Alternating Current and Direct Current Voltammetry with a Mercury Pool Electrode in Concentrated Hydrofluoric Acid A. M. Bond, T. A. O’Donnell and R. J. Taylor Department of Inorganic Chemistry, UniGersity of Melbourne, Parkcille 3052, Victoria, Australia Polarography in media, such as hydrofluoric acid, in which glass is readily etched, is severely restricted because the dropping mercury electrode must be constructed from material other than that normally used, vir. glass. Voltammetry in concentrated hydrofluoric acid or anhydrous hydrogen fluoride at a mercury pool electrode in a cell based on a commercially available moulded Kel-F tube is simple, and provides a convenient alternative electroanalytical procedure to conventional polarography in media in which glass i s etched. The reduction of thallium(1) has been investigated at a mercury pool electrode in about 50% HF by both ac and dc voltammetry. In this analytically important solvent, the limit of detection of thallium(1) was 5 X 10-6M and 1 x lO-%l by the ac and dc methods, respectively, with a reproducibility of &2%. Ac and dc voltammetry in anhydrous hydrogen fluoride are also shown to be practicable.
AQUEOUS HYDROFLUORIC ACID is used frequently for solution preparation prior to analysis because of its specific reactivity with silicate or because of the ability of fluoride to form very strong complexes with many cations. It has commonly been used to “open up” samples prior to the determination of metals in glasses and in mineralogical and metallurgical samples. It is effective, alone or in combination with other acids, in dissolving refractory samples such as the metals, alloys and oxides of zirconium, hafnium, titanium, tantalum, niobium, etc. In principle, once the sample is dissolved in concentrated aqueous hydrofluoric acid, any appropriate chemical or instrumental method of analysis can be used. However, fluoride ion interferes with many standard methods of analysis, so that frequently a second stage of sample treatment is necessary to’remove the interfering fluoride ion. This second stage is usually tedious and time consuming and is likely to reduce accuracy. There are obvious advantages in carrying out determinations directly on the solutions in hydrofluoric acid. Concentrated hydrofluoric acid, e.g. 50 is usually used for preparing solutions of samples for analysis. Aqueous H F is a weak acid whose dissociation is sufficient to permit its use as a supporting electrolyte for voltammetric determination of species dissolved in it. Thus for direct application of voltammetric methods, aqueous H F has the dual advantage of acting as solvent and supporting electrolyte. Under these conditions the presence of fluoride in this system does not interfere with the determination, because calibration curves are obtained from standard solutions, also prepared in hydrofluoric acid. Since H F is a weak acid, its absolute concentration need not be kept critically constant as the concentrations of hydrogen and fluoride ions are effectively buffered and polarographic methods for determination of electroactive species in aqueous HF have been shown to be relatively insensitive t o quite wide variations in H F concentration ( I ) . This is a n important factor in considering hydrofluoric acid as a solvent, as it would be difficult to maintain an absolutely
z,
--.__
41, 1801 (1) A. M. Bond and T. A. O’Donnell, ANAL.CHEM., (1969). 464
ANALYTICAL CHEMISTRY, VOL. 44, NO. 3, MARCH 1972
constant and known concentration of hydrogen fluoride because of its great volatility and reactivity. Polarography, the most common electroanalytical method of analysis, is severely limited in aqueous HF, because conventionally the technique employs a dropping mercury electrode (DME) with a glass capillary, which is etched by hydrofluoric acid. This leads to poor reproducibility of dropping characteristics and loss of accuracy. Replacement of the glass DME by one constructed from Teflon (Du Pont) (2-5) overcomes this problem. However, such electrodes are not particularly easy t o construct (2-5), nor are they commercially available ( 6 ) , and their use has therefore been limited. Alternatively, rapid polarographic methods have been used deliberately to prevent etching of a glass D M E ( I ) ; but this particular specialized technique is only reliably applicable t o solutions containing up to about 25 of hydrogen fluoride. Electroanalytical methods, which avoid the use of dropping mercury electrodes, would have considerable value in the field of electroanalysis in concentrated aqueous HF, providing they are comparable in sensitivity and reproducibility t o the D M E method. Solid HF-resistant electrodes provide possible alternatives t o the use of a glass DME. However, a simpler approach is t o retain the mercury, which has many advantageous properties as a n electrode material, as a pool electrode rather than a DME. For mercury pool voltammetry, applicable t o any HF concentration, construction of glass-free apparatus and the experimental procedures are simple. Initially in the present investigation, approximately 50 H F was chosen as the solvent and supporting electrolyte, and the TI(1) e S Tl(ama1gam) electrode reaction was studied to assess the applicability of the mercury pool electrode with both ac and dc voltammetric techniques. The feasibility of mercury pool electrode voltammetry in anhydrous hydrogen fluoride was also investigated.
