Determination of Antimony in Solder CLIFFORDL, BARBER,Kester Solder Co., Chicago, Ill.
T
HE determination of small amounts of antimony in
solder continues to be perplexing to the analyst. When a sample of purest commercially obtainable solder, which contains possibly 0.002 to 0.004 per cent of antimony and approximately 0.01 per cent of arsenic, is submitted for analysis to commercial laboratories, results ranging from 0.05 to 0.25 per cent or occasionally as high as 1 per cent of antimony are invariably reported. One investigation which may be considered typical and which clearly indicates the need for a sound, reliable method for the determination of antimony in solder, was made as follows: A number of 150-gram samples of solder were poured from the same batch of pure metal (these were used in all subsequent analyses and investigations). Seven of the samples were submitted for analysis t o Ieadin analytical and engineering laboratories (designated as A, B, D, E, F, and G), care being taken t o select well advertised firms of national and international standing in the field of metal analysis. To four additional samples sufficient antimony was added to make the antimony content exactly 0.12 per cent, the permissible limit for Class A solder alloy as defined by the American Society for Testing Materials; these were submitted t o laboratories A, B, C, and D. The results tabulated in Table I show antimonial contents of 0.05 to.0.51 per cent on solder which contains no antimony, and 0.18 to 0.27 per cent on Class A solder, thus eliminating both alloys in most cases from the preferred Class A status. TABLEI. ANTIMONY FOUND IN SOLDER
6
ANALYs'r
Pnan SOLDE~R (Sb 0.003%) %
A
0.13 0.07 0.20 0.05 0.51 0.18 0.21
B
C
D E F
c
CLASSA, A. 8.T.M. SOLDER (Sb 0.12%)
% 0.23 0.18 0.27 0.19
Unfortunately, the literature is of little assistance in this problem The ordinary gravimetric methods involving precipitation of the sulfides, followed by separation of the antimony from tin, are long, tedious, and generally conducive to inaccurate results. The volumetric methods, although somewhat better, are unreliable for small amounts of antimony, in part because of a comparatively large and variable blank titration and also because many methods make no provision for the removal of arsenic, which is subsequently titrated quantitatively along with the antimony. With either gravimetric or volumetric analyses the entire result is often accounted for by analytical error alone. A satisfactory method must not only determine antimony when present but also refrain from finding it when none exists.
EXPERIMENTAL A method involving the decomposition of antimonial solder in hot concentrated hydrochloric acid was first investigated. The significant fact was noted that pure tin-lead solder remains inert and apparently unattacked for long periods in boiling hydrochloric acid, while solder containing as little as 0.01 per cent of antimony is immediately attacked by boiling hydrochloric acid with resultant solution of the tin and lead and precipitation of the antimony in the form of voluminous black flakes. Andrews (2) and Yockey (8) proposed methods which involved treatment of the alloy in hydrochloric acid and potassium iodide, followed by recovery of the precipitated metal by filtration and washing. However, the freshly precipitated
voluminous metal dissolves on the filter paper on washing with water. In a number of tests the entire black precipitate was observed to disappear completely, giving a clear filtrate, This may be accounted for on the theory that the voluminous precipitate adsorbs tremendous proportions of hydrochloric acid which is capable of dissolving the metal when the latter is exposed to the oxidizing influence of the air. Nevertheless, an empirical method based on the decomposition of antimonial solder in hot hydrochloric acid was developed. First, a large test tube was drawn out to a fine capillary a t one end in such manner that a sample of decomposed antimonial solder, together with the acid, could be poured directly into the tube and the antimony allowed to settle out in the capillary portion, after which the length of the column of precipitated metal could be measured. It was found convenient to set the tube, with capillary end down, into a buret clamped upright and use the buret graduations t o secure an empirical reading. The tube was then calibrated by digesting in turn a number of prepared alloys of known antimonial content and noting the reading in each case. By plotting these readings against known antimonial contents, a curve was obtained by means of which an unknown alloy could be investigated. This method was found unreliable for alloys containing bismuth, which is precipitated to some extent along with the antimony. For alloys containing as much antimony as bismuth no measurable error is introduced, but when the bismuth content reaches five times that of the antimony an error of 100 per cent may result. Nevertheless, for certain control analyses in which the