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INDUSTRIAL AND ENGINEERING
CHEMISTRY
313
CONCLUSION Calcium, strontium, and cadmium are all co-precipitated with barium sulfate. Beryllium, magnesium, and zinc are not co-precipitated with barium sulfate. The data for cadmium indicate the preferential adsorption of iodide ion or of Cd14--.
(3) Heym, Ann. Phys., (9) 12, 443 (1919). (4) Johnston and Adams, J. Am. Chem. Soc., 33, 829 (1911). (5) Mellor, “Comprehensive Treatise on Inorganic and Theoretical Chemistry,” Vol. 3, p. 766, Longmans, 1923. (6) Nitchie, (3. ( 3 . 3 I N D . EXQ.CHEM.,21, 1 (1929). (7) Pollock, Sci. Proc. Roy. Dublin Soc., 11 (N.S.), 184 (1907). (8) Popoff and Neuman, IXD.ENG.CHEW,Anal. Ed., 2, 45 (1930). (9) Popoff, Waldbauer, and McCann., Ibid., 4 , 4 3 (1932).
LITERATURE CITED
RECEIVED April 16, 1933. Presented before the Division of Physical and Inorganic Chemistry at the 85th Meeting of the American Chemical Society, Washington, D. C., March 26 to 31, 1933. From the thesis of E. St. Clair Gants for the M.8. degree, State University of Iowa.
(1) Cornog, J . Am. Chem. Soc., 43, 2573 (1921). (2) Fajans, “Radio Elements and Isotopes,” P. 95, McGraw-Hill, 1931.
Determination of Calcium in Lead-Calcium Alloys of Low Calcium Content BEVERLY L. CLARKEAND LELAND A. WOOTEN,Bell Telephone Laboratories, Inc., New York, N. Y.
T
H E purpose of this work was the development of a rapid, high-precision method for determining calcium in lead-calcium alloys containing from 0.02 to 0.06 per cent calcium. Small amounts of several metallic impurities were present in the alloy. After the removal of the lead as sulfate from the nitric acid solution of the alloy, calcium and the impurities remain as the nitrates and sulfates. It is generally agreed that the best form in which to separate calcium for determination is the oxalate, and where speed is desired the volumetric evaluation of the calcium oxal’ate is obviously preferable to the gravimetric. Our problem was therefore that of modifying the classical method for determining calcium by the permanganate titration of the sulfuric acid solution of the oxalate, so that our requirements of precision and rapidity would be met. It was desired to run two concurrent determinations in an hour, with a precision of +0.002 per cent calcium. Shaw, Whitternore, and Westby (2), of the Western Electric Company, have published a method for this determination. They dissolve a 20-gram sample of the alloy in fuming nitric acid, add sulfuric acid, filter off the lead sulfate, and add successively ammonium hydroxide in excess, ethyl alcohol, and ammonium oxalate. The mixture is boiled for 2 minutes, and the precipitate allowed to settle for 5 minutes. The mixture is filtered through asbestos, the precipitate dissolved in sulfuric acid, and titrated hot with permanganate in the presence of the asbestos. A careful trial of this method led us to suspect compensating errors. Repeated determinations on the same sample were generally consistent when performed on the same day and by the same analyst; but were generally inconsistent when either of these conditions did not obtain. This observation pointed to the existence of uncontrolled variables. A careful search for such variables led to the discovery of the following sources of error and experimental difficulties: 1. Incompleteness of the calcium precipitation under the prescribed conditions. 2. Difficulty in selecting end point in presence of asbestos, and possible reducing action of the asbestos. 3. Failure to make an ammonium hydroxide separation of the Second and Third Group Metals before precipitating calcium as oxalate. 4. Solubility of calcium oxalate in the unneutralized hot wash water.
