in the absorption profiles of the various elements in the long pathlength system. While providing understanding of the physical bases which underlie the behavior of the various elements in the long pathlength cell system, these studies also indicate means by which to establish a more optimal analytical system for different elements. All data obtained for the elements under study have been categorized in terms of the Ll12, defined as that distance along the cell path where absorption will fall to one half of the maximal absorptivity. The Llp value of each element may be employed to select the optimal cell length for analysis. With the exception of cadmium, mercury, and zinc, the LllZ value for most of the elements is approximately 35-60 cm, and, hence, a length of 60 cm provides the optimal analytical sensitivity. Cells exceeding this length might well decrease sensitivity owing to the interference produced by the flame reactants in areas of the cell where specific absorption no longer takes place. At the temperature enployed here, long cells are not particularly advantageous analytically for the measurement of elements such as calcium and strontium which form involatile oxides and where absorption is limited to a small area of the flame as a consequence. The use of hotter flames for this purpose requires further study. The ring burner employed in these studies is very stable and, when used in conjunction with the absorption cell, virtually
isolates the flame path from the ambient air. As a consequence, outside air does not dilute the elemental vapor as in conventional open flames. Thus both greater efficiency and sensitivity are achieved while ready control of the oxidation reduction environment along the flame path is possible. Highly sensitive measurement of absorption in the far UV region, e.g. of As 1937 A (6), is also possible using a nitrogen or argon atmosphere with a minimal amount of air entrained to support combustion. These detailed studies have described the atomic absorption behavior of a number of elements as a function of the distance along the cell path. Several important parameters have been shown to determine absorption. These include the temperature, the vapor pressure of the various elements, and the chemical reactivity of the elements with other components of the combustion mixture. Additional studies of this type will be required to define completely the basic considerations pertinent to the atomic absorption of each element in the long pathlength system, and should lead to even greater applicability of the atomic absorption system for analytical purposes. RECEIVED for review January 22, 1970. Accepted April 20, 1970. Work supported by the International Lead Zinc Research organization, Inc., New York, N. Y .
ElectrochemicaI Behavior of Tr ipheny Itin Compounds and Their Determination at Submicrogram Levels by Anodic Stripping Voltammetry M. D. Booth and B. Fleet Department of Chemistry, Imperial College of Science and Technology, London, England
The electrochemical reduction of triphenyltin compounds in nonaqueous solution has been investigated by polarography, controlled potential electrolysis, and cyclic voltammetry and found to involve two 1-electron reductions. The first step involves the formation of a radical ion which is further reduced in the second step to the triphenyltin anion. The appearance of a prewave in the polarographic method indicated strong adsorption at the mercury surface of the radical ion. The phenomena has been utilized in the determination of trace amounts of triphenyltin compounds by anodic stripping voltammetry to a limit of detection of 10-8M. In addition, a procedure has been developed for determining triphenyltin compounds in potato crops.
IN RECENT YEARS, the interest in organotin compounds has been noticeably greater than in any other organometallic system ( I ) . They are finding increasing application as catalysts, stabilizers, and especially as biocides. The fungicidal properties of organotin compounds was discovered in 1954 (2) but the high phytotoxicity of the alkyl-tins prevented their use in agriculture. It was found, however, that the phytotoxicity of the corresponding aryl derivatives was much lower, and in (1) A. G. Davies, Chem. Brit., 4(9), 403 (1968). (2) G. J. M.Van der Kerk and J. G. A. Luijten, J . Appl. Chem., 4, 314 (1954).
