Direct Current, Alternating Current, Rapid, and Inverse Polarographic Methods for Determination of Tin(lV) A. M. Bond Department of Inorganic Chemistry, University of Melbourne, Parkuille, Victoria 3052, Australia
Conventional dc, conventional ac, rapid dc, rapid ac, inverse dc, and inverse ac polarographic methods for the determination of tin(lV) have been investigated. Hydrochloric acid has been found to be a suitable supporting electrolyte for all methods. Analytical use in this media is made of the Sn(ll) G Sn(0) electrode reaction at the mercury electrode. Conventional ac polarography is recommended as the best method for the determination of tin(lV) in the concentration range 10-3-10-6~. Inverse ac polarography (ac anodic stripping analysis) is recommended for trace analysis in the concentration range 10-6-10-8M. Detailed discussion, limitations, sensitivities, and various characteristics of each of the six methods are given. DETERMINATION OF TIN in geological samples is at present somewhat tedious and difficult (1). A simple, rapid and reliable method for determination of tin in rock samples was needed, however, in this laboratory for both determination of tin in tin ores and for trace analysis for tin in rock samples. Conventional dc polarographic methods for determination of tin have been employed in the past by several workers (2); however since the time of these reports, a number of new polarographic methods have been developed which in general have several useful advantages over the well known dc polarographic method. For instance, increased accuracy, higher degree of freedom of interference, and several other advantages can be readily obtained by ac methods (3). Very low limits of detection are possible by inverse or anodic stripping polarography (voltammetry) and trace analysis is conveniently carried out by this method. Rapid polarographic techniques with very short controlled drop times and fast scan rates of potential offer advantages of convenience and short analysis times. Consequently, a variety of polarographic methods of analysis for tin have been investigated in a n endeavor to provide rapid, convenient, and reliable methods for both quantitative and qualitative analysis for tin. A subsequent paper ( I ) will report the application of these findings t o problems of determination of tin in rock samples by polarographic methods.
EXPERIMENTAL Standard tin(1V) solutions were prepared from ammonium chlorostannate, (NH4)SnCl6,or stannic bromide, SnBrc. Standard tin(I1) solutions were prepared from stannous fluoride, SnF2. All acids used as supporting electrolytes were of reagentgrade purity. All measurements were made at 25 rt 0.1 “C and solutions were all deaerated with oxygen-free nitrogen. Polarograms were obtained using the Metrohm Polarecord E 261. Alternating current polarography was carried out -
Bond, R. J. W. McLaughlin, T. A. ODonnell, and A. B. Waugh, unpublished work, University of Melbourne, 1968-69. (2) I. M. Kolthoff and J. J. Lingane, “Polarography,” Vol. 11, 2nd Ed., Interscience, New York, 1952, pp 523-528. (3) B. Breyer and H. H. Bauer, “Chemical Analysis, Vol. XIII, Alternating Current Polarography and Tensammetry,” Interscience, New YorkfLondon, 1963, pp 128-131. (1) A. M.
using the Metrohm A.C. Modulator E 393 with a n ac voltage of 10 mV, rms at fifty cycles/sec. To minimize cell impedance, the modulating ac voltage was applied through a n auxiliary tungsten electrode. Rapid polarographic techniques and controlled drop times were achieved with a Metrohm Polarographie Stand E 354. For the inverse or anodic stripping polarography, the Metrohm hanging drop mercury electrode BM 5-03 was used. RESULTS AND DISCUSSION The polarography of tin in various media has been studied widely. Two common oxidation states, tin(I1) and tin(IV), exist. However, in solution, tin(I1) is readily air-oxidized t o tin(1V) and to maintain tin(I1) in solution, it is necessary t o carefully remove all dissolved oxygen and to maintain the tin(I1) in oxygen-free conditions. In most media tin(I1) produces two polarographic waves. One, a cathodic wave Sn(I1) 2e $ Sn (amalgam) is usually found t o be reversible and particularly well defined for analytical use. The other, a n anodic wave Sn(I1) eSn(1V) 2e, varies markedly in degree of reversibility and suitability of definition for analytical application with the particular media. Tin(IV), which is a n air stable form, does not produce polarographic waves in many media because the Sn(1V) species is extensively hydrolyzed and precipitation of hydrous stannic oxide or basic salts occurs even in highly acidic media. However, in acidic chloride or bromide media which can presumably stabilize high order halide complexes, two waves can be observed