Metrohm 746 VA Trace Analyzer User Manual

Page 547

7 Safety, Errors, Troubleshooting, Diagnosis, GLP

746 VA Trace Analyzer / 747 VA Stand

7-48

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Determination of lead and thallium

With a supporting electrolyte of pH = 1, Pb and Tl peaks overlap greatly.

Changing the pH to 13 separates the two peaks. (The separation of lead

and thallium can also be achieved by subsequent electrolysis or in acetate

buffer with EDTA).

Supporting electrolyte:

pH = 1

pH = 13

Analysis solution:

0.5 mg/L Pb; 1 mg/L Tl

Standard addition:

with 10

g Pb and 10

g Tl

Electrode:

SMDE

7.5.9

Calibration with chemically non-isoformal standards

With all three possible calibration techniques, it must be ensured that the standards

used for calibration are chemically isoformal with the analytes. The standard sub-

stances must therefore have the same valency (e.g. with Fe, Al) or complex forma-

tion form (e.g. with As, Cr, Se) as the substances already present in the analysis

solution. If this is not the case the calibration can produce completely wrong results

owing to the different peak voltages and sensitivities.