Iodometric Estimation of Zinc in Magnesium
Allovs J
CLYDE C. CAST0 AND ALBERT J. BOYLE Basic Magnesium, Incorporated, Las Vegas, Nevada
sulfate and boil the solution for 30 minutes. The manganous sulfate which is added ensures the precipitation of all manganese present as a filterable manganese dioxide. Filter the hot solution through a Selax No. 3001 or similar porcelain filtering crucible into a 500-ml. Pyrex suction flask. Wash thorou hly, controlling the final volume to not more than 150 ml. A8d 10 ml. of 10 per cent sodium or ammonium citrate solution for each milligram of iron present in the sample. Add ammonium hydroxide dropwise to the alkaline color change of methyl orange. Usually a turbidity occurs at this Occasionally the indicator is oxidized and more must r:dded. Add 1 to 2 sulfuric acid until the solution is distinctly acid to methyl orange. The volume a t this point should not exceed 200 ml. Place the flask in a cooling bath to bring the solution to room temperature. Add 1to 2 grams of potassium iodide and let stand for 10 to 15 minutes. Add 2 or 3 ml. of 1 er cent starch solution. To discharge the glue coloration due to iodine liberation which may develop, add sodium thiosulfate, making certain that no excess is introduced. Add potassium ferricyanide reagent in 1- to 2-ml. portions and titrate slowly with standard sodium thiosulfate after each addition until the starch-iodine color is faint, Continue this procedure until no additional color is. developed on addition of potassium ferricyanide reagent. A few more drops of sodium thiosulfate solution will discharge the remaining iodine and complete the titration. A sulfur-yellow end point is obtained which is sharp to within one drop of 0.1 N sodium thiosulfate. The end point should persist for a t least 30 seconds, although in most instances it remains much longer. Upon standing, however, the potassium ferricyanide reagent liberates iodine slowly, bringing back the blue color. A light green end point indicates the presence of copper or iron that has not been completely removed or complexed. In the event the alloy contains only a few tenths of a per cent of zinc, the following hydrogen sulfide procedure is used: Weigh a 10-gram sample of the alloy into a 60.0-ml. beaker, dissolve the sample in 100 ml. of 1 to 1 sulfuric acid, and dilute the solution to 300 ml. Neutralize the excess acid with ammonium hydroxide. Add 6 N sulfuric acid dropwise until just acid to methyl orange. Pass hydrogen sulfide through the solution a t a rate of a t least eight bubbles per second for a period of 45 minutes. Filter through close-textured paper. It is not necessary to wash the precipitate. Return the filter paper and contents to the original beaker. Add 5 ml. of concentrated nitric acid and 10 ml. of 70 per cent perchloric acid. Place on a hot plate, and boil to destroy the g p e r , taking the solution to dense fumes of perchlorio acid. ermit the solution to cool. Dilute to a volume of 100 ml. with distilled water and add 5 to 7 ml. of concentrated sulfuric acid. As an alternative procedure for the destruction of filter paper and conversion of zinc sulfide to a soluble zinc salt a combination of 5 ml. of nitric acid and 10 ml. of sulfuric acid may be used. The procedure from this point on is identical with that descnbed above, except for the omission of the persulfate step, since no manganese is present.
By modification of Lang's iodometric procedure, zinc may be determined in the presence of iron, after removal of manganese by persulfate oxidation and of copper as free copper by boiling with test lead. A direct procedure is outlined for alloys containing more than 2 per cent of zinc, and a concentration method employing hydrogen sulfide is used for alloys containing less than this amount.
T
HE increasing demand for magnesium alloys containing
aluminum and zinc has stimulated investigation of an iodometric method for the determination of zinc as proposed by Lang ( 8 , 3) and later by Gamzulov (1). Modification of the original procedure makes possible the determination of zinc in the presence of iron, after the removal of manganese as manganese dioxide by persulfate oxidation, and of copper as free copper by boiling with test lead. The general principle of the Lang method involves the liberation of iodine by the addition of potassium ferricyanide to a slightly acid zinc sulfate solution containing excess potassium iodide. The zinc precipitates as K2Zns[Fe(CN)s]r according to the equation:
2Fe(CN)s---
+ 21- e 2Fe(CN)s---- + 11
The iodine liberated is titrated with standard sodium thiosulfate. After the precipitation of zinc is complete, the rate of liberation of iodine from potassium iodide by potassium ferricyanide becomes sufficiently retarded to make possible an extremely accurate estimation of zinc. The method is advocated for magnesium alloys containing zinc in excess of 2 per cent. The inconvenience of precipitating zinc as the sulfide with hydrogen sulfide gas is eliminated, making possible the determination of a larger number of samples in much less time. The accuracy obtained is equal to or greater than that of the conventional methods for zinc. If alloys of lower zinc composition are encountered, it is necessary to concentrate the zinc from a larger sample by a preliminary hydrogen sulfide separation, or to add standard zinc sulfate solution to the sample before titration.
