Table 11. Determinations of Atomic C/H Ratios in Various Materials C/H ratio C/H ratio Sample found expected n-Hexane (test sample) 0.468-0.469 O.47la Isooctane (test sample) 0.447-0.445 0,444" Benzene (test sample) 1.0064.993 1 .OOo" C7HsS (test sample) (25.8% S) 0.871 0,875" Blend of gaseous hydrocarbons 0,424-0.414 0 .422b Distillate, top fraction 0.499 0 .498c Distillate, bottom fraction 0.536 0.531' Waxy blend (0.37 S) 0.577 0,579" Lubricating oil (2.50% S) 0.575 0.570" Viscous residue (1.34 S) 0.600 0 . 592c Pyrolysis gasoline 0.733 0 .738c a From Theory. * From GLC data. By a gravimetric method.
Results of carbon-to-hydrogen ratio determinations obtained on standard compounds and on various samples from practice are given in Table 11. The latter are compared with data from a gravimetric method. The standard deviation of the ratio is in the order of 0.3%. A limitation of the technique is that the hydrogen determination is not really specific since any combustible con-
taminant present will also consume oxygen and thus affect the first pressure drop. However, if the nature and the amount of the contaminant as well as its reaction(s) with oxygen are known, corrections may be applied. For instance, in many hydrocarbon samples, sulfur is present, sometimes in concentrations of up to several per cent. It may conveniently be trapped as silver sulfate on silver gauze, which is kept at a temperature of about 500 "C at the end of the combustion tube. Hence, when S is the percentage of sulfur present (which, in general, is known in the case of samples from plant streams), the corrected formula for the carbon-to-hydrogen ratio is given by: (B - C ) ( A 2B
4(A - B ) -
+
- 3C)S
(2)
100 - 0.75 S
Unfortunately, no reliable corrections are possible for contaminants that react with oxygen in an unpredictable way, such as nitrogen compounds or oxidizable metals on the catalysts. In all other cases, however, the two methods described in this note have proved very useful, especially in the fields of platforming and hydrotreating. RECEIVED for review June 8, 1971. Accepted August 23, 1971.
Alternating Current and Direct Current Polarography in Concentrated HydrofluoricAcid Solutions with a Teflon Dropping Mercury Electrode A. M. Bond and T . A. O'Donnell Department of Inorganic Chemistry, University of Melbourne, Parkville 3052, Victoria, Australia
HYDROGEN FLUORIDE shows a specific reactivity towards silica and silicates, and is a source of the strongly complexing fluoride ion. Therefore, it is frequently used in conjunction with other substances in the dissolution of refractory samples for analysis. The resulting fluoride in the prepared solution can cause serious interference to the subsequent analytical procedures in many instances. Many entities can be determined accurately by polarography in hydrofluoric acid solution without the necessity of rigorously controlling the hydrogen fluoride concentration ( I ) . Furthermore, if rapid polarographic techniques are used, analyses can be carried out ( I ) without etching the glass dropping mercury electrode (DME) in aqueous solutions containing H F concentrations up to 25 to 30x. However, if conventional polarographic techniques are to be used, or if the HF concentration is greater than the level stated above, etching of the glass DME leads to irregular formation of mercury drops and quite unacceptable polarographic response. If, for analytical or other purposes, polarographic studies are to be made in concentrated HF solutions, it is necessary to construct the DME from an HF-inert material. Raaen (1) A. M. Bond and T. A. ODonnell, ANAL.CHEM.,41, 1801 (1969). 590
ANALYTICAL CHEMISTRY, VOL. 44, NO. 3, M A R C H 1972
and coworkers (2-5) have reported construction of a DME from Teflon (Du Pont). This type of DME has been used in 0.1M NHdF-O.1MHF solutions by Headridge et al. (6) and in 0.1 to 28M (48%) hydrofluoric acid by Raaen as reported in various communications (4, 7-9). However, the real advantages of the Telon DME have not yet been exploited systematically in highly concentrated hydrofluoric acid. Concentrations of about 50% are commonly used for dissolution of solid refractory samples. Direct polarographic analysis in such media are reported here. The electrode reactions of Pb(II), Cd(II), Tl(I), and Sn (11) in moderately high concentrations of HF have been shown previously (1, 7) to be close to reversible. This work shows that reversibility is maintained in concentrated hydrofluoric acid (up to 50 %) and ac polarography is shown to be partic(2) H. P. Raaen, R. J. Fox, and V. E. Walker, U.S.At. Energy Comm. Rept. ORNL-3344, Nov. 30,1962. (3) H. P.Raaen and R. L. Clark, ibid., ORNL-3654, August 1964. (4) H. P. Raaen, ANAL.CHEM.,34, 1714 (1962). (5) H. P. Raaen, Chem. Instrum., 1, 287 (1969). (6) J. B. Headridge, A. G. Hamza, D. P. Hubbard, and M. S. Taylor, "Polarography 1964", G. J. Hills, Ed., Proc. 3rd International Conf., Southampton, London, 1966,Vol. 1, pp 625-633. (7) H. P.Raaen, ANAL.CHEM., 37, 1355 (1965). (8) H.P.Raaen, Anal. Chim. Actu, 44, 205 (1969). (9) Ibid., 48,427 (1969).
ularly useful for the determination of the four elements listed above. In addition to the analytical implications, ac polarography is very useful for studying the nature of electrode reactions in the reactive solvent. Some features of the design and operation of the Teflon DME were modified, although the basic design and construction of the Teflon DME as reported by Raaen and coworkers has been retained.
