Determination of Bismuth, Copper, and Lead in Aluminum Alloys GEORGE NORWITZ, SAMUEL GREENBERG,
AYD
FRED4 B.4CHTIGER
Industrial Test Laboratory, Philadelphia Naval Shipyard, Philadelphia 12, Pa.
An accurate method is presented for the determination of bismuth, copper, and lead i n aluminum alloys, which is much shorter than the method of the Aluminum Company of America, since it eliminates a double hydrogen sulfide separation and two ignitions. The proposed procedure depends upon the insolubility of the bismuth i n hydrochloric acid. Bismuth is determined colorimetrically as the iodide after precipitation as the Oxychloride and the copper and lead are determined electrolytically from a nitric acid solution.
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glass beads to the first filtrate and boil down to a volume of about 3 ml. Filter the osychloride precipitate through a S o . 44 K h a t m a n filter paper into the boiled down filtrate. Swab the beaker and wash precipitate with hot water. Dissolve the precipitate with 40 ml. of hot nitric acid (1 to 1) into a 1-liter volumetric flask. Dilute to about 500 ml. with cold water and add 4 ml. of sulfurous acid (1 part of saturated sulfurous acid solution to 3 parts of miter) and 10 ml. of potassium iodide solution (20%). (For very accurate work fresh potassium iodide solution and fresh sulfurous acid should be used, since slightly inaccurate results were obtained iyhen these solutions were more than 2 days old.) Dilute t'o the 1000-ml. mark and read the bismuth colorimetrically. The authors used a Klett-Summerson colorimeter (test tube model), filter S o . 42. To the combined filtrates containing the copper and lead add 30 ml. of concentrated nitric acid and boil down to a volume of about 10 ml. Add 15 ml. more of nitric acid and repeat the operation. Immediately add 10 ml. of concentrated nitric acid and dilute to about 50 nil. with hot vater. Transfer the solution into a 200-ml. tall-form electrolytic beaker and electrolyze for 30 minutes a t 2 amperes, using a platinum gauze anode and cathode. Add 2 ml. of sulfuric acid (1 t o 1) and electrolyze for 30 minutes longer. Remove the electrodes and wash them first with water g n d then x i t h alcohol. Dry the cathode in an oven a t 100" t o 105' C. for about 5 minutes and weigh as metallic copper, as soon as cool. D r y the anode for 30 minutes a t 210" C. and weigh as lead dioxide, which has a calculated factor of 0.866. A working curve for bismuth can be drawn up by using a standard bismuth solut,ion. This solution is best obtained by dissolving 0.4000 gram of metallic bismut,h in 20 ml. of nitric acid (1 to l), transferring to a 2-liter volumetric flask, and diluting to the 2-liter mark. Appropriate amounts of this standard solution are measured out into 1-liter flasks and then 40 ml. of nitric acid (1 t o 1) and 5 ml. hydrochloric acid (1 to 100) are added. (Since the method calls for a 2-gram sample, 10 ml. of this solution are equivalent to 0.10% of bismuth.) The solution is diluted to about 500 ml., sulfurous acid and potassium iodide solution are added, and the bismuth is read as usual.
S R E C E S T years aluminum allbys containing copper, bis-
muth, and lead have become increasingly important, especially in the manufacture of screw products ( 1 ) . The method commonly used for the determination of copper, bismuth, and lead in these alloys is that of the Aluminum Company of .Imerica (3). This procedure, although accurate, is long and complicated, involving two hydrogen sulfide precipitations and two ignitions. An investigation x a s conducted for the purpose of developing a shorter method for the analysis of copper, bismuth, and lead in aluminum alloys. Among the methods tried was the separation of bismuth from copper and lead by means of pyrogallol ( 2 6 ) . The authors found it unsatisfactory because complete quantitative precipitation of bismuth was not' obtained. The separation of lead as lead sulfate follon-ed by a manganese dioxide precipitation, using potassium permanganate and manganous sulfate, t o separate the bismuth from the copper (14) was also tried and found inaccurate, probably because of incomplete separation of the bismuth sulfate from the lead sulfate (18). Vsing the same separations as outlined in the proposed method the thiourea colorimetric. procedure (8, 9) v a s tried instead of the bismuth iodide ( 7 ) . I t offered no special advantage and was consequently abandoned. The method described in this paper was finally adopted as the most practical and workable procedure. It was found t o be as accurate as the method of the Aluminum Company of America and it requires only one half the amount of work. Advantage is taken of the fact t h a t the bismuth is insoluble in (1 to 1) hydrochloric acid ( 4 , 10, 1 7 ) . The bismuth is determined colorimetrically as the iodide after precipitation as the oxychloride, and the copper and lead are plated out simultaneously in a nitric acid solution. This proposed method has been primarily designed for commercial aluminum alloys of copper, lead, and bismuth. These alloys usually contain 3.5 to 6% copper, 0.2 to 0.6% lead, and 0.2 to 0.6% bismuth. The procedure, however, might be adapted for t h e analysis of other percentages of these elements in aluminum alloys.
