Disturbances at the hmde through gas formation – Metrohm 797 VA Computrace User Manual

Page 283

9.6 Voltammetric problems

797 VA Computrace – Software

271

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 prepare the
acetate buffer using ultrapure ammonia and ultrapure acetic
acid).

Supporting electrolyte:

0.01 mol/L HClO4; pH = 2

acetate buffer pH = 4.64