1970
Anal, Chem. 1983, 55, 1970-1973
LITERATURE CITED (1) Andersen, C. A.; Hinthorne, J. R. Science 1972, 175, 853-860. (2) Newbury, D. E. Scanning 1980, 3 , 110-118. (3) Krauss, A. R.: Krohn, V. E. I n "Mass Spectrometry, Vol. 6"; The Royal Society of Chemistry: London, 1981; pp 118-152. (4) Turner, N. H.; Colton, R. J. Anal. Chem. 1982, 5 4 , 293R-322R. (5) Ganjei, J. D.; Morrison, G. H. Anal. Chem. 1978, 5 0 , 2034-2039. (6) Smith, D. H.; Christie, W. H. Int. J . Mass Spectrom. Ion Phys. 1978, 26,61-76. (7) Ganjei, J. D.: Leta, D. P.; Morrison, G. H. Anal. Chem. 1978, 50, 285-290. (8) Havette, A.: Slodzian, G. J . Phys. Left. (Orsay, F r . ) 1980, 41, L247L250. (9) Burns-Bellhorn, M. S.;File, B. M. Anal. Biochem. 1979, 92,213-223. (10) Zhu, D.; Harris, W. C., Jr.; Morrison, G. H. Anal. Chem. 1982, 5 4 , 4 19-422. (11) Harris, W.C., Jr.; Chandra, S.; Morrison, G. H. Anal. Chem. 1983, 5 5 , 1959-1963.
(12) Nadkarni, R. A. Radlochem. Radioanal. Left. 1977, 3 0 , 329-340. (13) Nadkarni, R. A,; Morrison, G. H. J . Radioanal. Chem. 1978, 4 3 , 347-369. ... (14) Ross, G.D.; Morrison, G.H.; Sacher, R. F.; Staples, R. C. J . Microsc. 1883. 129. 221-228. (15) Andersen, C. A.; Hinthorne, J. R. Anal. Chem. 1973, 4 5 , 1421-1438. (16) Tamura, H.; Ishitanl, T.; Izumi, E. Shltsuryo Bunseki 1981, 29, 81-87. (17) Carmer, S. G.;Swanson, M. R. J . A m . Stat. Assoc. 1973, 68. 66-74. (18) Burns, M. S. Anal. Chem. 1981, 5 3 , 2149-2152. (19) Newbury, D. E. I n "Secondary Ion Mass Spectrometry 11"; Benninghoven, A., Evans, C. A., Jr., Powell, R. A,, Shimizu, R., Storms, H. A,, Eds.; Springer-Verlag: New York, 1979; pp 51-57.
RECEIVED for review April 8, 1983. Accepted July 10, 1983. This work was funded by the National Institutes of Health under Grant No. R01 GM-24314.
Voltammetric Determination of Water in an Aluminum Chloride-N-n -Butylpyridinium Chloride I onie Liquid Saeed Sahami and Robert A. Osteryoung* Department of Chemistry, S t a t e University of New York a t Buffalo, Buffalo, New York 14214
The electrochemical behavior of water has been investigated in an ambient-temperature molten salt, aluminum chloride-Nn -butylpyridinium Chloride. I t was found that throughout the entire range of melt composition, water undergoes a chemical reactlon to generate HCI, which can be eiectrochemlcaliy reduced at a platinum electrode. By use of rotating platinum disk experiments, a caiibratlon curve was obtained In the basic melt. This calibration curve was found to be linear up to a water concentration of 50 mM.
The N-n-butylpyridinium chloride (BuPyC1)-aluminum chloride mixtures are molten at ambient temperatures (-30 O C ) over a wide compositional range varying from 0.75:l to 2:l (mole ratio of AlC1,:BuPyCl) (1,2). The Lewis acid-base properties of these ionic liquids change as the mole ratio of AlCl, to BuPyCl changes. In acidic melts there is excess A1C13 while for basic melts BuPyCl is in excess. It has been shown potentiometrically that equilibrium 1provides an adequate description of the system over the entire composition range (2). 2AlC1,- e A1,Clf 4- C1-
(1)
Due to the low melting point, aprotic, and relatively anhydrous nature of the AlC1,BuPyCl melts, they have been employed as solvents for electrochemical and spectroscopic investigations of both organic and inorganic species (1-13). Although various authors have assumed that AlC1,:BuPyCl melts are totally anhydrous (1, 6, 12, 13) and even explained the remarkable stability of some radical cations in these melts due to the absence of water ( l ) ,no unambiguous evidence exists t o support this assumption. The present work was intended to obtain information on the electrochemistry and analytical determination of water in these melts. No direct investigations on the electrochemistry of water in A1C13-BuPyC1 are reported in the literature. In a recent electrochemical study of oxide and water addition in the basic
AlCl,-BuPyCl melts containing titanium chloride, it was suggested that the addition of water resulted in removal of oxide from the melt (9). Tremillon et al. (14) have studied the electrochemistry of water and HCl at a platinum electrode in a basic melt of AlC1,-NaCl at 210 "C. They observed two waves on the voltammogram which were assumed to be due to the reduction of dissolved HCl and dissolved water.