the last ten years triphenyltin compounds have been widely used as fungicides (3). Methods of analysis for triphenyltin compounds have mostly been based on the determination of inorganic tin after breakdown of the complex (4), but a direct spectrophotometric procedure has been described by Hardon et al. (5) Electrochemical methods have been applied but again only for the determination of inorganic tin following breakdown of the complex (6-9). For the purposes of fungicide residue analysis this type of procedure is nonspecific. Triphenyltin compounds are reduced directly at the dropping mercury electrode, and this offered the possibility of a direct procedure. A general survey of the electrochemistry (3) K. R. S. Ascher and S. Nissim, World Rev. Pest Conrr., 3 (4),
188 (1964). (4) K. Burger, Report of the Farbewerke Hoescht A.-G., Gendor1958. ( 5 ) H. J. Hardon, H. Brunink, and E. W. Van Der Pol, Analyst, 85, 847 (1960). (6) S . Gorbach, and R. Bock, 2.Anal. Chem., 163,429 (1958). (7) J. Vogel and J. Deshusses, Helv. Chim. Acta, 47. 181 (19641. . , (8) Thompson-Hayward Chemical Company, Pest.. Anal. Manual Vol 11, 1967. (9) P. Nangniot and P. H. Martens, Anal. Chim. Acta, 24, 276 (1961). ANALYTICAL CHEMISTRY, VOL. 42, NO. 8, JULY 1970
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of organotin compounds has beencarriedout byDessyeta1. (10) using dimethoxyethane as solvent with tetrabutylammonium perchlorate as supporting electrolyte. These authors observed two well defined waves for the reduction of triphenyltin chloride with the second wave occurring at extremely negative potentials. They attribute the first wave to a one-electron process to form the radical which subsequently dimerizes, Pha Sn C1 e_,Ph3 Sn.
- 1.6V
L
+ C1-
Ph3 Sn Sn Ph3 The second wave they consider to be due to a 2-electron reduction of the dimer to form the triphenyltin anion Phs Sn-. The polarographic behavior of triphenyltin fluoride has also been studied (11)and in aqueous ethanol was found to give a one-electron, irreversible wave. These workers observed the adsorption of the electrolysis product and its resulting dimerization to form hexaphenylditin. In view of the importance of this class of compounds, it was felt that a more detailed investigation of the electrode process was justified, primarily with the aim of developing a more sensitive method of analysis. EXPERIMENTAL
Reagents. Samples of triphenyl stannic acetate (98% purity) and hydroxide (99%) were supplied by the Ministry of Agriculture, Fisheries and Food, Plant Pathology Laboratory, Harpenden, Herts, and the Ministry of Technology, Laboratory of the Government Chemist. All reagents used were of analytical reagent grade. Acetonitrile (Hopkin and Williams Ltd.) was purified by redistillation. Hexaphenylditin was purchased from Alpha Inorganic Chemical Inc. A 5.10-3M stock solution of each compound was prepared in absolute ethanol. Apparatus. Direct current polarograms were recorded on a Radelkis polarograph type OH-102 (Metrimpex, Hungary). A Kalousek cell with a separated saturated calomel electrode (SCE) was used. Capillary characteristics at 0.0 V in a 0.1M KC1 solution were t = 3.34 sec, m = 1.65 mg./sec at h = 55 cm. Alternating current polarograms were measured with a Univector and general purpose polarograph (Cambridge Ltd., London), with the Metrimpex polarograph being used as the current output recorder. Polarograms were recorded in a Heyrovsky cell with a Hg pool anode, Peak potentials were measured in a Kalousek cell. The characteristics of the dropping mercury electrode, measured as before, were t = 4.41 sec, m = 1.88 mg./sec at h = 75 cm. Cyclic voltammograms were measured with a Chemtrix polarograph (Model SS.P-2), Beaverton, Ore., using a three-electrode cell with a hanging mercury drop electrode (Inverse Polarography unit with micrometer adjustment, E410, Metrohm, Switzerland). A vitreous carbon electrode (Chemtrix) was also used. Controlled-potential electrolysis for microcoulometric measurements was carried out using the Radelkis polarograph as potentiostat, and a modified H cell (12). For identification of the electrolysis products and determination of n values, controlled-potential electrolysis at a mercury pool electrode was performed using a Harwell 2000 series type (10) R. E. Dessy, W. Kitching, and T. Chivers, J. Amer. Chem. Soc., 88, 453 (1966). (11) A. Vanachayangkul and M. D. Morris, Anal. Lett., 1 (14),
