A NEW QUALITATIVE TEST FOR ALUMINUM* K. KERSHNER AND R. D. DUFF
MIssounr Scxoo~OF MINESAND METALLURGY, ROLLA, MISSOURI
A n ether solution of purpurin i s used to detect aluminum by means of a stabilizing effect of the dye lake on the foam produced by agitation of a n ammonium hydroxide-ether mixture. The test is performed on the solution obteined by pouring 6 N NHpOH mer the mized hydroxides on the .Pter. 0.5 mg. of A1 can be detected in the presence o f 50 mg. of Cr, Fe, or other metals. I n the presence of minute quantities of other elements, 0.001 mg. of A1 can be detected without preliminary treatment.
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The use of organic dyes to detect minute quantities of metals in solution has grown rapidly during the past decade. The detection of some of these metals, such as aluminum and magnesium, is dependent upon the formation of a lake with the metal hydroxide. Several tests of this nature have been developed for aluminum. The "aluminon" test (1) (ammonium salt of aurin tricarboxylic acid) and Atack's test (2) (alizarin) are among the more successful of these tests. The authors believe that they have developed a test which compares favorably with those mentioned.
Preparation of the Reagent Dissolve 0.40 g. of purpurin and 0.01 g. of gum sandarac in a liter of ether. Ordinary gum sandarac and compercial ether will suffice. Alcohol tends to lessen the sensitivity, but the amount present in commercial cthcr * does not prevent detection of traces of aluminum. Gum sandarac is but slightly soluble in ether, and 0.01 g. is most conveniently added by first dissolving the gum in a cc. of alcohol. Purpurin is prepared ( 3 ) by oxidizing alizarin with manganese dioxide and sulfuric acid. The reagent will keep well in a glass-stoppered bottle, and may be dispensed for class use from a stoppered buret attached to the wall of the laboratory.
Procedure Add N ammonium hydroxide to 1 cc. of the solution to be tested until it is alkaline. Boil to coagulate the precipitate, and filter. Wash the drained precipitate three times with hot distilled water. Now pour 5 cc. of 6 N ammonium hydroxide over the precipitate and collect the filtrate in a clean test tube. Add 10 cc. of N ammonium chloride solution and 1 cc. of the dye reagent to the filtrate. Place a clean thumb or cover glass over the month of the test tube and vigorously shake the contents. A pink foam collecting above the solution and remaining for several minutes confirms
* Presented at a meeting of the Columbia, Mo.. section of the American Chemical Society. December 11, 1931. 1271
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the presence of aluminum. Avoid a second agitation as this tends to . disrupt the layer of stabilized foam. For class use, where the aluminum hydroxide is isolated on the filter, one washing with distilled water will suffice. When traces of aluminum are to be detected, N ammonium hydroxide is used in precipitation to avoid undue loss of aluminum. Six N ammonium hydroxide is used for the wash because it is the usual concentration found in a set of qualitative reagents, and because it will easily dissolve sufficient aluminum hydroxide for the test, according to the following equation: AI(OH)r
+ NEGOH --+ NH4AIOa+ 2HxO
The purpose of the ammonium chloride is to serve as a buffer and to return the ammonium aluminate to aluminum hydroxide to form a lake with the dye. Other ammonium salts serve equally well. Ammonium nitrate acts in some unknown manner t o make a somewhat redder foam. Gum sandarac is used to aid in stabilizing the foam. More than the specified amount is t o be avoided as it tends to form a colorless foam even in the absence of aluminum. Other substances, such as sugar, glucose, starch, dextrin, gum tragacanth, gum copal, and gum dammar can be used as stabilizers. The gas evolved from the agitation of the ammonia-ether mixture produces a foam which rapidly disintegrates unless i t is further stabilized by the aluminum-purpurin lake. The pink foam is composed of minute bubbles and frequently persists for a day or more. Comparison with Atack and Aluminon Tests I'
Small quantities of various elements tend to interfere with the Atack test by forming lakes, particularly upon long standing. In the "aluminon" test, iron and beryllium produce lakes which are difficult to distinguish from the aluminum lake. In the ether-purpurin test, certain other metals produce foams which have less stability and a darker color, but if the outlined procedure is followed, no interference is encountered from any of the elements included in the usual scheme of qualitative analysis, nor from Be, La. PI, Nd, or Ce. Silicic acid, borates, phosphates, fluorides ( 4 ) , and other anions do not interfere even though present in the solution when the test is made. Phosphates interfere with the "aluminon" test. Atack's test is sensitive to 1 part A1 in 10,000,000 parts of water in the presence of minute quantities of Cr and Fe, though often an hour or more must elapse before the lake forms. The "aluminon" test is sensitive to lo-= moles of aluminum, but detection becomes difficult as the limit is approached. The ether-purpurin reagent will detect 0.5 mg. of A1 in the presence of 50 mg. of Cr or Fe or other metals when the outlined procedure is followed. I t will give an extreme sensitivity of 0.001 mg. of A1 when only minute quantities of other elements are present and precipitation and
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washmg are omitted. Cr and Fe do not greatly interfere even when actually present during the test, as 0.1 mg. of Al can readily be detected with 3 mg. of Fe or 5 mg. of Cr. The ether-purpurin test has given satisfactory results as a qualitative reagent for class use over a period of two years and offers a considerable saving of time over other methods of testing for aluminum. Literature Cited HAMMETT AND SOTWRY, "A New Reagent for Aluminum." J. Am. C h ~ m Soc., . 47, 1 4 2 3 (Jan., 1925). ATACK,"A N e w Reagent for the Detection and Colorimetric Estimation of Aluminum," J. Soc. Chenr. Id.,34,936-7 (Sept.. 1915). DELALANDK, Ber., 7, 1545 (1874). STONE,"Detection of Fluorides Using the Zirconium Lake of Alizarin," J. CHEM. Eouc.. 8,347 (Feh., 1931).
