MAY 15, 1936
ANALYTICAL EDITIO?;
Literature Cited (1) Aoree and Kline, Bur. Standards J . Research, 5, 1063 (1930). (2) Auerbach and Bodlander, 2.angew. Chem., 36,602 (1923). (3) Cajori, J . B i d . Chem., 54,617 (1922). (4) Caldwell and Doebbeling, Ibid., 110,739 (1935). (5) Caldwell, Doebbeling, and Manian, J . A m . Chem. Soc., 58, 84 (1936). (6) Gobel, J . Biol. Chem., 72,801 (1927). (7) Linderstr$m-Lang and Holter, Compt. rend. trav. lab. Carlsberg, 19,No. 14, 1 (1933).
183
(8) MacLeod and Robinson, Biochem. J., 23,517 (1929). (9) Quisumbing and Thomas, J . A m . Chem. Soc., 43,1504 (1921). (IO) Sherman, “Organic Analysis,” 2nd ed., pp. 78-85, New York, Macmillan Co., 1917. (11) Sherman, Caldwell, and Adams, J. Am Chem. Soc., 50, 2529, 2535, 2538 (1928). (12) Sherman, Caldwell, and Boynton, Ibid., 52, 1669 (1930). (13) Sherman, Kendall, and Clark, Ibid., 32,1073 (1910). (14) Treadwell-Hall, “Quantitative Analysis,” Vol. 11, 6th ed., p. 554,New York, John Wiley & Sons, 1924. R B C ~ I V EJanuary D 28, 1936.
De termination of Phosphorus in Stainless Steels A Rapid Method Using Perchloric Acid CHARLES D. SUSANO‘ AND J. H. BARNETT, JR.,ZKnoxville, Tenn.
I
N THE course of recent months, the materials testing
laboratories of the Tennessee Valley Authority have been called upon to make a large number of analyses of stainless steels. Particular stress was placed upon the rapidity with which the analyses could be made. The analysis in general presented no unusual difficulties, but since time was an important factor, a more rapid method for the determination of phosphorus was sought to replace the one now in general use. The method now used requires the action of hydrochloric-nitric acid mixture to effect solution of the sample, oxidation of phosphides to phosphate, and the subsequent removal of hydrochloric acid, a t best a time-consuming operation fraught with many difficulties. Perchloric acid (60 per cent) was found to be suitable for replacing the above-mentioned acid mixture, thus eliminating the necessity of removal of hydrochloric acid. It is known that perchloric acid interferes neither with the precipitation of ammonium phosphomolybdate nor with its estimation by alkalimetric methods (6). It is shown below that phosphides in steel are completely oxidized to phosphate by the action of hot 60 per cent perchloric acid. The use of weaker concentrationti of perchloric acid is not recommended. Lundell advises that from 10 to 15 per cent of the phosphorus may be lost if a 50 per cent acid is used (6). The speed of the method lies in obviating the necessity of removing chlorides and in circumventing oxidation by nitric acid and potassium permanganate as practiced in the normal phosphorus procedure (4). It is known that perchloric acid may be: used for the removal of chlorides, when present, as hydrochloric acid in phosphorus determinations (7), but the power of perchloric acid to oxidize phosphides to phosphate has apparently not been described. The authors’ conclusions with regard to the accuracy and the precision of the suggested method are based on the results of repeated analyses of several Bureau of Standards samples and a comparison with the certified values given therewith. The data given below show close agreement with Bureau of Standards phosphorus values. To show that perchlorates do not interfere with the precipitation of ammonium phosphomolybdate, a series of determinations on pure diammonium phosphate was made in the presence of normal concentrations of perchloric acid. The results (Table 111) gave close checks with the calculated theoretical phosphorus content of the salt in each case. In the determination of phosphate in iron ore, Willard has substituted perchloric acid for nitric acid in the procedure for the removal of chlorides after silica dehydration. He re1 2
Tennessee Valley Authority, Knoxville, Tenn. Present address, The Reilly T a r & Chemical Corp , Chattanooga, Tenn.
ports that the presence of perchloric acid introduces no error (11).