885-890 (1968). (12) B. Fleet and P. Zuman, Collect. Czech. Chem. Commun., 32, 2066 (1967). 826
ANALYTICAL CHEMISTRY, VOL. 42, NO. 8, JULY 1970
potentiostat (13) in conjunction with an integrator based on a GEC voltage to frequency converter (14). A HewlettPackard 3439A digital voltmeter equipped with a High Gain/ Auto Range unit type 3443A was used for current read-out, and the output pulses were counted on a Harwell 2117B Scaler. The coulometry cell consisted of the stirred mercury pool (Area = 10 cm2) as working electrode with platinum counter and SCE reference electrodes. For anodic stripping, the Chemtrix polarograph was used to supply both the applied potential for the pre-electrolysis and the voltage sweep for the stripping process. The electrode used was a rotated mercury coated platinum wire electrode, 1.6 cm long, A = 0.40 sq. cm. This was prepared by coating a rotating platinum electrode (Metrohm type EA 682) by the method of Joyce and Wescott (15). (Complete coverage of the electrode surface was checked voltammetrically by the absence of a significant current when the electrode was polarized to - 1.0 V os. SCE.) The cell employed for stripping has been described previously (16). The relative pH values of buffer solutions containing 50% v/v ethanol were measured with a Vibron pH meter model 39A (E.I.L. Ltd., Surrey). Nuclear magnetic resonance spectra were measured on a Perkin-Elmer R14 NMR spectrometer operating at 100 MHz. Infrared spectra were measured with a Perkin-Elmer Infracord spectrophotometer Model 137. Procedure. Solutions for dc and ac polarography and for cyclic voltammetry were made up in constant ionic strength buffer medium (acetic acid, ammonia, sodium hydroxide) containing 50% v/v ethanol. For dc polarographic and cyclic voltammetric measurements, 0.002% Triton X-100 was added. Solutions were deoxygenated with a stream of nitrogen for three minutes prior to measurement. For coulometric measurements at the mercury pool electrode, the solution contained 5.10-4M of the triphenyltin compound at an apparent pH of 7.3 (0.1M acetic acid/O.lM ammonia). The solution was deoxygenated prior to the electrolysis and a stream of nitrogen was passed through the cell throughout the experiment which was continued until the current had decayed to a nominally small and constant value. The product of the electrolysis was ether extracted and analyzed by NMR and IR spectrometry. For stripping voltammetry, aliquots of the appropriate stock solution were added to 0.3 ml of buffer solution (0.1M acetic acid 0.1M ammonia), 0.3 ml of 0.1% Triton X-100 and diluted to 15 ml so that the final solution contained 50 v/v ethanol. Solutions were deoxygenated by passage of nitrogen for 10 minutes prior to measurement. Procedure for Analysis of Potato Samples. A sample of five potatoes was taken from the batch supplied for analysis and the skins were removed. The skins were weighed and dried at 90 "C for 1 hour (melting point triphenyl stannic acetate 124 "C), to remove most of the water. They were then macerated with 200 ml of acetonitrile for 5 minutes, the liquor extract decanted off, and a further extraction with 100 ml of acetonitrile for 2 minutes carried out. The extracts were combined and passed down a 2-cm diameter column containing a 5-cm layer of anhydrous sodium sulfate to remove any remaining water and 2.5 cm of alumina to remove oils and starchy material. The flow rate through the column was of the order of 100200 ml/min. The column was finally washed with a further 50 ml of acetonitrile. (13) G. Phillips and G. W. C. Milner, Analyst, 94, 833 (1969). (14) General Electric Company, U.S.A., Transistor Manual, 7th Ed., p 346, 1964. (15) R. J. Joyce, and C. C. Westcott, Symposium on Trace Characterization, National Bureau of Standards, Washington, D. C., 1966. (16) M. J. D. Brand and B. Fleet, J . Polarog. SOC.,13 (2), 77 (1967).
I
c
Figure 1. pH dependence of 5.10-4M triphenyltin compound in 50% v/v ethanol, 0.002% Triton X-100 as maximum suppressor pH (1) 3.05; (2) 4.75; (3) 7.3; (4) 9.6; (5) 12.45; (6) 1M NaOH. Start potentials: (1) -0.3; (2 and 3) -0.4; (4, 5, and 6) -0.5 V us. SCE
S I
200 mV
3.0
I
I
0.6 p A
The volume of the eluent was reduced to less than 10 m l using a Kuderna evaporator. Further 100-g samples were taken from the first 1/4-inchsurface layer and from the bulk of the potato, and treated as before.