+
EXPERIMENTAL Reagents. Baker Analyzed (49.0%) and BDH Analar Grade hydrofluoric acid were used. Thallium(1) was added as the carbonate or nitrate salt. Apparatus. All volumetric equipment (pipets and burets) were made of polyethylene, as were the containers in which HF solutions were prepared and maintained. All mercury was purified by distillation, in the normal manner, prior to use. Voltammograms were recorded with a Metrohm Polarecord E 261. Ac voltammetry was achieved with the Metrohm A C Modulator E 393 and used with a n applied ac voltage of 10 mV rms at a frequency of 50 Hz.
(2) H. P. Raaen, R. J. Fox, and V. E. Walker, U.S. At. Eiiergy Coinn7. Rep., ORNL-3344,Nov. 30,1962. (3) H. P. Raaen and R. L. Clark, ibid., ORNL-3654,Aug. 1 , 1964. (4) H. P. Raaen, ANAL.CHEM., 34,1714 (1962). (5) A. M . Bond and T. A. ODonnell, ANAL.CHEM., in press. (6) H. P. Raaen, “Analysis Instrumentation 1965,” Plenum Press, New York, N.Y., 1966, pp 219-228.
f4,'-
Sort bridge to reference electrode
Auxilicry
Kei-F td>e
Tef Ion sinter
R Kel-F
D
Alumilium
Figure 1. Mercury pool electrode and cell constructed from Kel-F
-045
-065
Figure 3. Variation of dc peak height with square root of scan rate
-025
volt vs SCE
Figure 2. Dc voltammogram of 10-4Mthallium(I) in 49 % H F Scan rate
=
1.0 volt per minute
To minimize the ohmic IR drop in dc voltammetry, a Metrohm IR Compensator E354 was used with a three-electrode system. All potentials are reported relative to a saturated calomel electrode and the third or auxiliary electrode was a tungsten wire electrode. Ac voltammetry was also carried out with the same threeelectrode system. In this case, however, the three-electrode system was used to minimize cell impedance. No IR compensation was provided for the dc potentials reported in conjunction with the ac voltammetry. Construction of Cells Containing the Mercury Pool Electrode. In the present work, the cell (Figure 1) containing the mercury pool electrode was constructed from an HF-resistant moulded Kel-F tube of uniform bore (6 X 0.75 in.). Obviously, materials other than Kel-F could have been used, such as Teflon or polyethylene, although oxidizing solutions must be avoided with the latter. Uniformity of bore was necessary so that a constant surface area of mercury would be exposed to the H F solution, and results would be independent of the quantity of mercury used. However, as a safeguard against having a non-uniform bore, and also to furnish a convenient method of mercury transfer, a buret supplying a constant volume of mercury was used.
The reference electrode was connected to the test solution via a Kel-F salt bridge fitted with disks of sintered Teflon. The salt bridge was filled with an acidic potassium fluoride solution, and one end of the salt bridge and the reference electrode were dipped into a solution of saturated potassium chloride. In principle, and probably in practice, it is better to introduce the platinum wire contact for the Hg pool by a compression seal as in Figure 1. However, a simpler construction for the mercury electrode contact was also tried. A piece of platinum wire was forced into the Kel-F and sealed with epoxy resin (Araldite). This approach proved satisfactory, no H F leakage being observed over a period of twelve months. The arrangement shown in Figure 1 is obviously superior from the safety angle; but it would appear that if facilities for constructing the more elaborate contact are not available, then the simple approach is adequate. All solutions were degassed with argon or nitrogen. No thermostating of equipment was attempted because of the low heat transfer properties of Kel-F. Temperatures given in various stages of the text are equilibrium temperatures of a similar cell containing water and exposed to the atmosphere. All scans reported in this work were made in the negative direction; they were not commenced until the system reached electrochemical equilibrium. RESULTS AND DISCUSSION
Potential Range. The available potential range in both dc and ac voltammetry in about 50% H F was approximately 0.3 to -0.9 volt us. S.C.E. The limit of positive potential was imposed by oxidation of mercury and that of negative potential by evolution of hydrogen. DC Voltammetry of Thallium(I). At mercury electrodes, thallium(1) is reduced according to the electrode process e e Tl(ama1gam). Tl(1) In aqueous HF, polarographic reduction of thallium(1) at a dropping mercury electrode had been found to be reversible, and this could also be anticipated for the mercury pool electrode. Figure 2 shows a dc voltammogram obtained with the mercury pool electrode. For reversible reduction at a stationary electrode, relationships of the form (7)
+
i, = kn3/2ACa(Dav) ll2 (1) (7) R. S. Nicholson and I. Shain, ANAL.CHEM., 36,706 (1964).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 3, MARCH 1972
465
Table I. Variation of DC Peak Potential with Scan Rate Scan rate, -Ep, Volt volt/min us. SCE 0.5 0.594 1.0 0.592 2.0 0.580 3.0 0.568 Concentration of thallium(1) = 10-sM, T = 13.6 i 0.4 "C. [HF]= 49.0%.
