Photometric Determination of Tin in Copper-Base and Lead-Base Alloys C. L. LUKE Bell Telephone Loborafories, lnc., Murray Hill, N. J.
b A photometric phenylfiuorone method for the determination of 0.1 to 25% of tin in copper- or lead-base alloys has been developed. No separations are required because none of the constituents present in such alloys interfere when present in the concentrations normally encountered. The proposed method can probably be adapted to the analysis of several other nonferrous alloys.
T
STANDmethods used for the determination of tin in copperand lead-base alloys are time-consuming ( 1 ) . The rapid photometric phcnylfluorone method for tin ( 7 ) can be used for these analyses ( 4 ) . By making sure that antimony is in the pentavalent state and by keeping the acidity relatively high during the color development, the method can be made sufficiently specific si) that no separations are required.
HE
REAGENTS
Sulfuric Acid Solution, 15%. Mix 3 liters of sulfuric acid with 17 liters of water. Cool t o room temperature, dilute t o a 20-liter volume, mix, and cool. Standard Tin Solution. Prepare a 20% sulfuric acid solution containing 0.5 7 of tin por ml. ( 7 ) . If the solution bcconics cloudy on long standing, discard it. Just before calibration, dilute 5.0 ml. of the solution to 500 ml. in a volumetric flask with the 15% sulfuric acid. Gum Arabic Solution. Dissolve 2.3 grams of fincly powdered gum arabic (gum acacia) in 250 ml. of boiling water. Cool, then add as a preservative 0.5 gram of benzoic acid dissolved in 5 ml. of methanol. Filter through a fine-textured paper on a platinum cone with the aid of suction. Replace t h e paper once or twice if thc rate of filtration becomes too slow. Keep stoppered when not in use. This solution can be used for a t least :I month when stored at 25” C.
ml. of standard tin solution (5 y of tin per ml.) to 50-ml. volumetric flasks and add enough 15% sulfuric acid from a burct to make a total of 10 ml. in each flask. Working individually, add 10 ml. of p H 5 buffer solution, 1 ml. of gum arabic solution, and 10.0 ml. of phcnylfluorone solution, swirling after each addition ( 7 ) . Dilute thc solution to 49 ml. with water, mix well, and allow the flask to cool in a 25” C. mater bath for about 8 minutes. Dilute the solution to the mark, mix. and measure photometrically at 510 mp, using water as the reference solution, 10 & 0.5 minutes after the addition of the phenylfluorone solution (7). Analysis of Sample. Transfer 0.0500 t o 0.2000 gram of the sample t o a covered 125-ml. conical flask. When analyzing copper-base alloys, add 2 ml. of nitric acid; when analyzing lead-base alloys, add 2 ml. of perchloric-nitric acid mixture ( 5 1). Warm the mixture gently to effect complcte dissolution of the samplc, then wash and removc the cover. Add about 0.5 ml. of pcrchloric acid, if this acid is not already prrscnt, and 17 ml. of sulfuric acid. While swirling, hcat the solution nioderatcly on a Mekcrtype flame to expel volatilc acids and to redissolve any lead sulfate that ni:ty lw prcsent. Findly, fume tlic solution vigorously to cxpcl nll traces of perchloric and nitric acids and to reduce its volume to 15 ml. Cool to room tcrnperaturr, add 15 ml. of wntcr, heat j u s t to boiling on tlic flame, and then cool ngnin. Transft,r the solution to a 100-ml. volumctric flask, washing thc c:onical fl:tsk ontx, or tivice with lt5(X sulfuric acid from a wnsh tmttlc. \l’liilv swirling, dilute with wttrr to \vitliin :tl)out 2 ml. of the mark, cool to room tcsmpcratiirr, dilutr to tht, mark with w:itcr, and mix. If load sulfatc is prcscnt, : d h v thc pr?cipitrkr to stttl(s i i n t i l tlw sollition in thr upper part of the fl:tsk is kiirl!. v1r:tr e.g., 10 minutes. Trnnsfer 5.0 or 10.0 ml. of tlic sitpc’rnatnnt solution to a 100- or 200-ml. volumttric flask that hns hrcn sh:ikm frce of :my cxccss water. Dilutc to tlir mark with 15c/, sulfuric :wid 2nd mix. rI’ransfrr 10.0 ml. of thc solution to a 50-nil. volumetric flask and procccd :is in prrpnration of calibration curve.