2.5
RESULTS AND DISCUSSION
General Polarographic Behavior. The reduction of both triphenyl stannic acetate and the hydroxide in the protogenic ethanol-water medium shows the same polarographic behavior, three waves being observed over a wide pH range (Figure 1). A large maximum initially obscures the third wave (Figure 2) but this is easily suppressed by the addition of a surface active agent-e.g., Triton X-100. The pH dependence of the waves is shown in Figure 1. The height of the first two waves is virtually independent of pH. The third wave, initially observed at pH 4.75, reaches a maximum at pH 7.3 and then decreases in the form of a dissociation curve, being accompanied beyond pH 9.6 by the appearance of a pronounced minimum at about -1.65 V. Addition of more Triton X-100 has no significant effect on this minimum. All waves are found to be relatively time independent over the pH range 5-8. An optimum pH of 7.3 was chosen for further study in order to characterize the electrode process and for the development of an analytical procedure. At this pH, the first wave is concentration dependent over the range 10-5 and lO-4M and independent above 10-4M, and shows a linear relationship between current and mercury reservoir height. The second wave is only present above 10-4M and is concentration dependent up to lOU3M. The third wave exists over the whole concentration range and shows a linear dependence on concentration up to 5.10+M; above this concentration, the height of the wave reaches a limiting value. The second and third waves were shown to be diffusion controlled by the dependence of the wave height on the square root of the height of the mercury reservoir. The dependence of the half-wave potential on pH is shown in Figure 3 for a 5.10-4M solution of triphenyl stannic hydroxide in 50 v/v ethanol. Both waves I and I1 show some pH dependence. Wave 111, however, is pH independent, indicating that the reaction involves electron transfer prior to hydrogen ion addition. An electrocapillary curve is shown in Figure 2. The lowering of the surface tension which is characteristic of adsorption processes occurs over the potential range -0.6 to - 1.4 V, the latter potential as will be shown later coinciding with the formation of the second electrolysis product. The adsorption character of wave I was further substantiated by the characteristic shape of the i-f curve.
Figure 2. Electrocapillary curves: A 0.1M acetic acid/O.lM ammonia buffer (pH 7.3) in 50% v/v ethanol. o 5.10F4M triphenyltin compound in 50% v/v ethanol, pH 7.3. Polarographic waves of 5.10-4M triphenyltin compound in 50% v/v ethanol at pH 7.3. Start potential, -0.0 V. us. SCE
-1.4
-1.3
.-m 5
. I -
-g
g c _m
I
-1.2 -1.1 -1.0 -0.9 -0.8 -0.7 -0.6 -0.5
-
~
Wave Ill
-
-
:i!L-0.1
3
4
5
6
7
8
9 1 0 1 1 1
PH
Figure 3. Dependence of half-wave potentials on pH
It is therefore concluded from the concentration and reservoir height dependence, together with the shape of the electrocapillary curve, that the first and second waves (I and 11) correspond to the same electrode reaction and that the product of this reaction (the adsorption wave I precedes the diffusion wave 11) is adsorbed onto the electrode surface. The influence of several cations and anions on the shape of the polarographic waves was next studied. Lithium, barium, and lanthanum ions (0.1M as chloride) had no effect but the addition of tetrabutylammonium ion resulted in the disappearance of wave I11 and caused wave I1 to become more drawn out. Since the tetrabutylammonium ion is specifically adsorbed on the electrode and populates the inner Helmholtz layer, then this effect may be accounted for in terms of a competition between Bu4N+ and PhlSn. for the available electrode surface. The anions chloride, bromide, and iodide had no effect on the waves. The effect of varying the conANALYTICAL CHEMISTRY, VOL. 42, NO. 8, JULY 1970
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Mechanism of Electrode Process. From the evidence obtained, it is possible to postulate a scheme for the electrode process.
c-c
I
200 mV
1.6PA
Ph3Sn+
A
-Ph3SnH
/-
Figure 4. Coulometry at -1.55 V ES. SCE 5.10-4M triphenyltin compound in 50% v/v ethanol at pH 7.3 with 0,002z Triton X-100 as maximum suppressor. Polarographic waves before (I, 11, 111) and after coulometry (IV). Start potential, +0.2 V us.SCE
Table I. Coulometrically Determined Number of Electrons Consumed in Reduction of Triphenyl Stannic Hydroxide Depolarizer in 0.1M acetic acid/O.lM ammonia buffer; 0.002% Triton X-100; 50% ethanol
Working electrode Mercury pool DME
Reduction 1 (Ph3Sn++ Ph3Sn.) n 1.03 0.99 0.98 1.19
Overall reduction process n 1.91 1.88
...
