DETECTION OF FLUORIDES USING THE ZIRCONIUM LAKE OF ALIZARIN
The color reaction between fluorides and the red zirconium lake of alimrin is utilized as a test for fluorine. This is a specific and sensitive test easily applied. The preparation and use of a stable reagent and test @ p e n are described. These test @pers, although less sensitive than the reagent solution, form a convenient and simple method for performing the test. A note on the interference of fluorides i n the colorimetric determination of aluminum using Alizarin S is included.
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During an investigation of some metallic derivatives of alizarin sulfonic acid De Boer (1) found a characteristic reaction for zirconium (the insolubility of its alizarinate in concentrated hydrochloric acid). This test, however, could not he performed in the presence of fluorides, even mere traces interfering. He therefore suggested this reaction as a possible test for the fluoride ion. It was found that the use of alizarin ( 1 2 dihydroxy-anthraquinone), Color Index number 1027 (Z),instead of alizarin sulfonic acid, gave a more
sensitive reagent which was less likely to react with interfering substances. This paper reports a simple method for preparing the test reagent, the interferences, sensitivity, and the preparation and use of convenient test papers. To prepare the reagent, dissolve 0.5 g. of alizarin in 200 cc. of alcohol with the aid of heat. Dissolve 1.5 g. of zirconium chloride in about 75 cc. of alcohol. Mix the two solutions and allow the precipitated lake to settle. Filter or centrifuge and wash twice with alcohol. The moist precipitate is then suspended in suffiaent alcohol to make the volume 25 cc. Five cc. of this suspension is added to 100 cc. of water and shaken thoroughly. This more or less colloidal suspension is the reagent for use in the test and should be shaken before use. After some preliminary experiments the following procedure for performing the test was adopted: to 2.5 cc. of the neutral or slightly acid solution to be tested add an equal volume of concentrated hydrochloric acid. Then add 0.5 cc. of the reagent and mix. Allow to stand for not longer than fifteen seconds. If the test solution contains more than 0.3 347
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mg. of fluorine the red color of the reagent will turn yeUow practically immediately. Fifteen-hundredths mg. of fluorine takes about five seconds to decolorize the reagent, while 0.03 mg. requires fifteen seconds. This is the practical limit of the test. It is possible to make this reaction more sensitive by using less reagent and allowing to stand longer, but this is done at the expense of its specificity; ions such as sulfate, phosphate, and oxalate then decolorizing the reagent. In the known absence of interfering substances one may detect 0.0015 mg. of fluorine in 5 cc. by using 0.1 cc. of the reagent and allowing to stand five minutes. Under the conditions of the test as given, the ions found to interfere were the strong oxidizing halogen salts, such as chlorates, bromates, and iodates, which liberate the free halogen on acidification. Any combination of salts which liberates free halogens will produce the same interference. The liberated halogen bleaches the color of the reagent. In order t o obviate this interference add a small excess of a freshly prepared saturated solution of sodium sulfite tothe test solution before acidificationand proceed with the test as usual. Colored ions and compounds giving a precipitate on acidification interfere in a physical manner by masking the color change of the reagent. In the presence of interfering colored ions the method of isolating the fluoride ion will, of course, depend on the nature of the interfering substance. Various methods can easily be devised for each particular case, but in general it is usually more practical to have recourse to one of the well-known inorganic tests when the separation is at aU lengthy. Oxalates, sulfates, and phosphates have a tendency to decolorize the reagent on long standing, but under the conditions given do not interfere. High concentrations of phosphates cause the colloidal reagent to form large flocs, but they are not decolorized in the time allowed. Complex ions containing fluorine as silicofluorideor borofluoride give a positive test. A convenient though less sensitive method of performing the test is by means of test papers which may be prepared as follows: 2 g. of zirconium chloride is dissolved in 100 cc. of alcohol. Hardened filter paper, such as No. 50 Whatman's, is immersed in this solution and allowed to dry. The papers are next immersed in a solution of 0.1 g. of alizarin in 100 cc. of alcohol and hung up to dry. When dry, cut off and discard the edges of the paper and cut the remainder into strips 5 mm. wide. To perform the test, add an equal volume of concentrated hydrochloric acid to the neutral or slightly acid solution to be tested. Immerse a strip of the test paper for five to ten seconds. The reddish color of the paper will turn a pure yellow if fluorides are present. Orange tints of the paper do not indicate a positive test. The reaction is sensitive to about 0.3 mg. of fluorine per cc., the yellow color appearing in about ten seconds. Higher concentrations of fluoride ions effect a much more rapid color transition. The color of the test papers should be noted before the expiration of about twenty seconds.
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If allowed to stand longer high concentrations of sulfate and phosphate are liable to give misleading tints. Free halogens in the test solution may be treated with sodium sulfite as given above. In the determination of aluminum by the method of Atack, using Alizarin S Red (sodium salt of alizarin sulfonic acid), as described by Yoe (3) the interference of fluorides is not mentioned. It was found that as little as 0.3 mg. of fluorine in the final solution containing large amounts of aluminum produced complete decolorization of the aluminum lake, rendering any colorimetric comparison completely in error. The objection to the use of the aluminum lake as a test reagent for fluorides is its solubility in mineral acids. Literature Cited DB BOER,C h m . Weekblad. 21, 404 (1924). (2) "British Color Index of the Society of Dyen and Colorists," Leeds. England, 1924, p. 248. (3) YOE,"Photometric Chemical Analysis.'' Volume 1, John Wiley & Sons. Inc., New York City, 1928, p. 119. (I)
