Metrohm 757 VA Computrace - Manual (Page 174)

8 Troubleshooting
757 VA Computrace – Software
164
8.5 General rules for VA trace analysis
Chemicals and equipment
1. The purity of the reagents plays an important role in determin-
ing the results. Extremely pure chemicals should be used for
determining lower concentrations (see VA Application Note V-
49).
2. Measuring vessel, electrodes and all other equipment in con-
tact with the sample solution must be clean and free of con-
tamination substances.
Electrolytes
1. The pH during a determination plays an important role (e.g.
for Zn, Cd, Pb, Cu it should be approx. 4.5). Acetate, Ammo-
nium acetate or PIPES buffer are often used. For more infor-
mation see the Application Bulletins.
2. The electrolyte must be sufficiently conductive and concen-
trated.
3. The purity of the electrolytes and the cleanliness of the re-
agent bottles is very important.
4. The working life of the electrolytes is limited, particularly for
organic additives (buffer substances, complex formers). It
may be necessary to make up fresh solutions every day.
Standard solutions
1. The standard solutions should be made acidic (approx. pH =
1…2) and stored in plastic bottles.
2. Diluted standard solutions (ppb range) are very unstable and
must be freshly made. They must also be made sufficiently
acidic.
3. The concentration of the standard solutions must be arranged
so that a volume between 20 and 500 µL has to be added.
4. Standard additions are recommended. The peak height after
the last addition should be 2…5 times higher than the sample
peak.
5. 1000 ppm solutions (self-made or commercially available) are
often used as stock solutions. They are stable over long peri-
ods of time. Dilutions have to be made with dilute acids.