EXPERIMENTAL SECTION Anhydrous aluminum chloride (Fluka A.G.) was purified by sublimation at 220 "C from a mixture containing a small amount of sodium chloride and aluminum wires in a Pyrex tube sealed under vacuum. N-n-Butylpyridinium chloride was prepared by refluxing n-butyl chloride and pyridine (Fisher ACS). These procedures as well as the preparation of melts have been described elsewhere (1). Water was repurified by the use of a Milli Q purification system (Millipore Corp.). Before water is taken into the drybox, it was deaerated by bubbling with prepurified argon. Water was introduced into the stirred melt with a gastight microsyringe. Upon addition of water to the melt a local white precipitate formed which redissolved on stirring. All measurements were made after complete dissolution of the precipitate. Hydrogen chloride gas was obtained from Linde. The aluminum wire (Alfa Products) was cleaned in a 30:30:40 volume mixture of H2SO4:HNO3:H3PO4, rinsed with water, and dried. Chemicals were stored and all experiments performed under argon atmosphere in a Vacuum Atmospheres Co. drybox. A Metrohm glass cell was employed. It was covered with a Teflon lid which had several holes for reference and counterelectrode compartments, working electrode, and thermometer. The entire cell assembly was placed in a simple furnace and the temperature was controlled at 40 f 1 OC by a Thermo Electric Selector 800 temperature controller. Reference and counterelectrode compartments were aluminum wires dipped into the 2:1 AlC1,:BuPyCl melt and both were separated from the working compartment by fine glass frits. Glassy carbon (GC), tungsten (W), and platinum (Pt) disks were used as working electrodes. The GC (0.071 cm2) and W (0.077 cm2) disk electrodes were polished with 0.3-pm alumina (Buehler) by using a polishing cloth and Milli Q water as lubricant. The Pt disk electrode (0.049 cm2), which was obtained from Pine Instrument Co., was polished with successively finer grades of 1.0,0.3, and 0.05 pm alumina, treated
O003-2700/83/0355-1970$01.50/00 1983 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 55,NO. 12,OCTOBER 1983
1971
75
50
1 +A
25
I
I
I
I
05
0
-0 5
-1 0
E, V vs. AI/AI(III)
Figure 1. Cyclic voltammograms in a basic melt at different electrodes: scan rate = 50 mV/s; T = 40 OC; solid lines, 0.8:l melt; broken lines, 0.8:l 10.5 mM H,O; GC (a), W (b), and Pt (c) electrodes.
+
with hot 1:l HNO, and rinsed with Milli Q water. It was then electrochemicallycycled in 0.1 M HCIOl between -0.3 V and +1.% V (vs. SCE) by using a triangular wave with a scan rate of 5 V s-l for about 10 min. Thle electrode was then held at 0.2 V for -30 s to remove any residual oxide (15). All working electrodes were rinsed with water and air-dried prior to transfer to the drybox. All voltammograms were obtained by use of a EG&G PARC 175 Universal Programmer with a PARC 173 potentiostat and a Houston Omnigraphic 2000 recorder. A Pine Instrument Co. electrode rotator (Modell ASR 2) was used for rotating dislk electrode (RDE) studies.
