Owner manual

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

, 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

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

; 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)