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Measure fluorosilicic acid inline – precise, maintenance-free, in real time


LiquiSonic® continuously determines the concentration of hexafluorosilicic acid (H₂SiF₆) in water directly in the process. No sampling, no wear parts, no time delay.

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Real-time measurement

Continuous monitoring without delay

Maintenance-free

No moving parts or wear components

Maximum accuracy

Precise measurement even under difficult conditions

Reduced operating costs

Cost-efficient solution with a short payback period

Fluorosilicic acid (or hexafluorosilicic acid) is usually generated in the fertilizer industry as a by-product of phosphoric acid production. When phosphate rock is digested with sulfuric acid, hydrofluoric acid is formed, which reacts with the silicates contained in the ore to form H₂SiF₆. Since the composition of the phosphate rock naturally varies, the resulting fluorosilicic acid concentration also fluctuates—sometimes from batch to batch, sometimes continuously during the running process.

However, for further processing, for example into aluminum fluoride, fluorosilicates, or for use as a dosing chemical in water treatment, an exactly known concentration is crucial. It determines yield, reaction behavior in downstream processes, and, in dosing applications, the actual amount of active substance delivered. At the same time, H₂SiF₆ is highly corrosive and toxic, making conventional analysis with manual sampling and titration both time-consuming and risky for personnel. By the time a lab result is available, the process has often already moved on.

This is exactly where inline concentration measurement from SensoTech comes in: The inline pipe sensor provides the concentration directly in the running process, continuously and without any personnel contact with the medium.

How LiquiSonic® the fluorosilicic acidconcentration measures

The LiquiSonic® Rohrsensor basiert auf dem Messprinzip der Schallgeschwindigkeit. Aus der Schallgeschwindigkeit im Medium sowie der gemessenen Temperatur berechnet das System in Echtzeit die aktuelle H₂SiF₆-Konzentration. The Verfahren ist unabhängig von Leitfähigkeit, Farbe oder Trübung der Flüssigkeit und funktioniert auch bei erhöhtem Gasblasenanteil, ein typisches Merkmal von H₂SiF₆-Strömen, zuverlässig.

  • Design: fully encapsulated, without seals or moving parts
  • Materials: corrosion-resistant versions, for example in Hastelloy, tantalum, or with fluoropolymer coating (PFA/Halar)
  • Installation: directly in the pipeline, without a separate bypass
  • Maintenance: no consumables, generally maintenance-free
  • Interfaces: 4–20 mA as well as digital fieldbuses (including Profibus, Modbus) for connection to the process control system
  • Explosion protection: ATEX- and Ex-certified versions available

Real time instead of delay

Concentration values are available continuously, not only after the lab sample returns.

Less handling risk

Automated measurement reduces direct personnel contact with highly corrosive and toxic acid.

Maintenance-free in continuous operation

No wear or consumable parts in the corrosive medium, reducing maintenance effort over the plant's operating life.

Application examples

The following industrial processes typically face the challenge of reliably keeping fluorosilicic acid concentration under control, each with its own measurement requirements. They all share one basic problem: Because H₂SiF₆ is usually obtained as a by-product from variable phosphate rock, the concentration varies continuously in the process. Lab analyses lag behind these fluctuations and provide only isolated spot samples instead of a continuous picture.

In addition, H₂SiF₆ is highly corrosive and toxic. Manual sampling requires protective equipment and poses a real handling risk for personnel. Conventional sensors corrode relatively quickly in the aggressive medium and drift over time. Especially in dosing applications, the monitoring effort increases because the dosing amount must be continuously adjusted to the fluctuating concentration of each batch.

The inline sound velocity measurement with LiquiSonic® meets these challenges with continuous real-time values ​​directly in the process, a fully encapsulated sensor without seals or moving parts and exact concentration values ​​as the basis for correct, documentable dosage.

Fertilizer industry

Operators of phosphoric acid production

Process engineers who separate and concentrate H₂SiF₆ as a by-product from the wet process need reliable feedback on fluoride content as the basis for yield, further processing, and plant design.