solder is known to be free from bismuth or where it is desired to know whether the antimony is above a given maximum, the method is fast and reliable. It depends for its success on an exact duplication of technic. Of all the available methods of antimony determination, the potassium permanganate, modified after Low (4),and the potassium bromate, modified after Gyory (8) and Rowel1 (5), are most used. The further investigation of these methods was directed towards the elimination of the high variable blank titration which one secures even when antimony is not present. For this work pure solder (containing approximately 0.003 per cent of antimony and 0.01 per cent of arsenic) was used as the sample. That the solder used was practically free from antimony was established in the following way: Three of the 150-gram samples of pure solder previously mentioned were converted to 0.01, 0.02, and 0.03 er cent antimonial alloys, respectively, by the addition of the tKeoretica1 amounts of antimony; a fourth sample used in the test was not altered. One gram of sawings was then taken from each sample, laced in 250-cc. covered beakers with 75 cc. of concentrated hyd!ochlorio acid, and boiled. The 0.03 per cent antimonial alloy went completely into solution (with the exception of a few black flakes of precipitated antimony) in 8 minutes, the 0.02 per cent in 11 minutes, and the 0.01 per cent in 15 minutes; the pure sample required 95 minutes of alternate boiling and digestion on the steam bath to effect solution. A quantitative investigation was then made by digesting 5-gram samples of the three antimonial alloys and making a curve as previously described. This slightly hyperbolic curve was extrapolated to the axis. A 5-gram sample of pure solder was then digested, the reading referred to the extrapolated portion of the curve, and the corresponding maximum antimonial content noted. This analysis is, of course, open to the theoretical objection that it was necessary to assume the exact shape of the curve
443
444
ANALYTICA L EDITION
a t the extreme end, and that some antimony may have gone into solution in the course of the long boiling and digestion; however, the fact that the addition of antimony to the extent of 0.01 per cent produces such a marked change in behavior indicates that the sample originally did not contain this metal in amounts approximating this figure. , An attempt was fist made to decompose a 1-gram sample of solder in concentrated sulfuric acid and, after the addition of hydrochloric acid and water, to boil and cool the mixture and then titrate direct with 0.05 N potassium permanganate as recommended in a number of standard methods. This procedure was found wholly unsatisfactory, as large volumes of potassium permanganate were consumed in the titration and a sharp end point could not be secured. A fairly sharp end point was obtained by first filtering off the precipitated lead sulfate, but there was still a blank titration of 0.6 to 0.8 cc. of 0.05 N potassium permanganate which, on the basis of the 1-gram sample used, would correspond to 0.18 to 0.24 per cent of antimony. A still higher blank titration was obtained by the potassium bromate method, modified by decomposing the 1-gram sample with hot sulfuric acid, adding water and hydrochloric acid, heating the mixture until all was in solution, and then titrating hot, using methyl orange indicator. However, the blank was reduced somewhat by prolonged boiling before titrating. The regular potassium bromate method, involving treatment with hydrochloric acid and bromine followed by reduction with sodium sulfite, did not prove satisfactory for solder, owing to the precipitation of lead sulfite and the formation of a cloudy mixture which did not give a sharp end point. No particular advantage could be gained by using a 5gram sample (and increasingly more reagent) instead of 1 gram, as with either the modified bromate or the permanganate method the amount of blank titration is somewhat in proportion to the amount of reagent used. It was therefore decided that one requirement in the solution of this problem lay in keeping the amount of reagents as low as possible and that a certain amount of blank, apparently due to chemicals, wm unavoidable. After some experimentation involving judicious choice of apparatus, it was found that a 2-gram sample could be satisfactorily analyzed with 12 cc. of sulfuric acid, 30 cc. of hydrochloric acid, and 350 cc. of water. The blank titration due to these reagents alone is 0.20 cc. of 0.05 N potassium permanganate and must be subtracted from the total titration in making an analysis. Since arsenic is titrated quantitatively by potassium permanganate in dilute acid solution after digestion with sulfuric acid, it must be removed completely before titrating for antimony. This is accomplished by boiling for a few minutes with a little water and hydrochloric acid after digestion with sulfuric acid, very much as recommended by the American Society for Testing Materials (1). A