In this laboratory, this method has resulted in more than 50 per cent saving in operator’s time in the analysis of stainless steels. Fewer losses by splattering occur and, in general, greater accuracy has been obtained. While 60 per cent perchloric acid is among the more expensive analytical reagents, the saving in operator’s time more than compensates for the additional cost. The cost of the chemicals for this method amounts to approximately 7 cents per determination.
Experimental
REAGENTS.The reagents for this method are essentially those required in the normal determination of phosphorus in steel, plus 60 per cent reagent grade perchloric acid (1). Procedure To a 2.00-gram sample placed in a 500-ml. Erlenmeyer flask, add 20 ml. of 60 per cent perchloric acid and warm to effect complete solution. Cover with a funnel, the stem of which has been removed, and boil for 30 minutes or until the chromium is completely oxidized. Here a modified still head, as described by Smith, may be used to advantage (8). The solution should be a deep red to brown color. Cool, dilute t o 100 ml., and add ammonium hydroxide (specific gravity 0.90) until a slight precipitate is formed. Approximately 12.5 ml. will be required. Dissolve the brown precipitate by the addition of nitric acid (specific gravity 1.20). Here 20 t o 25 ml. will suffice. Add 1 to 5 ml. of 1 to 10 ammonium bisulfite, sufficient to reduce the chromium and vanadium present (ferrous sulfate may be used for this reduction). Boil to remove nitrous oxide fumes and free chlorine (2). Allow the solution t o cool slightly, add 50 ml. of ammonium molybdate solution, and agitate for 5 minutes. Allow the precipitate t o settle at room temperature for 2 hours or more. Filter the yellow precipitate and wash from the containing vessel, first with 2 per cent nitric acid five to ten times to remove iron salts, and subsequently with 2 per cent potassium nitrate solution until the filtrate is no longer mid to litmus paper. Place the precipitate with the filter pa er in the original precipitation vessel, dilute with about 50 cc. oFwater, and then add 8 drops of phenolphthalein indicator. Add an excess of standard sodium hydroxide solution and agitate to effect complete solution of the yellow precipitate. Titrate the excess of alkali with standard acid solution.
If the standard solutions are adjusted to 0.149 N , 1 ml. of alkali consumed by the yellow precipitate will be equivalent to 0.01 per cent of phosphorus in the original 2.00-gram sample. The alkali was standardized against Bureau of Standards potassium acid phthalate. The acid was then,
INDUSTRIAL AND ENGINEERING CHEMISTRY
184
in turn, standardized against the alkali and the theoretical factor (1 to 23) was used in the titrations according to the reaction (4)
VOL. 8, KO. 3
tion as magnesium ammonium phosphate and ignition to the pyrophosphate.
TABLE 11. DETERMINATION OF PHOSPHORUS Certified Numbe? of PhosDetermina- phorus tions Value
Bureau of Standards Sample
Discussion and Notes on Procedure The presence of vanadium in quantities up to 0.25 per cent seems to present no difficulty in this method since it appears in the precipitation reaction in the reduced state. If, however, the operator fails to reduce this element, error will be admitted in the form of apparent high phosphorus percentage. Bureau of Standards samples, No. 73 stainless steel, No. 101 18-8 chrome-nickel steel, and No. 30c chrome-vanadium steel, all contain vanadium in appreciable quantities, yet the results in Table I are in keeping with the certified phosphorus values. The quantity of silicon normally encountered in stainless steels failed to affect the results noticeably when alkalimetric methods were used in the final estimation. Gravimetric procedures, of course, call for the removal of silica before precipitation and this may be readily done when perchloric acid is used for initial solution, since silica appears in its most tractable form when dehydrated with this acid. In fact, silicon may be determined by the Willard and Cake method ( l a ) on the same sample originally weighed for phosphorus. Smith and Goehler have proved that vacuum-distilled perchloric acid is free from phosphorus (9). Blank determinations of perchloric acid used in these tests confirmed their findings. It is therefore unnecessary to run blank determinations if it is known that the perchloric acid is purified by vacuum distillation. For plain carbon steels (Tables I and 11) this method may also be applied. The authors’ conclusions in regard to its use in the analysis of plain carbon steels have been confirmed in parallel tests by Willard and Delp (IO). Likewise, highphosphorus iron may be analyzed. Nothing will be gained, however, by the use of perchloric acid in the analysis of any material which is normally soluble in nitric acid. An effort was made to eliminate the neutralization of perchloric acid by ammonia and the reacidification with nitric acid by the addition of ammonium nitrate directly to the reaction mixture. The results were not consistently good. The method seems applicable to phosphorus concentrations over a wide range, Bureau of Standards sample No. 56, brown phosphate rock, was checked by this method and the results were comparable with those obtained by the usual alkalimetric method (3). The analysis of several Bureau of Standards samples indicates that the accuracy and precision of the method using perchloric acid is good. Table I shows results obtained with the perchloric acid method using acid-alkali titration.
