ORBECO SP600, Instruction Manual, Page: 209 / 334

207
SP600 Spectrophotometer 01/2008
1.1 Methods
Notes:
1. For photometric determination of pH-values only use PHENOL RED tablets in black
printed foil pack and marked with PHOTOMETER.
2. Water samples with low values of Alkalinity-m (below 35 mg/l CaCO
3
) may give wrong
pH readings.
3. pH-Values below 6.5 and above 8.4 can produce results inside the measuring range.
A plausibility test (pH-meter) is recommended.
4. The accuracy of the colorimetric determination of pH-values depends on various
boundary conditions (buffer capacity of the sample, salt contents etc.).
5. Salt error
Correction of test results (average values) for samples with salt contents of:
Indicator Salt contents
Phenol red 1 molar
- 0.21
2 molar
- 0.26
3 molar
- 0.29
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 %