I n the blue multiplet a weak iron line a t 427.2 mp coincides with the chromium 427.5-mp line. If iron is suspected, its presence in the extract can easily be identified by the simultaneous appearance of iron emission lines a t 356.5, 357.0, 361.9, 363.2, and 364.8 mp in the vicinity of the chromium multiplet in the ultraviolet and a t 432.6 mp immediately beyond the CH band head at 431.3 mp (Figure 2). Unless these iron lines are present in the emission spectrum, no spectral interference results with the chromium lines a t 357.9 and 427.5 mp. The results shown in Table I for NBS samples lOOa and 152 were obtained in the presence of 0.1 gram of iron per aliquot portion extracted and, for the latter sample, with slit widths varying from 0.030 to 0.060 mm. The use of a recorder simplifies locating and measuring the chromium emission lines.
except very large amounts of iron. The flame spectrophotometric method can nevertheless be used in the presence of iron. The selective extraction avoids the introduction of high concentrations of diverse ions into the flame, and thus eliminates any error which might arise from their presence ( 7 ) or the need for a series of correction curves as has been reported by Burriel and coworkers (1). I n addition, the use of an organic solvent gives a fifty fold increase in the radiant power of chromium when compared to aqueous solutions of equal concentration. Consequently, accurate measurements can be made a t chromium concentrations as low as a feiv tenths of a microgram per milliliter. Extraction also permits concentration of the chromium in the organic phase, the attendant increase in sensitivity is valuable in trace analyses. ACKNOWLEDGMENT
SUMMARY
Extraction with 4-methyl-2-pentanone isolates chromium in the hexavalent state from all other elements
H. illden Bryan is indebted to E. I. du Pont de Nemours 8: Co., Inc., and Davidson College for a summer grantin-aid which made this study possible
and to the University of Tennessee for generously offering its facilities and supplies. LITERATURE CITED
(1) Burriel,
F., Ramiree-Muiioz,
J.,
Asunci6n-Omarrementeria, AI. C., Mtkrochim. Acta 1956,362. (2) Collier, H. E., Jr., Serfass, E. J., Moravian College, Bethlehem, Pa.,
private communication. (3) Dean, J. A , Lady, J. H., A N A L . CHEM.28, 1887 (19513). ( 4 ) Ghosh, C., 2. Physilc 78, 521 (1932). ( 5 ) Heyes, J., Z. Elektrochem. 42, 532 (1936). (6) Huldt, L., Lagerqvist, A., Arkiv Fysik 5 , 91 (1952). ( 7 ) Ikeda, S., J . Chenz. Soc. ( J a p a n ! , Pure Chem. Sect. 77, 463 (1956). (8) Lundegardh, H., “Die Quantitativ: Spektralanalyse der Elemente, Vol. 11, Fischer, Jena, 1934. (9) Rusanov, A. K., Kunina, S. I., Zavodskaya Lab. 9, 183 (1940). (10) Thanheiser, G., Heyes, J. Arch. Eisenhiittenw. 11, 31 (1937). (11) Vallee, B. L. Bartholomay, A. F., ANAL.CHEW28, 1753 (1956). (12) Weinhardt, A. E., Hixson, A. N., Znd. Eng. Chem. 43, 1676 (1951). (13) Wever, F. Koch, W., Wiethoff, G., Arch. Eisenhuttenw. 24,383 (1953). RECEIVEDfor review Fehruary 4, 1957. Accepted April 17, 1957.
Polarographic Behavior of the Uranyl-Cupferron System PHILIP J. ELVING and ALAN
F. KRlVlS
University of Michigan, Ann Arbor,
Mich.
Unusual current and potential phenomena disclosed in a previous polarographic examination of the uranylcupferron system in 10% sulfuric acid solution prompted the present more detailed investigation. The technique essentially involved polarographing series of solutions in which the concentration of one of the active species was held constant as the other was varied. Other pertinent experimental factors were also examined. At low concentrations of both uranium and cupferron two waves appear, corresponding to uranium(V1)-uranium(V) and cupferron-phenylhydrazine reductions. The variations in height and in Eliz of these two waves with increasing uranium and cupferron concentrations are basically related to the formation of an insoluble film of uranium(lV) cupferrate a t the mercury surface. Presence of such a film hinders the approach of uranyl ions to the electrode, facilitates the reduction of uranium(V1) past uranium(V) to uranium(IV), converts cupferron to more difficultly reducible cupferrate, and affects the capillary electrode performance de-
1292
0
ANALYTICAL CHEMISTRY
pending on the size of its lumen. The formation of the film postulated was verified by experiments in the presence of a known solvent for uranium(lV) cupferrate.
