A SCHEME of QUALITATIVE ANALYSIS, INVOLVING the USE of ORGANIC REAGENTS* J. T. DOBBINS, E. C. MARKHAM AND H. L. EDWARDS University of North Carolina, Chapel Hill, North Carolina
M
ANY schemes for the analysis of the metallic elements have been devised, all of them based upon the arrangement of the ions into groups, depending upon their behavior with certain group reagents. By far the most frequently used of these analytical procedures makes use of gaseous hydrogen sulfide as an important group reagent. While this method is invaluable as regards the teaching of the fundamental chemical principles involved, it, nevertheless, presents some difficulties in its practical application in the laboratory. The use of hydrogen sulfide as a precipitating agent is always fraught with difficulties, especially in the hands of an inexperienced operator. Many laboratories are not equipped with suitahle hoods, and cases of poisoning by the escaping gas are not uncommon. Also, the method, as ordinarily practiced, is time-consuming, and, because of the use of a fairly insoluble gas as a precipitating agent, is extremely inefficient, much of the gas being lost in the process of precipitation. In this investigation the authors propose to make use of the reactions of certain organic compounds, in a new scheme of analysis of the cations. This has been done in order to avoid the use of hydrogen sulfide, with its attendant objectionable features. Upon preliminary investigation concerning the applicability of 8-hydroxyquinoline, cupferron, nitroso-&naphthol, and some other organic compounds, pyridine seemed to show the greatest promise. This reagent appears to be especially suitahle on account of four properties which it possesses: (1) In aqueous solution, pyridine produces a mildly alkaline reaction, basic enough to d e c t the complete precipitation of the very insoluble hydroxides of iron, bismuth, lead, antimony, aluminum, chromium, and tin, and yet not sufficiently basic to precipitate the more soluble hydroxides of copper, cobalt, nickel, cadmium, and zinc. Furthermore, it is not sufficiently alkaline to dissolve the amphoteric hydroxides of lead, aluminum, chromium, antimony, and tin; (2) i t forms complex ions with certain of the other cations; (3) with the thiocyanate ion, these complex ions form insoluble compounds with the elements
manganese, copper, cobalt, nickel, cadmium, and zinc, the structure of which may be represented by the formula:
and, (4) the insoluble complexes thus formed are decomposed by ammonia, and are converted into the corresponding soluble ammonia complexes. An exception occurs in the case of manganese, which is oxidized, and precipitates as hydrated managnese dioxide during the treatment with ammonia. This permits the separation of copper, cobalt, cadmium, nickel, and zinc from iron, bismuth, lead, aluminum, chromium, antimony, tin, and manganese, which have been precipitated as the hydroxides by the pyridinethiocyanate reagent. In brief, the method employed in the present scheme may be outlined as follows: In case the color of the original solution indicates the presence of either a chromate or a permanganate, it is acidified with nitric acid and hydrogen peroxide is added to reduce them to chromic and manganese ions, respectively. After this treatment, Group I, consisting of those metals which form insoluble chlorides, is removed, as in the older method, by the addition of hydrochloric acid. This method has proved so satisfactory that i t has seemed neither wise nor desirable to attempt any improvement upon it. After the removal of this group the solution is boiled with concentrated nitric acid so that arsenic, iron, tin, and antimony may be oxidized to their higher valence states. The elements of Group 11, arsenic and divalent mercury, are identified in small portions of this solution. The remainder of the solution is then made almost neutral with ammonia, and Group 111, consisting of iron, bismuth, lead, chromium, aluminum, antimony, tin, manganese, 'copper, cobalt, nickel, cadmium, and zinc, is precipitated by the pyridine-thiocyanate reagent. From the filtrate from "Taken in part from a thesis submitted by H. L. Edwards, Group 111, Group IV, which consists of barium, in partial fulfilment of the requirements for the degree of Doctor of Philosophy at the University of North Carolina, strontium, and calcium, is precipitated, as in the older August, 1937. scheme, by the addition of ammonium carbonate. The 14
GENERAL SCHEME OX ANALYSIS Solution contains all the metals in the form of salts. Acidify with HNOZand add Hx02to reduce (Cr04))-- and (MnOJ-. add dilute HCI and filter. PreciPitate: AgC1' PbCIs' HgC1
Filtrate: Groups II-V.
Add couc. HNOa and heat, to oxidize As+++, Fe++,Sb+++, and Sn++.
To one portion add ammonfum molybdate. Yellow ppt. indicates
Group I
To remainder of solution add NH40Huntil alkaline, then HCI until just acid; then add Py-CNS reagent.
AS+++++.
To another portion add SnCL. White ppt. turning gray indicates Hgf+.
II
Then
Precipitate:
Filtrate: Groups IV-V Make alkaline with NH,OH add (NH&COa and heat.
Fe, Bi, Pb, Al, Cr, Sb,
Sn (hydrox~des)
Cu, Co, Ni, Cd, Zn, Mu (Py-CNS complexes)
Group I1
1
Group 111
filtrate from Group IV is examined for Group - V,. which consists of magnesium and the alkalies. It has been found this scheme to replace ordinary filtrations by use of the centrifuge, wherebv the se~arationof precipitates is effected much more rapidly, of tieprocedure can be carried out in the same tube, thus avoiding the loss of preci~itate as as loss due to the use of filter paper. In the event that a Centrifuge is not available, all of the procedures outlined in this scheme may be readily ca,+ed outby ordinary atration methods. LABORATORYPROCEDURE Section I . Preparation of fhe Solutionfor Analysis.-A quantity of material is used for the analysis which contains approximately 50 mg. of each metallic ion present. If the substance is watersoluble, it is dissolved in 20 cc. of water; if insoluble, it is dissolved in the appropriate acid, the solution evaporated almost to dryness, and the residue redissolved in 20 cc. of water, t o which a small amount of nitric acid is added, if necessary. If the color of the original solution indicates the presence of either chromate or permangnate ions, they are reduced to chromium or manganses ions, respectively, by treating the acid solution obtained above with 2 cc. of three Der cent. hvdroeen pcruride and boilmg for t h x e or four minurcs. If the solution is culorlr~s,the nbsenc* of both
