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V O L U M E 2 6 , NO. 7, J U L Y 1 9 5 4 tetraethylammonium bromide (TEAB) as a supporting electrolyte. Methylcellulose and tetradecyltrimethylammonium bromide both proved to be qatisfactory maxima suppressors. Gelatin was not compatible with the systems under investigation. All attempts to xork M ith buffer solutions containing sodium or potassium salts and tetraethylammonium bromide were unsuccessful owing to interference by reduction of the alkali metal ions. I n this respect, tetraethylammonium bromide reduces a t a lower negative voltage than either phosphonium salt. The tetraethylammonium bromide supporting electrolyte showed small reduction waves not proportional to concentration. These came about 0.20 to 0.40 volt before the phosphonium ion reductions. A plateau is obtained between the two onium ion waves, and thus no interference is noted in the reduction properties of phosphonium salts. The polarographic results obtained are given in Table I. Half-wave potential values have been corrected for the small I R drop across the cell. I t is seen that the El82 values do not change significantly with concentration changes in the region investigated. These reduction waves show diffusion current values proportional to concentration within a t least 5%. Slope analysis considerations (Table I ) indicate that the polarographic reduction of phenyltrimethylphosphonium iodide proceeds in two successive reversible 1-electron steps. I n the case of
tetra-o-tolylphosphonium iodide, a slope value equal to approximately 0.03 indicates that the reduction occurs by a single 2-electron reversible process. Rave-height comparison-that is, comparing IdlC values under the same conditions with known ( 5 ) phenyl iodonium salt 1-electron reductions-indicates the same conclusions in regard to the number of electrons involved in these phosphonium ion reduction waves. LITERATURE CITED
(1) Ronner, W.A , , and Kahn, J. E., J . Am. Chem. Soc., 73, 2241 (1951). (2) Colichman, E. L., Ibid., 72, 1834 (1950). (3) Ibid., 74, 722 (1952). (4) Colichman, E. L., and Love, D. L., J . O r g . Chem., 18, 40 (1953). (5) Colichman, E. L., and Naffei, H. P., J . Am. Chem. Soc., 74, 2744 (1952). (6) Fenton, G. W., and Ingold, C. K., J . Chem. SOC.,1929, 2342. (7) Inaold. C. K., Shaw, F. R., and Wilson, I. S.. Itid., 1928, 1280. (8) Pech, J., Collection Czechosloo. Chem. Communs., 6, 126 (1934). (9) Pope, W. J., and Gibson, C. S., J . Chem. Soc., 1912, 735. (10) Van Rysselberghe, P., and NcGee, J. M., J . Am. Chem. Soc., 67, 1039 (1945). (11) Willard, H. H., Perkins, L. R., and Blicke, F. F., Ihid., 70, 737 (1948). RECEIVED for review January 18, 1954. Accepted Bpril 7 , 1954.
Investiga-
tion sponsored by the Research Corp.
Extraction of Niobium into Diisopropyl Ketone From Hydrochloric Acid Solutions H. G. H I C K S and R. S. GILBERT California Research and Development Co., Livermore, Calif.
