Metrohm 757 VA Computrace Uživatelský manuál stáhnout pdf (Strana 185)

8.6 Voltammetric problems

757 VA Computrace – Software

175

• Mercury determination at the gold electrode

On prolongation of the enrichment time from 30 s to 240 s,

side peaks appear during standard addition:

Enrichment time: 30 s Enrichment time: 240 s

Analysis solution: 100 µg/L Hg

Supporting electrolyte: HClO4/HCl

Standard addition: with 1 µg Hg

Electrode: Au-RDE

Disturbances at the HMDE through gas formation

Gas formation at the HMDE during the deposition phase can lead

to drop fall or to a contact break in the Hg capillary. The following

example illustrates such a case:

• Determination of zinc in deionised water

If the zinc sample is acidified to pH 2 with HClO

or another

acid, hydrogen is also formed at the voltage selected for en-

richment. In the present example this leads to premature re-

lease of an Hg drop in the 2

standard addition thus making

an evaluation impossible. If acetate buffer (pH = 4.64) is

used, no such difficulties arise. However, a requirement for

this is that chemicals of the highest purity are used to keep

the zinc blank value as low as possible (it is advisable to pre-

pare the acetate buffer using ultrapure ammonia and ul-

trapure acetic acid).

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Metrohm 757 VA Computrace Uživatelský manuál Strana 185