Fluorine chemistry

Manufacturers of aluminum fluoride & fluorosilicates

For the further processing of H₂SiF₆ into secondary products, a precisely defined raw material content is a prerequisite for stable reaction and consistent product quality.

Water treatment

Operators of drinking water fluoridation systems

Where H₂SiF₆ is used as a dosing chemical, the exact concentration of the delivered batch determines the correct dosing amount and therefore compliance and safety.

Trade & logistics

Quality assurance during filling & transport

When filling and loading different concentration levels, fast, independent inline concentration measurement helps avoid off-spec batches and complaints.

Over 35 years of experience in concentration measurement

Since 1990, SensoTech has been a leading provider of inline measurement technology and has established itself as a specialist in precise concentration and density measurements.

Our systems are used worldwide, including in critical applications.

35+

Years of experience

50+

Countries worldwide

ISO 9001

Certified quality

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Frequently asked questions

Why is a laboratory analysis for the fluorosilicic acid concentration not sufficient?

H₂SiF₆ is usually produced as a by-product from variable phosphate rock, causing the concentration to change continuously during the ongoing process. Manual sampling and titration, on the other hand, provide only selective, time-delayed values. By the time the laboratory result is available, the process state has often already moved on, so deviations are detected only with a delay.

How does concentration measurement using sound velocity work technically for H₂SiF₆?

The sensor sends an ultrasonic signal through the liquid and measures its transit time over a defined measurement path. Since the calculated sound velocity depends clearly on the concentration and temperature of hexafluorosilicic acid, the concentration can be determined in real time. The method is independent of the medium’s conductivity, color, or turbidity and does not require electrodes or optical windows.

In what concentration and temperature range does the sensor operate for H₂SiF₆?

The measurement accuracy is guaranteed at ± 0.05 wt% within a concentration range of 5 to 25 wt% hexafluorosilicic acid in water at process temperatures between 20 and 30 °C.

What material is the sensor made of and why is it suitable for H₂SiF₆?

For highly corrosive and fluoride-containing media such as H₂SiF₆, SensoTech uses an inline sensor made of corrosion-resistant plastics (other special materials are also available). These plastics are significantly more resistant to fluoride ions than conventional metal alloys, which can be attacked by fluorine compounds. Combined with a fully encapsulated, seal-free design without moving parts, the sensor is built for permanent contact with aggressive media.

Does the sensor also provide reliable measured values in the presence of gas bubbles or solids?

Yes, LiquiSonic® is designed for use with elevated gas bubble content in the liquid and tolerates typical process fluctuations, such as those that can occur when stripping H₂SiF₆ from phosphoric acid production.

Does the sensor need to be calibrated or maintained?

No, the measurement system is completely maintenance-free; there are no mechanical wear parts, seals, or optical windows that can be attacked by the aggressive medium. Regular recalibration is not required, and the sensors remain stable for years.

Can the sensor be installed in existing pipelines?

Yes, the inline sensor is installed directly in the process line and does not require a separate bypass. This simplifies integration into existing plants and reduces additional connection points.

What output signals does the system offer for connection to process control systems?

The LiquiSonic® measurement system provides both classic 4–20 mA outputs and common digital interfaces such as Profibus DP, Modbus, and Ethernet, allowing it to be integrated easily into existing automation and process control systems.

Is the sensor available for hazardous areas?

Yes, SensoTech sensors are also available in ATEX- and Ex-certified versions and are therefore suitable for use in hazardous plant areas.

In which industries and processes is H₂SiF₆ concentration measurement typically used?

It is particularly relevant for the fertilizer industry in phosphoric acid production, for manufacturers of fluorochemicals such as aluminum fluoride or fluorosilicates, for operators of drinking water fluoridation plants, and for quality control during the filling and transport of H₂SiF₆.

Can the measurement principle also be used for similar media?

Yes, hydrofluoric acid (HF) is chemically closely related to fluorosilicic acid and occurs in similar process environments, such as during phosphate rock digestion or in etching bath applications. The LiquiSonic® measurement system can also be used for inline concentration measurement of hydrofluoric acid based on the same corrosion-resistant, maintenance-free principles.

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