Metrohm 757 VA Computrace - Manual (Page 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
4
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
nd 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).
Hg
Hg