Spectrographic Studies of Coprecipitation Fourth-Period Elements with Barium Sulfate, and Copper and Zinc with Lead Sulfate LOUIS WALDBAUER, F. W. ROLF, AND H. A. FREDIANI, State University of Iowa, Iowa City, Iowa
All barium sulfate precipitates for spectrographic analysis were obtained by the method of Waldbauer and Ganta (11). Purified salts were used in the preparation of spectrographic standards, the range of which was 0.01 to 1 per cent of each cation. The spectrographic standards had the same barium sulfate and lead sulfate content as the precipitate under examination.
The coprecipitation of iron, cobalt, nickel, chromium, and manganese with barium sulfate and copper and zinc with lead sulfate was studied spectrographically. All precipitates of barium sulfate were prepared by the method of Popoff and Neuman, which consists of addition of the sulfate ions to the solution of the barium ions. It was found that cobalt and nickel do not coprecipitate with barium sulfate, whereas iron, manganese, and chromium do. Copper and zinc coprecipitate with lead sulfate.
Apparatus The apparatus for the preparation and study of precipitates is essentially that used by Popoff and Keuman (7) and Popoff, Waldbauer, and McCann (8). Platinum Gooch crucibles were used in the barium sulfate precipitations, while Jena glass crucibles were used in the study of lead sulfate precipitations in order that the precipitates studied gravimetrically could be used in the spectrographic study without the danger of contamination with asbestos. It was necessary to use asbestos in the case of barium sulfate, since no Jena crucibles suitable for use with barium sulfate were available. The graphite electrodesowere found to show a slight lead contamination. Line 2614.2 A. appeared when a photograph of the spectrum, emitted by the pure electrodes alone, was taken. This is one of de Gramont's "raies ultzmes" for lead (1). No traces of the other elements involved were found.
I
N QCAKTITATIVE analysis it is of extreme importance that the precipitate used in the determination of any element or group be as pure as possible. I n practically all cases,
General Methods of Analysis
other ions are present in addition t o the one being determined. It is, therefore, of prime importance to know whether the elements or groups present will coprecipitate with the substance being precipitated and t o what extent the weight of the resulting precipitate will be affected. Since the elements in the fourth period are frequently encountered in the determination of sulfate as barium sulfate and since lead is commonly determined as the sulfate in brasses with both copper and zinc in solution, the problem deserves careful study. Spectrographic methods as carried out by Popoff, Waldbauer, and McCann (8) and Kaldbauer and Gantz (11) were used in this study.
BARIUMSULFATEPRECIPITATIONS. Two hundred fift milliliters of water were added to an acidified (3 to 4 ml. of 6 dhydrochloric acid) solution containing a 10-ml. excess of barium chloride and that amount of cation impurity equivalent to all the barium to be precipitated as the sulfate. The barium chloride solution was kept just below the boiling point by means of a hot plate while 40 grams of the standard sulfuric acid were added dropwise with constant stirring. The precipitates were digested for one hour at 65' C. on a constant-temperature air bath. The mother liquor was decanted, and the precipitate was washed with four 15-ml. portions of water, and then washed into the crucible. The precipitates were heated for one hour at LOO" C. For spectrographic determination, the electrodes and solutions were prepared as previously debcribed (8). An equal number of drops of standard, or of solution to be analyzed, were placed in each electrode, and the electrodes dried in an oven for 8 hours a t a temperature of 250' to 300" C. The high temperature is necessary to dry the electrodes completely, for they do not burn well when there is even a small quantity of sulfuric acid in the graphite. LEADSULFATEPRECIPITATIONS. Stock solutions of the nitrates of lead, copper, and zinc were prepared. Equivalent portions of lead solution were then placed in each of four beakers. The first and third portions were analyzed for lead sulfate content merely with the addition of 10 ml. of nitric acid followed by 5 ml. of concentrated sulfuric acid. The second and fourth portions contained 10 ml. of the foreign metallic solution (either cop er or zinc) added previous to the addition of the sulfuric acJ The method of preci itation of lead sulfate was essentially the same as described for farium sulfate. The solutions containing the precipitates were evaporated on hot plates until all the nitric acid was removed. The beakers and contents were then cooled to room temperature, each watch glass was rinsed off into its respective beaker with 50 ml. of redistilled water, and the solution and precipitate were warmed gently for 5 minutes and then cooled for one hour in an ice bath. The precipitates were then filtered through the Jena crucibles, and washed with 100 ml. of 2 per cent sulfuric acid solution and then with 30 ml. of the 50 per cent alcohol solution. Finally they mere dried in the oven between 130' and 140" C. for 4 hours, weighed, replaced in the oven for one hour, and reweighed. In no analysis was it necessary to dry the precipitates more than three times for them to reach constant weight. The rocedure for spectrographic determination was essentially that oPNitchie (6) and Popoff, Waldbauer, and McCann (8). The visual analysis of the plates was made with a bifocal microscope (magnifying power of X7) using transmitted light reflected from a sheet of white paper.
