Determination of Antimony in Tin-Base Alloys C. L. LUKE, Bell Telephone Laboratories, Inc., N e w York, N. Y.
The usual methods for determination of antimony in tin-base alloys yield low results in the presence of copper, due to catalyzed oxidation of antimony during solution of the sample in hot sulfuric acid solution (Low method); or to co-oxidation of antimony with copper following reduction with sulfurous acid (Rowell method). The low results can be eliminated b y using a Low-Rowell method in which the sample is dissolved in hot sulfuric acid and the traces of pentavalent antimony are reduced in hydrochloric acid solution with sulfurous acid under conditions where the amount of copper reduced is too small to cause appreciable co-oxidation of antimony.
THE
two methods most often used for the determination of antimony in tin-base alloys are those in which the sample is dissolved in hot sulfuric acid (Low's method, 9), or jn hydrochloric acid and bromine followed by reduction of antimony and arsenic to the trivalent state with sulfurous acid (Rowell's method, 2, 16); the arsenic is expelled from strong hydrochloric acid solution by boiling; and the antimony is titrated with bromate by the method of Gyory (4). (These are referred to below as the modified Low'and Rowell methods.) When the arsenic content is known and is small compared to that of the antimony it is sometimes simpler to titrate both the antimony and arsenic and then correct for the latter. This is usually done by solution of the alloy in sulfuric acid, dilution with water and hydrochloric acid, and titration a t 10' C. with permanganate. The above methods are satisfactory in the absence of such interfering elements as iron and copper. These elements are usually present, however, and their effects must be considered. Iron in small quantities is not harmful in the modified Low method, since the iron is oxidized to tile trivalent state in the hot sulfuric acid (16). Similarly, as Rowell has shown (16), in the Rowell method the reduction of iron in strong hydrochloric acid with sulfurous acid is slow and thus its deleterious effect is not too great. Iron in large quantities causes high results for antimony in both methods and separation must be resorted to. Fortunately, the iron content of most tin-base alloys is not high enough t o cause trouble. Copper will cause high results if it is in the cuprous state a t the time of the antimony titration. On the other hand, copper, when present in amounts greater than O.l%, may cause the results to be low, owing to its effect in catalyzing the oxidation of antimony previous to titration. This catalytic effect is usually small and can be ignored in most routine work, but in accurate work it cannot be ignored. The usual practice in such cases is to remove the copper before reducing and titrating the antimony. Because the methods of separation available are timeconsuming (I), it appeared desirable to investigate the catalytic effect of the copper in the hope of finding conditions under which the direct determination can be employed without interference from the copper. This investigation has resulted in the development of a method which yields satisfactory results in the presence of copper. The method consists essentially of dissolving the alloy in hot sulfuric acid, reducing the small amount of pentavalent antimony with sulfurous acid in strong hydrochloric acid solution, expelling arsenic and excess sulfurous acid by boiling, and titrating antimony with bromate. The method has been tested on Bureau of Standards tin-base alloys 54-a and 5 P b which contain about 7% antimony, 0.05% arsenic, 3% copper, and 0.03% iron. The results are very satisfactory (see Table VI).
