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Determination of Bismuth as Phosphate W. C. BLASDALE
AND
W. C. PARLE, University of California, Berkeley, Calif.
A
LTHOUGH the gravimetric method for the determination of bismuth as phosphate has been in use since 1905, and a number of papers have been published regarding it, the details of procedure recommended show a wide range of variation, and there is a decided lack of detailed quantitative data upon which a rational procedure might be based, The data here reported are designed to supply this deficiency.
Properties of Bismuth Phosphate The solubility of bismuth phosphate does not seem to have been determined. In order to ascertain its value a sample of the granular precipitate, separated from an acid solution, with a moderate excess of diammonium hydrogen phosphate was used. The well-washed precipitate was suspended in a large bottle of distilled water, which was kept immersed in a thermostat held a t 27" C. and shaken occasionally, for a period of 15 days. Twenty 50-cc. portions of the solution were then evaporated in a platinum dish and the residue was heated to 140" C. Its weight was found to be 0.0033 gram, indicating a molal solubility less than that of barium sulfate, Assuming complete dissociation and no hydrolysis, this would correspond to a solubility product of approximately 1.1 x 10-10. The residual precipitate showed no recognizable change in its physical properties and apparently had not undergone appreciable hydrolysis. When caused to separate under the conditions defined below as standard, the precipitate consists of coarse granules, which settle almost a t once and can be filtered and washed very rapidly, After drying a t 240' C. the precipitate did not change in weight appreciably when heated t o 1000" C. When caused to separate from solutions containing large concentrations of ammonium phosphate, an appreciable amount of the latter seems to be adsorbed. The evidence for this statement is the fact that when such precipitates were dried up to a temperature of 240" C. the weight found was slightly in excess of the correct value and the precipitate was decidedly hygroscopic. After ignition to about 800" C. the excess of weight was eliminated and the precipitate was no longer hygroscopic. In order to prevent the simultaneous separation of small amounts of basic salts it was found necessary that the solution should contain rather large concentrations of nitric acid and that both bismuth- and phosphate-containing solutions should be heated before mixing. Even under these conditions some of the precipitate which first separates is flocculent, but rapidly becomes granular a t 80" C. This may be due in part a t least t o an aging effect, but is largely due to the presence of small amounts of basic salts, which change into the phosphate if the solution contains sufficient acid and if the temperature is kept a t 80" C. The evidence for this statement is the fact that, with barely acid solutions and a t ordinary temperature, the rate of change in the character of the precipitate is less, the tests for nitrate ion in the precipitate disappear more slowly, and the deficiency in the total weight of precipitate found, in solutions containing known amounts of bismuth, is greater. Best Conditions for Precipitation In order to ascertain the effect of variations in the details of procedure a solution containing 20 grams of pure Bi (N08)8.5H20 and 50 cc. of concentrated nitric acid per liter was prepared and standardized by two methods. In the first, 50-cc. portions were evaporated in a platinum dish and the residue was heated cautiously, and finally brought to a temperature sufficient t o fuse the resulting bismuth oxide. The weights obtained were 0.2549, 352
0.2544, and 0.2553 gram. The average value (0.2549) corresponds to 0.3326 gram of BiP04. The bismuth present in 50-cc. portions of the solution was also determined by separating as basic carbonate with a slight excess of ammonium bicarbonate, digesting for a half hour, filtering on a Gooch crucible, and igniting. The results obtained were 0.2548, 0.2541, and 0.2546 gram. The average value (0.2546) corresponds to 0.3321 gram of BiPOd. A large number of preliminary quantitative determinations led to the adoption of the following standard procedure: The acid solution of bismuth was neutralized with ammonium hydroxide until a slight but permanent precipitate was produced; this was then dissolved in 2 cc. of concentrated nitric acid and the volume increased to 100 cc. The solution was heated to boiling, 50 cc. of 0.2 M diammonium hydrogen phosphate, also heated to boiling, were added slowly during the course of about 3 minutes, and the mixture was kept at 80' C. for an hour. The solution was filtered hot on a Gooch crucible, washed three times with 50-cc. portions of hot water, transferred to the crucible with cold water, dried, and ignited to dull redness for a half hour. The results obtained with 50-cc. portions of the standard solution were 0.3319, 0.3322, 0.3317, 0.3319, and 0.3322 gram. The average value is 0.3321 gram. When the filtrates from these determinations were neutralized with ammonium hydroxide and allowed to stand for 24 hours, no recognizable precipitate separated, nor could bismuth be detected by means of the sulfide test. Two additional determinations, in which the solution was made neutral towards methyl orange before the digestion at 80' C., gave 0.3311 and 0.3319 gram.
