The Dead-Stop Titration of Indium L. S. MARTENS1 and HERSCHEL FRYE Departmenf of Chemisfry, University of the Pacific, Stockton 4, Calif. b The familiar dead-stop titration method has been applied to the determination af indium by hexacyanoferrate(1l) with good results. Mole ratios of five indium(ll1) to four hexacyanoferrate(l1) were obtained, in agreement with ratios previously reported.
T
concept fundamental to the familiar “dead-stop” titration method was originated by Salomon ( 8 ) and developed by Foulk and Bawden (b), who coined the rame. Among a number of successful applications are those to zinc by Swinehart (9) and to gallium by Fetter and Swinehart (4). The simple apparatur, used by these workers is essentially that used in the indium determination. The method itself is uncomplicated because the analyst need not know either the potential applied across the identical platinum electrodes or the current flowing a t any time. The end point is indicatpd b) a considerable change in current, whil:h is apparent as an upswing of the galvanometer needle. The galvanometer used in the present work had a sensitivity of 1 X ampere per mm., resulting in a particularly rugged apparrttus. Theory of the method has been presented by Foulk and Bawden ( 5 ) , Reilley, Cooke, and Furman (Y),Bradbury ( I ) , Gaugin and Charlot (6), and more recently by Delaliay ( 3 ) . HE
EXPERIMENTAL
Standard solutions of indium(II1) and hexacyanoferrate(I1) were prepared. The indium solution was prepared by dissolving Fisher Scientific reagent grade indiuri metal (assay 99.99%) in G S reagent grade hydrochloric or nitric acid and diluting to volume. Baker and Adamson reagent grade potassium ferrocyanide was pulverized and dried to constant weight a t 100’ C. The anhydrous salt was then weighed and dieiolved in doubly distilled water; 2 grrims per liter of reagent grade sodium (‘arbonate (Baker and Adamson) was added to stabilize the solution, which wa,s then diluted to volume with water. 1 Present address, Oakland City College, Oakland, Calif.
Table 1.
a
Titrations
Potential, mv.
Concn.,” mole/liter
Tzmp., C.
200 400 400 300 300 400 500
0.001 0.001 0.01 0.001 0.01 0.01 0.001
27 27 25 27
Sweeping gas N, N; 0 2
None Xone 27 None 71 None Concentration refers to both indium(II1) and hexacyanoferrate(11). 70
Volumes of the standard indium(II1) solution were dispersed from a calibrated buret into the titration vessel, the mechanical stirrer was started, and current readings were taken within 1 minute after each addition of hexacyanoferrate(I1) solution; the latter solution was also added by means of a 10-ml. precision buret. No supporting electrolyte, in the polarographic sense, was used. After a sufficient number of readings had been taken to establish the shape of the curve, additional titrations were carried out recording only the volume of the titrant a t which the upswing of the galvanometer needle was noted. DISCUSSION
Table I summarizes the results of a number of the titrations. Typical conditions are listed, together with the calculated corresponding mole ratios of indium and hexacyanoferrate(I1) ; these latter data were obtained from the molarities of the reagents prepared as indicated above. From these data, taken over a wide variety of conditions and excluding the determination made with oxygen as the sweeping gas, the average mole ratio was calculated as 1 to 1.243. Standard deviation was ~k4.9.More carefully controlled conditions substantially increase the precision. Solutions of indium(II1) containing up to 2% by weight of ammonium sulfate and GN sulfuric acid showed no improvement in end point characteristics. At the potentials used in these determinations, the reduction of indium(II1) is irreversible, n-hereas that of hexacyanoferrate(II1) to hexacyanoferrate(I1) is reversible. Doubtless the solutions of hexacyanoferrate(I1) contain a little hexacyanoferrate(III), and current will flow when the hexacyanoferrate(I1) is in excess. The sharpness of the end point
Mole ratio 1:1.237 1:1.238 1:1.259 1 :1.245 1:1.254 1 : 1 ,237 1:1.248
is dependent on the continued lorn concentration of hexacyanoferrate( 11) because of the low solubility of the indium hexacyanoferrate(I1). As soon as the concentration of indium(II1) has been reduced sufficiently, current nil1 flow because hexacyanoferrate(I1) is being oxidized a t the anode and hexacyanoferrate(II1) is being reduced a t the cathode. From the data in Table I and those based on many similar titrations, the authors conclude that under appropriate, controlled conditions, indium may be determined by “dead-stop” hexacyanoferrate(I1) titration. The mole ratios indicated above are very close to five indium(II1) to four hexacyanoferrate(II), in agreement with Bray and Kirschman ( 2 ) . The authors also checked the method for the determination of zinc, with good results. In their experience, a higher potential must be maintained in the indium titration than in that of either zinc or gallium. LITERATURE CITED
(1) Bradbury, J. H., Trans. Faraday SOC. 49, 304 (1953). (2) Bray, U. B., Iiirschman, H. D., J Am. Chem. SOC.49. 2739 i1927). ( 3 ) Delahay, P., “New Instrumental Methods in Electrocheniietry,” pp. 258-64, Interscience, New York, 1054. (4) Fetter, N. R., Swinehart, D. F., ANAL. CHEM.28, 122 (1956). ( 5 ) Foulk, C. W.,Bawden, A. T., J . Am. Chem. SOC.48, 2045 (1926). (6) Gaugin, R., Charlot, G., A n a l . Chim. Acta 8 , 65 (1953). ( 7 ) Reilley, C. N., Cookc, W. D., Furman, X. H., ANAL.CHEX.23, 1226 (1951). ( 8 ) Salomon, E., 2. Physik. Chem. 24, 53 (1597). (9) Swinehart, D. F., AUAL.CHEV.23, 380 (3951). ~
RECEIVED for review October 22, 1062. Accepted April 25, 1963. VOL 35, NO. 8, JULY 1963
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