centration of ethanol was studied but no improvement in the shape of the waves was discernible. The number of electrons transferred in the first electrode process was roughly estimated by comparing the combined height of waves I and I1 with the wave for equimolar solutions of benzaldehyde (n = I) and benzil (n = 2) in 50% ethanol at pH 4.75 (acetic acid-ammonia buffer). The results indicated that n = 1. This was confirmed by coulometric determination of the number of electrons transferred at the mercury pool electrode. The values obtained for triphenyl stannic hydroxide are shown in Table I. Electrolysis at a potential corresponding to the plateau of wave I1 (- 1.1 V us. SCE) initially caused both waves I1 and I11 to decrease, while the height of the first wave (I) remained constant, This situation persisted until wave I1 had disappeared; waves I and I11 then decreased together. A polarogram of the solution after electrolysis showed no anodic wave. Similar results were obtained using microcoulometry at the dropping mercury electrode. Electrolysis at a potential of -1.55 V us. SCE (plateau of wave 111) at the DME and the mercury pool showed that both waves 11 and 111 decreased but in this case polarography of the resulting solution shows the appearance of an anodic wave ( E l l 2 -0.4 V us. SCE) Figure 4. By comparison with the total wave height of the original solution, this wave appeared to involve a 2-electron step, The height of this anodic wave decreased slowly on standing; a polarogram of this solution after 12 hours showed complete reversion to the original compound. 828
ANALYTICAL CHEMISTRY, VOL. 42, NO. 8, JULY 1970
+e
'c-e
Ph,Sn'
v.
%
Ph3Sn-
Ph3Sn-SnPh3 -2 - u +
The formation of the triphenyltin radical in the first step is in accordance with earlier observations (IO,11)and was confirmed by coulometric measurements as well as by the fact that the half-wave potential was independent of pH. This radical can be further reduced to form the triphenyltin anion Ph3Sn- but can also undergo a competing side reaction to form the dimer PhrSn-SnPh3. Evidence for the formation of the dimeric species was obtained from infrared spectrometric examination of the product obtained from controlled potential coulometry at a potential corresponding to the plateau of wave 11. The spectrum obtained was identical with that obtained from an authentic sample. It was impossible to differentiate between the monomeric and dimeric species using ultraviolet spectrometry (IO). As further evidence for the existence of a competing side reaction involving inactivation of the free-radical formed in the first step, it was observed that the concentration dependence of wave I11 reached a limiting value at higher concentrations, despite the fact that overall diffusion control of the wave had been demonstrated. The dimeric species was also shown to be electroinactive from dc polarographic and cyclic voltammetric measurements. This is at variance with the findings of Dessy et al. (IO),who attribute the second wave to the reduction of hexaphenylditin. Thus Dessy proposes an overall reaction scheme which can be represented as;
A7i C-D
the rapidly inactivated product of the first step reacts to form the dimeric species C which is electroactive in the same direction as the form A. The total limiting current of this type of system consists of two components; a diffusion controlled current due to the transition A+B and a current due to the transition C-tD. This latter step should exhibit some degree of kinetic control which should be reflected in the shape of the wave (17). In any case, it is very unlikely that the product of the first step would be electroinactive over the whole potential range. The present study has shown that the dimer itself is electroinactive and that the overall reaction scheme can more accurately be represented as;
A--LB-C
UD where the competing side reaction B+D is an irreversible chemical step. That the product is the triphenyltin anion Ph is confirmed from the one-electron nature of the reduction (dc polarography and coulometry) and by the pH independence of the half-wave potential of wave 111. (17) A. A. Vlcek, CoNect. Czech. Chem. Commun., 22, 1736 (1957).
