Talanra, Vol. 31, No. 12, pp. 1069-1073, 1984 Printed in Great Britain. All rights reserved 0039-9140/84 $3.00 + 0.00 Copyright 0 1984 Pergamon Press Ltd zyxwvut DIRECT LEV ELS DETERMINATION OF SUB-NANOMOLAR OF ZINC IN SEA-WATER BY CATHODIC STRIPPING VOLTAMMETRY CONSTANT M. G. VAN DEN BERG Department of Oceanography, University of Liverpool, Liverpool, England zyxwvutsrqponmlkjihgfe (Received 19 March 1984. Revised 3 May 1984. Accepted 19 June 1984) Summary-A novel technique for the determination of nanomolar levels of zinc in aqueous solution is presented. The zinc complex with ammonium pyrrolidine dithiocarbamate is adsorbed on a hanging mercury-drop electrode and the reduction current of zinc is measured by voltammetry. The detection limit for zinc is 3 x IO-“M. with IO-min collection time. A procedure is suggested for the simultaneous determination of Ni anh Zn in a single sample. The conventional electrochemical determination of trace amounts of zinc in aqueous solution is by anodic stripping voltammetry (ASV). The working electrode is then either a hanging mercury-drop elec- trode (HMDE), or a solid electrode covered with a mercury film. ‘x2 The limit of detection for zinc is 2-4 x 10e91ul after a plating time of 15-60 min with an HMDE,3,4 and lower by a factor of about ten if a rotating disk electrode (RDE) is used, with in situ plating of mercury.’ The determination of zinc is adversely affected by the formation of intermetallic compounds with Cu, Ni and CO,‘.~ especially when thin-film techniques are used. The half-wave potential of Zn’+ is at - 1.1 V (us. SCE) in non-complexing electrolytes,6 so the solution needs to be maintained at pH > 4 to prevent masking of the zinc oxidation current by the hydrogen wave. Recently it has been found possible to determine low concentrations of several trace metals in aqueous solution by means of adsorption voltammetry (AV). This technique is based on the tendency of certain complexes to adsorb on the surface of the HMDE. The reduction current of the metal ions in the ad- sorbed complex is used as a sensitive measure of the solution concentration. Thus low concentrations of Cu(II), U(VI), V(V) and Fe(II1) can be determined by complexation with catechol,7.8 and of Ni(I1) and Co(H) by complexation with dimethylglyoxime (DMG),9,‘o with limits of detection typically near lO_‘OM. The results of a study of the possible determination of trace quantities of Zn in sea-water by means of AV are reported in this paper. Several complexing ligands were tested in preliminary experiments and it was found that reduction peaks for both zinc and nickel in sea-water are obtained with &hydroxy- quinoline as complexing agent. However, the concentration-reduction current relationship was non-linear for both metals, although the sensitivity (reduction current/metal concentration) was suf- ficient for analysis of unpolluted sea-water. Com- plexes of zinc with catechol were also found to adsorb on the HMDE but the sensitivity was low (no peak from clean sea-water). Aqueous solutions of ammo- nium pyrrolidine dithiocarbamate (APDC) were found to produce a marked reduction peak in the presence of low concentrations of zinc. It is proposed in this paper to use this property for the detection of dissolved zinc in water and sea-water. EXPERIMENTAL Equipment Voltamperograms were recorded with a PAR 174 A polarograph in conjunction with a PAR 303 static mercury- drop electrode in the stationary-drop mode; the surface area of the mercury drop was 2.83 mm’. The internal clock of the polarograph &as altered to allow for a faster pulse rate of 10 Hz. The reference electrode was Ag/saturated AgCl, KCI. For some experiments a home-made-water-jacketed electro- chemical cell was used, containing 60ml of liquid, which allowed simultaneous pH-measurement. The solution was stirred with a Teflon-coated magnetic stirring bar. The pH was measured with a Radiometer PHM 64 Research pH- meter with a combined pH/calomel reference electrode, calibrated for the free hydrogen-ion scale with a solution of O.OlM hvdrochloric acid in 0.69M sodium chloride. The experiments were performed in a “clean room”, equipped with filtered-air conditioning. Reagents An aqueous O.lM APDC (“Spectrosol”, BDH) stock solution (100 ml) was prepared and traces of metal ions were removed’ by extra&on- with three lo-ml portions of 1.1.2~trichlorotrifluoroethane. A stock aqueous O.lM DMG , solution was prepared in 0.2M sodium- hydroxide. Buffer solutions were prepared from N-2_hydroxyethylpiperazine- N’-2-ethane sulphonic acid (HEPES), piperazine-NJ’- bis(Zethane sulphonic acid) (monosodium salt) (PIPES), and NJ’-bis(2-hydroxyethyl)-2-aminoethane sulphonic acid (BES): each stock buffer solution contained 1 mole of buffer agent and 0.5 mole of ammonia (“Aristar”) per litre. Metal contamination in these buffers was removed by addition of IO-‘M APDC and extraction with 1,1,2-trichlorotrifluoroethane. A stock buffer solution (1M boric acid/0.4M ammonia; pH 8.5) was used without further purification. Stock aqueous solutions of zinc, nickel and cobalt were prepared by dilution of BDH atomic-absorption (“Spectrosol”) standard solutions. Sea-water used for these 1069