Anal. Chem. 1992, 64, 1998-2000
1998
Selectivity and Sensitivity of Self-Assembled Thioctic Acid Electrodes Quan C h e n g and Anna Brajter-Toth' Department of Chemistry, University of Florida, Gainesville, Florida 3261 1-2046 The design of electrodes with controllable surface properties continues to be a major challenge. This study was undertaken to evaluate the possibility of self-assembly of thioctic acid (1,2-dithiolane-3-pentanoic acid) on a smooth gold electrode to form an organized monolayer with the carboxylic acid terminal in contact with solution controlling the response of the electrode. This surface is shown schematically in Figure 1. Li and Weaver have exploited spontaneous strong adsorption of thioctic acid on gold through its two sulfur atoms in order to attach pentaamminecobalt(II1) to the surface. The electrodes were designed to study intramolecular electron transfer, no particular effort was made to control surface roughness, and the electrochemical behavior of solution species a t thioctic acid modified gold was not reported.' Selfassembly of thioalkyl derivatives is emerging as a practical way to modify gold electrodes. Applications of the selfassembled monolayer electrodes (SMEs) to pH and metal sensing have been reported.2~3Sun et al. have shown that a self-assembled monolayer of 4-aminothiophenol can be used, by controlling solution pH, to electrostatically bind solution species.4 Electron transfer through the monolayers has been investigated, and a number of factors have been found to influence it including the chain length of the assembled derivatives6 and the density of packing of the monolayer.6 In this study, thioctic acid was assembled on agold electrode prepared on a single-crystal silicon wafer and the resulting SME was characterized to determine the quality of packing and the effect of solution pH on response. The results show that the electrode is permeable and stable. The charge of the monolayer controls the response to species in solution. The gold substrate also plays an important role in the response of the SME. Analytical implications of these results are discussed. EXPERIMENTAL SECTION Materials. Thioctic acid (1,2-dithiolane-3-pentanoic acid) was obtained from Aldrich. Hexaammineoruthenium(II1)chloride was purchased from Alfa Products; potassium ferricyanide trihydrate was obtained from Fisher Scientific. Sodium fluoride was obtained from MCB Co., sodium borate decahydrate was from Mallinckrodt, Inc., and Tris was purchased from Sigma. All chemicals were used as received. Aqueous solutions were freshly prepared from doubly distilled, deionized water and were purged with nitrogen for at least 5 min prior to use. Solutions of pH 1.5 were prepared from 0.1 M HClO4, pH 7.4 from 0.05 M Tris, and pH 9.1 from 0.05 M sodium borate. Apparatus. All electrochemical measurements were made with a Bioanalytical Systems electrochemical analyzer (BASloo), and data were downloaded to an IBM PS/2 Model 50 computer. A conventional three-electrode setup was employed, with a self-assembled monolayer electrode (SME) as the working electrode, platinum-wire auxiliary, and silver/silver chloride in saturated potassium chloride (Ag/AgCl) as the reference. All potentials are reported versus Ag/AgCl unless stated otherwise. (1)Li, T. T-T.;Weaver, M. J. J. Am. Chem. SOC.1984, 106,6107. (2) Hickman, J. J.;Ofer, D.; Laibinis, P. E.; Whitesides, G. M.; Wrighton, M. S. Science 1991, 252, 688. (3) Rubenstein, I.; Steinberg, S.;Tor, Y.; Shanzer, A.; Sagiv, J. Nature 1988,332, 426. (4) Sun, L.; Johnson, B.; Wade, T.; Crooks, R. M. J.Phys. Chem. 1990, 94, 8869. (5) Miller, C.; Cuendet, P.; Gratzel, M. J. Phys. Chem. 1991,95, 877. (6) Chidsey, C. E. D.; Loiacono, D. Langrnuir 1990, 6, 682. 0003-2700/92/0364-1998$03.00/0
Electrode Preparation. The Au electrodes on which thioctic acid monolayers were assembled were prepared by vacuum deposition of pure Au (99.99% maple leaf gold) onto a silicon wafer which was precleaned in a heated solution of 1:4 30% H202 in concentrated H2S04. Caution: A solution of H202 in H2S04 is a very strong oxidant and reacts violently with many organic materials. It should be handled with extreme care. The thickness of the deposited gold was determined through a mass change to be approximately 2 x lo3A. The silicon wafer was then cut into ca. 1-cm2pieces. Copper wire was connected to the gold with silver epoxy (Epo-tek 410E, Epoxy Technology), and the connecting point and the silicon side of the electrode were insulated with epoxy (Epoxi-patch,Hysol). Prior to use, the electrode was cleaned with a solution of 1:4 30% H202 in concentrated HzSOl followed by rinsing with distilled deionized water and absolute ethanol. The cleaning solution did not contact the electrode for more than 10 s to avoid erosion of the epoxy insulator. The geometric electrode area was determined by cyclic voltammetry (CV) in 5 mM Ru(NH3)e3+in 0.3 M sodium acetate at pH 2 with D,(Ru(NH3),j3+)= 5.7 X 10-7.7 Typical electrode areas were 0.29 cm2. The self-assembled monolayer was prepared immediately after cleaning by immersing the electrode for 24 h in the 0.1 % thioctic acid solution in absolute ethanol. The CV results on bare Au were obtained on a polycrystalline electrode of an area of ca. 0.28 cm2. The electrodes were cleaned using the procedure described by Oesch and Janata.8 Current densities were calculated from the geometric area which was determined by the procedure described above for vacuum-deposited gold electrodes.
