TECHNICAL NOTES
Controlled Potential Coulometric Method To Determine the Average Titanium Oxidation State of Titanium Chlorides in NaCl Charles E. Baumgartner GE Corporate Research and Development, Schenectady, New York 12301 INTRODUCTION 140
The electrochemical reduction of Tic14 to elemental Ti in molten chloride melts is complicated by both the presence of three discrete reduction steps, separated by approximately 0.5 V vs a C1p and a rather complex solution equilibrium. Particularly troublesome to the solution stability are the disproportionation equilibria which exist between the forms of intermediate oxidation state. For example, Chassaing e t al.3-5 and others6 identified four different heterogeneous reactions which occur simultaneously to establish the equilibrium solution conditions. 2TiC1,
TiCl,
+ TiCl,
4TiC1,
P
Ti + 3TiC1,
(1) (2)
2TiC1,
+ T i P 3TiC1,
(3)
Ti + TiC1,
(4)
P
2TiC1,
@
Additional complexity arises in molten NaCl where elemental Ti exists as a solid, the Ti salts of intermediate oxidation state possess an appreciable melt solubility, and Ti4+ exists in equilibrium between gaseous TiC14and a Tic&,- complex of limited melt stability. From the equations above it can be seen that T i electroreduction is complicated by two primary process inefficiencies: the volatilization of T i c 4 and the redissolution of deposited Ti (eq 3). Of these reactions, the forward reaction of eq 3 is the most important insomuch as the melt can support a highTiCl3 concentration. It is therefore important to be able to monitor the relative Tic12 and Tic13 concentrations, or the average Ti oxidation state, within the molten salt as a means of optimizing process efficiency. A controlled potential coulometric method based on an Fe*+/Fe3+ couple has been developed which, when coupled with the total Ti content determined by a separate analysis, is useful for calculating the average T i oxidation state in a solidified NaCl sample.
EXPERIMENTAL SECTION Reagents. Experimental Tic!, containing NaCl melts were prepared by intermingling known quantities of micrometer particle-sized Ti, powdered TiC13, and reagent grade NaCl in an inert atmosphere followed by heating under Ar at 900 “C for 24 h in a covered alumina crucible. A mixture of TiClz and Tic13 (1) Guang-sen, C.; Okido, M.; Oki, T. J.Appl. Electrochem. 1988,18 (l),80-85. (2)Guang-sen, C.;Okido, M.; Oki, T. J. Appl. Electrochem. 1987,17, 849-856. (3)Chassaing,E.;Basile, F.; Lorthioir, G. J . Less Common Met. 1979, 68,153-158. (4) Chassaing,E.;Basile, F.;Lorthioir, G. Ann. Chirn. 1979,4(4), 295299. ( 5 ) Chaasaing, E.;Basile, F.;Lorthioir, G. J . Appl. Electrochem. 1981, 11, 187-191. (6)Mellgren, S.;Opie, W. J. Met. 1957,2, 266-269. 0003-2700/92/0364-2001$03.00/0
c
L
/’
“t 120
d
100
v)
EXPERIMENTAL POINTS
40
20
0.1
I 0.2
I 0.3
I 0.4
1 0.5
I 0.6
I
I
0.7
0.8
TOTAL ml 1.673M TICI, SOLUTION ADDED
Figure 1. Method development using TiCi3 addition.
