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Anal. Chem. 1084,56,2392-2396
(11) Sawyer, D. T.; Roberts, J. L. “Experlmental Electrochemlstry for Chemists”; Wiley: New York, 1974. (12) Chrlstle, J. H.; Osteryoung, R. A. Anal. Chem. 1076, 48, 869-872. (13) Whltfleld, M., Jagner, D.,EdS. “Marlne Electrochemlstry”; Wlley: ChiChester, 1981; Appendix, pp 505-514. (14) Moffatt, J. M.; Zlka, R. G. Mar. Chem. 1083, 13. 239-251. (15) Wake, T. D.; Morel, F. M. M. Anal. Chem. 1083, 55, 1268-1274. (16) Whltfield, M. “Ion-selective Electrodes for the Analysis of Natural Waters”; Australlan Marlne Sciences Assoclatlon: Sydney, 1971; AMSA Handbook No. 2.
(17) Batley, G. E.; Florence, T. M. J . Electroanal. Chem. 1074, 55, 23-43.
for redew April 2, 1983. Resubmitted and accepted June 22,1984. Presented in part at the J. Heyrovsky Memorial Congress on Polarography,Prague, Czechoslovakia, 1980. This work was supported by the Department Of the Environment.
Examination of Interferences in the Stripping Voltammetric Determination of Trimethyllead in Seawater by Polarography and Mercury4 99 and Lead-207 Nuclear Magnetic Resonance Spectrometry A. M.Bond,* J. R. Bradbury, P. J. Hanna, G. N. Howell, H. A. Hudson, and S t a n Strother School of Sciences, Deakin University, Waurn Ponds, Victoria 321 7,Australia
M.J. O’Connor Department of Inorganic and Analytical Chemistry, La Trobe University, Bundoora, Victoria 3083,Australia
Detailed investigations using polarography and pulse Fourier transform multinuclear magnetic resonance spectrometry show that mercury( I I ) ions react with trimethyllead cations in seawater and other aqueous media according to the equation Hg2+ [(CH,),Pb]+ CH,Hg+ Pb2+ 2CH,.. This reaction may invalidate proposed methods for determining trimethyllead by anodic stripplng voltammetry at a glassy carbon electrode (in situ deposition of mercury) since mercury( I I ) ions are added in relatively high concentrations to the sample solution as part of the analytical procedure. While stripping techniques at the hanging mercury drop electrode do not suffer from this kind of Interference, under oxygen-free condltlons, transalkylation reactions at the electrode surface interfere with the direct determination of [(CH3),Pb]+ in the presence of inorganic Pb(I I ) and double standard addition experiments are required to simultaneously determine both forms of lead. These experiments are inherently inaccurate and chromatographic or other separation techniques are recommended as a precursor for determining [(CH3)3Pb]+in the presence of Inorganic Pb( I I ).
+
-
+
+
The concentration of lead in natural water systems such as seawater is extremely low (1-4);hence methods for lead determination require extremely sensitive analytical techniques. For this reason anodic stripping voltammetry (ASV) is commonly employed because it can be used directly to determine concentrations of less than 1 ppb. Lead can exist in organic and inorganic forms. The toxicity of lead is species dependent (5,6); for example, the alkylated lead compounds such as those used as additives in gasoline are more toxic than the ionic form. Thus considerable interest exists in developing techniques to distinguish between the various forms of lead (7-10). Electroanalytical techniques have been widely proposed for speciation studies of metals in general (11-14). In the particular case of lead some reports have been published (15-19) 0003-2700/84/0356-2392$01.50/0
which appear suitable for distinguishing inorganic from organic forms of lead. The most commonly employed ASV method for determining ionic lead (subsequently referred to as Pb(I1) without implying the form) in seawater was developed by Florence (3). In this method mercury(I1)ions are added to the solution prior to the electrolysis step, after which simultaneous deposition of mercury and lead occurs in situ on a glassy carbon electrode (GCE). This step is followed by anodic stripping of the lead. Since alkylated lead compounds are reduced at more negative potentials than inorganic lead (20),Hodges et al. (17, 18) have claimed that the Florence method can be modified by varying the deposition potential to determine the various forms of lead. However, a recent exchange of comments by various authors (21,22) has raised the issue of changing a system of metal species in natural waters by the addition of materials such as Hg(I1). Butin et al. (23)have described the redistribution reactions of organometallic compounds with Hg(I1). This information implies that exchange reactions might be expected to occur as a consequence of adopting the Florence method although no data are available in seawater to confirm this hypothesis. In the present investigation we have used the technique of lBHg and 207Pbpulse Fourier transform nuclear magnetic resonance (NMR) directly in seawater to ascertain if a reaction does occur between Hg(OAc)2and [(CH,),Pb]+. The NMR technique is restricted to high concentrations but this novel use of multinuclear NMR is invaluable in determining the nature of the products since both Ig9Hgand 207Pbhave a nuclear spin of 1/2 and have ideal nuclei for this technique now that Fourier transform techniques are readily available. Electrochemical methods were used to monitor the same reactions a t the much lower concentrations relevant to trace analyses and implications to the modified Florence method are described. In the absence of exchange reactions with deliberately added Hg(II), it may be possible to use a mercury electrode. However, a recent detailed investigation (20)of the electrochemical 0 1984 American Chemlcal Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 13, NOVEMBER 1984
2303
Table I. ao7PbNMR Data compound
solvent
(CH&PbOAc
glacial acetic acid glacial acetic acid HzO 0.59 M NaCl seawater seawater H2O CH2C12 seawater
(CH3)3PbCl PbClz
concn, M 0.8 0.8 0.2 0.02 0.02 0.05 0.6
saturated saturated
6(207Pb)," ppm
Wll2,b Hz
comments
+402 +408c +473 +442 +440 +439 +440d +432e -1163
50 6 39 60 38 31
decet, J Z O ~= ~ 78Q Hz~
= 78 HZ 78 HZ
J207pb-I~
J 2 0 7 p b . l ~=
119