x 1x 5x 1x 5x
10-4 10-4
10-6
10-6 10-6 T = 13.6 f 0.4 "C. Scan rate =
Q
!
5.lOOi /'
0
v./
i
i [TI]
0.600
0.594 0.594 0.592 0.592 1.0 volt per minute. [HF]
//
i
Table 11. Variation of DC Peak Potential with Concentration of Thallium(1) -Ep, Volt [TUX M us. SCE 1 x 10-8 0.613 5
*OC1
M
i
x
io4
4
6
Figure 4. Calibration curve. Plot of DC peak height us. concentration of thallium =
Scan rate
=
1 volt per minute
49.0%.
RT Ep = EI/Z- 1.109 nF
should hold, where
E p = peak potential of voltammogram ip = current corresponding to E p Em = polarographic half-wave potential k = constant A = electrode area n = number of electrons involved in the electrode pro(:ess. V = scan rate of dc potential Do = diffusion coefficient Co = concentration Other symbols are those used conventionally The validity of Equation 1 for the thallium electrode process at the mercury pool electrode was checked in part as ipwas shown to be proportional to the square root of the scan rate of dc potential, as shown in Figure 3. Table I shows that a slight variation of Ep occurs with increasing scan rate, although measurement of Ep at high scan rates was not particularly accurate, and some recorder distortion may have occurred. ip was found to be essentially proportional to concentration, although as Figure 4 shows, the relationship departs slightly from linearity at higher concentrations. The limit of detection of Tl(1) was found to be 5 X 10-6M. The limit was determined by the charging current contribution of H F rather than instrumentation limits. E p was found to be almost independent of concentration of thallium(1) as shown in Table 11. The slightly more negative values found at high concentrations may be the result of incomplete IR compensation associated with the relatively very high currents, or of kinetic complications. Table 111 shows some typical data used to test the reproducibility of E p and ip. Reproducibility of both parameters is extremely good and standard errors at all concentrations examined were less than
2 %. It is concluded therefore that the thallium(1) electrode reaction in 50% H F exhibits a high degree of reversibility, and 466
*
ANALYTICAL CHEMISTRY, VOL. 44, NO. 3, MARCH 1972
-0 b
-0 ' 6
- 0'4
volt vs S.C,E Figure 5. Ac voltammogram of lo-' M thallium (I) in 49% HF Scan rate
=
1.0 volt per minute
that thallium(1) can be determined voltammetrically at a mercury pool electrode, down to the 5 X 10-6M level with a reproducibility of at least k 2 %. Ac Voltammetry. Ac voltammetry of Tl(1) at a mercury pool electrode in 50% aqueous H F is particularly suitable as the wave occurs at a potential region where the charging current is almost at its minimum value. Figure 5 shows a typical ac voltammogram. It is particularly well defined and obviously highly suited for determination of thallium(1). With a scan rate of 1 volt per minute, a thallium concentration of 10-6M and a temperature of 13.6 f 0.4 OC, the summit potential, Ea, for the ac voltammogram was -0.585 f 0.002 V US. SCE. The height of the AC voltammogram, i p , was almost linearly dependent upon concentration of Tl(1) over the range to lW4M. Curvature was apparent at higher concentrations. ia- was independent of scan rate of dc potential up to 3 volts per minute, the fastest scan rate ob-
Table 111. Reproducibility of Ep and i, -Epr volt Run No. us. SCE i p sPA 1 0.5956 35.0 2 0.5944 34.6 3 0.5936 35.0 4 0.5920 35.3 5 0.5944 38.6 6 0.5964 37.4 7 0.5964 36.6 8 0.5980 36.0 9 0.5940 35.5 10 0.5928 35.4 11 0.5932 35.2 12 0.5948 34.9 13 0.6008 32.8 14 0.5936 38.2 15 0.5940 38.4 16 0.5940 33.9 17 0.5940 35.3 18 0.5946 35.6 Average 0.5948 35.8 Concentration of thallium = 1 x lO-4M. Scan rate = 1.0 volt per min. T = 13.6 -i: 0.4 "C. [HF]= 49.0y0