Effect of Varying Conditions The effect of variations from the standard conditions given above is shown in Table I. TABLE I. WEIQHT OF BISMUTH PHOSPHATE FOCXDBY MODIFYINQ STANDARD PROCEDURE Variations 1 instead of 2 cc. of concd. "Os used 4 instead of 2 cc. of conod. HNOa used 100 instead of 50 cc. of 0.2 M N H I ) ~ H P Oused I 4: 100 instead of 50 cc. of standar6 bismuth solution used 5 . 10 instead of 50 eo. of standard bismuth solution used, 6. Precipitate filtered without digesting at 80' C. 7. Precipitate filtered after digesting and allowing to stand 16 hours 8. 3 grams of KNOBadded 9. 3 grams of NaNOs added
1. 2. 3
BiPOl Found Gram 0 . 3 3 1 7 rtnd0.3310 0.3327 rtnd 0,3325 0.3318and0.3313 0.6659and0.6630 0.0653and0.0655 0 . 3 3 1 1 and0.3322 0.3310 and 0.3315 4.3315 and0.3323 0.3321and0.3311
These results suggest the following comments: Series 1 and 2 show that H. can be increased to unexpectedly large values (at least 0 . 4 N ) without decreasing the yield of precipitate. These findings are a t variance with the statement of Schoeller and Waterhouse (1) that the method is decidedly sensitive to variations in H-. Series 3, 4,and 5 indicate that the concentration of soluble phosphate used can be varied within rather large limits without detrimental results. The authors assume this to be due to the relatively slight solubility of the precipitate even in the presence of nitric acid. The much larger concentrations of soluble phosphate recommended by Schoeller and Waterhouse (1) seem unnecessary and result in a greater amount of adsorption of the precipitant used. They assume sufficient phosphate must be added to reduce H T t o a small value but the authors do not find this necessary. Series 6 and 7 indicate that the conversion of the basic nitrate into phosphate is usually nearly completed within a few minutes, if the solution is hot and contains sufficient H+. and that long standing does not appreciably affect the result. ,4 number of qualitative experiments, in which the rate of
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SEPTEMBER 1.5, 1936
ANALYTICAL EDITION
conversion of basic nitrate into phosphate was followed by testing the precipitate for nitrate ion, indicated the desirability of digesting the precipitate for an hour before filtering. Series 8 and 9 indicate but little error from occlusion of potassium and sodium and the feasibility of precipitating with phosphate of potassium or sodium rather than ammonium.
Separation of Bismuth from Other Ions The slight solubility of bismuth phosphate in dilute nitric acid suggests the possibility of separating it from a number of metals with which it is frequently associated. Table I1 gives experiments with 50-cc. portions of the standard bismuth solution, in the presence of certain nitrates. TABLE11. WEIGHTOF BISMUTH PHOSPHATE
a
(Found by the standard procedure in the presence of other ions) Added'" BiPOd Found Gram 0.3324 and 0.3317 Mg(N0s)t 0.3311 and 0.3323 Zn(Nod z 0.3353 and 0.3368 Cd(N0dz 0.3323 and 0.3324 Cu(N0s)z 0.3326 and 0.3341 Ca(NOs)z 0 , 3 7 7 0 and 0.3881 Pb (N0s)z 1 gram in each case.
Obviously the separation of bismuth from magnesium, zinc, copper, and calcium by this process offers no difficulty.
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The separation from cadmium gives a slight error, probably due to occlusion, and separation from lead, a t least under the conditions here suggested, is not possible.
