Metrohm 797 VA Computrace Instrukcja Użytkownika Pobierz pdf (Strona 280)

9 Troubleshooting

797 VA Computrace – Software

268

2. If organic components are present: carry out a UV digestion or

other suitable sample preparation.

3. Concentration of the analyte is too high: dilute.

4. Electrolyte solution too old: make up a new one. Its working

life with organic additives may be as short as 1 day or less.

With HMDE:

1. Standard addition solution containing metal complexing agents

need time to form the metal complex.

Spikes / signal jump in voltammogram

1. For MME: check the electrode.

2. Reduce the dynamic range of the potentiostat (see Potentio-

stat, section

3.3).

Oxygen interference

Oxygen can be electrochemically reduced and produces two waves

in the voltammogram, one of which is characterized by the ap-

pearance of a pronounced maximum. The oxygen reduction can

interfere for two reasons:

• The signals of the analytes are masked by the oxygen waves.

This becomes noticeable primarily in trace analysis as the oxy-

gen is present in a relatively high concentration in solutions

saturated with air (ca. 8 mg/L at room temperature).

• The hydrogen peroxide formed in the first step of the oxygen

reduction can react further with certain substances.

For these reasons, oxygen must be removed from the analysis solu-

tion before the polarographic analysis by saturation with inert gas

(usually nitrogen). With the inert gas flow rate of ca. 20 L/h set on

the 797 VA Computrace stand in the factory, a purging time of

3...5 min usually suffices.

Compare the curves (0.1 mol/L KNO3) before and after purging:

Before purging (Still oxygen in the solution):

1 2 ... 275 276 277 278 279 280 281 282 283 284 285 ... 294 295

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Metrohm 797 VA Computrace Instrukcja Użytkownika Strona 280