LaMotte SMART Spectro Spectrophotometer - Manual (Page 237)

APPLICATION: Drinking, surface, and saline waters; domestic and
industrial wastes.
RANGE: 0.00–2.50 ppm Silica
MDL: 0.03 ppm
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.
INTERFERENCES: Sulfi des and large amounts of iron interfere. Color
and turbidity may be removed by standardizing the
instrument with the original water sample. Since
silica is a component of glass waste and a common
contaminant, it is suggested to run a reagent blank
using silica-free water. The blank value is subtracted
from the sample concentrations.
SILICA, Low Range SMART Spectro Test Procedures 2.11
Test P
rocedures