Water Industry Journal

Monitoring fluoride in drinking water

Fluoride is a natural mineral found in rocks, soil, water and air. Although it is an organic mineral, it is also added to everyday items like toothpaste, mouthwash and water supply.

Fluoride helps prevent tooth decay and cavities. The enamel of a tooth should be solid and strong, but certain substances like sugar and refined carbohydrates create bacteria in the mouth that destroy tooth enamel. Fluoride neutralises these bad bacteria keeping enamel strong and healthy.

Government plans therefore to expand fluoridation schemes of the water supply across England, to improve children’s oral health, will be welcomed by many.

Monitoring fluoride in drinking water

The amount of fluoride in drinking water should be monitored regularly. The two most common ways to measure Fluoride using an Ion Selective Electrode (ISE) are; direct measurement and standard addition (recommended).
When performing a direct measurement, the ion-selective electrode is calibrated with standard solutions of the ion to be measured. This is done before the ion measurement itself, like the calibration of a pH glass electrode. The calibration can then be used for several determination series.
During a standard addition however, defined volumes of a standard solution of the ion of interest are added to a known volume of the sample solution in several increments. After each addition of the standard solution, the potential of the solution is measured. The ion concentration of the original sample solution can then be calculated from the difference between the initial potential and the potential measured after each addition.

The disadvantages of direct measurement are:

• A series of standards for calibration must be prepared which can be time consuming
• Matrix-dependent
• Change of electrode properties is only seen during calibration

The advantages of the recommended standard addition technique are:

• Matrix-independent
• No calibration necessary
• Only a single standard is needed
• Nearly no user interaction necessary
• Electrode is tested with each measurement

The Metrohm combined fluoride ISE integrates both the measuring and reference electrode as well as a temperature sensor (Pt1000) in a single housing. This design saves space, which makes this sensor ideal for measurements in small sample volumes. The ground-joint diaphragm with its large surface enables consistent electrolyte outflow on all sides. The signal is therefore more stable and there is a smaller risk of a blocked diaphragm. This benefit allows usage even in contaminated samples without negatively affecting the electrode performance.
The measured potential in the sample solution is temperature dependent. Therefore, it is important to determine the temperature for both the calibration and the sample to achieve highest accuracy per determination. The included Pt1000 enables a fast temperature measurement without needing an additional sensor. With this, handling errors can be reduced.
The combined F-ISE is available as either an analog or a digital version. The digital version can be used only with our latest titration platform OMNIS and has the advantage that it is automatically recognised by the software and therefore it can be monitored.

Online Fluoride monitoring solution

In addition to sample testing in the lab, Metrohm offer an online ‘continuous monitoring’ solution with the same standard addition ISE method. The 2026 Fluoride Analyser from Metrohm Process Analytics is the most straightforward and easy-to-use tool to do so online.
Fluoride is determined by Dynamic Standard addition (DSA) method over a defined calibration range. The detection is performed with a Metrohm Fluoride ion-selective electrode (F-ISE). The analyser can handle a wide range of Fluoride concentrations, from mg/L to %.

Save money and reduce risk by more accurately monitoring Fluoride content:

✓ In drinking water / (potable water)
✓ In wastewater treatment plants / (chemical)
✓ In toothpaste production / (food & beverage)
✓ In zinc – phosphating baths / (automotive)
✓ In etching baths / (semiconductor)

Contact Metrohm for more info

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