LaMotte SMART Spectro Spectrophotometer - Manual (Page 215)

APPLICATION: Drinking, surface, and saline waters; domestic and industrial
wastes.
RANGE: 0.00–2.50 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.
INTERFERENCES: Sulfides 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–LR 2/3 SMART SPECTRO TEST PROCEDURES 05.04