Figure 1. Teflon dropping mercury electrode used in this work
EXPERIMENTAL
Reagents. All chemicals used were of reagent grade quality. Cadmium(II), lead(I1) and thallium(1) were added as the nitrates and tin(I1) as stannous fluoride. Baker Analyzed Hydrofluoric Acid was used as the solvent and supporting electrolyte. Apparatus. Polarograms were recorded at 20 f 2°C in a cell constructed from Teflon. All polarograms were obtained with a Metrohm Polarecord E261. For the dc polarography, a Metrohm IR Compensator E354 was used, with a three-electrode system. For the ac polarography, a Metrohm AC Modulator E393 was used in conjunction with the Polarecord. An applied ac voltage of 10 mV, rms, at 50 Hz was used to obtain all ac polarograms. All potentials are reported relative to a Ag/AgCl (1M NaC1) reference electrode. The third or auxiliary electrode was tungsten. The reference electrode was separated from the test solution by a salt bridge constructed from Kel-F with isolating disks of sintered Teflon and filled with acidic K F solution. No correction was made for junction potentials as these were unknown, Construction of a Teflon DME. The Teflon DME, which consists of a Teflon portion and a glass capillary portion, was a slight modification of that described by Raaen et al. (2). The Teflon part of the DME, which dips directly into the test solution, was constructed exactly according to their report. Six Teflon pieces were made with pierced holes of diameter from 50 to 250 microns, as measured by an optical microscope. Fitting a glass capillary which controls the flow rate of mercury to the top of the Teflon via a tapered section as recommended by Raaen et al. caused serious problems. During the tapering of the glass, capillaries frequently became blocked by abrasive or powdered glass. Use of the tapered glass connection was then abandoned without disadvantage. Instead an unmodified glass DME (Metrohm) was fitted directly to the Teflon tip with a polyethylene adaptor to give the arrangement shown in Figure 1. As a result, DME's could be constructed rapidly and easily. Performance of the Teflon DME in 50 HF. Current-time curves were recorded in 1M NaCl, 50 HF, and other media at a constant potential of zero volt us. Ag/AgCl, using a Teflon DME with an orifice diameter of 250 microns. On two hundred consecutive drops, no variation of any currentdrop time curve from the mean was greater than 1 0 . 5 z . The diameter of this particular capillary was considerably larger than that used by Raaen, but performance and reproducibility were extremely good. Visually drops appeared to be excellent in shape and growth characteristics. With this capillary and with a mercury column height of 30 cm, a drop time of 8.0 sec and flow rate of mercury of 4 mg per sec were observed at open circuit in 5 0 z HF. At drop times of this length, a delay occurred between the detachment of one drop and the appearance of the next. This delay time can be seen by examination of the two plateau regions in the dc polarogram of Sn(I1) in 5 0 z H F as shown in Figure 2. The delay time, ascribed to back pressure, was also observed by Raaen ( 4 ) and has been observed with glass DME's. An ac polarogram of Cd(I1) in 50 H F is shown in Figure 3. Polarograms, both ac and dc, were highly reproducible,
W
- 0.75
- 035 vs
volt
Ag I A g CI
+GO5
Figure 2. Dc polarogram of Sn(1I) in 50
zHF
zz
-0 6
-05
-04
dt K A g I A g C I
all characteristics being identical with those obtained with a glass DME, as Raaen has reported in other media (10). RESULTS AND DISCUSSION
Potential Range. The polarographically usable potential range in 5 0 z HF was about +0.4 to -0.95 volt us. Ag/AgCl. The limit in the positive region was oxidation of mercury, and in the negative region, evolution of hydrogen. (10) H. P. Raaen, ANAL.CHEM., 36, 2420 (1964); 37,677 (1965).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 3, MARCH 1972
591
Cadmium(II). In 50 Cd(I1)
HF, the wave for the reduction
+ 2e e Cd(ama1gam)
Sn(I1)
*
*
Pb(I1)
+ 2e
Pb(ama1gam)
(2)
is characterized by a high but not complete degree of reversibility in aqueous HF. The ac wave was symmetrical in 50% H F with a halfwidth of 52 + 2 mV. E. was -0.408 volt us. Ag/AgCl and independent of concentration of metal ion and drop time. The limit of detection was about 5 X 10-6M and a plot of id us. concentration was linear up to lo-", curvature being observed at higher concentrations. Application of the method to the determination of lead in glass is now under way. Tin(I1). The polarography of tin(I1) in 50% H F is particularly interesting. Two extremely well defined waves are observed as shown in Figure 2. The more positive wave is an anodic wave with Eliz of -0.022 volt us. Ag/AgCl. The other more negative wave is cathodic with Elit of -0.536 volt us. Ag/AgCl. The ratio of the cathodic to anodic wave to 10-4M concentration range heights is 1.02 :1.00 in the indicating that the same number of electrons is involved in each electrode process. The value of (Eli4 - E3,4)for the anodic wave is 44 f 2 mV while that for the cathodic is slightly less, being 38 i 2 mV. The plot of Edo us. log (id - i)/i is slightly curved for the cathodic wave with a limiting slope approaching 30 mV at more positive potentials. This is consistent with a quasireversible two electron reduction