Materials Used Water, sulfuric acid, and nitric acid were purified by distillation. All water used in making solutions and in recrystallizations was redistilled from alkaline permanganate solution in an allPyrex still having a block tin condenser. Nitric acid and sulfuric acid were distilled in a Pyrex still. Upon examination in the spectrograph, the water and acids showed no trace of the elements to be studied. All salts were recrystallized from redistilled water to remove any impurities that were initially present. Fifty per cent alcohol was prepared from commercial 95 per cent ethyl alcohol decanted from silver nitrate, and distilled from calcium hydroxide. A sulfuric acid solution, made up from the distilled acid so that 40 grams of solution were equivalent to approximately 0.8 gram of barium sulfate, was used in all barium sulfate precipitations. This solution was standardized against Bureau of Standards potassium acid phthalate (7). The barium chloride solution was made to contain 21 grams per liter. When the presence of any of the cations was desired as an impurity in the solution of barium chloride, solid chlorides of the metals were added instead of stock solutions, to eliminate any change, such as hydrolysis, which would make it difficult to determine exactly the quantity of metal being added. In the case of iron, ferric ammonium chloride was used because it is a crystalline substance of definite composition, whereas ferric chloride tends to hydrolyze. The lead, copper, and zinc standard solutions were prepared by dissolving spectrographically pure metals in distilled nitric acid. Ten milliliters of each solution contained a weight of other metal equivalent to 0.5 gram of lead sulfate.
888
889
ANALYTICAL EDITION
December 15, 1941 Lines Used Copper Zinc Cobalt Nickel Iron Nanganese Chromium
2961.18 and 2961.89 A. 3282.3, 3302.6, and 3345 A. 2694 A. 2416 A. 2598 2576 2593.7 and 2605.7 A 2843:25, 283k.64, and 2830:47
quantitative spectrographic study was limited to iron, manganese, copper, and zinc for the reasons mentioned above. Standards were prepared for the elements in question, and by comparison with these standards the approximate per cent of the element coprecipitated was determined. The results are recorded in Tables I and 11.
A.
A.
Experimental Results
Discussion
Of the elements in the fourth period, titanium and vanadium were not studied because of the inherent difficulties in their use, no scandium salts were available, and potassium (7) and calcium (11) had previously been studied. The first procedure was to determine qualitatively whether or not the element in question did coprecipitate. The precipitates of barium sulfate were prepared from barium chloride solutions containing amounts of barium chloride and impurity, each equivalent to 0.8 gram of barium sulfate. Lead sulfate precipitates were prepared from solutions containing lead nitrate and impurity, each equivalent to 1.0 gram of lead sulfate. Spectrograms were taken, and the presence of elements being studied was determined by the presence or absence of the most persistent lines. The absence of the lines of the element in question was considered as conclusive evidence that the element was not present only after careful check by recognized analytical procedures. Cobalt and nickel were the first elements studied. Mellor (4) states that cobalt coprecipitates with barium sulfate, and Johnston and Adams ( 2 ) and Lange and Berger (3) report that nickel coprecipitates with barium sulfate. However, neither cobalt nor nickel could be found in the precipitates by spectrographic methods. When regular qualitative methods using organic reagents were used, no trace of either element could be found. The organic reagents used Rere: for cobalt, nitroso-beta-naphthol and for nickel, dimethylglyoxime (6).
I n the cases of iron, manganese, and chromium the weights of precipitates obtained were less than the theoretical, indicating that some phenomenon other than adsorption took place. This phenomenon may possibly be due to the formation of complex ions containing iron, manganese, and chromium similar to those discussed by Recoura (9, 10) in which iron and chromium are in the form of “ferrisulfuric acid” and “chromosulfuric acid”.
