LaMotte COD Plus Colorimeter 1922-EX2 - Manual (Page 112)

SILICA - LOW RANGE
HETEROPOLY BLUE METHOD • CODE 3664-SC
QUANTITY CONTENTS CODE
30 mL *Silica Reagent #1
*V-4466-G
30 mL *Silica Reagent #2
*V-4467-G
30 mL *Silica Reagent #3
*V-4468-G
10 g *Silica Reagent #4
*V-6284-D
1 Spoon, 0.1 g, plastic
0699
*WARNING: Reagents marked with a * are considered hazardous substances. Material
Safety Data Sheets (MSDS) are supplied for these reagents. For your safety read label
and accompanying MSDS before using.
Silicon dioxide, SiO
2
, commonly known as silica, occurs in all natural water.
Silica may be present as suspended, insoluble particles in a colloidal or
polymeric state. It may also be present in a reactive form as silicic acid or
silicate ions. Silica is a major nutrient for diatoms. A silica cycle occurs in
many bodies of water containing organisms, such as diatoms, that use silica in
their skeletal structure. The silica removed from the water may be slowly
returned to solution by the decomposition of the dead organisms. The major
source of silica in natural water is from the decomposition of silicate minerals
in the drainage basin from which the waters flow.
The presence of silica is particularly objectionable in water used for boiler feed
water purposes, as it may cause the formation of a hard, dense scale which has
unusually high resistance to heat transfer. Serious loss of turbine efficiency
results from insoluble silica turbine blade deposits caused by vaporization of
silica from boiler water.
APPLICATION:
Drinking, surface and saline waters; domestic and industrial
wastes.
RANGE: 0.0 – 4.0 ppm Silica
METHOD: Reactive silica forms a complex with ammonium molybdate
in an acidic solution to produce a yellow-green color in
proportion to the amount of silica present. Phosphate also
reacts with molybdate but the addition of oxalic acid
eliminates the molybdophosphoric acid complex. This silica
molybdate complex is then reduced by ascorbic acid to
produce an intense blue color.
SAMPLE
HANDLING &
PRESERVATION: Silica samples may be preserved by refrigeration at 4°C in
plastic containers up to one week without any change in
silica concentration.
COD PLUS TEST PROCEDURES 6/02 SILICA - LOW RANGE 1/3