due t o successive reduction of Sn(1V) --* Sn(I1) -+ Sn(ama1gam). In its natural state as found in geological samples, tin exists almost exclusively in the (IV) state. Methods of preparation of samples for analysis by fusions of various kinds usually ensure anyway that the tin present for analysis is in the (IV) state. Unfortunately, on the other hand, studies of polarographic waves of tin(I1) and tin(1V) which have been made many times by many polarographic methods, in many media (e.g. References 2-12), would indicate that use of the tin in the (11) state would provide the best method for polarographic analysis. Tin in its natural form, however, normally occurs in the (IV) state and t o convert it t o tin(I1) for polarographic
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(4) J. J. Lingane, J. Amer. Chem. SOC.,67, 919 (1945). ( 5 ) B. Breyer, F. Gutmann, and S. Hacobian, Australian J. Sci. Res., A4, 595 (1951). (6) P. W. West, J. Dean, and E. Breda, Collect. Czech. Chem. Commun., 13,1(1948). (7) W. B. Schaap, J. A. Davis, and W. H. Nebergall, J. Amer. Chem. SOC.,76, 5226 (1954). (8) J. B. Headridge, A. G. Hamza, D. P. Hubbard, and M. S. Taylor, “Polarography 1964, Proceedings of the Third International Conference, Southampton,” G. J. Hills, Ed., Macmillan, London/Melbourne, 1966, pp 625-633. (9) R. Naumann and W. Schmidt, Z . Anal. Chem., 240, 170 (1968). (10) R. Neeb and I. Kiehnast, ibid., 241, 142 (1968). (11) M. Francini, S. Martini, and C. Monfrini, Electrochim. Meral., 2, 3 (1967). (12) A. M. Bond and T. A. O’Donnell, ANAL.CHEM.,41, 1801 (1969).
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r k
-0.5 -0’3 VOLT Vs:Ag/AgCI
-0.7
-0.;
-0.7.
-0.~5 -0.j
V O L T Vs. Ag/AgCI
“OLT
-02
+
-0.5
-0.3
vs* Ag/AgC’
-0.’5
-0.3
=
1.50 X lO-*M
6. Conventional ac polarogram of tin in HCl/H&Oi
5M
[Sn (IV)] = 1.50 X 10-Vd. Same solution as for Figure l a
c . Rapid dc polarogram of tin in 5 M HCI/HzS04 [Sn (IV)] = 1.50 X lO-’M. Same solution as for Figure la. Drop time = 0.16 sec
d. Rapid ac polarogram of tin in H2S04
5M H a / -
[Sn (IV)] = 1.50 X lO-‘M. Same solution as for Figure la. Drop time = 0.16 sec analysis would require an additional step of reduction by either chemical or electrochemical means. Furthermore, the problem of subsequently maintaining the sample as tin(I1) and preventing air oxidation would need to be considered. Therefore, it was considered that polarographic analysis as tin(I1) was not suitable or convenient, as conversion to and maintenance as tin(I1) requires time-consuming and tedious extra steps which are not especially desirable. In addition the extra steps would undoubtedly lead to a loss of precision. Tin samples from rocks as prepared for modern methods of rapid analysis, almost invariably finish up as tin(1V) dissolved in fairly concentrated sulfuric acid, perchloric acid, nitric acid, or hydrofluoric acid media. An investigation of polarographic behavior of tin in these four media revealed unfortunately that no ac or dc waves of tin(1V) occur in any of these media. On the other hand, for tin(II), well defined and analytically usable Sn(I1) 2e e Sn(ama1gam) waves were observed in all cases and especially in hydrofluoric acid media,
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POLAROGRAPHIC METHODS WITH DROPPING MERCURY ELECTRODES
V O L T Vs. Ag/AgCI
Figure 1 a. Conventional dc polarogram of tin in 5 M HCI/H2SO~ [Sn (IV)]
as reported previously (12), the anodic wave Sn(I1) e Sn(1V) 2e was also particularly well defined. The results for tin(1V) meant that the desired direct polarographic analysis for tin(1V) was not possible in any of the acidic media described above. Lingane ( 4 ) has, however, reported that dc polarographic waves for tin(1V) are present in acidic chloride media. Consequently, hydrochloric acid was added to tin(1V) containing sulfuric, nitric, perchloric, and hydrofluoric acid solutions. Polarograms were then recorded, and in all cases polarographic waves were observed. This suggested that the various polarographic techniques of analysis for tin should be examined by preparing standard tin(1V) solutions in hydrochloric acid media. Concentrated hydrochloric acid (5M) was chosen as the electrolyte for this particular study. It was felt that use of this very concentrated hydrochloric acid medium would tend to “swamp out” or “saturate out” effects of other acids present in solutions of rock samples. For instance, it was hoped that the diffusion coefficient in 5M HCl would be almost the same as in, say, 5M HC1 plus 0.1M H2S04 and analytical results would not be affected by the method of preparation of tin samples which would mean that tin samples could be determined by direct comparison with calibration curves obtained in 5Mhydrochloric acid.