are formed within the NaCl matrix as a result of the above disproportionation equilibria. Following cooling, the recovered solidifed salts were stored in a dessicator. The total Ti content in the solidified salts was determined using ICP following acid digestion. The total concentration of oxidizable Ti salts was determined coulometrically (PAR Model 377A coulometry system) by dissolution within a Fe3+-containingsolution prepared as 1.0 M KCl, 4.0 X 10-3 M Fe2(S04)3,and 0.12 M HCl under NO. Procedure. A 20-mL aliquot of the above Fe3+solution is deoxygenated by flowing Nz through the gas inlet port for a minimum of 15min. A potential of +0.6 V vs a Ag/AgClreference is applied to the Pt working electrode until the coulometer indicates the absence of Fez+and other oxidizable species by the cessation of current flow. Optimum coulometric results are obtained by rapidly adding between 50 and 150mgof the solidified salt as a lump into the prepared solution. An anodic current is recorded almost immediately as Fez+,formed by the reaction of Fe3+with both Ti2+and Ti3+,is oxidized back to the trivalent form. Repetitive samples can be analyzed in the same solution. The average Ti oxidation state in the solidified NaCl sample is then determined using the following equation: Tiequiv = QMJFn
(5)
Tiequiv= equivalent weight of Ti+3 Q = coulombs passed/g sample M = MW ofTi F = Faraday’s constant
n = 1 electron
The value of Tiquivis then coupled with the total Ti concentration 0 1992 American Chemical Society
2002
ANALYTICAL CHEMISTAY, VOL. 64, NO. 17, SEPTEMBER 1, 1992
Table I. Controlled Potential Determination of Average Titanium Oxidation State in NaCl/TiCldTiCls SamDles av Ti oxidation state sample total Ti content (via ICP),% A
4.67
B
6.53
2.62
4.93
E
9.44
1.94 2.09 2.10 2.01 1.98 2.02 (av) 1.94 1.90 1.93 1.93 1.92 1.92 (av) 2.24 2.22 2.17 2.24 2.26 2.23 (av) 2.02 2.03 2.02 1.97 2.01 2.01 (av) 2.87 2.86 2.87 2.87 2.85 2.86 (av)
determined for the salt sample using ICP (or an alternate method) to provide the average oxidation state of the Ti present.
RESULTS AND DISCUSSION Initial method evaluation utilized a standard solution of 1.673 M TiC13 prepared by dissolving Tic13 powder into a deoxygenated dilute HC1 solution. Aliquots of this standard solution were analyzed coulometrically;the recorded coulombs compared favorably with the values calculated to correspond to that solution volume (see Figure 1). This standard solution was also utilized to confirm the invariance of the coulometric results to changes in KCl, HC1, and Fez(S04)3 solution concentration, provided that sufficient Fe3+ is present for reaction with the reduced Ti forms and sufficient HC1 is added to prevent precipitation of iron hydroxides. Table I shows the average titanium oxidation state for five separately prepared NaCl matrix samples calculated from the coulometric results and the total Ti content using eqs 5 and 6 above. These samples were synthesized from Ti, TiCl3,
1
2
3
4
5 6 TIME (WEEKS)
7
6
9
10
Figure 2. Average Ti oxidation state for samples stored under dry N2 or room ambient condltions.
and NaCl powders to contain total Ti concentrations ranging between 2.5 % and 9.5 % ,thereby covering the concentrations anticipated during Ti electroreduction from a chloride melt. Five aliquots were analyzed for each sample without intermediate solution changes to the coulometry cell. Replicate analyses showed excellent reproducibility. Total Ti concentrations within each of the NaCl matrices ranged from 2.61 % to 9.44% as determined by ICP. Initial sample preparations were such that four samples should theoretically favor Ti2+ formation and the fifth retain an appreciable Ti3+ content; these results were confirmed coulometrically. Sample B, which yielded an average Ti oxidation state of +1.92, likely possessed some unreacted Ti powder within the matrix which dissolved along with the rest of the sample in the coulometry cell. The ambient stability of the NaCl matrix encapsulated Ti salts was examined by selecting several salt pieces from sample A above which were approximately 150 mg in size. These pieces were stored over time either in a 50 OC oven under dry nitrogen or in a plastic sample bag under room ambient conditions. Sampleswere coulometrically analyzed using this technique over several weeks, and the results are shown in Figure 2. Moisture hydrolysis of Ti2+within the NaCl matrix during ambient storage results in a loss of oxidizable material which is demonstrated as an increase in the average titanium oxidation state for the salt. However, it is shown that samples can be maintained under dry conditions without impact on the Ti oxidation state. RECEIVED for review January 9, 1992. Accepted April 21, 1992. Registry No. NaC1, 7647-14-5; Ti, 7440-32-6; TiC12, 1004906-6; Tic&, 7705-07-9.
