A SYSTEMATIC ANALYSIS OF THE ANIONS. I ALKE D. DUSCHAX'AND M. C. SNEED, UMVERS~TY OF MINNESOTA, MINNEAPOLIS, MINNESOTA
Herewith is presented a systematic scheme for the qualitative separation and identification of the anions. The reagents used are inexpensive and the procedure is simple enough to be used by freshman and sophomore classes. By exercising proper care, the tests are suficiently accurate to detect one or two milligrams of any of the twenty-three more common inorganic anions in ten milliliters of solution in the presence of as much as 500 milligrams of another anion.
. . . . . . PART I
There is as yet no generally accepted system for the qualitative separation and identification of the anions, such as exists in the case of the positive ions. In the majority of qualitative procedures indication of the presence or absence of the more common anions is sought by a series of tests camed out on separate portions of the material under examination. Undoubtedly this method is useful in cases where there is some previous knowledge of the source of the material and it needs to be examined for only a few ions. However, in cases where the composition is entirely unknown, and especially when the amount of material available for analysis is small, it would be desirable to have a t hand a more systematic procedure by which tests for a large number of ions could be camed out on one rather small portion. A number of attempts have been made to systematize the analysis of the anions. One of the most complete of these procedures is that of A. A. Noyes (1). Others have been devised by George W. Sears (Z),G. L. Coyle. (3), Milobendeskii (4), Curtman & Hart (5), Curtman & Plechner (6), Karaglanov & Dimitrov (7), and Fernaudez & Gatti (8). None of these, however, have proved satisfactory. The purpose of these procedures has been to develop a systematic analysis which should make use of inexpensive reagents, and which should be simple enough to be used by large freshman or sophomore classes, and yet sufficiently accurate to detect one or two milligrams of any of the more common inorganic anions in ten milliliters of solution in the presence of as much as 500 milligrams of any other anion.
General Discussion The anions which are included in the system of analysis are the following: From a thesis submitted by Miss Alice D. Duschak to the Graduate School of the University of Minnesota in partial fulfilment of the requirements for the degree of Doctor of Philosophy. 1177
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Borate Bromate Bromide Carbonate Chlorate Chloride Chromate Cyanide
Ferricyanide Ferracyanide Fluoride Hypochlorite Iodate Iodide Nitrate Nitrite
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Perchlorate Phosphate Sulfate Sulfide Sulfite Tbioeyanate Thiosulfate
In the procedure for the detection of the anions as many ions as possible have been separated into groups based on the solubilities of their calcium, barium, cadmium, and silver salts, as follows: Ions whose calcium salts are insoluble in neutral solutions. Ions whose calcium salts are soluble, but whose barium salts are insoluble in neutral solutions containing 20y0 of acetone by volume. Group III Ions whose calcium and barium salts are soluble, hut whose cadmium salts are insoluble in neutral solutions. Group IV Ions whose calcium, barium, and cadmium salts are soluble but whose silver salts are insoluble in solutions slightly acid with HNOs. Group V Ions whose salts are practically all soluble. Group I Group I1
S i c e some of the ions form difficultly soluble salts with more than one of the group reagents, these ions may be found in more than one group. For this reason the separation of the anions into groups is not so definite and clean-cut as the separation of the cations. In the outline for division into groups (Table I), formulas enclosed in parentheses indicate precipitates which are formed only in solutions containing high concentrations of the anions concerned, and which, therefore, may not always be found in the groups under which they are shown. Discussions of the individual cases are included in the procetlures for the respective groups. In carrying out this system of analysis i t is assumed that the anions are present in solution in the form of soluble salts. In the analysis of an insoluble substance the solution may be prepared by any of the standard methods. S i c e the separation into groups depends on the precipitation of substances insoluble in water but in many cases soluble in acids, i t is essential that the solution a t the beginning of the analysis be neutral or slightly basic. If it is acid, it should be neutralized by a very slight excess of sodium carbonate, any precipitate formed being filtered off and rejected. If strongly basic, it should be nearly neutralized with acetic acid. In precipitating the first three groups the acetates of calcium, barium, and cadmium are used as precipitating agents for two reasons: 1. The acetate ion has neither an oxidizing nor a reducing action, and forms no insoluble salts with the ions used as group reagents, and, therefore, does not destroy any of the anions or interfere with the separation of any of the groups.
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ANALYSIS OF ANIONS.
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I
2. Solutions of the acetates of calcium, barium, and cadmium are slightly basic, due to hydrolysis, while the corresponding salts of the strong acids are slightly acidic. Since these group separations depend on the insolubility of the salts in neutral solutions, the acidity produced by hydrolysis in solutions of salts other than acetates may be sufficient to prevent complete precipitation. The general precautions to be observed in carrying out the analysis may be listed briefly as follows: 1. Keep solutions cool and neutral or slightly basic until after Group IV is precipitated. 2. Keep volumes as small as possible by using the smallest volume of each precipitating agent that will give complete precipitation and by rejecting all but one or two small washings in each case. 3. Some of the precipitates are very finely divided and have a tendency to pass through the filter paper. In cases where even a fine-grained paper will not retain the precipitate, filter paper pulp is found a very useful aid to iiltration. TABLE I Division into Groups Neutral solution containing all anions. Add Ca(CHsCO&
Filtrate: Add Ba(CHCO& and acetone
I-
Filtrate: Add Cd(CHsC02), Precipitate Bas01 Precipitate Filtrate. Add NiNOs Bas01 BaCr01 Ba(I0a)n NOa and HNOI [Ba(BrO&l [BaSzOal " [B@F~(CN)BI CIOr AgBr clor AgI AgCNS BrOh Ag&01 [IOsl Ag& Nor Nor [BOrl
I
- .-
-
PART I1 Procedure I. Analysis of Group I the neutral or slightly basic solution 1. Precipitation of the Calcium Salts.-To containing all the anions add a 2 N calcium acetate solution until the precipitation is complete. Stir or shake the mixture well and allow it t o settle. Filter the mixture, and wash the precipitate thoroughly with hot water.
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2. Identification of the Carbonate Ion.