Summary of Results Bismuth can be accurately determined as the phosphate, if separated from solutions which contain neither C1- nor SO1-- and are approximately 0.2 M as to nitric acid and approximately 0.065 M as to soluble phosphate. The method is accurate in the presence of moderate concentrations of Nai, Kf, Mg++, Zn++, CU++,and Ca++, gives slightly high results in the presence of Cd++, but is not accurate in the presence of Pb++. The chief source of error is the co-precipitation of basic salts; this can be avoided, in the presence of sufficient H+, by precipitating from a hot solution with a hot dilute phosphate solution and digesting for an hour a t 80" C. A second possible source of error is the occlusion of small amounts of ammonium phosphate; this can be eliminated by avoiding large concentrations of soluble phosphate, and by igniting the precipitate to 800" C. before weighing. Literature Cited (1) Schoeller and Waterhouse, Analyst, 45, 436 (1920).
RECEIVED June 16, 1936
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Microdetermination of Carbon and Hydrogen In Compounds Containing Arsenic, Antimony, Tin, Bismuth, and Phosphorus F. C. SILBERT A~%DW. R. KIRNER, Coal Research Laboratory, Carnegie Institute of Technology, Pittsburgh, Pa.
D
IFFICULTIES have been reported in the literature on the determination of carbon and hydrogen in compounds containing arsenic (IO),antimony (9),tin ( 6 ) ,bismuth ( 7 ) ,and phosphorus (1, 2, 3, 6, 8). In some cases these difficulties can be successfully overcome by the addition of suitable materials-. g., lead chromate, red lead, or copper oxide -to the combustion tube filling and to the sample in the boat. In certain cases these, and other subterfuges, fail and some investigators have determined the carbon only by a wet-combustion method. In many cases no attempt is made to determine either carbon or hydrogen, only the percentage of the metallic element present being determined. In connection with a study of the exhaustive chlorination of coal conducted in this laboratory, samples which contained appreciable quantities of antimony or phosphorus were submitted for analysis. Since a knowledge of the distribution of the elements of the coal among the reaction products was required in this work, it was important that the carbon-hydrogen content of the samples be known with some degree of accuracy. When the antimony-containing samples were analyzed for carbon and hydrogen by the ordinary microprocedure using a Pregl Universal tube filling, considerable amounts of a white crystalline substance (antimony oxide) condensed in the constricted end of the combustion tube and in the capillary and forechamber of the Dehydrite-filled absorption tube, which, of course, produced an error in the hydrogen determination. Further, a carbon-hydrogen determination on a pure, known compound made directly following such an analysis yielded erroneous results. In the case of the phosphorus-containing sample tlhere was no visible difficulty, but again a directly succeeding analysis on a pure, known
compound yielded false results, so that no confidence could be placed in the analysis of the phosphorus compound. Of the methods mentioned in the literature, the modified Dennstedt method recommended by Falkov and Raiziss (4), for the macrodetermination of carbon and hydrogen in organic arsenicals and mercurials, seemed most likely to succeed when applied to compounds containing antimony and phosphorus. It was found that a modification of their method could be successfully adapted to the microanalysis of such compounds. The micromethod was first tested on a pure, known organic arsenic compound and the results of Falkov and Raiziss were confirmed. The method was then extended to pure, known compounds containing, respectively, antimony, tin, phosphorus, and bismuth, and was finally applied to the abovementioned coal samples.
Experimental The analytical samples, unless otherwise designated, were pure compounds obtained from Eastman. The phosphorus and bismuth compounds were recrystallized from methyl. alcohol, the arsenic and antimony compounds from ethyl alcohol, and the tin compound from ether. Instead of using a Dennstedt catalytic combustion on platinum as recommended by Falkov and Raiziss, a portion of the copper oxide-lead chromate filling of an ordinary Pregl Universal filled combustion tube was removed and replaced by a 3-cm. cylinder made of 200-mesh platinum wire gauze filled with granulated red,lead and a 1-cm. plug made of platinum wire gauze. The red lead was prepared by igniting Merck's lead peroxide ( p r o analysi, granuliert nach Pregl), in a stream of oxygen in a microcombustion tube in an electric combustion furnace at the normal combustion temperature. To prevent any red lead dust from sifting
