1.1 Methods
Notes:
1. Temperature of the sample should be 15°C – 25°C.
2. If Silica or Phosphate is present a yellow colour is developed.
3. In this step any yellow colour due to Phosphate is removed.
4. Interferences:
Substance Interference
Iron large amounts interfere
Phosphate does not interfere at concentrations less than 50 mg/l PO
4
at 60 mg/l PO
4
the interference is approx. – 2%
at 75 mg/l PO
4
the interference is approx. – 11 %
Sulfide interferes at all levels
Occasionally water samples contain forms of silica which reacts very slowly with
Molybdate. The nature of these forms is not known.
A pre-treatment with Sodium hydrogencarbonate and then with Sulfuric Acid will
make these forms reactive to Molybdate (pre-treatment is given in “Standard Methods
for the Examination of Water and Wastewater” under “ Silica Digestion with Sodium
Bicarbonate”).
5.
SiO
2
Si
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