5 . Incomplete removal by washing of the excess ammonium oxalate in the calcium oxalate precipitate. The experimental basis for so characterizing the above follows : 1. The Western Electric Company method specified a cooling period of 5 minutes a t room temperature before filtration of the calcium oxalate precipitate. Since the completeness of the precipitation is a function of the final temperature of the solution, results obtained in summer will show a negative error from this cause measurably higher than in the case of those obtained in winter. Experiment confirmed this; microchemical examination of the filtrates from the calcium oxalate precipitation showed that cooling in an ice bath reduced the error by approximately 50 per cent. 2. The presence of asbestos in the solution being titrated was found definitely to obscure the end point, and to produce erratic results in check determinations. A positive error was introduced, caused either by a reducing action of the asbestos or by the tendency to overtitrate in turbid media, or by both. A certain alloy gave successive values of 0.051, 0.056, 0.053, and 0.055 per cent calcium by the Western Electric Company method. When the same alloy was analyzed by the method modified by the substitution of a Frittig glass filtering crucible for the asbestos Gooch, the values obtained by the same analyst were 0.040, 0.041, 0.042, and 0.042 per cent. 3. It was originally suspected that a source of error in the method lay in the failure to separate the Second and Third Group Metals as hydroxides before precipitating the calcium as oxalate. Upon investigation, however, it was proved that no measurable error was produced from this cause, at least in the case of the alloy in which we were primarily interested. This alloy contained on the average the following impurities: copper, 0.07 per cent; tin, 0.002 per cent; bismuth, 0.001 per cent; antimony, 0.001 per cent; arsenic, 0.002 per cent; and iron, 0.002 per cent. We found, however, that when the Frittig crucible was substituted for the asbestos mat, the hydroxides of these metallic impurities, if not previously removed, tended t o clog the Frittig crucible and render filtration prohibitively slow. For this reason, and also to guard against samples with abnormal amounts of impurities, it seemed desirable to filter off the ammonium hydroxide precipitate before precipitating the calcium. 4. Hahn and Weiler ( 1 ) state that calcium oxalate is appreciably soluble in hot water, but much less so in dilute ammonia solution. I n corroboration of this statement, on a
314
ANALYTICAL EDITION
synthetic lead-calcium solution containing 0.0396 per cent calcium, we obtained a value of 0.0344 per cent on washing with hot water, and 0.0381 per cent when the washing was done with cold 1 per cent ammonium hydroxide. The last washings were proved to be free from oxalate by testing with permanganate. 5. Tests with permanganate showed that washing a calcium oxalate precipitate three times with hot water does not remove all the excess precipitant. To some extent this error compensates that given in the preceding paragraph. It is clear from the foregoing that the Western Electric Company method, while doubtless satisfactory where speed is essential and a moderate precision sufficient, involves uncontrolled variables and compensating errors which make it unsuitable for work requiring the highest precision. We attempted to devise an improved method which would be free from those objections. The procedure follows:
PROCEDURE To a 20-gram sample of the finely milled and thoroughly mixed sample, contained in a 400-ml. beaker, add 100 ml. 1 t o 3 nitric acid. When the vigorous reaction has ceased, heat on a hot plate to hasten solution. When solution is complete, wash the sides of the beaker with hot water and add 20 ml. 1 to 1 sulfuric acid. Remove the lead sulfate by suction filtration through an asbestos mat and wash thoroughly with hot water. Reject the precipitate. At this point the volume of the solution should be between 150 and 175 ml. Neutralize the solution with ammonia and add 15 ml. in excess. Boil for 5 minutes and filter through a No. 42 Whatman filter or an asbestos mat, washing the precipitate with hot water. Reject the precipitate. To the filtrate add 10 ml. concentrated ammonium hydroxide, 30 ml. of 95 per cent alcohol, and stir well. Add 2 rams of Kahlbaum's "pro-analyse" ammonium oxalate. Boil for 3 minutes The volume of the solution should be about 300 ml. Remove from hot plate, rinse cover glass and sides of beaker with water, and allow to stand in an ice bath for 5 minutes. Temperature of solution after cooling should be 45" to 55" C. Filter the re ci itated calcium oxalate on a clean Frittig glass crucible 1&4), washing beaker six times with cold 1 per cent ammonium hydroxide. Wash precipitate six times with the 1 per cent ammonia solution, filling the crucible each time. Reject washings. Remove crucible from holder, and wash holder and outside of crucible thoroughly with distilled water t o insure freedom from oxalate. Replace crucible in holder and dissolve the calcium oxalate by pouring six 15-ml. portions of hot 10 per cent sulfuric acid throu h the crucible. Wash once with hot water after each addition of acid by filling the crucible Add to the filtrate 20 ml. 1 to 1 sulfuric acid and dilute to 300 ml. Heat the solution t o 80' to 90" C. and titrate t o a faint permanent pink with 0.05 N potassium permanganate.
&Ti
NOTESON PROC~DURE. 1. New Frittig crucibles should be allowed to stand overnight in dilute chromic acid mixture before being used. Crucibles should be kept in chromic acid mixture when not in use. Accumulated lead sulfate may be removed from the pores of the crucibles by washing with hot ammonium acetate solution. 2. All asbestos used should be treated with Concentrated hydrochloric acid and then washed free of chlorides with distilled water.