Conventional Direct Current Polarography. Conventional dc polarograms of tin(1V) were examined using a dropping mercury electrode (DME) with natural drop frequency and applying a scan rate of potential of 1 volt per 12 minutes. Supporting electrolytes consisting of 5MHC1, 5MHC1/H2SOi, 2e and 5M HCl/HF were used. The height of the Sn(I1) e Sn (amalgam) wave was examined as a function of tin(1V) concentration in each of these electrolytes. By conventional dc polarography, it was found that tin(1V) could conveniently be determined over the concentration range 10-3Mto 10-5M, as the calibration curves in all supporting electrolytes were identical. Half wave potentials were observed to vary somewhat with the composition of the supporting electrolyte but always fell in the range -(0.4-0.7) V us. AgiAgCl. The more negative potentials were associated with the presence of H2S04 or H F with the 5MHCl. Figure l a shows a typical dc polarogram in 5M HCl/ H S 0 4 . A maximum can be observed on this polarogram. The maxima in this medium were quite large at high tin(1V) concentrations but not observable at the lower concentrations. Plots of Ede us. log i / i d - i were observed to be linear with slopes of 29 i 2 mV. This indicates that the electrode reaction is reversible. Half-wave potentials, Ei12,were found to be independent of tin(1V) concentration except at very low tin(1V) levels where negative shifts were observed to occur. The assignment of the electrode reaction as Sn(I1) 2e Sn(ama1gam) was confirmed because addition of tin(I1) to a sample solution containing tin(1V) led to an increase in wave height without altering the half-wave potential. The lower limit of detection of tin(1V) was observed to be lO+M because at this level, the charging or background current and oscillations of the DME made the wave indistinguishable from the background. Calibration curves of concentration of tin(1V) cs. id were close to linear over the range 10-3M-8 X lOP5M,but marked curvature was observed at lower levels. The problem of
ANALYTICAL CHEMISTRY, VOL. 42, NO, 11, SEPTEMBER 1970
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measuring the limiting current (id) in the presence of the maxima could account for the nonlinearity. Conventional Alternating Current Polarography. The ac polarographic method of analysis using natural drop frequencies and scan rates of potential of 1 volt per 12 minutes was considerably more sensitive than conventional dc polarography and analyses for tin(1V) over the range (10-3-10-6)M were possible in the same supporting electrolytes as used for the dc work. Summit or peak potentials, E,, were independent of Sn(1V) concentration over the range studied. Figure 16 shows an ac polarogram in 5 M HCl/H2S04. This polarogram has a high degree of symmetry and the half band width is 45 f 2 mV. All these characteristics are consistent with a reversible electrode reaction. Calibration curves of concentration of tin(1V) us. wave height (id-) were linear over the entire range (10-3-10-6)M and particularly suitable for analytical purposes. E, values were coincident with the E,,Zvalues reported previously for the dc polarographic wave. This equivalence of E, and EIlzis expected for a reversible electrode reaction. The higher sensitivity, lower limit of detection, the linear calibration graphs, and all the advantages of ac polarography over dc polarography (3) meant that conventional ac polarography is a much preferred technique to conventional dc polarography for determination of tin(1V). Rapid Direct Current Polarography. The essential polarographic characteristic of rapid dc measurements using short controlled drop times of 0.16-0.32 second and fast scan rates of potential of 0.5-2 volts per minute were the same as with the conventional dc polarography. Plots of E d d e cs. log (i/h-i) were still linear with slope 29 i 2 mV and even at the short drop time of 0.16 second, the electrode reaction was still polarographically reversible. Figure I C shows a rapid dc polarogram in the 5M HCl/ H2S01electrolyte and comparison with the conventional dc polarogram (Figure l a ) shows that the maximum present at long drop times is not present under rapid polarographic conditions. This absence of maxima plus the considerably smaller oscillations of the current deflections with the short drop time make the rapid dc polarograms considerably easier to measure than conventional dc polarograms. Consequently, the rapid technique was also more reproducible than conventional methods. The sensitivity of rapid dc measurements was similar to conventional dc and analysis for tin(1V) between 10F3Mand 10-jM can be achieved. However, because of advantages reported earlier and the time saving offered by the method, it was found to be preferable to conventional dc polarography as a means of analysis for tin(1V). Half wave potentials from rapid polarographic measurements were very close to half wave potentials obtained from conventional dc polarographic measurements. Rapid Alternating Current Polarography. Figure I d shows a rapid ac polarogram in 5M HCl/HZSOa. Some loss of symmetry and a considerable loss of sensitivity has occurred compared with the conventional ac polarogram in Figure lb. Half-band widths, however, are still 45 =t2 mV and summit potentials are independent of Sn(1V) concentration, and equivalent to the half wave potentials measured from dc polarography. Calibration curves of Sn(1V) concentration 1;s. id- are linear but because of the lowered sensitivity compared with conventional ac polarography, only the tin(1V) range 10-jM can be determined by rapid ac techniques. The
