Silica and Phosphate Analysis by Colorimetry

How does colorimetry work?

In colorimetry, the substance to be measured is reacted with a reagent and converted into a colored compound. The intensity of the resulting color is measured through light absorption at a specific wavelength and converted to concentration using the Beer-Lambert law.

In silica measurement, molybdate reagent forms a yellow silicomolybdate complex with dissolved silica; the complex can also be reduced to a blue color, which increases measurement sensitivity. In phosphate measurement, phosphate forms a blue complex with molybdate and a reducing agent. In online analyzers, sampling, reagent addition, mixing, reaction time and measurement are carried out automatically in sequence.

What colorimetry measures and where it is used

Online colorimetric analyzers are used in the water-steam cycle:

• Monitoring silica slip in boiler feedwater, demineralized water and steam condensate
• Early warning at ion exchanger and reverse osmosis outlets
• Phosphate dosing control in boiler water
• Power plants and industrial steam boilers

Where colorimetry works well and where it struggles

In high-pressure boilers, silica can be carried into steam and form deposits on turbine blades; online measurement shows ion exchanger exhaustion early. Phosphate monitoring is one of the main tools of scale and corrosion control. The method is selective and can be used in the ppb–ppm range.

Limitations: there is reagent consumption, and parts such as reagents, tubing, pumps and valves need regular replacement. Measurement runs in intermittent cycles. Sample temperature and steady flow affect measurement quality. With colored or turbid samples, the measurement cell and zero setting need attention.

Choosing an instrument for colorimetry

When selecting, the parameter to be measured, measurement range, number of streams and the sample point in the water-steam cycle are determined. Reagent consumption and maintenance kits should be taken into account in advance when planning operating costs and downtime. During commissioning, Tetra configures the sample line, reagent feed arrangement and alarm outputs according to the plant's water chemistry program.

Tetra and colorimetric analyzers

Tetra supplies, installs, commissions and calibrates colorimetric process analyzers in Türkiye and services them with maintenance kits and spare parts. The product range includes the Thermo Orion 2230XP process silica analyzer, the Orion 8030cX silica analyzer and the Orion 2300 process phosphate analyzer. Tetra integrates these analyzers into SWAS water and steam analysis systems.