THECONSTANT ERROR A series of experiments was performed to determine the accuracy of the method. Solutions for analysis were prepared by dissolving 20-gram samples of commercial lead, containing the impurities stated earlier in this paper, in nitric acid and adding a measured amount of standardized calcium chloride solution. The lead was obtained from the St. Joseph Smelting & Refining Company. For the preparation of the standard calcium solution, Kahlbaum's purest grade of calcium carbonate was used. A spectroscopic examination of this material showed traces of barium, strontium, and magnesium only, the total impurities being estimated a t less than 0.03 per cent.
Vol. 5, No. 5
STANDARDIZATION OF SOLUTIONS. Calcium Chloride Solutions. Dry calcium carbonate was dissolved in c. P. hydrochloric acid and the solution diluted to the desired volume. The calculated calcium values of these solutions were found to agree very closely with those determined by the standard oxalate-permanganate method. Portions of the solutions (50 ml. or 100 ml.) were used for the standardization. In precipitating the calcium oxalate, the solution was allowed to digest overnight. Standard Potassium Permanganate Solution. Merck's c. P. potassium permanganate was used in the preparation of 0.05 N solutions which were prepared and stored with the usual precautions. The solution in use was frequently standardized against Bureau of Standards sodium oxalate. The data are summarized in Table I and show that the method invariably gives results which are in error in the sense of being too low, this error increasing somewhat as the calcium content decreases. The method is remarkably reproducible, however, as will be shown in the next section; and the existence of a constant error, if the source and magnitude of this error be certainly known, cannot be an objection. I n a search for the source of the constant error, the lead sulfate .and ammonium hydroxide precipitates were eliminated by spectroscopic and microchemical methods as containing negligible quantities of calcium. The filtrate from the calcium oxalate precipitate was found, however, to contain appreciable quantities of calcium. As a representative example, a synthetic solution containing the equivalent of 0.0397 per cent calcium gave an apparent calcium content of 0.0381 calcium, an error of -0.0016 per cent calcium. The filtrate from this determination was found by microchemical means to contain 0.34 mg. calcium, which is equivalent to 0.0017 per cent calcium on the 20-gram sample basis. The source of the constant error is therefore known, and cannot be eliminated without sacrifice of speed. The constant error for any given calcium content within the range covered can be obtained by interpolation from the data in Table I. TABLEI.
PER
CENT CALCIUX
ON
20-GRAM SAMPLE BASIS
AVERAQE DETERMIDEVIATION CONSTANT NATIONS ADDRD FOUNDFROM MEAN ERROR
%
%
%
%
0.0080 0.0041 fO ,0004 -0.0039 0.0100 0.0061 =to. 0006 -0.0039 0.0149 0.0136 fO .0004 -0.0014 0.0198 0.0181 11 ;to. 0004 -0.0017 0.0296 0.0277 f0.0006 -0.0018 16 0.0399 0.0381 f0. 0006 16 -0.0010 -0.0018 0.0492 0.0482 f O ,0003 7 0.0000 0 . 0005a fO.OOO1 +0.0006 7a a Blank determination on a 20-gram sample of commercial lead. 6 6 6
I n the last line in Table I are given the results of a series of blank determinations on 20-gram samples of the commercial lead. The value obtained of 0.0005 per cent calcium (0.1 ml. 0.05 N potassium permanganate) corresponds closely to the indicator error to be expected on a volume of 300 ml. in which the titration is usually made.
PRECISION OF METHOD Eight determinations were made on each of four alloys to determine the reproducibility of the method. The data are presented in Table 11. Since the correction applied for the constant error was obtained by the same method, it is sukject to the same accidental error. Even taking this into consideration, it is obvious from an inspection of the data in Tables I and I1 that the precision of the method is better than k0.002 per cent calcium. It should be noted that this high precision is obtainable only by skilled analysts, well practiced in the method. Such an analyst can run two concurrent determinations by this method in an average time of 55 minutes, which will agree within the stated precision. This time can be reduced by about 10 minutes if the use of fuming nitric acid for the solution of the sample is not objectionable.
September 15,1933
INDUSTRIAL AND ENGINEERING CHEMISTRY
TABLE 11. REPRODUCIBILITY OF METHOD ALLOY 1 1 1
1
1 1
1 1 2 3
4
AVERAGE AVERAGE DETERMI- APPARENT CORRECTED NATIONS CALCIUM ClLCIUM 1 1 1 1
1
1 1
1 8 8 8
% 0.0177 0,0189 0.0174 0.0187 0.0177 0.0177 0.0193 0.0177 0.0271 0.0406 0.0620
% 0.0197 0.0209 0.0194 0.0207 0.0197 0.0197 0.0213 0.0197 0.0291 0.0419
....
