What forms of silica do the Hach Silica methods and test kits analyze?

Document ID

Document ID TE925

Version

Version 4.0

Status

Status Published

Published Date

Published Date 07/26/2018
Question
What forms of silica do the Hach Silica methods and test kits analyze?
Summary
Hach methods silica forms
Answer
Hach’s silica methods determine molybdate-reactive silica. The terms 'molybdate-reactive' or 'molybdate-unreactive' does not imply reactivity, or lack of it, toward other reagents or processes. Occasionally a sample contains silica which either reacts very slowly with molybdate or does not react at all. In at least one of its forms, silica does not react with molybdate even though it is capable of passing through filter paper and is not noticeably turbid. It is not known to what extent such “unreactive” silica occurs in waters. “Molybdate-unreactive” silica can be converted to the “molybdate-reactive” form by heating or fusing with alkali, such as in a sodium bicarbonate digestion. See Standard Methods 4500-SiO2 for details.

Hach procedures for silica include the Silicomolybdate Method for high range measurement and the Heteropoly Blue Method for low range method. The Heteropoly Blue method is an extension of the Silicomolybdate method to increase sensitivity. In both methods the color formed is directly proportional to the amount of molybdate-reactive silica present in the original sample. Total silica can be determined by performing analysesusing theelectrothermal atomic absorption method (Standard Methods 3113B), the inductively coupled plasma mass spectrometric method (Standard Methods 3125) or the inductively coupled plasma method (Standard Methods 3120). Terms such as “colloidal,” “crystalloidal,” and “ionic” have been used to distinguish among various forms of silica but such terminology cannot be substantiated. Hach does not provide solutions or support for any of the methods for total silica.

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