RESULTS AND DISCUSSION Behavior of Water in Basic Melts. The electrochemicd behavior of water in basic melts (from 0.75:l to 0.99:l) has been investigated by cyclic and rotating disk voltammetry. Figure 1 illustrates cyclic voltammograms; the solid lines represent typical background in a basic melt at a glassy carbon, tungsten, and platinum disk electrodes. It is seen that at both glassy carbon and tungsten electrodes the voltammograms are featureless, whereas at platinum there are cathodic and anodic peaks. In all cases the cathodic and anodic limits are due to the BuPy+ reduction and chlorine evolution, respectively ( 1 ) . The broken lines in Figure 1show voltammograms after addition of 10.5 mM water to the melt. Cyclic voltammograms at glassy carbon and tungsten are essentially identical with those observed in the absence of added water. (In some instances, a wave, presumably due to water, can be detected near the cathodic limit at a W electrode.) At a platinum electrode in the presence of added water there is an increase in cathodic and anodic peak current a t EpC= -0.525 V and EpB = -0.370 V, respectively. The peak potentials are scan rate dependent; the cathodic peak shifts negative, while the anodic peak shifts positive with increasing scan rate. The large peak potential separation and the scan rate dependency of peak potentials areindicative of a slow charge transfer reaction (irreversible behavior). The ipa/ipcratio does not increase with increasing scan rate, i.e., apparently there is no following chemical reaction. The oxidation peak seems to be very sensitive to the electrode surface. If the platinum electrode is kept in the melt for a long time or scanned folr -20 times, the anodic peak with E,* = 4.370 will disappear. Upon polishing and pretreating the platinum electrode, one could again observe the oxidation peak. Increasing water concentration up to 70 mM did not have any effect on cyclic voltammograms obtained at the glassy carbon and tungsten electrodes, while cathodic and anodic
E, V vs AI/AI (111) Figure 2. RDE voltammograms for reduction of water in a 0.8:lmelt:
scan rate, 5 mV/s; rotation rate = 600 rpm; mM water added, 0 (l), 5.55(2),11.1 (3),16.65 (4), 22.2 (5),27.75(6),33.3 (7),38.85(8).
peak currents increased a t the platinum electrode. Cyclic voltammograms (especially peak currents) at the platinum electrode in the melt containing water were not terribly reproducible. For more accurate and quantitative measurements steady-state rotating disk voltammetry a t platinum electrodes was employed. Voltammograms of a 0.8:l melt and subsequent additions of water are shown in Figure 2. A single reduction wave with Eljz= -0.425 f 0.020 V (for 0.8:1 melt 11 mM water) is observed. From a series of experiments one can conclude: (1)increasing water concentration to 50 mM in a melt of constant composition changes the Ellzvalue -0.06 V more negative; (2) as the electrode rotation rate was increased from 400 to 3600 rpm in a given melt with fixed water concentration Ell? shifted -0.07 V negative; (3) changing the melt composition from 0.75:l to 0.99:l resulted in an Ellz shift -0.07 V more positive. To confirm that the reduction wave for water was diffusion controlled, limiting currents at E = -0.9 were plotted vs. the square root of the electrode rotation rate and were linear, passing through the origin. Based on the Levich equation (16)
+
Il = 0.62nFAD213~1/2v-1/6Co
(2)
for a convective diffusion controlled process, plots of I, vs. w1lz should be linear and pass through the origin. I ]is the limiting current in amperes, n the number of electrons transferred, F the Faraday constant, A the area of electrode in cm2,D the diffusion coefficient, w the rotation rate in radians/s, v the kinematic viscosity, and C, the concentration of the electroactive species in mol/cm3. To determine the amount of residual water present in the freshly prepared melts calibration curves were prepared. Plots of concentration of water added to the basic melt vs. the limiting current of the reduction wave (measured a t the plateau) resulted in a linear calibration curve up to -50 mM added water. However, as the concentration of added water increased above 50 mM, the slope of the calibration curve decreased (i.e., the observed limiting current was less than expected). Apparently above this concentration the solubility of HC1 diminishes (see below). Extrapolating the straight line in such plots to zero current gives residual water present in the melt (see Table I). Although the intercept of the calibration curve in each melt varies with the melt composition (Le., presence of different concentration of residual water), the slope of the calibration curve remains more or less the same (see Table I). Experiments performed with various batches of BuPyCl (synthesized a t different times) gave calibration curves with different intercepts, even for melts of
ANALYTICAL CHEMISTRY, VOL. 55, NO. 12, OCTOBER 1983
1972
Table I. Summary of Calibration Curve Data for Water in Basic A1,Cl:BuPyCl Melts 0.8:l mM H,O
fiA 21 35 45 60 76 81 89.5
0
6.56 13.12 26.24 32.80 39.36 54.28
mM H,O
0.9:l PA 15 29.5 37.5 42.5 50 58.5 68 73.5 88
0
5.55 11.10
16.65 22.20 27.75 33.30 38.85 49.95
mM H,O
0
6.56 13.12 19.70 26.25 32.75 39.40 46
I , ,' PA 8
mM H,O
31 48.5 55 62 67 83 90.5
14 21 28 35 42 49
I , , a PA
0
7
39.5 56 69 71.5 85 95
1.73 slope, 1.30 1.41 1.63 P AImM average slope, fiA/mM = 1.52 + 0.18 7.5 residual 20 14 11 water nDZ/3 C 5.84 x 10-5 4.65 x 10-5 5.05 x 10-5 5.84 x 10-5 a Limiting currents in PA, measured at 600 rpm. Residual water in &I, obtained by extrapolating the calibration curve for each melt to zero current. obtained, using slope and the Levich equation (1);see ref 1 0 for values of v. I
75 -
I
I
1
I
I
I 50
I
I
I
I
'
t
I
50 -
I,pA 25 -
;0 2
I
1
0
-0 2
-0 4
I
I
1
-0 6
-0 8
-4 0
E, V vs AI /AI (Ill)
Figure 3. RDE voltammogram for reduction of HCI in a 0.8:l melt: scan rate, 5 mV/s; rotation rate, 600 rpm; 0.8:l (a),0.8:l HCI (b).
