V O L U M E 21, NO. 6, J U N E 1 9 4 9 veloped, and no deflection is observed on the galvanometer. If one of the mercury surfaces is disturbed by gentle shaking or tapping, while the other is kept quiescent, an electromotive force is developed and a current' flows in t,he galvanomet'er, provided that there is a trace of oxygen present in the elect,rolyte to be reduced at, the still mercury surface. If, however, an electrolyte free of any trace of oxygen is used, no current is observed to flow. When a solution of potassium chloride, prepared from water deoxygenated according to the above procedure, mas used as t,he electrolyte in this cell, no current was observed to flow until air was admitted to the cell and then the usiual deflection of the galvanometer was obtained. I t is believed t,hat this electrical method provides an even more
753 sensitive qualitat,ive test than the Winkler method and confirms the efficacy of this technique for complete removal of oxygen from water. LITERATURE CITED (1) Taylor, R., J . Am. Chem. SOC.,52, 3576 (1930). (2) W i n k l e r , L. W., Ber. deut. chem. Ges., 22, 1764 (1889). (3) W i n k l e r , L. W., 2. anal. Chem., 53, 665 (1914). RECEIVED September 21, 1918. Abstract of a portion of a dissertation submitted t o t h e Chemistry Department of t h e Graduate School of t h e Johns Hopkins University in partial fulfillment of the requirements for the degree of doctor of philosophy.
Polarographic Determination of Pentavalent Antimony in the Presence of Pentavalent Arsenic I. M. KOLTHOFF AND R. L. PROBST School of Chemistry, University of Minnesota, Minneapolis, Minn. HIS work was started in 1940, but interrupted during the Twar. Since 1940 two publications on the reduction of pentavalent antimony have appeared. Kraus and S o v a k (2) found that pentavalent antimony is reduced in two steps in relatively concentrated hydrochloric acid. The first wave corresponds to the reduction of Sb' to Sb"' and the second to the reduction of Sb"' to antimony in an amalgam, The two waves are not distinguishable until the concentration of the acid is greater than 6 S. .It concentrations above 7 'Ir the ~ a v e are s well developed and the reduction is complete. FT'ithout being acquainted with this study, Iingane and Sishida ( 4 ) reported that a small reduction wave of pentavalent antimony is observed when the hydrochloric acid concentiation is 0.5 S,that in 4 LV hydrochloric acid two naves are obscived, and that in 6 &V acid the height of the first wave (Sb' d SbIII) is exactly 0.4 of the total diffusion current. The authors' work carried out before 1942 is in general agreement with the above studies. They were interested in making use of the polarogiapliic reduction of Sb' to SbO for the determination of antimony in the presence of arsenate. Although arsenate has been found not to be reducible a t the dropping mercury electrode, they found small reduction waves in relatively concentrated (6 A7 hydrochloric acid solutions. Large amounts of arsenate, therefore, interfere with the polarographic determination of pentavalent antimony in 6 hydrochloric acid. By changing the supporting electrolyte thcy have succeeded in eliminating this interferencc. EXPERlMEYTA L
Stantlard solutions of trivalent arsenic arid antimony were prepared and standardized in the usual way. By oxidation with bromine, standard solutions of arsenate and pentavalent antimony were prepared. The polarographic measurements were carried out in the absence of oxygen at 25" * 0.1 ' C. The characteristics of the three capillaries used were determined a t various potentials. In order to intercompare diffusion currents obtained with different capillaries the values of the diffusion current constant (3)I = i ~ / ~ n 1 ~ ' ~int 'which ' ~ , i,, is the diffusion current, c the millimolar concentration, V I the mass of mercury in milligrams flon-ing out per second, and t the drop time, are reported in the tables. The values of m 2 ' 3 t 1 ~ 6 in a potential range between -0.4 and -1.0 volt [ P S . saturated calornel ~lectrode(S.C.E.)]of the threp capillaries \'ere 1 . i 8 , 1.82, and 2.07, respectively. REDUCTION OF PENTAVALENT ANTIMOKY
I n agreement with Iiraus and Sovak (2) and with Lingane and Sishida (4)tivo waves were found a t hydrochloric acid concentrations equal to or greater than 6 LV hydrochloric acid. The first wave corresponds to a reduction of Sb' to Sb"' and the second t o that of Sh"' to Sb amalgam. Aktacidities smaller than 4 A'
only one wave is observed, the height of which decreases with decreasing acidity. Even in 4 hydrochloric acid the diffusion current is smaller than the value calculated for complete reduction of Sb' to SbO. KOreduction wave was observed in alkaline medium. The results are summarized in Table I. The second diffusion current id, refers to the total current (sum of the two wave heights). All values are corrected for the residual current. The half-wave potential T I / , refers to the saturated calomel electrode. The value of the diffusion current constant of 7.50 in 6 N hydrochloric acid is the same as the value reported by Lingane and hTishida (4). Both diffusion currents in 6 *V hydrochloric acid were found to be proportional to the concentration of pentavalent antimony in a range between 0.17 and 2 X 31. The diffusion current of trivalent antimony was determined in the same concentration range in 6 S hydrochloric acid. The ratio of the total diffusion current (id,) of SbVto that of Sb"' was found to be 5/3 X 0.91. If the diffusion coefficients of Sb" and Sb'" in 6 .V hydrochloric acid were the same the ratio would have been 5/3. The above results indicate that the diffusion coefficient of SbV (in 6 N hydrochloric acid) is smaller than that of Sb"'. I n 8 S hydrochloric acid the ratio was found equal to 5/3 X 0.94. Further investigations were made of the reduction of pentavalent antimony in 1 A; hydrochloric acid in the presence of varying amounts of alkali bromides and chlorides. As is evident from the
Table I. Reduction of 1.53 Millimolar Pentavalent Antimony Solution a t 25" + 0.1" C. a t Varying Acidities Concentration of Hydrochloric Acid,
.v
idl,
ra.
id!,
TI, i
Izn
11"
Pa.
__ 7.50 ..
.
!% Id
... ...
20.8 20.0 15.0
(US.
3.03
.,._ 1')
..
-0.11 -0.18
T.2 0.4 1.9
.. ..
3.C.E.)
-0.24
-0.3.5
