Cathodic stripping methods

Schwartz, J., Henze, H. and Thomas, F.G. (1995) Voltammetric speciation of butyltin compounds. 1. An adsorptive cathodic stripping method. Fresenius J. Anal. Chem., 352, 474 178. 

Many other variations of the stripping technique have been developed. For example, a number of cations have been determined by electrodeposition on a platinum cathode. The quantity of electricity required to remove the deposit is then measured coulometrically. Here again, the method is particularly advantageous for trace analyses. Cathodic stripping methods for the halides have also been developed in which the halide ions are first deposited as mercury(I) salts on a mercury anode. Stripping is then performed by a cathodic current. 

Adsorption stripping methods are quite similar to the anodic and cathodic stripping methods we have just considered. Here, a small electrode, most commonly a hanging mercury drop electrode, is immersed in a stirred solution of the analyte for several minutes. Deposition of the analyte then occurs by physical adsorption on the electrode surface rather than by electrolytic deposition. After sufficient analyte has accumulated, the stirring is discontinued, and the deposited material is determined by linear-scan or pulsed voltammetric measurements. Quantitative information is based on calibration with standard solutions that are treated in the same way as samples. 

Aleksandrova and Kletenik [49] proposed voltammetric method for the determination of chloride concentration in different water samples. They use renewable silver working electrode. Both direct anodic voltammetry and cathodic stripping voltammerty could be successfully used in 1 M sulfuric acid. In the case of cathodic stripping method, 3 min per electrolysis step was employed at —0.07 V to obtain silver chloride film at the electrode surface. 

Rue E. L. and Bruland, K. W. (1995). Complexation of iron (111) by natural organic ligands in the Central North Pacific as determined by a new competitive ligand equilibration/adsorptive cathodic stripping voltammetric method. Mar. Chem. 50,117-138. 

Banica et al. [7] studied the effect of adding Ni(II) as an aid in the detection of penicillamine. The catalytic-Ni(II) method enables detection of the analyte with better selectivity compared to other indirect cathodic-stripping-voltammetric methods. 

To date, a few methods have been proposed for direct determination of trace iodide in seawater. The first involved the use of neutron activation analysis (NAA) [86], where iodide in seawater was concentrated by strongly basic anion-exchange column, eluted by sodium nitrate, and precipitated as palladium iodide. The second involved the use of automated electrochemical procedures [90] iodide was electrochemically oxidised to iodine and was concentrated on a carbon wool electrode. After removal of interference ions, the iodine was eluted with ascorbic acid and was determined by a polished Ag3SI electrode. The third method involved the use of cathodic stripping square wave voltammetry [92] (See Sect. 2.16.3). Iodine reacts with mercury in a one-electron process, and the sensitivity is increased remarkably by the addition of Triton X. The three methods have detection limits of 0.7 (250 ml seawater), 0.1 (50 ml), and 0.02 pg/l (10 ml), respectively, and could be applied to almost all the samples. However, NAA is not generally employed. The second electrochemical method uses an automated system but is a special apparatus just for determination of iodide. The first and third methods are time-consuming. 

Luther et al. [92] have described a procedure for the direct determination of iodide in seawater. By use of a cathodic stripping square-wave voltammetry, it is possible to determine low and sub-nanomolar levels of iodide in seawater, freshwater, and brackish water. Precision is typically 5% (la). The minimum detection limit is 0.1 - 0.2 nM (12 parts per trillion) at 180 sec deposition time. Data obtained on Atlantic Ocean samples show similar trends to previously reported iodine speciation data. This method is more sensitive than previous methods by 1-2 orders of magnitude. Triton X-100 added to the sample enhances the mercury electrode s sensitivity to iodine. 

Van den Berg [131] used this technique to determine nanomolar levels of nitrate in seawater. Samples of seawater from the Menai Straits were filtered and nitrite present reacted with sulfanilamide and naphthyl-amine at pH 2.5. The pH was then adjusted to 8.4 with borate buffer, the solution de-aerated, and then subjected to absorptive cathodic stripping voltammetry. The concentration of dye was linearly related to the height of the reduction peak in the range 0.3-200 nM nitrate. The optimal concentrations of sulfanilamide and naphthyl-amine were 2 mM and 0.1 mM, respectively, at pH 2.5. The standard deviation of a determination of 4 nM nitrite was 2%. The detection was 0.3 nM for an adsorption time of 60 sec. The sensitivity of the method in seawater was the same as in fresh water. 

Platinum was determined in seawater by adsorptive cathodic stripping voltammetry in a method described by Van den Berg and Jacinto [531]. The formazone complex is formed with formaldehyde, hydrazine, and sulfuric acid in the seawater sample. The complex is adsorbed for 20 minutes at -0.925 V on the hanging mercury drop electrode. The detection limit is 0.04 pM platinum. 

Van den Berg [620] also reported a direct determination of sub-nanomolar levels of zinc in seawater by cathodic stripping voltammetry. The ability of ammonium pyrrolidine dithiocarbamate to produce a significant reduction peak in the presence of low concentrations of zinc was used to develop a method capable of achieving levels two orders of magnitude below those achieved with anodic stripping voltammetry. Interference from nickel and cobalt ions could be overcome by using a collection potential of 1.3 V, and interference from... 

Cysteine and cystine has been determined in seawater by a method based on cathode stripping voltammetry of the copper complex [327]. 

A method for Pb and other metals in seawater consists of an adsorption step of the complex with 8-hydroxyquinoline (3) followed by differential cathodic stripping voltametry LOD 0.03 nM Pb, after 10 min adsorption74. 

Stan Van Den Berg is a Professor of Chemical Oceanography at the University of Liverpool. His research interests focus on the chemical specia-tion of trace elements and organic compounds in natural waters and the redox chemistry of metals and sulfides. His research group has pioneered advances in analytical techniques using electroanalytical methods (cathodic stripping voltammetry and chronopotentiometry). Dr. Van Den Berg is a broad-based analytical chemist. 

CdSe CdSe was deposited on different substrates. The two-step method of the electrosynthesis of CdSe films, based on the initial chemical modification of polycrystalline gold surface with selenium overlayer was described [157]. In the second step, this overlayer was cathodically stripped as a Se in a Se(IV)-free electrolyte medium that was dosed with the requisite amounts of Cd(II) ions. 

Anodic (or cathodic) stripping voltammetric (SV) techniques can be used to measure traces of metal in the environment. These methods are very sensitive and their precision is in the order of l to 2%. Stripping voltammetry is carried out in two stages. 

Agemian and Bedak [42] have described a semi-automated method for the determination of total arsenic in soils. Chappell et al. [43] have described an inexpensive but effective method for the quantitative determination of arsenic species in contaminated soils. Chappell found that the extraction efficiency varied with the ratio of soil to acid and with the concentration of the acid. Rurikova and Beno [346] accomplished speciation of arsenic(III) and arsenic(V) in soils by cathodic stripping voltammetry. Wenclawiak and Krah [347] used reactive supercritical fluid extraction in speciation studies of inorganic and organic arsenic in soils. In this method, derivatisation with thioglycollic acid methyl ester was performed in supercritical carbon dioxide. Various other workers have discussed the determination of arsenic in soils [44-46]. 

Bern [210] has reviewed methods developed up to 1981 for the determination of selenium in soil. These methods include neutron activation analysis, atomic absorption spectrometry, gas chromatography and spectrophotomet-ric methods. Square-wave cathodic stripping voltammetry has been used to determine selenium in soils [212],... 

---