Measure silica - 0.002 to 0.2 ppm (2 to
200 ppb)
SILICA
The measurement of silica is extremely
important in a number of cases, the most
common being the analysis of make-up
water, feed-water condensate and
condensed steam, with the range of silica
measurement being 0.002 to 0.2 ppm (2 to
200 ppb).
A great deal of work on the problem of
measuring this low level of silica was
carried out by Morrison and Wilson of the
Central Electricity Generating Board's
Research Laboratories at Leatherhead,
Surrey, England and much of their
procedure has been transferred to
Tytronics'® Sentinel.
METHOD
The method for the measurement of higher
levels of silica is based upon the formation
of the molybdosilicic acid by reaction with
molybdate. There are two forms of the
molybdosilicic acid, the a- and b- forms.
These two forms of the molybdosilicic acid
have different absorbance characteristics,
but on reduction to the molybdenum blue a
different reduction, and hence complex, is
formed.
The method chosen as being the most
reliable was that which ensured the
formation of the b-molybdosilicic acid and
reduction of this acid with 1-amino-2-
naphthol-4-sulphonic acid to form the
molybdenum blue complex which is
measured at 810 nm.
If phosphates are present these will also
form a similar molybdophoshoric acid
which may also be reduced. To overcome
phosphate interference, tartaric acid is
used which results in the destruction of the
molybdophosphoric acid.
To obtain the required sensitivity a longer
pathlength probe, 4 cm in transflection is
used in place of the normal 2 cm.
The measurement of silica, especially at
these low levels, requires extreme
precautions being taken in the preparation
of reagents. Even deionized water must be
further "polished" by passing the de-ionized
water through a suitable ion-exchange
column.
All reagents must be prepared and stored
in polyethylene or polypropylene containers
as contact with even Pyrex vessels results
in minute traces of silica being extracted.
As mentioned above, typical ranges of
measurement are:
0 to 100 ppb
0 to 500 ppb
0 to 1000 ppb
Higher levels are measured by omitting the
reduction stage and simply measuring the
yellow colour of the b-molybdosilicic acid.
Higher measurement ranges are:
0 to 2 ppm
0 to 5 ppm
0 to 10 ppm
Use our Tytronics Sentinel for measuring silica
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