REAGENTS
STANDARD POTASSIUhl BROMATESOLUTION (0.1 N ) . Twice recrystallize potassium bromate from water, and dry at 180" C. to constant weight. Dissolve 2.784 grams of the potassium bromate in water and dilute to 1 liter in a volumetric flask. METHYLORANGESOLUTION.Dissolve 0.1 gram of methyl orange in 100 ml. of water. PROCEDURE
Transfer 1 gram of the finely divided sample (free of metallic iron) into a dry 500-ml. Erlenme er flask. Add 10 ml. of sulfuric acid and heat without cover &st on a hot plate and then on a Tirrill flame until complete dissolution of the sample is attained, and copious white fumes are being evolved. During the initial treatment with acid avoid heating the sample on a plate that is too hot; otherwise the sample may melt and complete dissolution will become very difficult. Cool. Add 10 ml. of water and 3 or 4 grains of 12-mesh silicon carbide to act as an antibump. Add 50 ml. of hydrochloric acid and warm the solution for a few minutes to dissolve all salts. Adjust the temperature to approximately 50" C. Add 25 ml. of sulfurous acid (6%) and place the flask on a hot plate with surface temperature of 275' to 300" C. Boil the solution without cover until the volume is reduced to 60 * 5 ml., remove from the plate, and dilute to 350 ml. with boiling water. Pass a fairly rapid stream of air or oxygen through the solution for 5 minutes. Titrate in the usual manner with 0.1 N potassium bromate solution, using methyl orange as indicator: (Ml. of KBrOa
- blank) X 0.609 = %' antimony DISCUSSION
CATALYTIC OXIDATION OF ANTIMONY IN PRESENCE OF COPPER. Copper has been used as catalyst in the reduction of arsenic and antimony (9, 10). On the other hand, it appears that under certain conditions copper can also act as catalyst in the oxidation of the two metals. Thus, it has been shown that low results for antimony are obtained in the Low or Rowell methods for the analysis of lead and tin alloys when the latter contain copper (6,16,17). Rowell and other workers have reported high results for antimony when copper is present, but it is probable that in these instances precautions were not taken to ensure complete oxidation of the copper to the cupric state by bubbling air through the solution before titration of the antimony. The author has recently studied the behavior of copper in the analysis of antimony in tin-base alloys by the Rowell and modified Low methods. Experiments indicate that in the Rowell method the low results are caused by co-oxidation of antimony with cuprous ion as the latter is oxidized by air or oxygen previous to the bromate titration. Cupric ion does not catalyze the oxidation of antimony in hydrochloric acid solution (17) (see Tables I and 11).
Table
No.
1.
Co-Oxidation Copper Added
Mu. lb
2b 3
..... ..... .....
..... 30 C u + + + +
+
b
448
Time of Bubbling Min. 0
0 5 5 5 6 6 60 Cu++ 10 10 7 120 c u 5 60 Cu: 8 9 10 120 c u 10 120 c u + 10 10 11 120 c u Exact amount of 8b in aliquot unknown. Solution waa not bubbled with oxygen.
4
0
of Antimony with Copper KB;Os Used"
M1. 13.70 13.73 13.73 13.71 13.70 13.73 13.74 13.62 13.40 13.46 13.45
ANALYTICAL EDITION
July, 1944
Table II. Co-Oxidation of Antimony with Copper Copper Added HCI Added Antimony I'uun(1'' Mo. Mi. MR. l b ... 1.0 15 2b 15 120 1.0 3b ... 50 0.2 46 120 50 0.2 5c ... 50.0 15 BC 15 50.0 7 ... 15 50.1 84' ... 15 49.9 9d 49.4 30 13 49.3 10d 120 15 49.1 11 120 15 120 12 15 49.4 13 ... 50 50.1 14 50.0 ... 50 50.0 15 120 50 16 49.9 50 120 17 50.0 50 120 18 * ... 50.0 50 190 50.1 50 20' 30 49.2 50 48.9 50 21. 60
Table
449
IV. Determination of Antimony in Tin-Base Alloys b y
KO.
.
.
I
...
From the foregoing it is evident that it should be possible to obtain correct results for antimony in the Rowell method if conditions can be found for completely reducing antimony with sulfurous acid without reducing the copper. It appeared that this might be accomplished by complexing the copper with hydrochloric acid. Experiments showed that reduction of copper with sulfurous acid is very slow and incomplete in a strong hydrochloric acid solution, where the copper may exist in the form of complex acids (11). The small amount of cuprous ion which is produced induces very little cc-oxidation of antimony during the oxidation of the former with oxygen previous to the bromate titration (see Table 11). Unfortunately, however, it was found that the rate of reduction of pentavalent antimony with sulfurous acid is, like that of copper, very slow in strong hydrochloric acid solution. Bromide catalyzes the reaction and permits rapid quantitative reduction of antimony in strong acid solution, but appenrs to catalyze the reduction of copper in the same manner, with the result that co-oxidation of the antimony takes place (see Tables I11 ond IV). Table No. 1
2
111.
Reduction of Antimony with Sulfurous A c i d Oxidant Used HC1 Added Antimony Found" M1. Ma. KBrOs 15 50.0 15 49.9
No.
Antimony present 50.0 mg 120 mg. of cop er a; CuSO,.iHaO added before oxidant. Only about 9 5 h of antimony was oxidbed with bromate., d After addition of acid exceas Bra was expelled by boding previous to reduction with sulfite. C
.
