PD ISO TS 17951 — Water Quality Determination of Fluoride using Flow Analysis (FIA and CFA) (Parts 1 & 2)

📐 PD ISO/TS 17951 is a BSI Published Document covering the determination of fluoride in water using flow analysis (FIA and CFA). Fluoride is a common ion in drinking water and the environment: at appropriate levels it is beneficial to dental health, but at excessive levels it may cause fluorosis. Accurate determination of fluoride in water is therefore essential for water supply safety, public health and environmental monitoring. Part 1 uses flow injection analysis (FIA) with off-line distillation and spectrometric detection; Part 2 uses continuous flow analysis (CFA) with automated in-line distillation. The series was prepared by BSI Technical Committee EH/3/2 and published in July 2016.

1. Scope and General Framework 📌

PD ISO/TS 17951 applies to the determination of fluoride in various water samples (drinking water, surface water, groundwater, wastewater, etc.). Part 1 specifies a method using flow injection analysis (FIA) with off-line distillation followed by spectrometric detection. Part 2 specifies a method using continuous flow analysis (CFA) with automated in-line distillation. Both rely on the reaction of fluoride with a colour-forming reagent (e.g. lanthanum-alizarin complexone or a similar system) to form a coloured complex that is quantified spectrophotometrically.

2. Principle 🔧

Both methods are based on the colour reaction of fluoride with a specific colour-forming reagent. In acid medium, fluoride undergoes a ligand-exchange reaction with lanthanum-alizarin complexone (or a similar metal-dye complex), changing the solution colour; the absorbance is proportional to the fluoride concentration. FIA injects a small sample volume into a continuously flowing carrier, where mixing and colour development occur in a reaction coil before entering a flow-cell detector. CFA delivers sample and reagents at a constant ratio via a peristaltic pump, passes through an on-line distillation and colour development stage, and enters the detector.

The distillation step converts fluoride to volatile fluorosilicic acid (H₂SiF₆), which is carried over in a steam flow, condensed and collected, thus separating fluoride from the matrix. The automated in-line distillation of Part 2 integrates this step into the flow system for full automation.

3. Interferences and their Removal 🔥

Main interferences in fluoride determination come from coexisting cations and anions:

• Iron, aluminium, calcium and other metal ions: form stable complexes or precipitates with fluoride, reducing measurable free fluoride and causing negative bias.
• Phosphate and silicate: at high concentration they may interfere with the colour reaction or compete for the colour reagent.
• High-salt and organic matrices: affect colour stability and distillation recovery.

Distillation pre-treatment effectively removes these interferences. In addition, the standard specifies pH control with a buffer, addition of masking agents, optimisation of distillation conditions, and the use of standard addition or matrix matching to remove residual matrix effects.

4. Reagents, Apparatus and Procedure 🔩

Key reagents include: sodium fluoride stock and working standard solutions, colour reagent (lanthanum-alizarin complexone, etc.), buffer solution, masking agents, and acid for distillation. For apparatus, an FIA system comprises an injection valve, peristaltic pump, reaction coil, flow cell and spectrophotometric detector; a CFA system comprises a multi-channel peristaltic pump, in-line distillation device, mixing coils, air-segmentation device and detector.

A typical procedure is: sample collection and preservation → distillation (off-line or in-line) → reaction with colour reagent → spectrophotometric detection → quantification against a standard curve → calculation and reporting. Each batch should include reagent blanks, duplicate samples and QC samples.

5. Engineering Practice Essentials 🛠️

• 🔍 Method choice: prefer CFA (Part 2) when high throughput and automation are needed; use FIA (Part 1) for flexible injection with smaller sample numbers.
• 🔧 Distillation verification: confirm distillation recovery for different matrices by spike recovery tests, adjusting distillation conditions where necessary.
• 📊 Quality control: use certified reference materials, duplicate samples and blanks, prepare and verify calibration curves, and ensure data reliability.
• 📋 Reporting: report results as fluoride (F⁻) mass concentration, stating the detection limit, working range and precision, and noting the pre-treatment method.

6. Frequently Asked Questions ❓