A
study (3, 15) indicated rather odd polarographic phenomena in solutions containing both uranium(V1) and cupferron. I n viea of the apparent lack of direct interaction between the uranium(T’1) and cupferrone.g., complex formation-a more intensive investigation of the system seemed warranted, as cupferron is a valuable analytical reagent for uranium-containing solutions. The polarographic behavior of uranium has been summarized ( 1 2 ) ; most pertinent to the present study is the work of Kolthoff and Harris (5, 6, 10). In 0.02M hydrochloric acid solution uranyl ion gives two waves: The first corresponds to a reversible one-electron reduction of U02++ to U02+; the second wave, which is approximately twice the height of the first, represents an irreversible reduction, probably uraRECEST
nium(V) to (111). As the hydrogen ion concentration is increased above 0.2M, the first wave height increases and the second correspondingly decreases to maintain the total current constant. In 6 M hydrochloric acid. the first wave height approaches that expected for a 2 e reduction, indicating that at high hydrogen ion concentration uranium(VI) is reduced to uranium(1V); this effect seems to be due to the increasingly rapid disproportionation of uranium(V). The optimum stability of uranium(V), with respect to its disproportionation to uranium(V1) and uranium(IV), is in the p H range of 2 to 4 (14). I n acid solution uranium(V) tends to disproportionate, because of the pH-dependence of the reactions: 2U02+
+ 4H+ UOn++
+ U+‘ + 2H20
(1)
or
2uO2++ 2H+
uo*+-+ GO-- + € 1 2 0
(la)
Ions which complex EO2++ increase the first wave height by shifting the disproportionation equilibrium (Equation l) to the right. T h e rate of disproportionation increases considerably with increasing uranyl concentration, as the amount of uranium(\*) produced per unit time is increased, thus raising il but not linearly with uranyl concentration. The rate is dependent on the square of the uranium(V) concentration ('7, 9). The presence of sulfuric acid, even in low concentration, results in apparently anomalous behavior (17)-e.g.) il for the first wave is larger than expected for a l e change. K h e n the sulfuric acid concentration is increased to 0.5 to l M j il increases to slightly less than that for a 2e reduction. The molar hydrogen ion concentration is lower than in the Kolthoff and Harris investigations (6, 10) and the increase in il must be due to the sulfate complex (17 ) .
The polarography of cupferron has been described (11, 15). I n 10% sulfuric acid, cupferron produces one polarographic wave representing its 6 e reduction to phenylhydrazine. Logarithmic analysis of the wave gives a single straight line whose slope indicates the irreversibility of the reaction. The polarographic behavior of uranium in the presence of cupferron has been briefly studied (3, 16).
EXPERIMENTAL
Apparatus. A Leeds 8: Northrup T y p e E Electro-Chemograph mas used in conjunction with a thermostated H-cell (25" i. 0.1' C.) containing a saturated calomel reference electrode (13); damping was equivalent t o galvanometer performance. S o correction was made for the cell iR drop, since t h e H-cell resistance, measured by a General Radio impedance bridge, was only 25 ohms. T h e capillaries (Corning marine barometer tubing) had t h e characteristics listed in Table I. Chemicals. Reagent grade cupferron (G. F. Smith Chemical Co.) TT as purified (15) b y recrystallization twice from alcohol after decolorization with S o r i t e , and then from alcohol alone; t h e crystals were stored over solid ammonium carbonate. Aqueous solutions were freshly prepared for each r u n n i t h boiled distilled water. Eimer 8: Amend C.P. grade uranyl sulfate was dissolved in water to give a 0.0640.V solution; the exact uranium concentration was determined by reduction with a Jones reductor, aeration, and titration with standard dichromate solution using sodium diphenylaminesulfonate as indicator. Nitrogen was passed through two chromium(I1) solutions to remove re-
Table I.
Characteristics of Capillaries
Used (Open circuit 25' C.) h, Cm.O
Wde-Bore (Lumen = 0.073 Um.) in Distilled Water 42 6 49 0 58.6
8 1 6 4 4 9
1 29 1 49 1 93
Narrow-Bore (Lumen = 0.058 hlm.)
in Distilled Water
48.6 57.4 68.7
4 5 3.7 3.1
1.76 2.12 2.59
I diminishes and then disappears completely, simultaneously, wave I1 increases in height and shifts to more negative potentials. Varying the uranium concentration at constant cupferron concentration gives only one wave until the uranium-cupferron ratio becomes very high, when a second wave appears. Extension of the polarograms into the positive potential region in order to investigate the high, apparently Faradayic residual currents-e.g., Figure 1,C-revealed no m-ave up to the mercury oxidation wave, ca. 0.2 volt; these residual currents are further discussed in the section on the narrowbore capillary.
+
Narrow-Bore in 10% Sulfuric Acid and 0.005ye Gelatin 53 6 68 7
4 0 3 1
1 9 2 4
Very Narrow-Bore (
