Hach DR/850 Procedures Manual page 396

SILICA, Low Range,
Interferences
Reagent Preparation
Summary of Method
continued
Interfering
Substance
Color
Phosphate
Iron
Slow reacting forms of
silica
Sulfides
Turbidity
To prepare Amino Acid F Reagent Solution, dissolve 11.4 grams
of Amino Acid F Reagent Powder in 100 mL of 1.0 N Sodium
Hydroxide Solution. The solution is stable for at least one month
if stored in a plastic bottle.
Silica and phosphate in the sample react with molybdate ion
under acidic conditions to form yellow silicomolybdic acid
complexes and phosphomolybdic acid complexes. Acid reduces
the yellow silicomolybdic acid to an intense blue color, which is
proportional to the silica concentration.
Interference Levels and Treatments
Eliminated by zeroing the instrument with the original
sample.
Phosphate does not interfere at levels less than 50 mg/L
PO . At 60 mg/L PO , an interference of -2% occurs. At
4 4
75 mg/L PO the interference is -11%.
4
Large amounts of iron interfere.
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 pretreatment.
Interfere at all levels
Eliminated by zeroing the instrument with the original
sample.
396