tainable with the instrument used. This observation is in accordance with theory for stationary electrodes, and with experimental findings at a mercury hanging drop electrode, for a reversible ac electrode process (8). Examination of the theory for ac reversible or diffusion controlled electrode processes at a stationary electrode shows that equations describing the shape of the reversible ac voltammogram are essentially the same as those obtained for ac polarography at a dropping mercury electrode. This being the case, it would be expected that the width of the ac voltammogram at half the wave height would be approximately 90/nmV. Without compensation for IR drop, with a scan rate of 1 volt per minute and with a Tl(1) concentration of lO-SM, a value of 95 mV was obtained, indicating a high degree of reversibility. At higher concentrations, the halfwidth became broader, presumably because of the increasing importance of the IR drop, as the current increased. That E, is slightly more positive than the dc E, value is also indicative of an electrode process with a high degree of reversibility. E, is closely related to the standard redox potential, E", where Eo occurs at approximately 85% of the dc wave height. It can be seen that, in agreement with the dc voltammetry, the ac electrode process has a high degree of reversibility and is very suitable for the determination in concentration aqueous H F of Tl(1) down to 1 X 10-6M. This limit of detection occurs because at this level, the residual current of the H F supporting electrolyte became larger than the current from the reduction process. The reproducibilities of E, and is- were similar to the equivalent ~
~~
(8) W. L. Underkofler and I. Shain, ANAL.CHEM., 37,218 (1965).
dc parameters, and no advantage can be claimed for either ac or dc voltammetry over the other when the Hg pool is used. Comparison with Polarography. The polarographic method of analysis can be used in 50x H F with a Teflon dropping mercury electrode (5). However, these electrodes are not available commercially and are extremely difficult to cotastruct by comparison with the mercury pool electrode, which can be assembled simply without any special equipment in a readily obtainable tube of Kel-F or other HF-resistant material. When the Hg pool method was used, the limit of detection of Tl(1) was found to be 5 X 10-6M with dc techniques, and 1 X 10eeM with ac voltammetry. Thus the detection limits of voltammetry with a mercury pool and with polarography using a Teflon dropping mercury electrode are quite similar(5). The reproducibility of analytical figures for a Teflon dropping mercury electrode is about the same as for a glass dropping mercury electrode, and is slightly better than data obtained with the mercury pool electrode. However, the reproducibility of better than =t2% possible with a mercury pool electrode is quite satisfactory for most analytical purposes. The overall performance of the mercury pool electrode should therefore make it an attractive alternative to the Teflon dropping mercury electrode for electroanalytical studies in media such as aqueous HF, which etch glass. It would prove especially advantageous if the highly specialized facilities for construction of a Teflon dropping mercury electrode, as hitherto reported, were not available. AC and DC Voltammetry in Anhydrous Hydrogen Fluoride. An extension of the work described above required some preliminary voltammetry in 100 % anhydrous hydrogen fluoride. Both ac and dc voltammetry, with a mercury pool electrode, were shown to be feasible, and to be relatively simply achieved with the following alterations to the experimental arrangement used previously in aqueous hydrofluoric acid. First, the cell needs to be connected to a vacuum line to maintain anhydrous conditions and to contain the toxic and hazardous HF vapor. The vacuum line was constructed from Kel-F to eliminate the use of glass. Second, the aqueous saturated calomel reference electrode must be eliminated. In this work, the reference electrode, Cu/CuF?(s) (0.1M NaF, HF) was chosen on the basis of some detailed work by Burrows and Jasinski (9). In accordance with their findings, the electrode couple was found to be reversible and Cu/CuFz was found to be a most convenient reference electrode. The reference electrode and platinum auxiliary electrode, used in work in anhydrous hydrogen fluoride, were fitted to the cell by means of compression seals based on Teflon glands. RECEIVED for review July 23, 1971. Accepted October 5, 1971. (9) B. Burrows and R. Jasinski, J. Electrochem. SOC.,115, 348 (1968).
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