DISCUSSION
Analysis of many filtrates from the hydrochloric acid-insoluble material showed t h a t bismuth is insoluble in hydrochloric acid (1 to l), but that copper and lead are partially soluble. In the case of the 11s standard (5.11% copper and 0.48% lead) 0.2 to 0.3% copper and 0.2 to 0.3% lead were found to have gone into solution. This solubility of copper and lead obviously eliminated the possibility of discarding the hydrochloric acid filtrate. I n order to check the colorimetric method for bismuth three curves were made. The first curve Tvas drawn up by using aluminum samples of known bismuth content; the second, by using high-copper aluminum alloys to which had been added weighed amounts of metallic bismuth. I n order t o maintain siplilar conditions known amounts of lead nitrate were added to the solution obtained by dissolving the insoluble hydrochloric acid residue in nitric acid. I t was found impracticable to add metallic lead a t the very outset, since the undissolved metallic lead in the residue
PROCEDURE
Dissolve a 2-gram sample in a 400-ml. beaker with 50 ml. of hydrochloric acid (1 to 1) without applying heat. Immediately after the reaction ceases add 50 ml. of distilled water and filter through a S o . 40 Whatman filter paper, collecting the filtrate in a 600-ml. beaker. Wash with hot water. Dissolve the precipitate back into the original 400-ml. beaker with 40 ml. of hot nitric acid (1 to 1) and wash with hot water. iidd 1 ml. of ferric nitrate solution (1%)and then ammonium hydroxide until the solution is just turbid. If the end point is overstepped make the solution acid again with nitric acid and again add ammonium hydroxide. Add 1 ml. of hydrochloric acid (1 t o I ) , dilute to 250 to 300 ml. %-ithhot water, and let stand on asteam bath for 45 minutes. While t'he bismuth oxychloride is coagulating add 40 ml. of concentrated nitric acid and several
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ANALYTICAL CHEMISTRY
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absorption of the ye1loLv iodobismuthite solution is a t its maximum a t 460 millimicrons wave length. However, the S o . 42 Sample S o . Copper Lead Bismuth filter used for the method outlined in this paper, although having % 76 76 a maximum transmission of 425 millimicrons, has proved entirely 1 4.16 0 37 0.42 satisfactory. 2 4.18 0.39 0.43 3 4.19 0 39 0.42 I n neutralizing the solution n-ith ammonia before the precipi0 39 4 4.17 0.43 tation of the hismuth as the oxychloride, a n opalescence would ap5 0.37 4.19 0.42 6 0 40 0.42 4.19 pear even if no ferric ion \vere present. Hoivever, the authors 7 0.40 4.19 0.41 8 0 39 4 20 0.42 recommend the addition of the ferric nitrate before the neutraliza9 0 39 0 42 4.23 tion, because they found that the turbidity produced by the ferric 4.17 10 0 38 0 42 A r. 4 19 0 39 0 42 hydroxide n-as much sharper than the turbidity that n-ould be Av. deviation 1 0 01 i.0 01 *o 00 produced otherwise. If so desired, bismuth can he determined gravimetrically by clightly'modifying the regular procedure. The solution obtained could not readily be dissolved off the filter paper with nitric acid. hy dissolving the hydrochloric acid-insoluble material into the The third and most rapid working curve, described under Pruoriginal 400-ml. beaker is filtered into another 400-ml. beaker and cedure, was drair n up by the use of standard bismuth nitrate soluwashed with hot lOy0nitric acid. The oxychloride is precipitated tion. The three curves followed Beer's law and were practically as usual, then filtered through a tared Gooch crucible and dried identical, proving conclusively the validity of the method. for one hour at 105' to 110" C. S o exhaustive study was made In \\ orking with synthetic solutions i t was found that a tiace oi of this modification, since t h e colorimetric method is faster and hydrochloric acid is essential to stabilize the color, especially just as accurate. Gsing the gravimetric modification, the R hen the sample contains more than 0.40% bismuth. Hov ever, folloir-ing bismuth results n-ere obtained on sample 142X which if too much hydrochloric acid is present (more than 1 nil. per contains 27, nickel and 1.5% silicon: 0.43, 0.42, and 0.44%. liter), the colorimetric readings are lolver than they should be. ;Is can be seen from Table 111, the average value for this sample This is borne out by the literature, which cites interference when t)y the Aluminum Company of America procedure is 0.42%. too much chloride ion is present (12). All metals (8, 11) that inThe Aluminum Company of America states ( 2 ) t h a t it is best terfere \\ ith the colorimetric determination of bismuth, such aq not to plate out copper in the presence of large amounts of iron, copper, lead, silver, tin, and antimony, are either not present in nickel, and aluminum. However, for several years, this laboraaluminum alloys or are removed by the hydrochloric acid-oxytory, using the method of Sloviter (Is),has been plating out copchloride separation. The sulfurous acid added reduces any oxidizper in aluminum alloys without any preliminary separations and ing agents, such as ferric iron, that might interfere with the dehas obtained excellent results. This method is not applicable t o termination. S o interference Tvhatsoever is encountered in the copper, lead, and bismuth alloys because bismuth partially plates colorimetric procedure of bismuth due to the presence of nickel. on the cathode and partially on the anode ( 5 ) However, in the Aluminum Company of America method the A41thoughmanganese is not present in commercial copper, lead, bismuth results will tend to be high, if a large amount of nickel is and bismuth alloys, a study was made, using synthetic samples, of the possible interference of this element. It was found that present, unless considerable care is taken. The yelloiv color of the bismuth complex (BiI3.3KI or BiI,.some manganese plated on the anode, if the manganese content of 2KI.4H20) (15), as developed in this method, is very stable. The the sample were above 0.05%. Therefore, if a high percentage color develops immediately and does not change upon standing, manganese alloy were encountered, after electrolysis, the anode for 24 hours. Ir-ould have to be stripped with nitric acid (1 to 1) and the lead determined as lead sulfate. An alternate procedure would be t o Wiegand, Lann, and Kalich (19) have shown t h a t the light weigh the deposit and determine and correct for manganese. An attempt \?as made t o take the comTable 11. Comparison of Results for Type 11s Alloy bined filtrates, containing the copper Aluminum Company of Amerlca Procedure Suggested Procedure and lead, down to fumes of perchloric Sample S o Copper Lead Blsmuth Copper Lead Bismuth
