Most water contains an abundance of silicon and its derivatives, silica (SiO 2) or silicates (SiO 4 – and SiO 3 2–)

 What is Silica?

Silica, also known as silicon dioxide (SiO 2) or mineral quartz, is a derivative compound of the chemical element silicon (Si) and makes up approximately 59% of the Earth's crust and a large part of common sand. Silicon links up with oxygen to form silicates, which are the most common suite of minerals.

Silica is found as dissolved silica or suspended silicate particles in most natural waters. Silica is used in many industrial applications, including food and pharmaceutical production, as an additive and for water filtration.

Why Measure Silica?

Most water contains an abundance of silicon and its derivatives, silica (SiO 2) or silicates (SiO 4 – and SiO 3 2–). Silica concentration in water is commonly less than 30 mg/L, although concentrations higher than 100 mg/L are not unusual. Concentrations exceeding 1000 mg/L are possible in brines and brackish water.

Silica in Water can Cause Significant Problems for Industrial Water

Silica and silicates are added to water as conditioners, detergents and corrosion inhibitors. However, silica in water can cause significant problems for industries, primarily in boiler and steam applications. High pressures and high temperatures cause silica deposits on boiler tubes and heat exchangers.

These glassy deposits lower the efficiency of heat transfer and lead to premature failure. Silica deposits on steam turbine blades decrease efficiency and necessitate costly downtime for cleaning. Silica levels must be kept below 0.005 mg/L (5 ppb) in very high-pressure applications.

Measuring silica in water is useful when the efficiency of demineralizers is monitored. Testing for silica (one of the first impurities detected when the exchange capacity of a demineralizer is exhausted) provides a sensitivity check of demineralizer performance.

Analytical Procedures and Methods for Silica

Analytical procedures for silica include the Silicomolybdate Method for high range measurement and the Heteropoly Blue Method for low range measurement. The Heteropoly Blue method is an extension of the Silicomolybdate method to increase sensitivity.

Silica methods from Hach ® determine molybdate-reactive silica. The terms 'molybdate-reactive' or 'molybdate-unreactive' do 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 being present in dissolved or colloidal form. 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-SiO 2 for details.

In both Silicomolybdate and Heteropoly Blue 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 analyses using the Electrothermal 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.

At Hach, find the testing equipment, resources, training and software you need to successfully monitor and manage silica levels in your specific process application.

Quick Links

Why Measure Silica?

Featured Products

Related Processes

Methods for Monitoring

FAQs

Shop Now

Featured Products to Measure Silica

5500sc Silica Analyser

The 5500sc Silica Analyzer requires only two liters of reagent for the analyzer to perform unattended for up to 90 days; twice as long as the predecessor Series 5000 instruments.

Spectrophotometers

Hach is known for spectrophotometry innovations and offers many of the leading spectrophotometric instruments in the water analysis 

Silica Chemistries, Reagents and Standards

Hach chemistries are rigorously tested in combination with our packaging and instruments to ensure the highest possible system performanc

DR900 Colorimeter

The DR900 Portable Colorimeter allows for quick and easy access to your most frequently used testing methods in less than four clicks.

Shop Now

EZ Series Silica Analysers

The EZ Series Online Analyzers offer multiple options to monitor silica in water. Typical applications are steam and power generation, surface water and process water.

Shop Now

 

Which Processes Require Silica Monitoring?

Power & Steam

Boiler Water: Scale formation due to hardness, silica, and corrosion product deposits can decrease the heat exchange capacity and global efficiency of the boiler. Steam quality varies with the concentration of impurities in the boiler water and the potential mechanical carryover of contaminated water droplets into the steam. The higher the operating pressure of the boiler, the higher the purity requirements since contaminant carryover is promoted. Silica measurements (drum boiler blowdown): silica build-up in the boiler depends on the drum pressure, < 5 ppb.