RESULTS Capacitance Measurements. Cyclic voltammograms were run from -0.2 to +0.2 V, and the capacitance was calculated from the sum of the cathodic and anodic current measured at 0 V divided by 2 times the scan rate and the electrode area. The procedure was similar to that of Miller et al., who used it to determine the relative dielectric constant of a hydroxy thiol m ~ n o l a y e r In . ~ their approach, the inverse of the capacitance determined for a series of hydroxyalkanethio1 homologues, HO(CH2),,SH, was plotted as a function of film thickness. From the slope of the plot, the relative dielectric constant, c, of the monolayer was determined from
C = Eedd where C is the capacitance per unit area, €0 is the permitivity of the vacuum (8.85 X 10-14F/cm), and d is the film thickness. In this work, the reported relative dielectric constant of alkanethiol, t = 2.6: was used in eq 1 to determine film thickness d from C values obtained from CV. The results are summarized in Table I. The capacitance of thioctic acid SME determined from CV is much larger than that reported for monolayers of hydroxy thiol of similar alkyl chain length, HO(CHz),SH where n = 6.5 Consequently, the film thickness determined from eq 1is much smaller than the value of 6.5 A calculated by assuming 1.3 A 9 for each of the four methylene and a methine group in thioctic acid. In addition, the film capacitance shows a dependence on solution composition. The results indicate that the assumption that the electrode (7) Elson, C. M.; Itzkovitch, I. J.; McKenney, J.; Page, J. A. Can.J. Chem. 1975,53, 2922. (8)Oesch, U.; Janata, J. Electrochim. Acta 1983,28, 1237. (9) Porter, M. D.; Bright, T. B.; Allma, D. A.; Chidsey, C. E. D. J.Am. Chem. SOC.1987, 109,3559.
0 1992 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 64, NO. 17, SEPTEMBER 1. 1992 * 1998
Au substrate Flpure 1. Diagram of the thloctlc acld monolayer on the Au substrate.
Table I. Results of Capacitance Measurements scan rate C d' electrode electrolyte (V/s) (fiF/cm2) (A) thioctic acidb 0.1 M HC108 5.12 10.3 2.2 0.1 M KCI +
HO(CHdcSHI
10 mM Trisd 0.1 M KCI + 10 mM borate* 0.1 M KCI 0.1 M NaF 0.1 M KC1 + 10 mM Tris
10.24 5.12 10.24 5.12 10.24 5.12 5.12 5.12 ~~
~~
9.W ~~
12.7 11.5 13.5 12.3 8.5 8.5 3.7
2.4 1.8 2.0 1.7 1.9 2.7 2.7 6.2 ~~
The monolayer thickness was calculated from the equation C = ttddwithr = 2.6. ThiocticacidmdifiedAuelectrde. Theelectrode areawas0.29cm2.cpH1.5.dpH7.4.'pH9.1.~Fromref5.
