电化学分析伏安分析法.ppt

Electrodes in stripping voltammetry ● Hung mercury electrode ● glass carbon electrode mercury plating (镀汞膜的玻璃态石墨电极） Problems ■ Compared with other polarographic methods, how about the sensitivity of square wave polarography? ■ what advantages do mercury hung electrode and mercury membrane electrode have ? §5-11 单指示电极安培滴定（Amperometric titration with single indicating electrode, polarographic titration) Amperometric titration: keeping potential constant Potentiometric titration: keeping current constant or at null-current a: analyte is electroactive (able to be reduced on electrode), but the titrant is not b: analyte and titrant are all able to be reduced on the polar electrode Electrodes Reference electrode: calomel electrode, mercury pool electrode Working electrode: dropping mercury electrode, solid microelectrode Advantages ? Solid microelectrode can be used ? wide linear range. 0.1 ~ 10-4mol.L-1 Using a electrode with a big area, DL can be down to 10-6 mol.L-1 Back titration can increase upper limit of determination ? some electro-inactive materials can also be measured using electro-active titrant. Example Th4+ Al3+ Fe3+(added) Titrated by F- ThF62- AlF6- Fe3+(added) Titration continued ThF62- AlF6- FeF63- §5-12 双指示电极安培滴定（永停滴定） （Dead-stop end point titration） §5- 13 双指示电极电位滴定 （potentiometric titration with double indicating electrodes) 自 学 其 它 相 关 内 容 0.1M DME Calomel electrode Corresponding to section ac Before voltage applied ≈ + + + + + - - - - - ≈ ≈ + + + + + - - - - - G G G Null potential point 电容电流：是由于汞滴表面与溶液间形成的双电层，在与参比电报连接后，随着汞滴表面的周期性变化而发生的充电现象所引起的。此电流与滴汞电极的电位有关。对于微量组分(如＜10-5M)的测定，虽然注意到所用试剂的纯度并经过仔细的除氧，但由于电容电流的存在，仍有微量的残余电流(约为10-7A数量级)通过（Why some 10-7A? Please refer to page 157~158, in the teaching book)， 这已足以起较大的测量误差。所以电容电流的存在是提高极谱分析灵敏度的一个主要障碍。 2、 迁移电流（migration current） 极谱分析电解