Reagents Potassium ferricyanide, c. P. (0.2 molar) ; test lead, granular; ammonium persulfate, c. P.; sodium or ammonium citrate, c. P. (IO per cent solution). Manganous sulfate, c. P. (1 gram of manganese per liter of solution); potassium iodide, c. P.; starch solution (1 per cent); sodium thiosulfate, 0.1 N , and hydrogen suliide.
TABLE I. RECOWRYOF ZINC FROM STANDARD ZINC SULFATE SOLUTIONS Zinc in Sample
Zinc Recovered
Gram
Gram
0.0125 0.0250 0.0500
Procedure Transfer 2 grams of the alloy to a 400-ml. beaker, and add 50 ml. of distilled water and 35 ml. of 1 to 2 sulfuric acid in small portions. During this operation keep the beaker covered as much as possible with a watch glass to avoid loss by mechanical spray. In the event copper is present, add 5 to 10 grams of test lead and boil the solution for 15 minutes. Filter through a fast paper and wash the residue five times with hot distilled water. To the filtrate, add sufficient manganous sulfate solution to ensure the presence of approximately 10 mg. of manganese in the sample. Add not more than 1 gram of ammonium per-
0.0127 0.0250 0.0502
Zinc in Sample Zinc Recovered Gram
Gram
0.0625 0.1250 0.2500
0.0627 0.1254 0.2507
Calculation Each milliliter of 0.1 N sodium thiosulfate solution is equivalent to 9.97 mg. of zinc: M1. of 0.1 N Na&Ot X 0.00997 X 100 % Zn Wt. of sample
623
I N D U S T R I A L A N D E N GI I N E E R I N G C H E M I S T R Y
624
Results and Discussion The interference of iron present in the sample may be effectively prevented by the addition of citrate. Unless this precaution is taken before the addition of potassium iodide to the solution, free iodine is liberated and the ferric iron is reduced. The resulting ferrous iron is partially reoxidized during the titration, thereby producing low values for zinc. Gamzulov (I) noted this reduction but did not find subsequent interference from the ferrous iron. If copper is present, even in small amounts, it produces a green color which interferes with the true sulfur-yellow end point. If present in large amounts, copper gives high values for zinc. The test lead treatment will remove at least 10 mg. of copper satisfactorily. Granular aluminum and aluminum foil were not so effective as lead. Manganese present in the sample leads to high values for zinc. Removal of this element as manganese dioxide by persulfate oxidation is extremely effective. Approximately 10 mg. of manganese must be present to avoid the formation of a colloidal precipitate. Cadmium produces high results for zinc and consequently must be removed if present. Preliminary investigation indicates that it may be precipitated with 40-mesh aluminum ( 4 ) from a slightly acid solution. Granular aluminum also successfully removes copper if a small amount of cadmium is present. Lang (2) advocates complexing cobalt with potassium cyanide end eliminating the effect of nickel by the addition of sodium potassium tartrate and aluminum sulfate. However, these elements occur in such small amounts in magnesium alloys that they do not interfere. The sodium thiosulfate solution waa standardized against weighed amounts of c. P. potassium dichromate, dried at 110' C. The empirical zinc factor of 9.97 mg. of zinc per ml. of 0.1 N sodium thiosulfate was employed. This is the figure found by Lang and was verified by the present work. Results of testa to determine the accuracy of the iodometric method are shown in Table I. A standard zinc sulfate solution
T u r n 11. DETERMINATION OF ZINCBY IODOMETRIC METHOD (In solutions containing magnesium, aluminum, manganese, cadmium, nickel, iron, and oopper 98 aulfates. Zinc present, 0.1250 gram) M?gAluMannesium minurn ganese Iron Copper Cadmium Nickel Zina Reaent Present Present Present Present Present Present Recovered Qrama
... ... ... ... ... ... ... ... ...
... ... ..... . ... ... ... ...
... ... ... ... ... .*.
.... .. ... ... 3.7
Gram
0.10 0.25 0.50
.... .... .... .... .... ....
..... ... .... .... .... .... .... .... .... .... .... .... ....
.... ....
.... .... ....
Gam
.... .... ....
0.004" 0.004
0.012" 0.010 0.020" 0.020 0.040" 0.040 0.100
.... .... .... ....
.... .... .... .... .... .... .... .... .... .... ....
Gram
.... .... .... .... .... ....
....
.... .... ....
Gram
.... .... ....
Gram
.... ....
....