capacitance is determined by the alkanethiol region of thiol must not be valid. Cyclic Voltammetry of Fe(CN# and R U ( N H & ~ +at the B a r e Au Electrode and the Thioctic Acid SME. CV of Fe(CN)$ and R u ( N H ~ ) was ~ ~ +run to test the effect of solution pH on the response of the probes. Since for thioctic acid pK. = 5,'O the carboxylic acid terminus of the monolayer should be fully protonated a t low pH and have a negative charge in neutral and basic solutions. The cyclic voltammograms of 1mM probes were run a t the bare Au electrode and the SME in acidic (pH 1.5), neutral (pH 7.4). and basic (pH 9.1) solution. The potential window used for Fe(CN)& was +0.6 to -0.1 V, while R u ( N H ~ ) was ~ ~ +run from +0.3 to -0.5 V. The scan rate was 0.1 Vis for both probes. Fe(CN)$ and Ru(NH&3+ were chosen as probes because they have opposite charge and reasonably fast kinetics on gold. Figure 2 displays the results obtained at the SME in solutions of different pH. The response of Fe(CN)& is only observed a t low pH. In neutral solutions, where the carboxylic acid head groups can be expected to be partially dissociated, a small response is observed a t -0.2 V, which decreases in basic solutions where the dissociation of the acid head groups is complete. While the response to Ru(NH&3+ is essentially the same in neutral and basic solutions, i t is significantly smaller a t low pH. High proton concentration a t the surface, indicated by the large CV background currents a t negative potentials in the absence of Ru(NH3h3+in solution, may accountforthis behavior. TheCVresultsshowthat inneutral and basic solutions the response of the anion is effectively suppressed. The response of the cation is significantly lower at low pH while the response of the anion remains unchanged under these conditions. Following the CV experiments, the SME was transferred to a probe-free buffer solution of the same pH. The only detectable response was obtained for R u ( N H ~ )at ~ ~pH + 9.1. At this pH the negative charge of the SME due to the carboxylic acid terminus should be highest. The background-corrected cathodic peak currents and the AE,values for the probes were measured from the cyclic voltammograms a t the SME and the bare Au electrode. Eo'was
measuredas theaverageofE,andE,. Theresultsareshown in Table 11. For Fe(CN)& . " a t low nH and for R u (. N H M + in neutral and basicsolutions, AEp and ,i values are essentially the same a t the SME and the bare Au electrode. Uncertainty in background correction and the roughness of the bare electrode contribute to the differences in ,i a t the SME and the bare Au electrode. The E O ' values for Fe(CN)&P are the same a t the SME and bare Au electrode. However, EO' of R u ( N H ~ ) ~ ~is+ less / ~ +negative a t the SME. Stability of t h e Thioctic Acid SME. The potential window in which the SME is stable depends on the solution pH. For example a t pH 1.5 the anodic background current begins to increase at approximately 0.7 V and the cathodic, at -0.2 V. The anodic limit is similar at higher pH. The cathodiclimit isapproximately0.2Vmorenegativeinneutral solutions. In the potential window of +0.6 to -0.2 V the background currents do not exceed 3 pAlcm2. Electrode response and the background current do not change as long as the electrode is used in this potential window. The SME is damaged when the potential exceeds the limits, allowing large oxidation or reduction currents at positive and negative potentials, respectively. The damage is accompanied by the appearance ofthe characteristicreduction peak ofgold oxide.8
DISCUSSION The considerable amount of work with self-assembled monolayers has been primarily aimed a t producing ordered, tightlypacked films which decreaseelectroactivity ofsolution probes. Measurement of capacitance has been used as a method to verify the quality of packing.s,6J'J Typically, when the films are tight, capacitance is independent of solution composition. The selectivity of thioctic acid SME supports a conclusion that the thioctic acid monolayer is reasonably well ordered and, therefore, that thioctic acid can be selfassembled a t the smooth gold surface. However, it can be seen from the results summarized in Table I that the capacitance of thioctic acid electrode depends on the electrolyte and is higher than predicted from calculations which consider alkanethiol as theonlydielectric in an ideal capacitor. We, as well as others: attribute this to the ability of the electrolyte to penetrate the layer. The similar negative potential window a t the SME and the bare gold electrode points to the permeability of the layer. As our results show, permeability does not affect the stability of the SME. The response of the SME is reproducible in the potential window where the monolayer is stable. The charge of the monolayer determines the response of the SME to the solution analytes. The carboxylic acid group in direct contact with solution plays an important role in determining the monolayer charge. As a result, the response of the Fe(CN)63- anion is effectively eliminated when the monolayer charge is negative at high pH where the carboxylic acid head group is dissociated. A significant decrease in the response to the Ru(NH3)2+cation occurs when [H+l in solutionis high, whichresultsinapositivechargeofthemonolayer due to a high concentration of H+ in the film. Comparison of the AE, and i, values in Table I1 obtained from CV at the SME and the bare gold electrode reveals some interesting points. Since Fe(CN)& and Ru(NH&3+ display essentially the same AEp a t the SME and the bare gold electrode, the presence of the film must not noticeably affect the kineticsof the probes. More importantly, the small AEp values show that the kinetics are fast at the SME. The kinetics of R u ( N H ~ ) a~t~this + SME are different from the slow kinetics reported by Chidsey and Loiacono a t H02(10) Wagner,A. F.;Folkers, K.;VitarninsondCoenryrnes;Intemcienee:
New York, 1964; p 247.