Figure 2 a. Inverse ac polarogram of tin in 5M HCl -1.0 [Sn (IV)] = 2 X 10-6M
-0.2
-0.6
VOLT
VS.
Ag/AgCI
b. Inverse dc polarogram of tin in 5 M HCI
[Sn (IV)]
=2 X
10-6M
-0.8 VOLT
- 0.4 Vs. A g / A g C l
0.0
method however, requires only exceptionally short analysis times and maintains all the ac advantages obtained over dc methods (3). POLAROGRAPHIC METHODS WITH HANGING MERCURY DROP ELECTRODES
Alternating Current and Direct Current Inverse (Anodic Stripping) Polarography. With inverse or anodic stripping polarographic (voltammetric) methods, a sufficiently negative potential is applied to the hanging mercury drop electrode (HMDE) system to reduce the tin(1V) to tin(0). By controlled electrolysis at this potential, the tin(0) can be concentrated into the HMDE by forming an amalgam. The tin amalgam can then be stripped of its tin by changing the potential of the HMDE in a positive direction until a potential is reached at which the tin diffuses from the amalgam back into the solution. The peak height of the current-voltage curve obtained in this manner is proportional to the concentration of the tin. In this work, hanging mercury drops 0.52 mm in diameter and with a drop surface area of 1.38 f 0.04 mm2were used. The tin(1V)was electrolyzed to tin(ama1gam) in the presence of 5M HC1 at a potential of -1.0 V 1;s.Ag/AgCl. An electrolysis time of 3 minutes was used at this potential. The 3 minutes consisted of 2 minutes with stirring and a further minute without stirring to allow equilibrium conditions to be attained. The tin amalgam was then stripped by scanning the dc potential by ac or dc methods over the range - 1.O to 0 V us. Ag/AgCl at a scan rate of potential of 1 volt per minute. Figures 2a and 26 show the inverse or anodic stripping polarographic scans of a tin(1V) solution by both the ac and dc methods. The peak potential, Ep, from the inverse ac scan and the EP:2value from the inverse dc scan are in very close agreement. The peak potential of the dc scan differs slightly from peak potential of the ac scan however, as is expected. The inverse dc Epizand ac Ep are as theoretically predicted, approximately 28/n mV more anodic than the El,z and E$ values found previously for the electrode reaction Sn(I1) 2e e Sn(ama1gam) by dc and ac polarography. Thus, the peak is assumed to be due to the anodic electrode reaction Sn(ama1gam) Sn(I1) 2e. The direction of current flow with dc scans verifies that an
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anodic wave is being observed, as is expected, but with ac stripping, it is not possible to distinguish between cathodic and anodic electrode reactions. EDvalues in both ac and dc stripping were found to be independent of Sn(IV) concentration. Peak heights of both dc and ac scans were investigated as a function of Sn(1V) concentration. Calibration curves were linear over the tin range studied except near the lowest limit of detection where curvature was observed. Tin(1V) could be determined from 10-3-10-7M by dc stripping and from 10-8M by ac stripping. The higher sensitivity of the ac method was due to the much lower background current observed compared with the dc method. Alternating current stripping analysis, because of the lower limit of detection and higher sensitivity would appear to be more suitable than dc stripping analysis of tin(1V). Reproducibility studies of both methods were similar and no advantage in this direction was found for either method. Although ac stripping analysis of tin(1V) can be carried out in the range 10-3-10-8M, only the range 10-6-10-8 should be analyzed by this method. The range 10-3-10-6M would best
be done by conventional ac polarography as this method is much faster, more accurate, convenient, and reproducible than stripping analysis. CONCLUSIONS
Tin(1V) solutions can be analyzed polarographically down to 10-8M in hydrochloric acid supporting electrolytes. Inverse ac (anodic stripping) polarography with a hanging drop mercury electrode is recommended for analysis for tin in the range (10-6-10-8)M. For higher concentrations ( l o + 1OP6)M,conventional ac polarography with a dropping mercury electrode of natural drop frequency is recommended. Other polarographic methods of ac and dc rapid ,polarography, dc conventional polarography, and inverse dc polarography have also been investigated but for analysis for tin(1V) were not as useful as the above-mentioned methods. RECEIVED for review September 8, 1969. Accepted March 16, 1970.
