Semimicro Detection of the Silver Group of Anions1 J. STANTON PIERCE and EVELYN HAZARD University of Richmond, Richmond, Virginia
HE trend toward semimicro methods in industry furic acid. Compare the treated and untreated porand research is causing a similar trend in the tions of the solution. A blue-green color or blue teaching of elementary qualitative analysis. Serni- precipitate in the treated solution indicates femmiao methods based on standard procedures are cyanide. ~articularly adapted to the teaching of this new 5. Sulfide. (If a good test is obtained for ferritechnique since at the same time the student learns cyanide, sulfide is not likely to be present.) To the the reactions involved and develops an acquaintance remaining precipitate from 2, add 4 to 8 drops of 18 N sulfuric acid and 3 to 5 drops of N silver nitrate. Boil with the new methods. This paper takes up the semimicro detection of the gently for 1 to 2 minutes. A dark precipitate indicates silver group of anions. Most of these tests are semi- sulfide. Ferricyanide also gives a dark precipitate micro modifications of standard procedures, but some but the latter has a tinge of yellow or orange. possess novelty in the fact that the interference of 6. Iodide. To one-fifth of the filtrate obtained other anions is cut to a minimum by separation before in 2, add 2 ml. of M ammonium sulfate, 10 drops of the final tests are carried out. Only two tests involve 15 N ammonium hydroxide, and 3 drops of N silver the use of complex organic reagents. nitrate. (If i t is desired to test for one-fifth of a drop or less of 0.1 N iodide in this aliquot portion and if it is known that ndther the bromide nor thiocyanate PROCEDURE ROR THE TESTS is over ten times the iodide couceutra1. To a few drops of the unknown, add 1drop of N concentration tion, a turbidity a t this point can be taken as a test silver nitrate. Let stand for several minutes. If a for iodide.) Centrifuge. Discard the filtrate. Shake precipitate is formed, add 1 drop of 6 N nitric acid. the precipitate with 4 ml. of 4 N ammonium hyIf a precipitate remains, some member of the group droxide. If the precipitate does not dissolve iodide likely is present. No precipitate indicates the absence is present. of all members of this group. 7. Bromide. Transfer one-fifth of the filtrate 2. If a positive test was obtained in 1, add to 3 ml. from 2 to a small beaker, add 5 ml. of water, 10 drops of the unknown solution, 5 drops of N sodium acetate, of 6 N sulfuric acid, and 3 drops of 6 N potassium and 5 to 10 drops of N cadminmnitrate. If no precipinitrite (the latter only if iodide is present), and evapotate is formed, sulfide, femcyanide, and ferrocyanide are absent. If a precipitate forms, centrifuge. Test rate slowly to about 3 nil. Transfer the solution to a for complete precipitation. Save the filtrate. If the small test tube, add 4 to 8 drops of N potassium filtrate is cloudy, filter or centrifuge until i t is clear. permanganate, and heat for several minutes on a water Paper pulp sometimes helps to clear up a cloudy bath, passing the gas evolved through fluorescein filtrate. Wash the precipitate by stimng with a few paper in the mouth of the test tube. A pink color of the paper indicates bromide. drops of water and centrifuging. Discard the wash[Note: This paper is prepared by soaking filter ings. Stir the precipitate with 10 drops of water. paper in a solution of fluorescein in a water-alcohol 3. Ferrocyanide. To one-half of the suspension (1: 1) mixture, and drying. The paper is moistened of the precipitate from 2, add dilute hydrochloric with distilled water just before being used. Freshly acid until the solution is strongly acidic. Add 10 prepared paper should be used.] drops of 0.1 N ferric chloride. A blue precipitate in8. Chloride. Dilute one-fifth of the filtrate from dicates ferrocyanide. Centrifuge. Test for complete 2 to about 15 ml. Add 24 drops of 6 N nitric acid. precipitation. Save the filtrate. If i t is not clear, Evaporate slowly to about 7 ml. Continue heating centrifuge again or filter. 4. Ferricyanide. To one-half of the filtrate from and add 4 drops of M manganous nitrate and 5 drops of 3,add 5 drops of freshly prepared saturated solution N potassium permanganate. Evaporate to 2 ml. Centrifuge. Discard the precipitate. Add 2 drops of ferrous sulfate, which is distinctly acidic with sulof N silver nitrate to the filtrate. A white precipitate 'Acknowledgment is made to John Anderson and K . E. indicates chloride. A slight cloudiness is not a test. Calender, former students of the senior author at Georgetown 9. Cyanide. To one-fifth of the filtrate from 2, College, Georgetown, Kentucky, for their work in the earlier add 10 drops of 0.1 N sodium chloride and 3 to 5 development of these procedures, and to Dr. Warren C. Johnson, drops of N silver nitrate. Centrifuge. Stir the prewho supervised their testing at the University of Chicago. 126
