402
Estimated Detection Limit
The estimated detection limit for program 88 is 3.0 mg/L SiO
2
.
For more information on the estimated detection limit, see
Section 1.
Interferences
Occasionally a sample contains silica which reacts very slowly
with molybdate. The nature of these “molybdate-unreactive”
forms is not known. A pretreatment with sodium bicarbonate,
then sulfuric acid will make these forms reactive to molybdate.
The pretreatment is given in Standard Methods for the
Examination of Water and Wastewater under Silica-Digestion
with Sodium Bicarbonate. A longer reaction time with the sample
and the molybdate and acid reagents (before adding citric acid)
may help in lieu of the bicarbonate treatment.
Summary of Method
Silica and phosphate in the sample react with molybdate ion
under acidic conditions to form yellow silicomolybdic acid
complexes and phosphomolybdic acid complexes. Addition of
citric acid destroys the phosphate complexes. Silica is then
determined by measuring the remaining yellow color.
Interfering Substance Interference Levels and Treatments
Color Eliminated by zeroing the instrument with the
original sample.
Iron High levels of Fe
2+
and Fe
3+
interfere.
Phosphate Does not interfere below 50 mg/L PO
4
3-
. At 60 mg/L PO
4
3-
, a negative 2% interference occurs. At 75 mg/L PO
4
3-
the
interference is negative 11%.
Sulfides (S
2-
) High levels interfere.
Turbidity Eliminated by zeroing the instrument with the
original sample.
SILICA, Ultra High Range, continued
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