592
(3)
The anodic wave, involving the same number of electrons must therefore be the electrode process
(1)
was observed to be completely reversible. From dc polarograms, plots of Eda us. log (id - i)/i were linear with slopes of (29 =k 2) mV. The Eliz value was -0.500 volt us. Ag/AgCl, and independent of drop time and concentration. Ac polarograms were completely symmetrical, the summit or peak potential, Ea, was the same as Ell2being -0.500 volt us. Ag/AgCl and ac waves had half-widths of (48 2) mV, indicating reversibility. A detection limit of about 10-6M in cadmium was found in the analytical application of ac polarography. Plots of ac wave to height us. concentration were linear over the range M in cadmium. Above this concentration, curvature was observed, presumably because of the increasing importance of ohmic IR drop at the higher currents associated with higher concentrations. Ac polarographic determination of Cd(I1) in 50 % H F is therefore particularly favorable. A particularly interesting feature of reduction of Cd(I1) in concentrated H F is that the value of Elizis about 100 mV more positive than values reported in less concentrated H F or in other fluoride media ( I ) . This suggests a major difference in solvation or other coordination depending on the concentration of the aqueous H F solution. Lead(I1). The lead(I1) dc wave, with an Ell2 of -0.404 volt us. Ag/AgCl was particularly well defined in 50z HF. The slope of the plot of Edo us. log (id - i)/iwas 33 1 mV. As in previous work ( I ) and from data given by Raaen (7) the electrode process,
+ 2.9 e Sn(ama1gam)
Sn(1I) e Sn(1V)
+ 2e
(4)
For the anodic wave a plot of E d c us. log (id - i)/i was close to linear with slope of 42 2 mV, indicating that this electrode process is more irreversible than the cathodic one. The ac polarography is entirely consistent with the observations of the dc polarography. The cathodic ac wave was symmetrical and had a half-width of 54 f 2 mV. The anodic wave was also symmetrical, but the half width was approximately 66 mV. The summit potentials of the anodic and cathodic waves were -0.020 and -0.540 volt us. Ag/AgCl, respectively in good agreement with Ell2 values. However, the ratio of the ac wave heights at tin concentrations of 10-4M are 1 : l o . The much smaller anodic wave height relative to that of the cathodic wave is consistent with the rate of this electrode process being slower, and the conclusion from the dc polarography that the anodic electrode process is more irreversible than the cathodic is confirmed. In the analytical application of ac polarography, the cathodic wave was more sensitive giving a limit of detection of about 10-6M, while for the anodic wave the limit of detection was 10-5M. Consequently, the cathodic wave is more favorable for determination of tin(I1). However, since the anodic wave is observed at potentials considerably more positive than for many other species in HF-e.g., Cd(II), Tl(1) and Pb(I1) this wave would prove extremely useful for determination of tin(I1) in the presence of species whose waves overlap the cathodic tin(I1) wave. Thalliurn(1). Both the dc and ac waves for reduction of thallium, according to the electrode process
ANALYTICAL CHEMISTRY, VOL. 44, NO. 3, MARCH 1972
*
Tl(1)
+ e e Tl(ama1gam)
(5)
were extremely well defined in 50% HF. The dc E112and the ac E, values were both -0.560 volt us. Ag/AgCl. The dc and ac electrode processes were reversible as shown by dc linear plots of Edo us. log (id - i)/i of slope 56 k 2 mV and ac halfwidths of 92 f 2 mV. The ac calibration curve of peak height US. concentration of Tl(1) was linear from the detection limit of 5 X 10-6M to M. The determination of thallium(1) by ac polarography in 50% H F with a Teflon DME is therefore particularly convenient. ACKNOWLEDGMENT
The assistance of Mr. E. B. Taylor in the fabrication of the Teflon DME is gratefully acknowledged.
RECEIVED for review June 7, 1971. Accepted August 23, 1971. The authors wish to acknowledge a grant from the Australian Research Grants Commission for purchase of the polarographic equipment used in this work.
