Hach DR/700 Procedures Manual page 169

SILICA, HR,
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 and acid reagents, before the 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 yellow silicomolybdic acid complexes and
phosphomolybdic acid complexes. Addition of citric acid preferentially
destroys the phosphate complexes. Silica is then determined by
measuring the remaining yellow color.
REQUIRED REAGENTS
High Range Silica Reagent Set (100 tests) . . . . . . . . . . . . . . . . . 22443-00
Includes: (2) 1042-66, (1)14548-99, (2) 1041-66
Description
Acid Reagent Powder Pillows
for High Range Silica. . . . . 1 . . . . . . . . . 100/pkg . . . . . . . 21074-69
Citric Acid Powder Pillows . . . 1 . . . . . . . . . 100/pkg . . . . . . . 21062-69
Molybdate Reagent Powder Pillows
for High Range Silica . . . . . . 1 . . . . . . . . . . 100/pkg . . . . . . . .21073-69
REQUIRED APPARATUS
Clippers, for opening
powder pillows . . . . . . . . . . 1 . . . . . . . . . each . . . . . . . . . . . 968-00
DR/700 Filter Module
Number 42.01. . . . . . . . . . . 1 . . . . . . . . . each . . . . . . . . . . 46242-00
OPTIONAL REAGENTS
High Range Silica Reagent Set
(100 Tests) for 25-mL samples . . . . . . . . . . . . . . . . . . . . . . . 22443-00
Includes: (2) 1042-66, (1) 14548-99, (2) 1041-66
Silica Standard Solution, 10 mg/L . . . . . . . . 500 mL . . . . . . . 1403-49
continued
Quantity
Per Test
42-85
Unit
Cat. No.
Cat. No