Thermo Scientific AQUAfast AQ3700, User Manual

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Thermo Scientific Orion AQUAfast AQ3700 Colorimeter User Guide
1.1 Methods
Notes:
1. For photometric determination of pH values only use PHENOL RED tablets in black
printed foil pack and marked with METER.
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 content
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 %