Nitrazine Yellow, a New Indicator HENRYWENKER,425 Cherry St., Elizabeth, N. J.
M
A S Y indicators have been introduced for the purpose of refining the methods of pH determination. While these newer indicators fill an actual need, their individual usefulness is necessarily restricted to a limited pH range, which in many cases lies well below or above the neutral point of p H 7. Litmus solution and litmus paper, because of their favorable p H range, still are widely used for the titration of acids and alkali to neutral, and for rapid tests whether a solution has an acid, alkaline, or neutral reaction. A new indicator which has certain advantages over litmus is 2,4dinitrobenzene-azo-l-naphthol-3,6disulfonicacid, a yellow dye for which the author suggests “nitrazine yellow” as a convenient name. Nitrazine yellow forms red crystals which dissolve easily in water and in dilute acids and alkali. The acid solution has a bright yellow color; the alkaline solution is bright deep blue. The solution in distilled water has a red-brown color, due to partial conversion of the dye into its blue ionogen form. The dyestuff also dissolves readily in 80 per cent alcohol, but is almost insoluble in 96 per cent alcohol. The solution in concentrated sulfuric acid is bright blue. The p H range of nitrazine yellow was determined by the use of buffer solutions made from 0.1 M primary potassium phosphate solution (13.620 grams per 1000 cc.) and 0.05 M borax solution (19.110 grams of N a ~ B 4 0 ~ . 1 0 H per z 0 1000 cc.) according to Kolthoff (1). For 10 cc. of buffer solution, 0.2 cc. of 0.05 per cent dye solution was used (buret). TABLEI. PH RANGEOF NITRAZINE YELLOW PHOBPHATE SOLUTION
cc*
8.77 8.30 7.78 7.22 6.67 6.23
BORAX SOLUTION
PH
COLOROF SOLUTION
CC.
1.23 1.70 2.22 2.78 3.33 3.77
6.0 6.2 6.4 6.6 6.8 7.0
Yellow Yellow, slightly duller Pale yellow Pale gray Pale blue Blue
The pH range of nitrazine yellow, according to Table I, is 6.0 to 7.0, with a sharp neutral point a t p H 6.6. However, the color change between p H 6.0 and 6.4 and between 6.8 and 7.0 on the other side is so small that for practical titration purposes almost the entire color cha.nge takes place between p H 6.4 and 6.8. Dye solutions of the concentration given above show no dichroism; in more concentrated solutions, however, dichroism is pronounced, the color of the solution changing from yellow through purple and violet shades to blue, especially in thick layers. I n titration tests, 0.5 cc. of 0.1 per cent dye solution was used for 250 cc. of distilled water. Complete color change from bright yellow to bright blue and vice versa was caused by 0.3 cc. of 0.1 N caustic soda and 0.1 N hydrochloric acid, respectively. The same result was obtained in concentrated solutions of neutral salts, such as sodium chloride, sulfate, or nitrate. Because of its sensitivity and sharp color contrast nitrazine yellow is naturally well suited for the preparation of a highgrade test paper. For the same reasons, the preparation of nitrazine paper is a difficult process, requiring considerable care and experience to obtain uniform high quality. During these experiments it has been found that by addition of a certain amount of phenolphthalein to the dye solution the sensitivity of the resulting paper can be greatly increased. This is due not to the indicator properties of phenolphthalein,
but to the fact (2) that it reduces the porosity of filter paper, so that drops do not run out but remain for a long time on the paper. The red color of phenolphthalein shows only at a p H well above 8, causing a violet coloration of the paper with such strong alkaline solutions. A test paper prepared in this way from a dye solution which has been carefully adjusted to the right p H has a slate-gray color. Using the same buffer solutions as in Table I, t,he paper shows the following colors: COLOROF PAPER
PH 6.0 6.2 6.4 6.6 6.8 7.0
Yellow Yellow Yellow-green Green Blue Dark blue
-
As can be seen, the paper has the same sensitivity as the dye solution. Dichroism is not noticeable in these tests; the color changes continuously from yellow through green to hlue. The limit of sensitivity of this paper is the same for acid and for alkali; it lies a t 0.00001 N for hydrochloric acid and for caustic soda. At this dilution, the color change appears only in the drop, not on the paper which forms a neutral gray background; after 1 minute both the acid and alkaline drop show a definite change, to yellow and blue, respectively, compared with a drop of distilled water. (The water used for making up the caustic solution must be free from carbon dioxide.) It is of theoretical interest to note the rearrangement which the dye molecule undergoes when changing from the yellow to the blue form. According to the theory of Hantzsch in its modification by Kolthoff (3), we have to assume that the dye in acid solution consists of the azo form:
I n a n alkaline medium this form, which is a pseudo acid, changes into the aci- or ionogen form: 0
This change causes the formation of two quinoid systems: Kot only does the benzene ring rearrange to a p-quinoid form, but simultaneously an o-quinone is formed in the naphthalene ring. This conjugated system of two quinoid rings explains the color intensification of the dye in alkaline solution. It has been found that all dyes made by coupling diazotized 2,kdinitroaniline with azo components in mineral acid solution show a similar color change. The l-naphthol3,6-disulfonic acid was selected as azo component because the dye derived from it shows the brightest colors and has the most favorable p H range of the dye series as far as it has been investigated. LITERATURE CITED (1) Kolthoff, I. M., “‘Indicators,” p. 147, Wiley, 1926. (2) Ibid., pp. 224-5. (3) Ibid., p. 245.
RECEIVED October 10, 1933.
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