Thermo Electron Orion 95-02, Инструкция по эксплуатации

Страница: 28 / 43

26
Interferences
Volatile weak acids are potential electrode interferences.
Concentrations of these interfering species that cause a 10% error at
10
-3 M (44 ppm CO
2
or 100 ppm CaCO
3
) and at pH 4 and 5 are listed
in Table 5.
Effects Of Dissolved Species
Water vapor is a potential electrode interference. Water can move
across the membrane as water vapor, changing the concentration of
the internal filling solution under the membrane. Such changes will
be seen as electrode drift. Water vapor transport is not a problem if
1) the total level of dissolved species in solution (osmotic strength)
is approximately equal to that of the internal filling solution and
2) electrode and sample temperatures are the same. Addition of
carbon dioxide buffer to samples of low osmotic strength
automatically adjusts them to the correct level. Samples with osmotic
strengths greater than 1 M should be diluted before measurement.
Dilution should not reduce the carbon dioxide level below 10
-4
M.
Samples with osmotic strengths above 1 M that cannot be diluted can
be measured by adjusting the osmotic strength of the internal filling
solution. To adjust the total level of dissolved species in the internal
filling solution, add 0.425 g reagent grade NaNO
3
to 10 mL internal
filling solution.
Table 5
Levels of Interferences Causing A 10% Error At 10
-3
M CO
2
Interferences pH 5 pH 4
NO
2
- (NO
2
) 3.5 x 10 -3 M 5.3 x 10 -4 M
(160 ppm) (24 ppm)
HSO
3
- (SO
2
) 5 x 10
-3
M 7.5 x 10
-4
M
(320 ppm) (48 ppm)
HOAc 6.2 x 10
-3
M 3.6 x 10
-3
M
(Acetic acid) (0.37 g/100 mL) (0.22 g/100 mL)
HCOOH 2.0 x 10
-2
M 7.5 x 10
-3
M
(Formic acid)