Silica (SiO₂) is a common constituent of natural waters. It exists in dissolved, colloidal, and particulate forms. Determining silica concentration is important in water analysis, especially for boiler feed water, cooling systems, and industrial processes, where excessive silica can form hard scales on equipment surfaces and reduce efficiency.

The molybdosilicate method is a reliable colorimetric technique for measuring dissolved silica. In this method, silica reacts with ammonium molybdate under acidic conditions to form yellow molybdosilicic acid.

SiO₂+12(NH₄)₂MoO₄+2H⁺+H₂O → H₄[Si(Mo₁₂O₄₀)]+24NH₄⁺+2H₂O

Chemicals used-

• Ammonium Molybdate Solution-Reacts with dissolved silica in acidic medium to form yellow molybdosilicic acid complex. This is the key reagent responsible for colour development.To prepare Ammonium Molybdate Solution dissolve 10 g ammonium molybdate water with stirring and gentle warming, and dilute to 100 ml.
• 1:1 Hydrochloric acid-Provides the acidic medium necessary for the formation of the molybdosilicic acid complex. Ensures proper reaction conditions.Used to acidify the sample. Add 1 ml HCl in 1 ml distilled water to make 1:1 HCl.
• Silica Standard Solution-Used to prepare calibration standards.Dissolve 4.73 g of sodium metasilicate nonahydrate to 1000 ml distilled water. Take 10 ml from it and dilute to 1000 ml distilled water.
• Oxalic acid solution-Dissolve 7.5 g of oxalic acid in distilled water and dilute to 100 ml.Here we have used all the following chemicals/reagents of Researchlab  company. You can use the same company or any other companies.
• Distilled Water-Used for reagent preparation and dilution. It must be silica-free to prevent contamination and false high readings.The conductivity of distilled water should be <5 μS/cm to prevent contamination.

Apparatus Used-

• Volumetric Flasks(1000ml,100ml)-Used to prepare accurate standard and reagent solutions. Works on the principle of delivering precise volume at a specific calibration mark.
• Pipettes(2ml)-Used for transferring exact volumes of reagents and samples. Plastic is preferred to avoid silica leaching from glass.
• Plastic Beakers(250ml)-Used for carrying out the reactions safely. Non-glass materials prevent contamination from silica present in glassware.
• Spectrophotometer-Measures the absorbance (intensity of yellow colour) at a wavelength of around 410 nm.

Process –

Sample & Std preparation	Take 50 ml sample in a beaker. Add 1 ml 1:1 HCl, 2 ml ammonium molybdate & 2 ml oxalic acid solution in it. Prepare std solution as sample.
Taking Absorbance & Factor Calculation	Measure within 2 minutes both std solution & sample absorbance at 410 nm. Run a blank sample(distilled water) in the same way. Perform the test at least in 2 set to get accurate result. Factor calculation procedure mentioned previously.
Calculation	Silica(mg/L)= (Sample abs-Blank abs) x F/Sample taken

Purpose of Silica Testing in Water-

• Prevent Boiler and Turbine Scaling-High silica levels form hard, glassy deposits on boiler tubes and turbine blades, reducing heat transfer and efficiency.
• Protect Cooling Systems-Silica scales can block cooling water lines and heat exchangers, leading to overheating and maintenance issues.
• Ensure Water Treatment Efficiency-Monitoring silica helps evaluate the performance of demineralization and reverse osmosis (RO) systems.
• Maintain Product Quality-In industries like electronics, pharmaceuticals, and power generation,WTP,Food processing industries,rubber & polymer industries very low silica levels are required to ensure product purity and equipment safety.
• Environmental Monitoring-Silica testing helps assess natural water quality of water sources.

Handling Mistakes during Silica Testing in Water-

• Contaminated or old reagents-Impurities or degraded reagents affect colour development and absorbance.
• Improper timing during colour development– Over- or underdeveloped colour causes inaccurate absorbance readings.
• Not using a reagent blank- Instrument baseline errors may go incorrected, causing false readings.
• Dirty or scratched cuvettes- Affects light transmission, resulting in inaccurate absorbance.

Conclusion-

The ammonium molybdate method is a reliable and accurate colorimetric technique for determining silica concentration in water. It helps ensure the quality of water used in industrial, domestic, and environmental applications. By forming a measurable yellow molybdosilicic complex, this method provides a direct relationship between colour intensity and silica content.To perform silica test at a water manufacturing company or laboratory should ensure by following standard procedures mentioned above to accurately determine the silica and assess the quality of the drinking water or any type of water like Waste-water,ETP,STP etc. with availability of the apparatus & chemicals and also the above real-time photo attachment will help to understand the testing process completely. In addition, if expert guidance is needed ,please reach out to Pro Research & Testing Laboratory.

FAQs Related to Silica Testing in Water-

1. What is the principle of silica testing by the ammonium molybdate method?
Silica reacts with ammonium molybdate in an acidic medium to form yellow molybdosilicic acid. The intensity of the yellow colour, measured spectrophotometrically, is directly proportional to the silica concentration.

2. Why is it important to test silica in water?
High silica levels cause scaling in boilers, turbines, and cooling systems, reducing heat transfer and efficiency. Testing ensures water treatment systems operate effectively and prevents damage to equipment.

3. Why should glassware be avoided in silica testing?
Glass contains silica, which can dissolve into the sample and produce false high readings. Hence, plastic or teflon containers are recommended.

4. What wavelength is used for silica measurement?
The yellow molybdosilicic acid complex is commonly measured at a wavelength of 410 nm using a spectrophotometer.

5. Which reagent is used to remove phosphate interference?
Oxalic acid is used to eliminate phosphate interference, ensuring only silica reacts with ammonium molybdate.

6. How can we ensure accurate results?
Use fresh reagents, handle samples in silica-free containers, maintain correct reaction timing, and always use a reagent blank for calibration.

How We Verified This Testing/Research Procedure :

This testing is done under qualified analysts in our lab.Continually monitored by expertise.Repeatedly testing is always done to get accurate result.

Written by
Riya Ghosh (M.Sc. Food Technology, MAKAUT)
Designation – Chemist

Reviewed by
Anwesha Das (M.Sc Microbiology,BU)
Designation – Microbiologist

Verified By
Dr. Jyotirmoy Kumar Dey (Phd,Chemistry)
Designation – Senior/Chief Chemist
Experience – 25 Years +