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PROCEDURE
Preparation of Calibration Curve. Transfer 0, 2.5, 5.0, 7.5, and 10.0
DISCUSSION
I n order to minimize interference
due to other metal ions, it is necessary to maintain the acidity of the solution as high as practical at the time of color development. The conditions chosen in the present investigation (about p H 1.2) ensure good specificity while at the same time make it possible to achieve fairly rapid color developnicnt. Specificity tests on metals that may be expected to be encountered in the analysis of copper and lead alloys have shown that as much as 5 mg. or more of the following metals can bc prcscmt in the 50-ml. volumetric flask :it color development without influencihg the tin results: arsenic(V)! tc~lliiriuin, sclenium, copper, zinc, tiiting:iiicsc, cadmium, nickel, aluminiiiii, and magnesium. Cobalt causcs thc rvsults to be about 5% higher due to the pink color of the cobalt salt \Jut this (’an easily be compensated for by cnrr).iiig a second aliquot through tile pliotonictric tlctermination and ornit,tiiig the addition of the plicnylfluororic:. l‘mces of permanganic acid, prodiiccd as a result of fuming with perchloric acid, do not interfere in the proposcd method sincc the mangaricsc is suhsequently reduced by the pli(~iiylfluorone solution. ‘I’hc fact tli:tt s~idi metals as manganese, cobalt, riickcl, cadniium, zinc, aluminuni, and ni:ignesium show littlc or r a iritcift:rciict. in the proposed tin mctliod suggchsts that it should be possible to :idapt this method to the direct analysis of ovcr 0.17, of tin in these metals arid tlicir alloys . Several metals tend to (:ausc Iiigti results in the photometric tin rricttiotl. Not more than about 100 y of bisiiiiith or iron or 50 7 of titanium c:in be tolerated at color dcvclopmciit. In the proposed method the interfcrcmc of antimony is prevented by convcrting it to the pentavalcnt state by funiiiig with perchloric acid. ‘I’mces of ant.imony that may escape oxidation ( 6 ) do not cause any measurable error. Unfortunately, there is a tendency for the pentavalent antimony to bc rt:duced by the organic matter prcscrit at color development, rspccially :tt elevated temperatures. For this rcason no more than about 25 y of this VOL. 31, NO. 1 1 , NOVEMBER 1959
1803
metal can be present in the 50-ml. volumetric flask when color development is made at 25' C. Hovlever, if the solution is cooled in a 15" C. water bath during color development, as much as 100 y of antimony can be tolerated. If this practice of cooling to 15°C. is followed, it will be nrcessary to calibrate in the same manner bemuse the extent of color development
Table 1. Analysis of NBS Copperand Lesd-Base Alloys
Tin, % Sample Analyzed Present Found Sheet braes No. 37-h 0 99 1.01 0 99 1.01 Manganese bronze No. 62-b
0 99
1 .OO
0 96
0.96 0.96 0.96
0.96
0 96 Ounce metal No.
124-h
Cast, bronze No. 52-b
Phosphor bronze No. 63-b
4 93 4 93 4 9.7 800 800 8 00
4.94 4.96
9 78 9 78
9.80 9.75
5 16 5 16 5 16 5 16 34 88
5.20 5.19
4.98 8.00
7.86 8.02
had-base alloy No. 53-c
Solder No. 127
5 . 14a
5.174 34.40 34.88
34 88
Tin-base alloy No. 54-C
86 33 86.00 Color developed at 15" C. in standards
and smnple.