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cipitate thoroughly with about 1.5 ml. of 2 N ammonium hydroxide and add 15 drops of N potassium bromide. Acidify with concentrated sulfuric acid. Heat for several minutes in a water bath, passing the evolved gas through freshly prepared, moist, strongly alkaline (with sodium carbonate) picrate paper. The test for cyanide is an orange color. If i t is known that sdfite and thiosulfate are not present in appreciable amounts, this test may be carried out without the precipitation of silver cyanide (along with silver chloride) and solution in ammonium hydroxide. 10. Thiosulfate. To the remainder of the 6ltrate from 2 add 5 drops of N silver nitrate. Centrifuge and test for complete precipitation. Wash the precipitate by stimng with a few drops of water and centrifuging. Discard the washings. Add 5 drops of 4 N ammonium hydroxide to the precipitate. Shake well. Black particles of silver sulfide indicate thiosulfate. Save precipitate and solution. 11. Thiocyanute. To the precipitate and solution from 10, add 2 drops of saturated potassium bromide and 2 drops of 18 N sulfuric acid. Centrifuge. To the filtrate add 1 drop of 6 N femc chloride. An orange-red color is a test for thiocyanate. DISCUSSION OF RESULTS
Tests were carried out by careful students to determine the sensitivity and reliability of the tests. Unknowns containing one drop of 0.1 M solution of each2 of the anions gave tests for each anion. For convenience in semimicro work, the total volume of unknown usually was approximately 3 ml. However, tests were obtained for each of the anions in solutions containing one drop of 0.1 M anion solution in a total volume of 30 ml. Sulfide, ferrocyanide, and femcyanide were precipitated as cadmium salts, filtered, and tested for as usual. The filtrate was evaporated to about 5 ml. for the rest of the tests. Ferricyanide is the only anion in this group which is an oxidizing agent. Femcyanide is an oxidizing agent in alkaline solution, and in slightly acid solution it reacts readily with cyanide, iodide, and thiosulfate, but i t does not react with these anions so readily in sodium acetate solution. Therefore, the separation of ferricyanide from the above anions is camed out a t the outset of these tests so that there 'Sulfide reacts readily with ferricyanide in acid, neutral, or alkaline solution, so the coexistence of appreciable amounts of each of these anions is incompatible. Therefore, both newr should be put in the same unknown.
will be little opportunity for reaction. When the unknowns were tested soon after they were made up, so that mutual reactions did not take place between the anions, or so that atmospheric oxidation did not take place, careful students usually obtained tests for one drop of 0.1 M solution of each of the anions in the presence of 10 drops of 0.1 M solution of each of the other anions. For this work the tests were compared with controls. Due to the reactivity of some of the anions, changes in composition take place with extreme readiness in certain combinations. For example, high concentrations of femcyanide and high concentrations of reducing constituents facilitate the formation of ferrocyanide, so frequently a positive test for ferrocyanide was obtained although i t was not initially present. Sulfide in solution is oxidized readily by atmospheric oxygen so that a thiosulfate test is obtained. Some c. p. sulfides contain appreciable amounts of thiosulfate. Therefore, i t is advisable to test sulfide solution for thiosulfate when i t is made up, and later from time to time, unless i t is kept from exposure t o air. If the laboratory supply of sulfide contains thiosulfate, a solution of sulfide free of thio-auIfate can be made by passing hydrogen sulfide into 0.1 N sodium hydroxide solution. Since several of the anions in this group are reducing agents, the presence of a high concentration of oxidizing agent would be expected to decrease appreciably the sensitivity of some of these tests. To see the effect of typical oxidizing agents, tests were made in the presence of chromate and nitrite. One drop of 0.1 M solution of each of the anions in this group usually gave a test in the presence of 10 drops of 0.1 M solution of each of the other anions in this group and chromate and nitrite. However, slight modifications of the tests were necessary in a few cases. For example, in the bromide test, chromate was reduced before the acid solution was heated to remove iodide, or bromide was lost. Sodium sulfite was a satisfactory reducing agent. In the final ferrocyanide test an extra washing of the cadmium precipitate cut down the tendency of the oxidizing agent to react with ferrocyanide when the solution was acidified. In the procedures given above, only one-fifth of the solution is used for each of the anions which forms a soluble cadmium salt. While one drop of 0.1 M solution of each anion in the unknown is sufficient to give a test, as has been shown repeatedly by the work of careful students, it is advisable to use somewhat larger amounts in student unknowns.