Bureau of Standards Sample N o 7.1 Rtainless steel
l o f 1S;chromium 8-nickel steel No. 8d bessemer steel No. 200 A. 0. H. steel No. 30c chrome-vanadium steel No. 70 cast iron
%
%
0 108 0 042
0 101 0 042
The results of several aliquots of pure diammonium phosphate are given in Table 111. In each case the aliquot was equivalent to 0.235 mg. of phosphorus, a quantity in the same order as those quantities occurring in the Bureau of Standards samples used in this investigation. TABLE 111. RESULTS WITH PUREDIAMMONIUM PHOSPHATE Sample
Number of Determinations
Phosphorus Present
%
%
%
%
(NHdzHPOa
5
0.235
0 232
0.235
0 226
Phosphorus Found Average High Low
It is likely that this method may be applied to the analysis of phosphor-bronze with considerable savings in operator’s time. Work on such a method is now in progress in this laboratory. Summary The phosphorus content of difficultly soluble steels, such as stainless steels, may be quickly determined by dissolving and subsequently boiling the sample in 60 per cent perchloric acid before subjecting it to normal analytical procedures, Perchlorates do not interfere with the determinations of phosphorus either in volumetric or gravimetric methods.
Acknowledgment The authors wish to acknowledge with thanks the helpful criticisms and suggestions of G. Frederick Smith of the University of Illinois, H. H. Willard of the University of Michigan, and G. E. F. Lundell of the Bureau of Standards, the cooperation and encouragement of P. J. Freeman, principal materials engineer, Tennessee Valley Authority, and the helpful assistance of E. H. Spreen and Charles Barton, chemists, Chemical Testing Laboratory, Tennessee Valley Authority.
Literature Cited (1) Am. SOC.Testing Materials, Part I, Metals, 33, 340 (1933).
RECEIVED
%
%
%
0.023
0.024
0.022
3 3
0.011 0.099
0.044
0 , 0 1 1 0.012 0.097 0.098 0 . 0 4 2 0.043
0.096
4
0.019 0.78
0,019 0.80
0.022
0.018 0.80
0.80
% 0 105 0 042
0.010
% 0.023
3
% 0.099
0 044
0.042
Certified Phosphorus Phosphorus Found Value Average High Low
4
3
3 2
(2) Cain and Tucker, J. IND.ENQ.CHEM.,5, 647 (1913). (3) Hillebrand and Lundell, “Applied Inorganic Analysis,” p. 563, New York, John Wiley & Sons, 1929. (4) Hundeshagon. F., 2. anal. Chem., 28, 141-72 (1889). (5) Lundell, G. E. F., private communication, January 31, 1936. (6) Lundell, Hoffman, and Bright, “Chemical Analysis of Iron and Steel,” p. 212, New York, John Wiley & Sons, 1931. (7) Ibid., p. 219. (8) Smith, G. F., ”Mixed Perchloric, Sulfuric and Phosphoric Acids and Their Applications in Analysis,” 1st ed., Fig. l A , p. 28, Columbus, Ohio, G. Frederick Smith Chemical Co., 1935. (9) Smith and Goehler, IND. ENQ.CHEW,Anal. Ed., 3, 54 (1931). [IO) Willard, H. H., private communication to C. D. Susano, January 21, 1936. (11) Ibid., March 5, 1935. (12) Willard and Cake, J. Am. Chem. SOC., 42, 2208 (1920).
WITH PERCHLORIC ACIDMETHOD TABLEI. RESULTS
Number of Determinations
No. 8d bessemer steel No. 200 A. 0. H steel
Phosphorus Found Average High Low
Table I1 gives the results of analyses performed in the same manner as given above, except that the phosphorus is determined finally as in the usual gravimetric method by precipita-
February 17, 1936.
Q
