Catalytic hydrogen evolution in cathodic stripping voltammetry on a mercury electrode in the presence of cobalt(ii) ion and phenylthiourea or thiourea

Abstract

The system Co(II)-phenylthiourea (PTU)-borax buffer was investigated by cathodic stripping voltammetry (CSV) at a hanging mercury drop electrode. The results of the voltammetric measurements showed that the presence of both PTU and Co(II) gives rise to a new irreversible peak at about -1.5 V. Based upon our previous results obtained in the study of other sulfur compounds and the sulfide ion itself, the peak was ascribed to the catalytic hydrogen evolution superimposed on the reduction of the coordinated Co(II) ion. The catalyst itself is a Co(II) complex with the sulfide ion produced by the decomposition of the analyte during the deposition step. The influence of PTU and cobalt concentration, accumulation conditions and stripping parameters was investigated and complementary data on thiourea are included. The results showed that the measurement of the catalytic hydrogen evolution peak current can be used as a basis for a simple, accurate and rapid method for the determination of PTU within the concentration range 10-100 nM. The catalytic method is relatively free of interferences and could be a suitable alternative for cases in which the stripping peak due to mercury ion reduction in the accumulated mercury compound is disturbed by some interference.

Determination of Ethylenethiourea (ETU) at Trace Levels in Water Samples by Cathodic Stripping Voltammetry

“…Proof for this hypothesis was obtained by investigating the voltammetric behavior of ETU in the presence of Co(II) ions. As already reported by Spã taru and Bã nicã [36], the catalytic hydrogen evolution occurs in the presence of both thiourea and Co(II) ions, which give rise to a new irreversible peak at about À 1500 mV similar to that produced by the sulfide anion itself [41]. The same catalytic effect can be also observed for phenylthiourea [36] and methylthiohydantoin-glicine [37] in the presence of Co(II) ions.…”

“…As already reported by Spã taru and Bã nicã [36], the catalytic hydrogen evolution occurs in the presence of both thiourea and Co(II) ions, which give rise to a new irreversible peak at about À 1500 mV similar to that produced by the sulfide anion itself [41]. The same catalytic effect can be also observed for phenylthiourea [36] and methylthiohydantoin-glicine [37] in the presence of Co(II) ions. According to the authors, the catalyst itself is a Co(II) complex with the sulfide ion produced by the decomposition of these analytes during the deposition step.…”

“…The cathodic stripping voltammetry (CSV) at the hanging mercury drop electrode (HMDE) allows the TU determination at trace levels, as already described by Stará and Kopanica [34] and Smyth and Osteryoung [35]. Alternative methods for the determination of TU and its derivatives by CSV based on the catalytic hydrogen evolution in the presence of Co(II) were described recently by Spã taru and Bã nicã [36,37]. However, to the best of our knowledge, there is no paper devoted to the stripping voltammetry to determine ETU.…”

“…The interference of other metal ions like Zn(II), Pb(II), Fe(II), Mn(II), Ni(II), Co(II) and Cr(III) was also tested, since metal ions may split the stripping peak due to the metal ion assisted reduction of mercury in the surface layer [36,50]. However, no interference of these metal ions was observed on the ETU stripping signal in concentrations up to 500 mg L…”

New Electrocatalytic Reactions at a Mercury Electrode in the Presence of Homocysteine or Cysteine and Cobalt or Nickel Ions

“…In support to the hypothesis of the catalysis by a metal sulfide, it should be pointed out that, under similar conditions, a peak alike to K was detected with solution containing the HS À ion [39], thiourea [35,40] or thiourea derivatives [40,41] (that act as sources of sulfide ion). However, due to its higher lability, thiourea yields such a peak at a concentration about 100 times lower than that of Hcy.…”