Hach DR/700 Procedures Manual page 741

SILICA, LR,
Sulfides and large amounts of iron interfere.
There is no interference from phosphate below 50 mg/L PO
60 mg/L PO , an interference of minus 2% is observed. At 75 mg/L the
4
interference is minus 11%.
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, Silica,
Digestion with Sodium Bicarbonate. A longer reaction time of sample
with the Molybdate 3 Reagent, before addition of citric acid, is often
helpful in lieu of the bicarbonate pretreatment.
SUMMARY OF METHOD
Silica and phosphate in the sample react with molybdate ion under
acidic conditions to form silicomolybdic acid complexes and
phosphomolybdic acid complexes. Addition of citric acid preferentially
destroys the phosphate complexes. An amino acid is then added to
reduce the yellow silicomolybdate to an intense blue color, which is
proportional to the silica concentration.
REQUIRED REAGENTS
Description
Amino Acid F Reagent
Powder Pillows . . . . . . . . . 1 pillow . . . . 100/pkg . . . . . . . 22540-69
Citric Acid Powder Pillows . . . 2 pillows . . . 100/pkg . . . . . . . 21062-69
Molybdate 3 Reagent . . . . . . . 1.0 mL. . . . . 50 mL SCDB. . . . 1995-26
REQUIRED APPARATUS
Clippers, for opening
powder pillows . . . . . . . . . . 1 . . . . . . . . . each . . . . . . . . . . . . 968-00
DR/700 Filter Module
Number 81.01 . . . . . . . . . . . . . 1 . . . . . . . . . each . . . . . . . . . . 46281-00
OPTIONAL REAGENTS
Amino Acid F Reagent Powder Pillows,
25 mL sample size . . . . . . . . . . . . . . . . . . 100/pkg . . . . . . . 22538-69
continued
Quantity
Per Test
81-67
. At
4
Unit Cat. No