T
HE extraction of niobium into diisopropyl ketone from mineral acid-hydrofluoric acid aqueous solution has been investigated ( 1 ) . The present work demonstrates the extraction of niobium into diisopropyl ketone from hydrochloric acid solutions in the absence of hydrofluoric acid. A radiochemical separation procedure based on these findings has been designed and found to be excellent. The authors were unable to keep sufficient tantalum in hydrochloric acid solutions, thus rendering cornparison b e h e e n tantalum and niobium unreliable. EXPERIMENTAL
Ten milliliters of concentrated hydrochloric acid were added to about 20 mg. of freshly precipitated niobium oxide containing 35-day niobium-95 and the mixture was digested in a hot water bath. The solution was cooled in an ice bath, saturated with hydrogen chloride gas, and digested hot. This procedure was repeated until the entire precipitate dissolved (usually twice was sufficient) to give a clear, slightly yellow solution which was adjusted to ION in hydrochloric acid. The solution was used as a “stock” solution for the extraction studies described below. A small amount of the stock solution was added to a known volume of standardized hydrochloric acid and the aqueous solution was stirred with an equal volume of diisopropyl ketone for 1 minute in a centrifuge cone. The technical grade diisopropyl ketone had been previously equilibrated with the proper concentration of hydrochloric acid. The layers were then centrifuged and separated, and equal aliquots of each phase were transferred to machined tetrafluoroethylene (Teflon) cups. These cups were covered with pressure-sensitive tape to avoid spilling. The gamma activity of the niobium-95 in the aliquots was counted with a single-channel scintillation pulse analyzer using a thallium-activated sodium iodide crystal. Organic and aqueous phases of the same extraction were counted consecutively in the same position relative to the crystal to minimize errors. The results are shown in Figure 1. 1 Present address, University of California Radiation Laboratory, Livermore, Calif.
In order to check that equilibrium was attained, extractions were performed using mixing times up to 15 minutes. No change was noted in the amount of niobium extracted. No attempt was made to keep ionic strength constant, hence no information was obtained concerning species in solution. DISCUSSION
The data in Figure 1 shovi that niobium extracts well into diisopropyl ketone from 10-11 hydrochloric acid and passes back into the aqueous phase when the diisopropyl ketone is equilibrated with 6M hydrochloric acid. This extraction behavior has been incorporated in the radiochemical separation
a
ag
50
W 0
c W
6: a
0 4
a
IP
HOl MOLARITY
Figure 1. Extraction of Niobium into Diisopropyl Ketone from Hydrochloric Acid Solutions
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ANALYTICAL CHEMISTRY
procedure outlined below. To test the specificity of the procedure, niobium fractions were isolated from fission product mixtures; the radiochemical separation procedure given below UBUally gave about a 60% recovery for the niobium. Measurement of the radioactive decay characteristics of the separated niobium fraction showed that the procedure had provided decontamination from all fission products by a factor of a t least 1000. The procedure has proved reliable for more than 40 radiochemical analyses. The sequence of steps used in the separation is as follows: Dissolve the metal target in hydrochloric acid containing carrier niobium a8 the oxalate complex, precipitate niobium from the target solution by digestion with nitric acid, wash the precipitate
with hot concentrated nitric acid, dissolve the niobium hydrated oxide in hydrochloric acid, extract niobium into diisopropyl ketone from 10M hydrochloric acid, back-extract the niobium into 6 M hydrochloric acid, precipitate the hydrated oxide with ammonia gas a t pH 9, slurry the precipitate with 5 ml. of concentrated nitric acid, dilute to 20 ml., adjust to pH 9 with ammonia gas, digest, wash precipitate with hot concentrated nitric acid, dry, ignite, and weigh as NbzOs. LITERATURE CITED (1)
Stevenson, P. C., and Hicks, H. G., ANAL. CHEM.,25, 1517 (1953).
RECEIVEDfor review September 2 5 , 1953. Accepted April 5 , 1954. performed under the auspices of the Atomic Energy Commission.
