engleski

Sensitivity, reliability, reproducibility and accuracy of mercury drop electrode for the trace metal determination and speciation

A new sensitive electrode systems and electrochemical methods for the determination of very low trace metal concentration and speciation in natural water systems, are developed and tested. The applicability of the new mercury drop electrode system (MDE-1) for the determination and the speciation of trace metals in natural water sample using differential pulse anodic stripping voltammetry is described. The results obtained by measurements at pH = 7.8 using quartz cell, with a plastic stirrer and a glass reference electrode showed significant adsorption of the dissolved trace metal. The adsorption mainly takes place at the surface of the cell assembly (a plastic stirrer and a reference electrode made of glass), whereas only 1.5% of dissolved copper(II) is adsorbed on the walls of the quartz cell. After replacing the material of the plastic stirrer and the glass tip of the reference electrode with the quartz one, the dissolved copper(II) ions adsorption is depressed from 60% (at copper(II) concentration of 1x10^-7 M) to less then 20% (at copper(II) concentration of 2x10^-8 M). Consecutive measurements of anodic peak current in acidified solution and in the concentration range between 10^-7 and 2x10^-10 M of cadmium(II) showed relative standard deviations between 0.37 and 1.73%, only. The sensitivity of the electrode is enhanced by changing the form of the rotating stirrer. The detection limit of 2.5x10^-11 M Cd2+ was achived. The quality of voltammetric measurements can be improved by the application of thallium(I) as an internal normalization indicator standard redox process. Current fluctuations caused by unstable experimental parameters such as the drop size of the hanging mercury drop electrode and the hydrodynamics of the system are depressed, when the ratio of the signals of the investigated and the indicator electrode processes is used for the analysis. This advantage can be attributed to the partial correlation of these two signals fluctuations. Results of repetitive measurements of copper(II) and lead(II), the standard addition method used for analytical determination of concentration of copper(II) in a sample, and pseudopolarographic measurements of copper(II), with the addition of thallium(I) as the internal standard redox process are presented. A test procedure, to decide whether the indicator process itself has undergone some systematic changes and is qualified for data normalization, is proposed. The limits of the proposed procedure are encountered when the two signals are too close and when the variations of the repeated measurements for the two signals are not correlated.

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