Table
II.
is diminished slightly a t this lower temperature. I n the analysis of alloys containing less than about 0.1% of tin or when excessive amounts of interfering metals are present, it will be necessary to resort to separations (2, 9 , 6,7). Following separation by distillation, the tin in an aliquot of the distillate can probably be converted to sulfate b y treatments with sulfuric acid and hydrogen peroxide (9) and then determined photometrically. The rate of color development and the intensity ultimately produced with a given amount of tin depend on the acidity of the solution a t color ievelop ment. The rate and intensity decrease with increasing acidity. I n order to obtain reproducible results it is necessary to control the acidity rather closely. I n the preparation of the standard tin solutions and in the analysis of samples, this can be done by making at least a tenfold dilution with 15% sulfuric acid taken from a large batch. Of course, each time a new supply of 15% sulfuric acid is made up i t will be necessary to recalibrate. I n the region of p H 1.2 most of the tin-phenylfluorone color is formed after 10 minutes but since the reaction approaches completion very slowly, i t is necessary to control the time of color formation rather closely in order to assure reproducible results. During the conversion of lead perchlorate or nitrate to sulfate, the rate of heating of the sample on the flame must be controlled so that excessive expulsion of sulfuric acid is avoided. After dilution of the fumed sample with 15 ml. of water i t is necessary to hest the solution t o boiling in order to
Analysis of Various Mixed Samples
Tin in 50-MI. Volumetric Flask, 7
Sample Analyzed 200 me. Pb 5 mg. Sn 200 mg. Ph 25 mg. Cu 5 mg. Sn 125 mg. Pb 75 mg. No. 53-c 100 mg. Ph 50 mg. No. 53-c 50 mg. Cu
+ ++ + + + 100 mg. Pb + 50 mK. No. 53-c + 50 50 mg. No. 53-c + 25 mg. Cu 170 mg. Ph + 30 mg. No. 127 120 mg. Pb + 30 mg. No. 127 + 50 150 mg. P h + 50 me. No. 62-b 150 mg. Ph + 50 mg. No. 124-b 150 mg. Ph + 50 ing. No. 52-b 150 mg. Pb + 50 mg. No. 63-h
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ANALYTICAL CHEMISTRY
37-b
cu
Present 50 0 50 0
38 7
25 8 25 8 30 8 25 8 52 3 52 3 52 3 4 8 24 6 24 6 40 0 40 0 49 0 49 0
Found 50 0 50 0 39.0 26 0 26 1 31 0 26 0
52 2
52 52 5 23 23
4
5 0 5 4
38 3
38 4 47 0 46 5
dissolve sulfates of iron and aluminum that might otherwise occlude mme of the tin. I n contrast to the analysis of traces of t i in lead metal (7), the loss of tin by occlusion in lead sulfate during the analysis of lead alloys containing over 0.1% tin is negligible (Table 11): On long standing, a precipitate is Seen to form in 15% sulfuric acid solutions of tin. For this reason, i t is best to prepare fresh dilute standard tin solutions each time a calibration is made. Recent work has shown that the phenylfluorone solution used ie not as stable as was formerly thought. For this reason, i t should be kept in a cool dark place when not in use and should be prepared fresh every three or four weeks. UPERIMENTAL
In order to test the accuracy and reproducibility of the method, several
NBS copper-base and lead-base alloys of known tin content were analyzed. Sample sizes varying from 0.0300 to 0.2000 gram were taken and suitable volumetric dilution was made to bring the tin content within the range of measurement. The results obtained are shown in Table I. I n the analysis of lead-base alloy No. 5342 about 50 7 of antimony was present at color development. In order to simulate the composition of a white metal or of a lead alloy containing less than 5% of tin, various mixtures of metals and NRS alloys were analyzed. All of the samples were dissolved in 2 ml. of perchloric-nitric 1). The results acid mixture (5 obtained are shown in Table 11. The values are satisfactory except in those cases where mixtures of lead and certain NBS copper-base alloys are analyzed. T o date no explanation for the low res u l b obtained with these samples has been found. Before using the proposed method in the analysis of white metals, it will be desirahle to ohtain a check against some independent method.
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LITERATURE CITED
(1) Am. SOC. Testing Materials, Phila-
delphia, Pa., "ASTM Methods of Chemical Analysis of Metals," 1956, pp. 208. 471. -. - - - - I
(2) Zbid., p. 333.
I S ) Farnaworth. M.. Pekola. J.. ANAL. - \ - I
I
.
C H E M . - ~ ~735 , (1954). (4) Zbid., 31, 410 (1959). f.51 Kallmann. S.. Liu. R.. Oberthin,. H.., Zbid., 30, 485 (1958j. ' (6) Luke. C. L.. Zbzd.. 28. 1273 (1956). \ - I
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(7) RECEIVEDfor review March 31, 1959Accepted July 16, 1959.