Work
Tit rimet ric Determination of Zirconium JAMES S. FRITZ and MYRON 0.FULDA lowa State College, Ames, lowa
M
OST of the recent progress in analytical methods for
macro amounts of zirconium has been due to the development of organic precipitants such as cupferron, mandelic acid, and arsonic acids. Of these, mandelic acid and its derivatives (6, 8, 9) are probably the most successful. They possess excellent selectivity for zirconium and under proper conditions form precipitates of definite composition. Because gravimetric methods are time-consuming, a rapid volumetric determination of zirconium is desirable. Kolthoff and Johnson ('7) have reported a direct amperometric titration of zirconium, thorium, tin, and uranium with m-nitrophenylarsonic acid. White (10) developed a titrimetric method for zirconium following the quantitative isolation of zirconiuni mandelate. Dhar and Das Gupta ( 1 ) determined zirconium by either colorimetric or titrimetric means following precipitation with oxalohydroxamic acid. The direct titration of thorium in acid solution with ethylenediaminetetraacetic acid (Versene) using Alizarin Red S indicator has recently been reported ( 2 ) . Zirconium interferes in this titration but cannot itself be titrated by the thorium procedure. Now, however, new indicators have been found which permit the rapid and accurate titration of zirconium with Versene. The method proposed is selective and should be of use in numerous cases. INDICATORS
The general reactions involved in the titration are:
+ HzY-z = ZrY + 2H+ + HaY-2 + (ab - 2)H+ = ZrY + aHJn Zr(1V)
ZrIn,
(1) (2)
where Y represents the Versene radical. After the bulk of the zirconium has reacted according to Reaction 1, the highly colored zirconium-indicator complex is destroyed (reaction 2), marking the end point. In addition to being highly colored, the zirconium-indicator complex must be less stable than the zirconium-Versene complex. Furthermore, Reaction 2 must be fairly rapid, so that the end point will not be overrun. Several indicators were tried. Alizarin Red S has been used for the colorimetric determination of zirconium ( 5 ) but is useless for the Versene titration because of the very slow reaction of the zirconium-indicator complex with Versene. Carminic acid, chloranilic acid, and Chrome Azurol S also are unsatisfactory. Both AlizarolCyanoneRC and Eriochrome Cyanine RC give Eriochrome Cyanine RC sharp and vivid end points. The Alizarol Cyanone end point is often slow if a direct Versene titration is attempted, but is
fast if a slight excess of Versene is added and back-titrated hot with standard zirconium. Either direct or back-titration is possible with Eriochrome Cyanine. but the end point in the direct titration should not be approached too rapidly. EFFECT O F pH
With either Alizarol Cyanone or Eriochrome Cyanine iridicator, satisfactory end points can be obtained in the pH range 1.0 to 2.0. Best accuracy is obtained, however, if the final pH is between 1.3 and 1.5. Results for zirconium are high above pH 1.5 and low below pH 1.3. EFFECT OF TE,MPERATURE
Heating the solution to 70" t o 90" C. speeds the reaction and permits a rapid back-titration of excess Versene with zirconium. h direct titration in hot solution is not, however, recommended owing to the possibility of hydrolyzing the zirconium In one experiment where the zirconium solution was heated for 15 minutes before titration, no end point was observed even with 100% excess Versene. Probably part of the zirconium precipitates and the jellylike precipitate absorbs the indicator so strongly that no end point is possible. REAGENTS AND SOLUTIONS
Alizarol Cyanone RC (National Aniline Division, Allied Chemical & Dye Corp.), 0.4oJ, aqueous solution. Eriochrome Cyanine RC (Geigy), 0.4% aqueous solution. Hafnium chloride, 0.5.V solution in 5% hydrochloric acid, standardized by evaporation and ignition to the oxide. The zirconium content was 2.1% by weight as determined by spectrographic analysis. Thorium nitrate, 0.5M solution prepared and standardized as outlined by Fritz and Ford ( 2 ) . Versene, 0.5M solution prepared from the disodium salt of ethylenediaminetetraacetic acid. This solution was standardized against calcium carbonate ( 4 ) or against pure zinc metal (3). Zinc amalgam, 20-mesh zinc lightly amalgamated with mercury. Zirconium chloride, 0.5M solution in 5% hydrochloiic acid, standardized by evaporation and ignition t o the oxide. The zirconyl chloride used to plepare this solution contained 100 p.p.m. or less of hafnium. PROCEDURES
Procedure A. The zirconium concentration of the solution taken for analysis should be about 0.003 to 0.005M. Adjust the pH to 1.4 with dilute ammonia, add 2 drops of Eriochrome Cyanine indicator, and titrate with 0.05M Versene t o disappearance of a pink color. If iron is present, add 10 grams of amalgamated zinc to 50 ml.
