THEORY AND APPLICATION OF MULTIPLE SEMIDIFFERENTIAL ELECTROCHEMICAL STRIPPING ANALYSIS WITH THIN MERCURY FILM FORMED IN SITU

A theory describing the curves of 1.5 th order derivative e' and 2.5 th order derivative e of the current i vs. the electrode potential E in anode stripping processes at thin mercury film electrode formed in situ on the rotating glassy carbon disk electrode is presented. The equations for 1.5 th order derivative and 2.5 th order derivative were derived from the semidifferential equation. ALGOL-121 language was used for programming the equations, and the results were processed by a digital computer. In the e' and e vs. E curves, the heights between the negative peak and the positive one are e'_(pp)=0.426((n~(3.5)F~(3.5)v~(2.5)At_pD_o~(2/3)N~(1/2))/(v~(1/6)R~(2.5)T~(2.5)))/C_o and e_(pp)=0.658((n~(4.5)F~(4.5)v~(3.5)At_pD_o~(2/3)N~(1/2))/(v~(1/6)R~(3.5)T~(3.5)))/C_o the positive peak potentials are E'_(pp)=E_(1/2)+0.496((RT)/(nF))+1.15((RT)/(nF))logH and E_(pp)=E_(1/2)-0.273((RT)/(nF))+1.15((RT)/(nF))logH the widths between the negative peak and the positive one are W'_(pp)=2.12((RT)/(nF)) and W_(pp)=1.46((RT)/(nF)) The ratics of e'_(pp) and e_(pp) to the peak current i_p are (e'_(pp)/(-i_p))=0.924((nFv)/(RT))~(1.5) and (e_(pp)/(-i_p))=1.43((nFv)/(RT))~(2.5) Experimental results on the analog circuit for multiple semiderivative are in agreement with the theory. A method based on this 1.5 th order derivative for the determination of trace Cu~(2+), Pb~(2+), Cd~(2+) and Zn~(2+) in sea water is proposed. The e'_(pp) values are in linear proportion to the trace concentration of the heavy metals, when the other parame- ters remain constant. The e'_(pp)/i_p values for Pb (Hg), Cd (Hg) and Zn (Hg) at a scan rate of 100mV/s at 25℃ were 20, 19 and 18 respectively. These values are close to the theoretical values of 20. Comparing with the results obtained previously by us using the anodic stripping semidifferential electroanalysis, this method gives much better sensitivity, resolution and shorter time of pre-electrolysis. The relative standard deviation obtained were less than ±3%.