I n the modified Low method copper catalyzes the oxidation of antimony and arsenic during solution of the sample in hot sulfuric acid. This oxidation does not appear to be associated with cuprous ion, for the amount of oxidation is proportional to the time of heating (17). Recent work has shown that the amount of oxidation is also controlled by the concentration of the sulfuric acid used in the solution of the alloy. Thus, when a 1-gram
Rowell Method Oxidant Used
Antimony Found"
% 54-b KClOs 6.08 54-b KClOa 6.20 54-b KCIOI 6.f10 54-b 7.35 nrz 54-b 7.37 Rrz 6 5a-b 7.35 Rrz 54-b 7c Ijrz 7.33 54-a 8 7.24 Brz 54-a 7.22 9 Brz 54-a 7.22 10 Brz a Certificate value for 54-a = 7.32 and for 54-b = 7.39. b 100 mg. of KBr added just before reduction with sulfite. C Solution of alloy and reduction with sulfite performed under a reflux condenser.
1 2 3) 4 5
Table
V. Determination of Antimony in Tin-Base Alloys b y
No.
Sample
Modified Low Method Antimony Found'
Notes
% 1 54-a 7.22 Wet flask 2 54-a 7.26 Wet flask 3 54-a 7.24 Wet flask 4 54-a 7.28 Acid fumed 5 54-a 7.28 Acid fumed 6 54-b 7.32 Wet fiask 7 54-b 7.37 Wet flask 8 54-b 7.35 Dry flask but acid was 9) 54-b 7.35 Dry flask but acid was 10s 54-b 7.36 Dry flask but acid waa llb,C 54-b 7.39 Dry flask but acid was 12 54-b 7.40 Acid fumed Acid fumed 13 54-b 7.42 Acid fumed 14 54-b 7.41 a Certificate value for 54-a c 7.32,and 54-b 7.39. b 1 gram of KBr added before distillation of arsenic. C Sample treated with HzSOr as directed in Procedure.
not not not not
fumed fumed fumed fumed
-
sample of tin-base alloy is dissolved in 10 ml. of ordinary concentrated sulfuric acid t o which a few drops of water have been added, rapid and complete solution of the alloy occurs and catalyzed oxidation of the antimony is pronounced. On the other hand, if the sulfuric acid has been fumed free of excess water before addition of the sample, dissolution is more difficult and may even be somewhat incomplete. The copper is precipitated as the anhydrous sulfate and causes little or no oxidation of antimony (see Table V). Because of the difficulty of controlling conditions with variou? types of alloys, however, it does not appear practical t o use a method which depends upon elimination of error by precipitation of the copper as sulfate. Instead, it should be better t o dissolve in the presence of sufficient water to ensure complete dissolution of the alloy and then reduce the small amount of pentavalent antimony present. It is obvious that nothing would be accomplished by attempting to reduce the antimony in a hot sulfuric acid solution with the aid of the usual carbon or sulfur compounds. It has been found that the reduction can be accomplished successfully with sulfurous acid in strong hydrochloric acid solution without appreciable reduction of copper (see Table VI). Apparently the reduction of very small amounts of antimony is complete Table
b
Sample
VI.
Determination of Antimony in Tin-Base Alloys b y Proposed Method No. Sample Aniimony Founda
%
1 2 3 4 5 6 7
54-a 7.34 54-a 7.30 54-a 7.33 54-a 7.31 54-b 7.41 54-b 7.42 54-b 7.43 8s 54-b 7.42 Qb 54-b 7.41 10) 54-b 7.41 11) 54-b 7.43 12b,C 54-b 7.41 13C 54-b 7.41 14d 54-b 7.42 Certificate value for 54-a 7.32 and 54-b 7.39. b 10 mg. of A s + + +added before addition of HISOX. C 1 ml. of water added with HISO,. d 100 rather than 50 ml. of HC1 added.
-
-
INDUSTRIAL A N D ENGINEERING CHEMISTRY
450
under conditions where highly incomplete reduction is obtained with large amounts of antimony (see Table IV, experiments 1 to 3). From the above it would appear that in the absence of bromide it might be possible to obtain correct results in the Rowell method by reducing first in dilute acid solution and then, after oxidation of the cuprous ion, repeating the reduction in strong hydrochloric acid solution.
