A SCHEME fir QUALITATIVE CHEMICAL ANALYSIS EMPLOYING SPOT TESTS W. CULE DAVIES University College, Cardiff, Wales
T
HE present scheme of analysis for basic radicals is a complete one It adopts as a basis the hydrogen sulfide method of separation into groups, but then uses "special" reanents for the identification of the members of each group, instead of the often very complicated chemical separations of the older methods. This scheme is an adaptation of the very comprehensive
system of chemical analysis in use a t the Technical University of Delft.' In its present form the scheme only deals with the identification of the more commonly occurring basic radicals, and in many cases the special
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1 Nr~uwsmunc AND DULPER,"A short manual of systematic qualitative analysis by means of modern drop reactions,u Amsterdam, 1935.
TABLE 1 GROUP1. Add hydrochloric acid t o the hot solution of the substance. Residue (Group 1A). AgC!, Hg&: washinas. Wash with ammonla. Residue,Black-Hg,++.
Filter.
Wash with hot water, rejecting the
Filtrate: Allow to cool. Filter. White, crystalline residue (Group 1B). ( f i n f i r m hv T e s t 2
PbCL.
Washings: Make slightly acid with hydrochloric acid. Precipitate: AgCI. (Confirm by Test 1.)
GROUP2. Filtrate Groups 2, 3, 4, 5 and 6.. Oxidize with bromine water, bail, bring acidity to about 2 N hydrochloric acid. Pass in hydrogen sulfide for five mmnutes, then ddute m t h water until acidity is about 0.2 Nhydrochloric acid (test with methyl violet paper), snd pass in hydrogen sulfide for fifteen minutes. Filter. Residue (Group 2). Wash with very dilute ammonium nitrate solution, rejecting the washings, then with hot 2 Nsodium hydroxide. Residue, HgS, PbS, CuS XS, and Bi2Sa: Treat with hot nitric acid (1:l). Washings: Divide into 2 parts. (1) Add an excess of ammonia: blue coloration indicates copper. ( C o b by acidifying a drop with sulfuric acid, and continuing with Test 4.) Filter. Test a portion of the residue for bismuth by Test 5, and wash the remainder with very dilute sulfuric acid, followed by water. A residue indicates l e d . (Confirm by Test 2.) (2) Add excess sodium hydroxide. Filter. Wash residue with water and test it for cadmium by Test 6.
Residue, HgS: Dissolve a small portion in bromine water. and apply Test 3.
Gxoup 3. Filtrate Groups 3, 4. 5 and 6. Boil in order to remove hydrogen sulfide and 10 reduce the bulk. Add bromine water and rcmove the exces bv boiline. Add 1 e. of solid ammonium chloride and excess of immonH (sp. gr. 6880). Warm until the mixture just commences t o boil. Filter.
Alkaline washings: Add an equal volume of cancentrated hydrochloric acid, gently warm for some time. Filter. Residue, As& (Confirm by Test
7.)
GROW5. A c i d i i Groups 5 and 6 with glacial acetic acid and boil t o remove hydrogen sulfide. If necessary 6Iter-rejecting any residue.' Then make slightly alkaline with ammonia and add ammonium carbonate. Warm for a few minutes. Filter. Residue (Group 5). BaC08, CaCOa, SrCO,: Pour a small amount of Warm the wash- - dilute acetic acid over the residue. ines to drive off carbon dioxide. ~ d d oatassium t dichromate saiutiou and some sodium acetate. Filter. ~
Residue (Group 3). Fe(OH)r, AI(OH)S. Cr(OH)r (possibly some Mn(0H)S. Treat with excess hat 2 N sadtum hydroxtde. Residue. &(OH)*, Cr(OH)r. (and M?(OH)*). Test small oortmns for . . . . .-----~~ iron by Test 10, for chromium by Test 11, and for manganese by Test 12.
Alkaline washings: Apply Test 13 for aluminium. Note: Some of the chromium mav dissolve here; it does not, howeve;. interfere with the aluminum test,but can be reprecipitated by further boiling of the alkaline washings.