O F IRON, MANG.4NESE, TABLEI. CONTAMINATION CHROMIUM IN BARIUMSULFATE
Element None
Difference from Theoretical
Barium Sulfate Calculated Found Gram Gram 0.8236 0.8315 0.8015 0.7860
-
0.8234 0.8317 0.8015 0.7861
Av. Iron
0.8604 0.8195 0.8624 0.8128
0.8512 0.8096 0.8521 0.8024
hlanganese
0.8115 0.8165 0.8144 0.8567
0.8099 0.8146 0.8220 0.8542
Chromium
0.8859 0.8075 0.8060 0.7957
0.8795 0.8008 0.7975 0.7868
Mg. 0.2
+ 0.2 + 00 .. 01 + 0.025 -
9.2 9.9 -10.3 -10.4 Av. - 1 0 . 0
Av.
Av.
- 1.6 - 1.9 - 2.4 - 2.5 - 2.1 - 6.4 - 6.1 - 7.5 - 8.9
- 7.4
AND
E 1em en t in BaSOa by Spectrograph
-%
.. .. .. ..
0.75 0.75 0.75 0.75 0.01 0.01 0.01 0.01
.... .. ..
I n like manner iron, manganese, and chromium with barium sulfate and copper and zinc with lead sulfate were studied. Iron and manganese were found to be coprecipitated with barium sulfate. Because of the juxtaposition of other lines with the L‘raiesultimes” of chromium, the spectrographic study of the coprecipitation of this element was considered of little value. Indications, however, pointed strongly toward coprecipitation of chromium. It was itlso found that copper and zinc were coprecipitated with lead sulfate. The gravimetric study, therefore, was devoted to iron, manganese, chromium, copper, and zinc; the
TABLE 11. CONTAMINATION OF COPPER AND ZINC IN LEAD E SULFATE Difference from Actual Mg.
Element in PbSO4
0.1 0.1
... ...
... ...
0,4124 0.4120 0.4121
1.8 1.4 1.5
1.7 1.3 1.4
0 2875 0.2875
0.2876 0 2878
0.1 0.3
... ...
0.4 0.3 0.3
0 2875 0.2875
0.2888
1.3 1.5
1.1 1.3
PbSOa Calcd.5 Gram
PbSOa Found Gram
0,4106 0,4106
0.4107 0,4107
0.4106 0.4106
0.4106
Pure P b Zn present
Pure P b Cupresent
Difference from Calcd.
0,2890
iWg.
7%
...
...
0.4 0.4
0 Calculated PbSOa value was determined electrolytically by depositing lead in a known weight of standard lead nitrate solution as PbOr on anode.
The data seem t o indicate that zinc contamination of lead sulfate could be due to mixed crystal formation, since zinc sulfate and lead sulfate crystallize in the orthorhombic system. It does not seem probable, however, that copper contamination was due to mixed crystal formation, since the pentahydrate of cupric sulfate crystallizes in the triclinic system. It is more likely that adsorption, occlusion, or possibly postprecipitation occurred in these cases.
Conclusions Iron, manganese, and chromium are coprecipitated with barium sulfate. Cobalt and nickel are not coprecipitated with barium sulfate. The data for iron, manganese, and chromium indicate the formation of complex ions containing these elements. Copper and zinc are coprecipitated with lead sulfate. The data for zinc indicate the possibility of mixed crystal formation, whereas the data for copper indicate the possibility of adsorption, occlusion, or postprecipitation.
Literature Cited (1) Gramont, A. de, Compt. rend., 17’1, 1106 (1920). (2) Johnston and Adams, J. Am. Chem. SOC., 33, 829 (1920). (3) Lange and Berger. Z.Elektrochem., 36, 176 (1930). (4) Mellor, “Comprehensive Treatise on Inorganic and Theoretical Chemistry”, Vol. 3, p. 766, New York, Longmans, Green and Co., 1923. (6) Nitchic, C. C., IND. ENG.CHEM.,ANAL.ED., 1, 1 (1929). (6) “Organic Reagents for Metals”, 2nd ed., pp. 33 and 67, London, Hopkin and Williams, 1934. (7’) Popoff and Neiinian, IND.ENQ.CEEM.,ANALED., 2, 45 (1930). (8) Popoff, Waldbauer, and McCann, Ibid., 4 , 4 3 (1932). (9) Recoura, Ann. chim. phys., (7’) 4, 494 (189.5). (IO) Recoura, Z.Elektrochem., (S) 11, 278 (1907). (11) Waldbauer and Ganta, IND.ENQ. CHEM., ANAL. ED., 5, 311 (1933). EXTRACTS from theses submitted by Frederick Rolf and Harold Frediani to the State University of Iowa for the degree of M.S.
