Hach DR2400 - Page 726

Silica
© Hach Company, 2003. All rights reserved. Printed in the U.S.A. 3/04 1ed
Page 6 of 6 Silica_PTRL_ULow_HTB_Eng_Ody.fm
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Summary of Method
A number of modifications are necessary to adapt the Low Range Silica method
for analyzing trace levels in the Ultra Low Range method. It is absolutely
necessary to use the one-inch Flow-Thru Cell and liquid reagents. The Flow-
Thru Cell increases the reproducibility of the optics and reduces the instability of
the readings that result from moveable sample cells. Liquid reagents produce
more reproducible readings and lower blank values by eliminating slight
turbidity that may remain when using powdered reagents. In addition, the
liquid reagents are directly used with Hach process analyzers for continuous
silica measurement.
Silica and phosphate in the sample react with molybdate ions under acidic
conditions to form yellow silicomolybdic acid complexes and phosphomolybdic
acid complexes. Addition of citric acid destroys the phosphate complexes.
Amino Acid F Reagent is then added to reduce the yellow silicomolybdic acid to
an intense blue color, which is proportional to the silica concentration. Test
results are measured at 815 nm.
Required Reagents
Quantity Required
Description per test Unit Cat. No.
Rapid Liquid ULR Silica Reagent Set ...........................................................................................................26785-00
Includes:
Amino Acid F Reagent Powder ................................................................................. 55 g.....................26511-55
Amino Acid Reagent Dilution Solvent ..................................................................... 475 mL...............23530-11
Citric Acid F Reagent.........................................................................1 mL ................ 500 mL...............22542-49
Molybdate 3 Reagent.........................................................................1 mL ................ 500 mL.................1995-49
Required Apparatus
Adapter, multi-pathlength cell ...............................................................1 ....................... each....................59466-00
Cylinder, graduated, 50-mL, poly ..........................................................1 ....................... each......................1081-41
Dispenser, fixed volume, 1.0-mL, w/bottle..........................................3 ....................... each....................21113-02
Flask, Erlenmeyer, 250-mL, PMP, w/cap ..............................................2....................... each....................20898-46
Pour-Thru Cell Kit....................................................................................1 ....................... each....................59404-00
Required Standards
Silica Standard Solution, 1-mg/L SiO
2
............................................................................ 500 mL.................1106-49
Silica Standard Solution, 10-mg/L SiO
2
.......................................................................... 500 mL.................1403-49
Silica Standard Solution, 500-µg/L SiO
2
......................................................................... 3.78 L .................21008-17
Water, deionized ................................................................................................................. 4 L ..........................272-56