+
similar composition. Therefore, the possible source of water in BuPyC1:A1Cl3 melts seems to be BuPyCl which is highly hygroscopic. Table I also includes an estimate of nD2i3obtained from calibration plots and the Levich equation. If n is taken as 1,a value of D of -4 X lo-' cm2 s-l is obtained, which is surprisingly small. It has been reported that water can react with A1Cl3:NaCl melts to generate HCl(14). We attempted to investigate the nature of the species that are being reduced in these melts. T o answer this question several other experiments were performed. In the first experiment, addition of hydrochloric acid (12 M in water) to the basic melt gave a single reduction wave with the E,,, value similar to that observed for water in this composition. In another experiment pure HC1 gas was bubbled into the 0.8:lmelt for a few minutes. A RDE voltammogram at platinum was recorded and is shown in Figure 3. It is seen that its behavior is similar to that of water with a similar E,,, value. Upon bubbling HC1 gas or adding water to the melt, the limiting current increases. These results clearly indicate that water and HC1 both have similar behavior and give a single reduction wave on platinum. Apparently, water can react with aluminum-containing species in the melt to generate HCl which seems to be soluble. Indeed, litmus paper showed that HC1 was formed upon addition of water. Therefore, one can conclude that the reduction wave observed in Figure 3 is due to the following reaction: 2HC1
+ 2e-
-
Hz+ 2C1-
(3)
Indeed, the absence of this reduction wave on glassy carbon and tungsten electrodes which are known to have high hy-
I
I
I
0
-0 2
-0 4
I
I
-0 6
-0 8
E, V v s Al/Al(lll)
Flgure 4. RDE voltammograms In a basic melt: scan rate, 5 mV/s; rotatlon rate, 600 rpm; 0.8:l 15 mM H,O (l), 0.8:l 15 mM H,O + H, (2).
+
+
drogen overpotentials is not surprising. To confirm that the reduction of HCl (hydrogen ion) is to hydrogen, H, gas was bubbled into a melt for a few minutes. A RDE voltammogram at platinum showed an anodic current at the bottom of the wave, where the current was zero in the absence of Hz(see Figure 4). This is indicative of H, oxidation. The oxidation current did not increase when more Hz was bubbled into the melt and the current decreased after bubbling was stopped. This is apparently due to the very limited solubility of the Hz gas in these melts and also the sensitivity of platinum surface. This limited solubility of H, ratio observed at platinum in Figure explains the small &"lipc 1. Turning now to the chemical reaction of water in the melt, as was mentioned previously in the basic melt water reacts with A1C14- to generate HC1 and possibly an aluminohydroxy or aluminoxy species. Tremillon et al. (14) have reported the following reaction in the basic high-temperature AlC1,:NaCl melts AlC14- HzO + 2HC1 AlOC12(4) This reaction (i.e., the formation of Al0Cl2-)is in disagreement with the observation of Linga et al. (9),in their electrochemical study of oxide and water addition to a basic AICl,:BuPyCl melt containing Ti(1V). They observed that addition of water to the melt seemed to decrease AIOC1,- concentration rather than increasing it. In fact, recent infrared investigations of addition of water to AIC13-l-methyl-3-ethylimidazoliumchloride melts indicate the formation of an aluminohydroxy species such as
+
+
ANALYTICAL CHEMISTRY, VOL. 55, NO. 12, OCTOBER 1983
E, V vs AI/AI IHI)
Cyclic voltammograms at different melt composltions: sweep rate, 50 mVls; T = 40 O C ; 1.2:l (a), 1.2:l + 7 mM H20 (b), 1.4:l + 7 mM H,O (c), 1.6:1 + 7 mM H,O (d), 2:l -k 7 mM H 2 0 (e). Figure 5.