method was finally developed in which the end point is sharp and distinct and can be determined accurately to a drop of 0.05 N solution. The method is fast and its high order of precision is indicated by reference to Table 11. No antimony would be reported in an alloy which contained none. Repeated analyses of pure solder (0.003 per cent of antimony and 0.01 per cent of arsenic) gave consistently a titration of 0.20 cc. of 0.05 N potassium permanganate, which is the blank on the reagents. A 0.21 per cent arsenic alloy, made by adding the theoretical amount of arsenic to a sample of pure solder, was also analyzed with the same result, proving the arsenic to have no effect on the titration. Both alloys (0.01 and 0.21 per cent of arsenic) were then analyzed as before, except that the step for the removal of arsenic was
Vol. 6 , No. 6
omitted, analyses of 0.01 and 0.21 per cent of arsenic, respectively, were obtained. TABLE11. DETERMINATION OF ANTIMONY BY PROPOSED METHOD KNOWNANTIMONIAL CONTBINT ANTIHONIAL CONTEINT FOUND %
%
0.03
0.04 0.04
0.03 0.04 0.n4
0.03 0.04 0.04 0.03 0.03 0.03 0.05
0.06 0.06 0.05
0.06 1.00
0.98 0.98 0.98 0.99 0.99 1.00
2.00
1.98 1.99 1.99 1.99
METHOD Make a preliminary qualitative test for antimony by treating 1 gram of filings in a covered 250-cc. beaker with 50 cc. of concentrated hydrochloric acid and gently boiling the acid. If the metal is immediately attacked by the acid with the liberation of small, fine bubbles of gas, and visual disintegration of the' sample, antimony is present. If after 10 minutes' boiling there is no visual indication that the acid is attacking the metal and the sawings appear t o lie inert in the bottom of the beaker in large aggregates formed from the coalescence of small adhering particles, antimony is not present and the analysis may be discontinued. The presence of 0.01 per cent of antimony in solder readily manifests itself in altering the behavior of the metd in boiling hydrochloric acid. Having demonstrated that antimony is present, weigh 2 grams of sawings into a loosely covered 250-cc. extraction flask, add 12 cc. of concentrated sulfuric acid, place the flask on a piece of perforated asbestos, and heat over a free flame until decomposition is complete and the preci itated lead sulfate is white; then continue t o boil gently for a k w minutes longer, occasionally shaking the flask. (A small, porcelain crucible, placed upright, makes a very convenient cover for the flask.) Allow the mixture to cool, add cautiously 15 CC. of water and then 15 cc. of hydrochloric acid, and boil for 10 minutes to expel arsenic. To the boiling solution add 50 cc. of water and 15 cc. of hydrochloric acid and continue to boil for a few minutes longer to insure the complete solution of antimon sulfate. Any insoluble residue consisting of small amounts oflead sulfate and particles of free sulfur may be ignored. Cool the flask thoroughly in running cold water and filter by suction through a No. 3 Buchner funnel, washing once by decantation and two or three times more on the filter with 10-cc. portions of sulfuric acid (1 to 10). Transfer the filtrate to a beaker, dilute t o 400 cc. with water which has been cooIed to approximately 10' C., and titrate slowly at 15" C. with 0.05 N potassium permanganate until the ink color tends to persist. The filtration should be done rapidyy and the titration made promptly, as there is an additional precipitate of the lead salt on standing. A titration of 0.20 cc., due t o a blank determination on reagents treated under the same conditions, must be subtracted from the total titration. Titration X normality X 3.04 = per cent of antimony
NOTESON
THE
ANALYSIS
The extraction flask recommended for the digestion of the alloy is a special type of Erlenmeyer which stands about 16 cm. high and measures about 7 cm. across the bottom. Such a flask has the advantage that a large amount of sample can be treated with a relatively small amount of reagent. A considerable amount of sulfur separates out during the digestion; this does not affect the results. Obviously an application of this method leads to the
November 15,1934
I N D USTR I A L A N D E N G I N EER I N G C H E M I STR
determination of arsenic, By subtracting the titration secured in the determination of antimony from the titration resulting from an analysis in which arsenic is not removed, one can secure a titration due to arsenic alone. Equal amounts of reagent, however, should be used. The pink color fades rapidly at the end point; the latter is reached when, on the addition of a drop of permanganate, the color does not fade instantly but tends to persist.
Y
445
LITERATURE CITED (1) Am. SOC. Testing Materials Standards, Serial Designation B-18-21,p. 504 (1921). (2) Andrews, J . Am. Chem. SOC.,17, 869 (1895). (3) Gyiiry, S.,2. anal. Chem., 32,415 (1893). (4) Low, W. W., J. Am. Chem. Soc., 29,1907,66(1907). (5) Rowell, H. W., J. SOC.Chem. Ind., 25,1181 (1900). (6) Yockey, J . Am. Chem. SOC.,28,646,1435(1906). RECEUEDJuly 24, 1934.