2000
ANALYTICAL CHEMISTRY, VOL. 64, NO. 17, SEPTEMBER 1, 1992 100:
a
b 50
0-
-
Q
Y
h - 50-
- 0.1
-100-
M HClOi
_ _ _ _ - -0.1 M KCI a n d 10 mM tris
. 800
..... 0.1 M KCI a n d 10 mM b o r a t e
600
200
400
- 0.1 M HClO, _ _ _ _ _ 0.1 M KCI a n d 10(pHm M1.5)t r i s
(PH 1.5)
0
( H 74)
QpH 9.1)
-200
-400
E/mV
..... . 0.1 M KCI a n d 10 m M borate
-15
0 -h -, , 400
-
,
( H 74) &H 9.1)
i m
I I II
200
0
-200
-400
-600
-800
E/mV
Flgure 2. Cycllc voltammograms of 1 mM Fe(CN)6s (a) and 1 mM Ru(NH3)e3+(b) on the thioctic acid modified Au electrode. The scan rate was 100 mV/s. The electrode area was 0.29 cm2. All potentials are reported vs Ag/AgCi/KCI.
Table 11. Cyclic Voltammetric Results for Fe(CN)6" and Ru("&.~+ on the Bare Au and Thioctic Acid Modified Electrodes
Drobea Fe(CN)e3-
electrode bare Aud thiocticacide Ru(NH3)s3+ bare Aud thiocticacide
DH 1.5 1.5 1.5 7.4 9.1 1.5
7.4 9.1
AE,
(mV) 65 63 66 70 78 f
74 71
E O t b
(V) 0.45 0.46 -0.07 -0.11 -0.12 f
-0.06
-0.08
ip,cc
(uA/cm*) 268 287 211
246 214
f
230 224
"Probe concentration was 1 mM. bFormal potential Eo' was calculated from Eo'= (Ep,c+ Ep,a)/2and was measured vs Ag/AgCl/ KC1. Cathodic peak current normalized by the electrode area. Scan rate was 100 mV/s. Electrode area was 0.28 cm2.e Electrode area waa 0.29cm2.f Poorly defied peaks could not be accuratelymeasured.
producing a high surface concentration of the probe and a resulting surface wave distorting the potential measurement. As shown from the CV results, in agreement with the proposed structure of the SME, attractive coulombic interactions can be confirmed when the probe electrostatically attached to the SME shows a response in probe-free buffer solutions.
CONCLUSIONS The results illustrate the desirable characteristics of the SME obtained by self-assembly of thioctic acid on the welldefined gold electrode surface. The results indicate that selectivity of the thioctic acid film can be controlled. Permeability contributes to the sensitivity of the film while selectivity is maintained by the charge of the monolayer a t the SME. The negatively charged terminal group plays a major role in screening the response of the anionic probe a t the SME while favoring the response of cationic probes. Permeability of the monolayer to H+ ions screens the response of cationic probes.
C(CH&oSH under similar solution conditions.6 The same and fast probe kinetics a t the bare Au electrode and the SME ACKNOWLEDGMENT indicate reasonable permeability of the fih1.~1 The essentially We want to thank Denise Merkle for the vacuum-deposited same i, at the SME and the bare gold electrode indicates that Au on silicon wafers and Mike Freund for helpful discusthe monolayer does not impede diffusion sufficiently to sions. This work was supported in part by the US.Army produce a different CV response a t the SME. Research Office through Grants No. DAAA 15-85-C-OOO34 The formal potential, E"', of Fe(CN)63-/4-at the SME and and No. DAA103-86-0-0001 administered by Battelle. the bare gold electrode is the same. For the R u ( N H ~ ) ~ ~ + / ~ + couple the formal potential in neutral and basic solutions shows a small shift to more positive values at the SME. This RECEIVED for review November 15, 1991. Accepted May points to attractive interactions of Ru(NH3)e3+with the SME, 26, 1992. Registry No. R U ( O H ~ ) 18943-33-4; ~~+, Fe(CN)&, 13408-62(11)Gough, D. A.; Leypoldt, J. K.Anal. Chem. 1979, 51,439. 3; Au, 7440-57-5; thioctic acid, 62-46-4.
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.