AlOHCl, rather than A10C12- (17). Unfortunately we are not in a position to clarify this problem. Attempts to do constant potential coulometry to obtain information on the stoichiometry of dissociation of water into HC1 failed. Coulometry a t E = -0.55 V (which corresponds to potential on the plateau of the reduction wave in Figure 2) at a platinum sheet of large area failed. The experiment seemed to be very complicated. There was no trend between passage of coulombs and decrease in the limiting currents when electrolysis was stopped a t various times and RDE: run a t platinum disk. Behavior of Water in Acid Melts. In acid melts (where the AlC1,:BuPyCl mole ratio is > l ) , the addition of water, especially in large quantities, leads to a strong exothermic reaction giving off HC1 and forming a white precipitate. This precipitate eventually dissolves in the melt to generate more gas. As shown in Figure 5, cyclic voltammograms of acid melts at platinum give cathodic and anodic peaks similar to thoce observed in the basic melts but a t more positive potentials. In general, the peak currents in acid melts are smaller compared to those in basic melts. Even the peak currents observed in acid melts containing water are less than that seen for the same concentration of water added to basic melts. As in basic melts, the electrochemical behavior of water and HC1 is identical. This implies that water apparently decomposes to HC1 and reduces electrochemically to H,.
1973
The electrochemical behavior of water in acid melts is more complicated than in basic melts. There are several problems associated with water reduction in acid melts including the following: (1)Cyclic voltammograms are not very reproducible. (2) RDE experiments give ill-defined waves which are not reproducible. When negative going voltammograms were switched to scan positive (especially after reaching the limiting current plateau), the current decreased drastically, reaching almost zero. Apparently the platinum electrode is poisoned. (3) When the acidity of a melt containing water increased, the peak currents decreased. (4)In an acid melt of fixed composition and constant water concentration, the peak currents seemed to diminish with time. For the purpose of graphical illustration and to give some idea of water or HCl reduction potentials, cyclic voltammograms of water were recorded in acid melt compositions of 1.2:l to 2:l and are shown in Figure 5. These voltammograms were obtained in each melt composition after addition of 7 mM water. At the tungsten electrode, upon addition of water to a 2:l melt, an ill-defined reduction peak with EPC= + 0.2 V was seen. The peak current seemed to decrease with time. When a glassy carbon electrode was used, no reduction or oxidation peaks were obtained for water or HCl gas in acid melts. Registry No. Water, 7732-18-5;hydrogen chloride, 7647-01-0; aluminum chloride, 7446-70-0; N-n-butylpyridinium chloride, 1124-64-7.
LITERATURE CITED (1) Robinson, J.; Osteryoung, R. A. J. Am. Chem. Soc. 1979, 101, 323. (2) Gale, R. J.; Osteryoung, R. A. Znorg. Chem. 1979, 18, 1603. (3) Gale, R. J.; Gilbert, B.; Osteryoung, R. A. Znorg. Chem. 1979, 18, 2723 (4) iussey, C. L.; King, L. A.; Wllkes, J. S.J. Electroanal. Chem. 1979, 102,321. (5) k h s o n , J.; Osteryoung, R. A. J. Electrochem. Soc. 1980, 127, ILL.
(6) Robinson, J.; Osteryoung, R. A. J. Am. Chem. Soc. 1980, 102, 4415. (7) Gale, R. J.; Osteryoung, R. A. J. Nectmchem. Soc. 1980, 127,2167. (8) Hussey, C. L.; Laher, T. M. Znorg. Chem. 1980, 20, 4201. (9) Linga, H.; Stojek, 2 . ; Osteryoung, R. A. J. Am. Chem. Soc. 1981, 103, 3754. (10) Nanjundlah, C.; Shlrnizu, K.; Osteryoung, R. A. J. Electrochem. Soc. 1982, 129,2474. (11) Cheek, G. T.; Osteryoung, R. A. J. Electrochem. Soc. 1982, 129, 2488. (12) Laher, T. M.; Hussey, C. L. Inorg. Chem. 1982, 27,4079. (13) Luer, G. D.; Bartak, D. E. J. Org. Chem. 1982, 47, 1238. (14) Trernillon, 8.; Berrnond, A.; Molind, R. J. Electroanal. Chem. 1978, 74,53. (15) Barr, S. W.; Guyer, K. L.; Weaver, M. J. J. Nectroanal. Chem. 1980, 111, 41. (16) Bard, A. J.; Faulkner, L. R. "Electrochemical Methods"; Wlley: New York, 1980; p 288. (17) Talt, S.;Osteryoung, R. A., State University of New York at Buffalo. unpublished work.
RECEIVED for review June 6, 1983. Accepted July 21, 1983. This work was supported in part by the Air Force Office of Scientific Research.
