The TESTING of ORGANIC REAGENTS for INORGANIC ANALYSIS HELMUT M. HAENDLER University of Washington. Seattle, Washington
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N A recent paper, Yoe (3) proposed a rapid method for the testing of organic reagents to determine their adaptability for colorimetric inorganic analysis. This method can he extended, with a few modifications, to include testing of organic reagents for use in qualita-
tive spot tests and in precipitation analysis. Important possibilities might easily be overlooked if the reagent is studied only from a colorimetric standpoint, and for the sake of completeness, spot tests, precipitation tests, and colorimetric tests should be run on each
new or suspected reagent. The same reagent and metal solutions can be used in each case. Practically all important metals can now be obtained rather reasonably, and, if necessary, a few of the rarer ones could be omitted. Each periodic group can be represented by a t least one member so that coverage is almost complete. Certain elements, rhenium for example, are difficult, if not impossible, to keep in cationic form. In such cases the most common anionic form, such as perrhenate, can be used. Solutions containing 5-10 mg. of the metal per ml. afford a better basis of comparison than solutions made from the same weights of different salts, and also more nearly approximate the average stoichiometric proportions, in particular if the reagent has a low solubility in the solvent employed, which, incidentally, should be miscible with water. When available, acetates are preferable, as they generally give neutral solutions; otherwise chlorides or nitrates will prove satisfactory in most cases. When possible, a one per cent. reagent solution, usually in water or alcohol, should be used. Should the solubility of the reagent be less, a saturated solution is best.
studied individually to determine the best conditions for possible qualitative use. PRECIPITATION TESTS
The results of the spot tests will sometimes give indications as to the elements which will precipitate with the reagent, but all metals should be tested, nevertheless. Precipitations can be carried out very satisfactorily in the spoon ends of small porcelain spatulas. A small filter is prepared by fitting a small bottle with a 2-hole rubber stopper bearing a glass elbow for connection to the suction line and a short length of vertical glass tubing, one end of which has been softened and flattened, without much constriction of the inner bore, by pressing it onto an asbestos plate. Two 15-mm. square pieces of filter paper are placed over the end of the tube, moistened with water, and the center is pushed down with a glass rod into the tube to a depth of 1-2 mm. A drop of the reagent and a drop of the metal solution are mixed in a spoon, allowed to stand for a minute or two, and then transferred to the filter, and washed with water. Gentle suction is sufficient. The true color of the precipitate can be noted and its solubility in acid, base, or organic solvent can be deterSPOT TESTS mined directly on the filter paper. If no precipitate is Complete spot tests of all the metallic ions in the formed in the spoon the solution can be made acidic periodic table can be made very rapidly by an adaption or basic to determine any possible influence of pH on the of Yagoda's method of quantitative spot tests (2). precipitation. Large, rectangular sheets of filter paper are ruled in the In any event, should the reagent form precipitates form of a periodic table, that of Werner ( I ) being ex- with one or more of the metallic ions these should be ceptionally well suited for later study and comparison. studied more in detail. This detailed study is someSquares about 25 X 25 mm. are a convenient size, what similar to that for colorimetric reagents, and and a 12-15 mm. diameter paraffined spot is placed should include: (1) identification of the compound in the center of the square corresponding to each metal formed, whether it is an inner complex, molecular, to he tested. The circles can be made by direct im- and so forth; (2) determination of the completeness of print of the paraffin with a warmed metal tube of suit- formation, for incomplete precipitation naturally bars able diameter and thickness, or by direct transfer of the i t from quantitative use; (3) determination of the wax from a sheet of paraffined tissue paper by means of stability of the compound, whether it is suitable for the warm metal tube. After cooling, a drop of re- direct weighing; (4) effect of pH; (5) sensitivity; (6) agent, or a corresponding amount if less than a one interfering ions; and (7) possibilities for indirect per cent. reagent solution is used, is put in the center of analysis. each circle, and the paper is allowed to dry, supported COLORIMETRIC TESTS out of contact with any surface. As soon as the reThese tests have already been discu~sed;~ in them agent has dried, the metal solutions are added and the colors noted. If desired, other sheets can be used to care must be taken to insure the presence of sufficient determine the color in acid or basic medium, the acid reagent to give a complete reaction with the metal of the metallic salt anion being used. Should any present. Thus a fairly complete picture of the analytical satisfactory indications be obtained, each spot can be possibilities of a new organic reagent can be deterWERNER. A , , "Neuere Anschauungen auf dem Gebiete der mined in the space of a few hours. The collection of anorganischen Chemie," Braunschweig, Vieweg & Sahn, 1923, such data should aid immeasurably the development p. 9. of more accurate theories of this branch of analytical YADODA, H.. Ifid. Eng. Chem.. Anal. E d . 9, 79 (1937). chemistry. S Y ~J. ~ H.., J. CHEM. EDUC.,14,170 (1937).