Vol. 16, No. 7
complete without loss of antimony on alloys of the type of Bureau of Standards tin-base alloys Nos. 54-a and 54-b. EXPERIMENTAL
Aliquot portions of a solution of antimony trichloride, each containing about 75 mg. of antimony, to ether with 20 ml. of hydrochloric acid and 20 ml. of (1 to 1) sulfuric acid were diluted to 300 ml. with boiling water. Measured amounts of copper sulfate or cuprous chloride were dissolved in the solution, a rapid stream of oxy en was bubbled through the solution for 5 or 10 REDUCTION OF PENTAVALENT ANTIMONY WITH SULFUROUSminutes, and t%e antimony was titrated with standard otassium ACID. The rate of reduction of antimony with sulfurous acid bromate solution (0.1 N ) . The results are recorded in $able I. is markedly affected by the acidity of the solution. Kurtenacker Aliquot portions of a solution, each containing 50 mg. of antiand Furstenau (8) have shown that the optimum acid concenmony as antimony trichloride, 5 ml. of hydrochloric acid, and tration for the reduction of antimony in hydrochloric acid solu5 ml. of sulfuric acid were treated in a 500-ml. Erlenmeyer flask tion is about 2 N . The rate of reduction is a t a minimum in with 15 or 50 ml. of hydrochloric acid and then diluted to 300 or (1 to 1) hydrochloric acid and complete reduction in such solu100 ml., respectively. Measured amounts of copper in the form tion is attained only by repeated treatments with sulfurous acid of copger sulfate were added and the solutions were then heated (7). Rohmer (13, 14) found that reduction is greatly accelerated to 60 C., treated with 2 grams of anhydrous sodium sulfite, by the presence of bromide. The author has confirmed this and boiled a t a moderate rate for 10 minutes, and diluted, bubbled, has shown that rapid and complete reduction of antimony in and titrated as directed in the Procedure {see Table 11). The strong hydrochloric acid solution can be attained if bromide results have been corrected for the blanks indicated. is resent (see Table 111). The above experiment was repeated, oxidizing the antimony ft appears probable that pentavalent antimony chloride can with a slight excess of various oxidants previous to the addition exist in strong hydrochloric acid solution in the form of Werner of the hydrochloric acid (see Table 111). complex acids such as HSbC16, H,SbClr, and. H,SbC18 (19). With One-gram samples of Bureau of Standards tin-base samples such compounds it is evident that reduction of the antimony 54-a and 54-b (containing ,about 7% antimony, 0.05% arsenic, with sulfurous acid will be difficult, since the tendency for the 3% copper, and 0.03% iron) were transferred into 500-ml. antimony t o accept electrons and become reduced is diminished Erlenmeyer flasks, warmed gently (so as not to lose too much because the chlorine atoms are sharing their electrons with the acid) with 50 ml. of hydrochloric acid until most of the sample antimony. had dissolved, and finally dissolved completely by the addition of When bromide is added to the solution before reduction with small portions of potassium chlorate or by adding 10 ml. of sulfurous acid it is probable that there is an intermediate formahydrochloric acid-bromine mixture (12 ml. of bromine dissolved tion of a certain amount of the corresponding complex bromide in 100 ml. of hydrochloric acid). Sufficient bromine or chlorate acid. This should be less stable than the chloride acid because was added to oxidize all the tin and copper. The samples were of the p e s t e r size of the bromine atom, and hence reduction of then cooled to 50" C., and treated with 35 ml. of sulfurous acid the antlmony should be easier. (8%) and 2 ams of anhydrous sodium sulfite, and the antimony The addition of bromide should be avoided wherever po!siblewas then regced and titrated as directed in the Procedure. The e.g., when reducin antimony in dilute hydrochloric acid Eolucertificate values for antimony are 7.32% for sample 54-a and tion-because the %romine produced in the subsequent titra7.39% for 54-b (see Table IV). The values given have been cortion with bromate reacts but slowly with the antimony and inrected for a blank of 0.02 ml. of 0.1 N potassium bromate for the dicator, and the blank is high (see Table VII). samples oxidized with chlorate and 0.06 ml. for those oxidized with bromine. Samples &a and 54-b were analyzed as directed in the Procedure, adding water rather than sulfurous acid previous to the Table VII. Quantitative Removal of Arsenic by Distillation distillation of the arsenic. Some of the samples were dissolved in 10 ml. of sulfuric acid containin4 small amounts of water. Volume after With others, the 10 ml. of sulfuric a a d were heated in a 500-ml. No. Arsenio Present HrSOa Added Boiling KBrOa Used hfo. hft. M1. hfl. Erlenmeyer flask t o copious white fumes to expel all water, and cooled near1 to room temperature before addition of the sample 0