Detection of Calcium in the Presence of Strontium and Barium EARLE R. CALEY,Frick Chemical Laboratory, Princeton University, Princeton, N. J,
W
HILE it has long been known that calcium hydroxide is more insoluble than either strontium or barium hydroxide, especially at temperatures near the boiling point of water, this fact has not found practical application as the basis of a qualitative precipitation reaction for calcium, owing to the usual carbonate content of alkali hydroxide solutions applicable as reagents. However, if hydroxyl ions are generated by the interaction of mercuric oxide and potassium iodide within a solution to be tested for calcium, under conditions that exclude the presence of any carbonate ion, the slight solubility of calcium hydroxide may be safely utilized for the detection of calcium in the presence of the other tu70 alkaline earth ions. The reaction involved in this method can be represented by the equation:
even when carefully dried, reacted with each other and absorbed carbon dioxide from the air on standing a comparatively short time. The standard calcium solution, containing 0,0010 gram of calcium per cc., was prepared by dissolving 2.4970 grams of pure calcium carbonate in the minimum necessary quantity of hydrochloric acid, boiling to expel all carbon dioxide, and cooling, and finally adjusting the volume to 1000 cc. The concentration of this solution was checked by igniting and weighing the calcium sulfate obtained by the evaporation of 100.0 cc. with a slight excess sulfuric acid in a platinum dish. Because the ordinary c., P. grades of barium and strontium chlorides were found unsuitable by reason of their calcium content, barium chloride dihydrate and strontium chloride CaCI2 HgO 4KI -/- H20+Ca(OH)2 2KC1 K2Hg14 hexahydrate included in DeHaen’s line of specially purified reagents were employed. These salts were stated to contain In practice a chloride solution known to contain only al- not more than 0.0005 and 0.01 per cent of calcium, respeckaline earth salts is reduced to small volume, rendered tively. Instead of using standard solutions, it was found slightly acid with hydrochloric acid, and transferred to a test more convenient to take weighed quantities of these salts, tube. The solution is boiled for a few minutes to expel all thus avoiding numerous evaporations. carbon dioxide, and while still actively boiling a suitable The test experiments were made in graduated hard-glass quantity of solid potassium iodide is added. When this has test tubes, generally used in pairs in order to compare a given dissolved, powdered mercuric oxide is added to the continu- sample directly with a blank or with another sample conously boiling solution, and if a white cloudy precipitate ap- taining a smaller or a larger amount of calcium. These pears as the mercuric oxide rapidly dissolves, the presence of tubes were mounted upright so that the solid reagents could calcium is indicated. This simple technic effectively excludes be dropped in without loss from adhesion to the sides. To carbon dioxide and a t the same time takes advantage of the promote smooth boiling in this position, when using ordinary fact that the solubility of calcium hydroxide is at its minimum gas burners, it was found necessary to place small spirals of a t this temperature, whereas the solubilities of the other two platinum wire in the test solutions. alkaline earth hydroxides are at their maxima.
+
+
+
+
EXPERIMENTS
MATERIALS AND APPARATUS Both reagents required for this test must be free from sulfates and from metals that can be precipitated as hydroxides, including calcium itself. In addition, a blank made with the quantities of potassium iodide and mercuric oxide used in the test should give a clear solution. Yellow mercuric oxide is to be preferred to the red form, since its smaller particle size leads to more rapid and complete solution during the reaction with potassium iodide, and furthermore, the precipitated form can usually be obtained in a higher state of purity. The usual laboratory grade of potassium iodide was found suitable. Preliminary experiments showed that the necessary complete and rapid solution of the mercuric oxide in convenient volumes of test solution occurred only when the weight of the potassium iodide approached ten times that of the oxide. Attempts to use an already prepared mixture of the two reagents were not successful, since the intimately mixed solids,
Where calcium was present alone (Table I), the necessary volumes of standard calcium solution were first pipetted into the graduated test tubes and acidified with one drop of 0.1 N hydrochloric acid. The solutions were next adjusted with distilled water to slightly more than the desired final volume in each case, and boiled down to the given fixed volume be- . fore adding the potassium iodide. Where the quantity of potassium iodide was large enough to make a significant difference in volume, the solutions were again boiled to the desired final volume before adding the mercuric oxide. The period of observation for judging the absence or presence of any cloudiness or turbidity, due to precipitated calcium hydroxide, was restricted to about a minute after the addition of the mercuric oxide in order to avoid error due to the decreasing volume of the continually boiling solution. In all cases where a precipitate was observed, the fact of its being calcium hydroxide, and not calcium carbonate due to carbon dioxide from the air, was confirmed by diluting the mixtures