Filtrate: Boil t o remove all hydrogen sulfide. ( ) Test 2 or 3 drops for anttmouy by Test 8. (2) Reduce 1 cc. with alumimum. Filter. Test a drop of the liltrate for tin by Test 9.
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Residue, BaCr0,-yellow. s
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Filtrate: Warm, add solid sodium carbonate. Filter, and wash the residue with water-reject the filtrate and washin&. Residue: Pour over the filtei'paper a small amount of dilute acetic acid. Test washings (a) for calcium by Test 17, and for stro~tiumby Test 18.
Filtrate GROW6. Divide into two parts. (1) Smaller part: Add ammonium phosphate and allow t o stand. The ~ r e c i ~ i t a of t e ammonium maenesium vhosuhate is apblv concollected andbmi&d with sodium hvdroxiae. ~ h & Ke~idue(Group 4). NiS. CoS, MnS, and ZnS (possibly some firmatory Test 19 for magnesium. (2) Larger part: Evaporate to dryness t o remove ammonium Cr&: Dismlve in a little concentrated hydrochloric acid (it r n a v l. w.nereswm add hvdroecn veroxide t o r o m ~ l e t e salts. Add water. Divide the aqueous extract into two parts. ~ to ~ - some , and test one Dart for botassium b v T e s t 20. and the other for Boil off most of the a&d agd then add a little witer. &tion). * Test portions of the resulting solution for nickel by Test 14, for sodium by T& 21. BoiPa portion of the original substance with sodium hydroxide cobalt by Test 15, for mangame by Test 12, for zinc by Test 16. solution. An ammonium sall is indicated by evolution of amand for chromium by Test 11. monia (odor and action on moist litmus paper).
GROW4. Filtrate Groups 4, 5 and 6. Pasrhydrogen sulfide for several minutes into the hot iiltrate. Filter.
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tests are adapted from those described in the publication, "The B. D. H. Book of Organic Reagents."= The method has several commendable features. It uses smaller quantities of original substance and is therefore especially economical in all the reagents used. It might be argued that explanations of some of the tests are obscure, but, on the other hand, others are very
'The
9 B. D. H. book of organic reagents," 6th ed., The British Drug Houses, London, England, 1937.
instruckve (complex formation, adsorption, and so forth). The analyst's main task, however, is identification, and the present scheme offers considerable advantages in rapidity and certainty. Some radicals, for example tin and antimony, which create much difficulty in the ordinary way, are now identified with the greatest of ease and with absolute certainty. Before commencing the systematic analysis, a preliminary examination should be made in the ordinary
way. The sample is prepared and a solution made for the systematic separations as usually recommended, but using not more than 0.3 gram of the material. Nitric acid, if required to bring the sample into solution, should be removed by evaporation of the solutiou to dryness. Then the residue is extracted with hot dilute hydrochloric acid. Table 1 gives the procedure to be followed for the group separations. The special tests then follow. These tests are camed out by spotting on to a strip of pure filter paper, or by mixing in a depression of a porcelain spotting tile, or in small test-tubes. The special tests given here have been selected with regard to their simplicity, convenience, and accessibility of the materials required. It is not suggested that they are the best tests available for each estimation, but each one has proved quite satisfactory in class work. New tests are frequently appearing, and revisions and replacements may sometimes be found desirable. The method now described has been used with complete success in the Analytical Laboratories a t University College, Cardiff. Students appreciate the speed and accuracy, and are impressed by the neatness of the tests. Furthermore, they are introduced to some of the simpler methods of microchemistry-a distinct advantage for later training. A marked improvement in the students' laboratory technic has resulted. Thus, in spite of the fact that several writers have recently criticized the use of qualitative analysis and especially of spot tests in the teaching of practical chemistry, the author has no hesitation in recommending a scheme such as that now described for the consideration of teachers. TESTS FOR METALLIC RADICALS TEST 1-Silw ( A ) Add a drop of two per cent. aqueous manganous sulfate, a dmo of ammonia. and a dron of sodium hvdroxide solution to some silver chloride spotted on to a strip of filt
