The role of thiourea as additive for solving medium-modification problems in potentiometric stripping analysis.
Thiourea (TU) has been studied as a possible additive for medium modification in order to modulate ion sensitivity in PSA (potentiometric stripping analysis). TU has three effects: (a) chelation with the metal ion and/or oxidizing agent; (b) specific adsorption at the mercury thin-film electrode (in the 10(-4)M TU range); (c) diffusional electron transfer during the electro-deposition and redissolution steps in a PSA measurement. These effects were studied by PSA, ASV (anodic stripping voltammetry) and CA (chronoamperometry). For TU concentrations higher than 10(-3)M, electron-transfer by the additive is the predominant factor affecting sensitivity; this effect depends upon the value of the ion-stripping potential. For TU concentrations lower than 10(-3)M, complexation with the metal ion and/or oxidizing agent is the main factor affecting sensitivity. Appropriate choice of the concentration of TU therefore makes it possible to enhance or to lower the specific ion sensitivity, and even to mask an ion with respect to other ions. Examples are given for Zn(2+), Cu(2+), Cd(2+), Tl(+), Bi(3+), and an extensive study was performed with Pb(2+).
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Laboratoire de Chimie Analytique et de Spectrométrie de Masse, Facultés Universitaires Notre-Dame de la Paix, 61, rue de Bruxelles, B 5000 Namur, Belgium.
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