.... .... .... .... .... .... .... .... .... . . . . . . . . .... .... . . . . . . . . .... .... . . . . . . . . .... . . . . . . . . . . . . .... .... . . . . . . . . .... .... . . . . . . . . .... .... 0.001" .... .... .... 0.003a . . . . . . . . .... 0.005" .... .... .... . . . . 0.0010" . . . . . . . . . . . . 0.0025' . . . . . . . . . . . . 0.0050n .... .... . . . . 0,0050 .... .... .... . . . . 0.02" . . . . .... . . . . 0.02 .... . . . . . . . . 0.10 .... .... . . . . . . . . 0.001 .... . . . . . . . . 0.002 . . . . . . . . . . . . 0.005 . . . . . . . . . . . . 0.010 . . . . 0.005" 0.020" .... . . . . . . . . . . . . .... .... . . . . . . . . .... .... . . . . . . . . . . . . .... . . . . . . . . .... 0.001' . . . . . . . . .. .. .. .. 0.001" . . . . . . . . 0.001" . . . . . . . . o.ooia . . . . . . . . .. .. .. .. 0.002" . . . . . . . .
0.20 0.004' 0.20 0.0125 0.20 0.020" 0.20 0.040' 0.10 0.10 1.9 .... 0.10 .... 1.9 1.9 0.10 0.0125 1.9 0.10 0.012" 0.10 0.0 Za 0.003" 1.9 Element removed or complexed.
a.7 3.7 3.7 1.9
Gram
........
Gram
0.1247 0,1246 0.1247 0.1245 0.1255 0.1251 0.1270 0.1246 0.1280 0.1246 0.1296 0.1330 0.1252 0.1249 0.1248 0.1251 0,1254 0.1253 0.1300 0,1250 0.1330 0.1860 0.1248 0.1252 0.1261 0.1293 0.1252 0.1246 0.1248 0.1248 0.1251 0.1251 0.1253 0.1250 0,1247 0.1248 0.1249
Vol. 15, No. 10
TABLE 111. ANALYSISOF MAGNESIUM ALLOYS" (Containing approximately 3% zinc, 6% aluminum, and 0.3% manganese) Zinc Found Zinc Added by as Standard Sample Iodometric Zinc Sulfate Total Zinc Weight Method Solution Recovered Error Grama
Gram
Gram
Mg.
Gram
0.1345 -0.1 0.0721 0.0625 0.0625 0.1354 -0.2 0.0731 -0.2 0.0625 0.1344 0.0721 0,0728 0.0625 0.1353 0.0 0.0743 0.0625 0.1366 -0.2 0.1274 0.0648 0.0625 $0.1 0.1315 0,0693 0.0625 -0.3 0,1371 -0.2 0.0748 0.0625 a All samples contained a small amount of copper, which was removed, and of iron, which WBB complexed.
was prepared by dissolving freshly ignited c. P. zinc oxide in sulfuric acid. Portions of this solution were used as standard zinc samples in all tests. Table I1 records the effect of various metallic ions on the recovery of zinc from solutions Containing a uniform amount of zinc and varying amounts of the metals. A series of analyses was run to test the applicability of the iodometric method to magnesium-zinc alloys. A 4gram sample of alloy was dissolved and divided into two equal parts. The original zinc content was determined on one aliquot, and the zinc content after the addition of a standard zinc solution was determined on the other. Table I11 shows the results of these tests. Table IV compares the zinc values obtained by two different methods. Twenty grams of alloy were dissolved in sulfuric acid and divided into two equal parts. Both solutions were gassed to concentrate the zinc as sulfide. The standard potassium ferrocyanide procedure, using uranium nitrate as an external indicator, was employed on one portion, the iodometric method on the other.
TABLE IV. ANALYSISOF MAGNESIUM ALLOYSO (Containing approximately 0.4% zinc, 4.5% aluminum, 0.3% manganese) Zinc Found b Zino Found by Ferrocyanide Method Iodometrio Metgod %
%
0.52 0.61 0.55 0.34 0.42 0.35 0.46 0.41 0.51 0.39 All samples contained a small amount of copper and of iron, which was complexed.
0.52 0.61 0.54 0.33 0.40 0.36 0.46 0.43 0.50 0.38 whioh was removed,
Summary A method is described for the iodometric determination of zinc in magnesium alloys. A direct procedure is outlined for alloys containing greater than 2 per cent of zinc, while a concentration method employing hydrogen sulfide is described for alloys containing less than this amount. The interference of copper is prevented by the removal of this element with test lead. Manganese is removed as manganese dioxide by oxidation with ammonium persulfate. Errors due to the presence of iron are eliminated by complexing the ferric iron with sodium citrate. The accuracy obtained by this method is equivalent to that of any standard procedure for zinc. The procedure is applicable to a wide variation of magnesium alloys containing zinc.
Literature Cited (1) Gamiulov, Ya. I., Zavodskaya Lab., 3,984-5 (1934). (2) Lang, Rudolf, 2.anal. Chem., 79, 16r-70 (1929). (3)Zbid., 93, 21-31 (1933). (4) Townsend, F. E., and Cade, G . N., Jr., IND.ENCI. CHEM.,ANAL. ED.,12,1634 (1940).
