Metrohm 746 VA Trace Analyzer - Manual (Page 544)

7.5 Vagaries of the chemistry
746 VA Trace Analyzer / 747 VA Stand
7-45
7.5.5 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
, hydrogen is also formed
at the voltage selected for enrichment. In the present example this leads to
premature release 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 prepare the ace-
tate buffer using ultrapure ammonia and ultrapure acetic acid).
Supporting electrolyte: Supporting electrolyte:
0.01 mol/L HClO
4
; pH = 2 acetate buffer pH = 4.64
Analysis solution: deionised water
Standard addition: with 50 ng Zn
Electrode: HMDE (enrichment 60 s at –1.2 V)
Zn
Zn