AVERAQE DEVIATION FROM MEAN
%
-0.0004 $0.0008 -0.0007 $0.0006 -0.0004 -0.0004 $0.0012 -0.0004 1 0 .0003 10.0004 10.0006
315
We are indebted to J. D. Struthers of this laboratory for performing most of the experimental work reported. LITERATURE CITED (1) Hahn and Weiler, 2. anal. Chem., 70, 1 (1927). (2) Shaw, Whitternore, and Westby, IND.ENQ.Cmix., Anal. Ed., 2,401 (1930). RECEIVED June 13, 1933. Presented before the Division of Physical and Inorganic Chemistry at the 86th meeting of the American Chemical Society, Washington, D. C.,March 26 to 31, 1933.
Silicon Tetrafluoride Volatilization W. D. ARMSTRONG, Laboratory of
Physiological Chemistry, University of Minnesota, Minneapolis, Minn.
H E volatilization of siliA highly simplijied apparatus, without gasand their connecting tubes. An scrubbing devices, for the quantitative evolution all-glass apparatus with only a con tetrafluoride as a trap a t -35" c. b e t w e e n the step in the determinaand collection of silicon tetrafluoride gives good digestion flask and the receiver Of fluorine has reached its results since the apparatus can be thoroughly also to give complete reh i g h e s t state of development dried and possesses no dead spaces* BY tempera- covery of fluorine under various in the t h o r o u g h s t u d i e s o f lure control, the loss of fluorine caused by hyconditions of p r o c e d u r e , A and Jacob (6) Reynolds, ROSS, and Shuey (6). These writers, drolysis of the tetrafluoride is usually prevented w h i t e , frost-like s u b l i m a t e and, should hydrolysis occur, the Juorine-conformed in the top of the trap and together with Adolph (1) and in the bends of the connecting Wagner and Ross (8)' adetaining products can be driven into the receiver tubes when scrubbers were used, quate reviews of the subject. Treadwell and Koch (7), wagner by the application of heat to the delivery tube. in spite of all p r e c a u t i o n s to eliminate water from the appaand Ross, and Casares (4) have reported the quantitative recovery of fluorine. However, ratus. This material may have been the same as the "strange Reynolds, Ross, and Jacob recovered from calcium fluoride deposit" first mentioned by Treadwell and Koch (7) in conan average of only 93.5 per cent of the calculated fluorine. nection with their gasometric method for measuring silicon Furthermore, the results of Shuey with sodium fluoride varied tetrafluoride. They, however, observed it only when working from $9.77 to 100.36 per cent and averaged 95.72 per cent. below atmospheric pressure. It seems likely that this deposit Willard and Winter (Q),using a modification of the Casares was silicic acid and hydrofluosilicic acid produced by the hyprocedure, obtained good recovery with inorganic fluorides drolysis of silicon tetrafluoride by water retained on the glass but found low results on the determination of fluorine added walls or formed in the reaction bulb. The rinsings of the trap and tubes were acid in reaction, and after several deterto feed ash. Previous workers, except Casares and others who have minations a white solid accumulated in the apparatus a t the employed a gasometric technic, have found it necessary to points where the original deposit collected. This substance was insoluble in water scrub the silicon tetrabut dissolved in boiling fluoride before it enalkali apd was therefore tered the receiver, in probably s i l i c i c acid. order to remove subI n an all-glass apparastances, chiefly acids, tus designed to collect which interfered with the analysis of the res i l i c o n tetrafluoride c e i v e r c o n t e n t s for quantitatively f r o m fluorine. Various so d i u m silicofluoride decomposed by heat a types of wash bottles similar deposit was obcontaining sulfuric acid served, and the results or s u l f u r i c acid plus of the titration of the silver sulfate or c h r o m i u m trioxide receiver contents f o r h a v e been used t o hydrofluosilicic a c i d a b s o r b hydrochloric were low. The washings of the connecting acid, sulfur d i o x i d e , tube of the apparatus and oxides of nitrogen. D required an additional Some o b s e r v a t i o n s a m o u n t of standard made by this writer inalkali sufficient to give dicate that these wash a quantitative recovery bottles cause a loss of f l u o r i n e chiefly on of fluorine f r o m t h e a c c o u n t of the diffid e c o m p o s e d silicoculty of drying them FIGURE1. SILICON TETRAFLUORIDE EVOLUTION APPARATUS fluoride.
T
