Alizarin as a Reagent for Ammonium Ion in Elementary Qualitative Analysis' HUBERT FRINGS West Virginia Wesleyan College, Ruckhannon, West Virginia usual test for NH4+ ion in elementary qualitaTHE tive . analysis calls for warming the unknown solu-
paper in the usual techniques, either macro or semimicro. The sensitivity of the alizarin paper test was compared with that of the litmus paper test, on the semimicro scale, using a solution of (NH4)&04as the source of NH4+ and 4N NaOH to form the NH3, with the following results. Litmus paper, under the conditions of this experiment, gave a clearly defined test with as little as 5 y of NHr+, but, a t this level, a comparison patch was needed for clear distinction. Alizarin paper gave a clear-cut test with as little as 4 y of NH4+. No comparison patch is ever needed with alizarin paper for the color change is sharp. Alizarin paper is, thus, just as sensitive as litmus paper and a t low concentrations of NH4+gives a much clearer test. This method for NHa+ was tested for student preference in a course in semimicro qualitative analysis. The students were given mimeographed instructions for the soluble cation group with the directions for NH4+ calling for the use either of litmus paper or alizarin paper, without specifying either. There were no indications that alizarin paper was not a usual test. By the end of the course, all of the students were using the alizarin paper test instead of the litmus paper test, because they preferred its sharper color change. Alizarin paper is easily prepared, i t is as readily stored and used as litmus paper, i t is equally sensitive, and i t has the followiug advantages: (1) the color change with alizarin paper is sharper than that with litmus paper, and (2) the color developed with NHa is readily dissipated by drying the paper, thus allowing clear-cut differentiation in cases of spattering or in cases of suspected contamination of the paper.. (This is also possible with litmus, but the change from alkaline blue to neutral blue is less sharp and, with small quantities of impurities, would beuseless.) It is worth noting that alizarin paper, prepared for the NH4+ test, can also be used in a confirmatory test for Al+++, following the directions given by Feigl,¶ or as given in one scheme of elementary qualitative analysis.8 This is an excellent supplement to the usual "Aluminon" test, or i t could be used in place of that test with some advantapes.
tion or solid with NaOH or KOH solution and testing for the liberation of NHa by the use of a piece of moistened neutral or red litmus paper. While this is a satisfactory test, I have found that alizarin paper used as an indicator for the liberated NHa has certain advantages over the litmus paper. Alizarin paper can be readily prepared by soaking quantitative grade filter paper or spot test paper in a saturated solution of alizarin in ethyl ether or methyl alcohol and allowing i t to dry. While ordinary technical grade alizarin paste (20 per cent) serves perfectly well for the purpose, Alizarin Indicator gives more uniform results and is easier to use, for the paste requires the removal of the water either by evaporation or washing with alcohol. It is important to have the filter paper or spot test paper of high grade, for alizarin reacts with extremely small quantities of many metallic ions to give colored compounds. Dry alizarin paper, prepared as above, can be stored and handled like litmus paper. Untreated alizarin paper is light yellow in color; when moistened and exposed to NH3 i t assumes a purple color. This color change is, therefore, shaxper than the color change of litmus paper. Drying the ammoniated alizarin paper drives off the NH, and restores the original color, while similar drying of alizarin paper which has been acted on by NaOH or KOH does not bring about a color change. This allows the differentiation between Na+ or K + knd NH4+ in cases of suspected spattering of the NaOH or KOH solutions onto the paper. Also, if there is suspicion of contamination of the paper itself with metallic ions i t is simply necessary to heat the alizarin paper after developing the purple color, for color changes produced by other than NH3 are not reversible on drying. While litmus paper also loses its blue color developed by NHa on drying, the change is from the basic blue to the neutral bluish tint, a change which is not too sharp. With small quantities of NHa, or with small quantities of impurities on the paper, the alizarin paper allows much clearer testina. There is no need to give detailed directions for the w ,, ..Quditative Analysis by Spot Tests,o 2nd English use of alizarin paper in the scheme for elementary quali- Edition, Nordemann Publishing Company, N ~ W York. 1939, tative analysis, for i t is simply substituted for litmus P P 115-18. a ENGELDER, DUNKELEERGER. AND SCHILLER. "Semi-micro
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Prsented at the 1944 meeting of the West Virginia Academy of Science at Fairmont. West Virginia, M a y 5. 1944.
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Qualitative Analysis," 2nd Edition, John Wiley and Sons, Inc., New York, 1940, p. 184.
