1.1 Methods
Notes:
1. For photometric determination of pH values only use THYMOLBLUE tablets in black
printed foil pack and marked with PHOTOMETER.
2. pH values below 8.0 and above 9.6 can produce results inside the measuring range.
A plausibility test (pH-meter) is recommended.
3. The accuracy of the colorimetric determination of pH values depends on various
boundary conditions (buffer capacity of the sample, salt contents etc.).
4. Salt error
Correction of test results (average values) for samples with salt contents of:
Indicator Salt content
Thymolblue 1 molar
– 0.22
2 molar
– 0.29
3 molar
– 0.34
The values of Parson and Douglas (1926) are based on the use of Clark and Lubs
buffers. 1 Mol NaCl = 58.4 g/l = 5.8 %
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