides of the jars should be painted black, except for a 5×8 cm. window to allow the light to pass through. The prepared filter is placed in position between the colorimeter and light, and the instrument “set” in the usual way with the standard in both cups. Readings, which should agree within 0.1 mm. are taken with the left cup set at 1, 5 and 8 mm.
The standard solutions are made by adding 1 cc. of 0.02% phenol red and brom cresol purple, and 0.03% brom cresol green to 20 cc. of the alkaline buffer solutions. The buffers used should have pH values which will completely transform the indicator to the alkaline form.
Get full access to this article
View all access options for this article.