9 " I
200 mV
c t
I
Figure 5. Cyclic voltammogram of 5.10-4M triphenyltin compound at pH 7.3 in 50% v/v ethanol, 0.002%Triton X-100. Start potential, -0.1 V DS. SCE. Scan rate, 200 mV/sec
The identification of triphenyltin hydride as the product of controlled potential electrolysis at a potential corresponding to the plateau of wave I11 was confirmed by NMR spectrometry (18); the second step in the reduction is therefore a oneelectron transfer to form an anionic species which subsequently undergoes protonation. Electrolysis of this solution at a potential corresponding to the anodic wave for the triphenyltin hydride regenerates the original triphenyltin cation. The apparent irreversibility of wave I1 would appear to be due to a distortion of the wave caused by the adsorption process. Values of the transfer coefficient (a)which range from 0.66 to 0.90 also reflect this anomaly. The appearance of a minimum at pH values above 9.6 can be attributed to inhibition of the electrode process due to adsorption of the dimer formed in the first step. Cyclic Voltammetry. Cyclic voltammetric measurements at the HMDE were also used to confirm the mechanism of the electrode process (Figure 5). A steady state voltammogram with a depolarizer concentration of 5.10-4M and a scan rate of 200 mV sec-1 shows several cathodic and anodic peaks. Peaks Ia and Ib correspond to the adsorption/desorption of the free radical product formed in the first step. The peaks IIa and IIb show a peak separation of 70 mV which is slightly greater than the theoretical value for a reversible one-electron reduction. This slight irreversibility in the electrode process would also appear to be a consequence of the adsorption of the radical. Peak I11 is due to the one-electron reduction of the triphenyltin radical to form the triphenyltin anion PhBn-. The rapid deactivation of the species causes the overall reaction to be irreversible. The anodic peak (IV) corresponds to the two-electron oxidation of triphenyltin hydride to form the original triphenyltin cation. Analytical Determination. The preliminary investigation of the electrochemical reduction of the triphenyltin ion indicates the optimum conditions for the determination of this species by several electrochemical techniques. Direct Current Polarography. The general polarographic behavior shows that the best definition of the waves for analytical determination are obtained in a 50% v/v ethanol(18) M. L. Maddox, N. Flitcroft, and H. D. Kaesz, J. Organometal. Chem. 4, 50 (1965).
5
I
Figure 6. Dependence of ac peak height on concentration for triphenyltin compound at pH 7.3 in 50% v/v ethanol
Table 11. Dependence of Limiting Current on Concentration for DC Polarographic Waves of Triphenyltin Compounds at pH 7.3 in 50% V/V Ethanol Containing 0.002% Triton X-100 Concentration, Wave height, pA Molar I I1 2 x 10-6 0.04 ... 4 x 10-5 0.06 ... 6X 0.08 ... 8X 0.12 ... 1 x 10-4 0.14 ... 2 x 10-4 0.21 0.21 4 x 10-4 0.21 0.66 6X 1.11 0.21 8 x 10-4 0.21 1.56 1 x 10-3 0.21 1.93
water medium containing 0.1M buffer (0.1Macetic acid, 0.1M ammonia) and 0.002% Triton X-100. The apparent pH of the medium is 7.3. Both the adsorption pre-wave and the first reduction step (11) can be utilized. The pre-wave shows a rectilinear dependence of wave height upon concentration over the range l.10F5 to l.10-4M. Wave I1 shows a concentration dependence over the range 1.10-4 to l.lO-aM Table 11. Alternating Current Polarography. The reversibility of the first step in the reduction indicated the possible utility of the ac technique. Employing the same solution conditions as for dc polarography except for the omission of the surface active agent, the ac polarograms showed two peaks over the concentration range l.10-3 to l.10-4M, with peak potentials -0.68 and -0.98 V us. SCE. These peaks are attributed to a tensammetric adsorption process (peak I) and peak I1 to the first reduction step (dc wave 11). The concentration dependence of the two peaks is in accordance with earlier dc polarographic observations, i.e., the height of peak I is independent of concentration whereas peak I1 shows a rectilinear dependence over the concentration range studied (Figure 6). Below 10-4M, peak I1 disappears and the height of tensammetric peak (I) shows a rectilinear dependence on concentration down to a lower limit of detection of 5.10PM. The use of the two peaks thus enables analytical measurements to be made over the concentration range 5.10+ to l.10-3M (Figure 6). Stripping Voltammetry. Anodic stripping voltammetry ANALYTICAL CHEMISTRY, VOL. 42, NO. 8, JULY 1970
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