Two New Total Alkalinity Indicators

Abstract

ries have been suggested for the total alkalinity titration of water (1). One of these undergoes an orange-red to bluish-purple transition, while the second changes from a deep yellow to a muddy hue. . Among the advantages claimed for these dyes is that their greatest color responses occur in a pH range where methyl orange fails to function. Moreover, the color responses are of an order which can readily be reproduced by most analysts. As Cooper (2) and other investigators have shown, methyl orange is sometimes unsuitable for the determination of low alkalinities. Schroeder (3) states that the phenolphthalein-methyl orange titration is not accurate in boiler-water solutions whose concentrations of carbonates are below 60 ppm. At 30 ppm. of carbonate the maximum possible error which may be expected is 26 per cent. This error arises largely from the fact that the first perceptible color change with methyl orange occurs at a pH of 4.6. According to Cooper (2), the pH prevailing at the end-point of an acid-carbonate titration of 30 and 50 ppm. should be 4.85; for 70 and 100 ppm. the pH should be 4.75; for 150 ppm., a pH of 4.6; and for 250 ppm., a pH of 4.5. Thus, methyl orange under the most favorable circumstances is only justified for alkalinities in excess of 150 ppm. Many individuals possess imperfect color sensibility, however, rendering impossible the proper identification of he faint orange characteristic of methyl orange at a pH of 4.6. Often the sample is overtitrated to a deeper orange representing a pH of 4.5 or 4.4, with a consequent sacrifice in accuracy. Other considerations also contribute