Use of Polarographic Methods for the Determination of Tin in Geological Samples A. M. Bond, T. A. O’Donnell, and A. B. Waugh Department of Inorganic Chemistry, University of Melbourne, Parkville, Victoria, 30.52, Australia R. J. W. McLaughlin Department of Geology, University of Melbourne, Parkville, Victoria, 30.52, Australia
Quantitative methods for determination of tin in geological samples, by polarographic methods, have been established. Conventional alternating current polarography was found to be highly selective and specific for tin in rock samples examined, enabling quantitative determinations to be achieved rapidly and directly without any prior separation of the tin. The method has been used for the geochemical survey of tin on a large number of samples. A comparison was made of several methods of fusion used to prepare tin solutions prior to polarographic analysis. Results obtained by inverse ac and dc polarographic methods using a hanging drop mercury electrode are also described for determination of tin in trace amounts. Use of hydrofluoric acid as a solvent, and rapid ac or dc polarographic techniques provided rapid, simple, and reliable estimations of tin. MOSTEXISTING methods for determination of tin in geological samples are tedious and time consuming as they require preliminary extraction and concentration of tin ( I ) , and are subject t o considerable error because they are nonspecific and because of the multistage processes necessary t o complete the analysis. Tin exists in rock samples almost exclusively in oxidation state (IV). Fusion techniques, such as sodium carbonatesodium peroxide fusions, however, ensure that tin present in (1) M. Farnsworth and J. Pekola in “Treatise on Analytical Chemistry,’’ Part 11, Vol. 3, I. M. Kolthoff and P. J. Elving, Ed., Interscience, New York/London, 1961. 1168
solution is in oxidation state (IV). In conventional chemical analysis, tin(1V) is reduced t o tin(I1) and determined volumetrically against a standard solution of iodate (1-3). The volumetric method is not specific for tin, being subject t o interference from a considerable number of species ( I ) unless specific preliminary procedures are used to prepare fairly pure tin solutions. Gravimetric methods ( I ) and colorimetric methods ( I ) are in use for determination of relatively small amounts of tin; but these methods lack selectivity and sensitivity, and again preliminary separation is generally required. Methods based on atomic absorption ( 2 ) , emission ( I ) , and X-ray spectroscopy ( I ) have been used. Bowman ( 2 ) has applied the atomic absorption method to tin ores and concentrates and has used it for routine analysis. Heggen and Strock ( 4 ) and Mitchell (5) have used spectrographic methods of analysis of tin in geochemical and other samples; but again prior enrichment and separation were necessary ( I , 4 , 5). Recently, neutron activation analysis has been used (6) for determination of tin in geological samples.
(2) J. A. Bowman, Anal. Chim. Acfa, 42,285 (1968). (3) I. M. Kolthoff and E. B. Sandell, “Textbook of Quantitative Inorganic Analysis,” 3rd Ed., Macmillan, New York, 1952, p 604. (4) G . E. Heggen and L. W. Strock, ANAL.CHEW,25, 859 (1953). (5) R. L. Mitchell, Analysf,71,361 (1946). (6) 0. Johansen and J. Richardson, ibid.,94,976 (1969).
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