⚗️ Chlorate, chloride and chlorite in water of low contamination — ion chromatography with conductometric, UV or amperometric detection

Physicochemistry PN-EN ISO 10304-4

Overview

ISO 10304-4:2022 specifies a method for determining dissolved chlorate, chloride and chlorite ions in water of low contamination (e.g. drinking water, raw water, swimming-pool water). Because of the variety of suitable sets of equipment and of the procedural steps that depend on them, the standard gives a general description. Suitable preparation of the sample (e.g. dilution) and the use of a conductometric (CD), UV or amperometric (AD) detector make possible the working ranges of table 1: chlorate 0.03–10 mg/l (CD), chloride 0.1–50 mg/l (CD), chlorite 0.05–1 mg/l (CD), 0.1–1 mg/l (UV, 207–220 nm) and 0.01–1 mg/l (AD, 0.4–1.0 V); the range is limited by the ion-exchange capacity of the column, and the lower limit of 0.05 mg/l for chlorite was obtained in calibration checks, but interlaboratory trials showed that it is reached with difficulty and only with great care. Interferences: organic acids (mono- and dicarboxylic) and disinfection by-products (e.g. chloroacetic acid); dissolved organic substances that react with the working electrode of the AD (a loss of sensitivity); fluoride, bromide, carbonate, nitrite and nitrate (table 2 — for example 1 part of chlorate against 500 parts of nitrate, 1 part of chlorite against 1000 parts of carbonate or chloride for the CD); elevated chloride and bromide interfere with the determination of chlorite and chlorate and are removed on special exchangers; solid particles and organic compounds (mineral oils, detergents, humic acids) shorten the life of the column and are removed before the analysis. In Poland it is in force as PN-EN ISO 10304-4:2022-08 (English version; it replaced PN-EN ISO 10304-4:2002).

Chlorite and chlorate are the main by-products of disinfection with chlorine dioxide (the disproportionation of ClO₂ in a neutral and alkaline medium, photochemical decomposition); as strong oxidants they can cause changes in the blood, and therefore their sum in drinking water is limited (the PWr textbook: it should not exceed 0.7 mg/dm³). In laboratory practice they are determined by ion chromatography on the same apparatus as the anions of part 1 (ranges: chlorate 0.03–10 and chlorite 0.01–0.1 mg/dm³; a 350 µl loop for chlorite, chlorate and bromate; calibration 0.01–1.0 mg/dm³). Ion chromatography is the basic technique for determining inorganic disinfection by-products in waterworks laboratories, compatible among others with the EPA 300.0/300.1 methods.

Method principle

The chromatographic separation of chlorate, chloride and chlorite is carried out on a separation column with a low-capacity anion exchanger as the stationary phase and usually aqueous solutions of the salts of weak mono- and dibasic acids as the mobile phase (the eluent; the choice — for example based on sodium carbonate, sodium or potassium hydroxide, with organic modifiers — depends on the column and the detector; the eluents are degassed and protected from becoming saturated with air again, for example by purging with helium or with an in-line degasser). Detection: conductometric with or without a suppressor (the eluent has to have a sufficiently low conductivity, and therefore the CD is often combined with a suppressor — a cation exchanger that lowers the conductivity and converts the ions into acids), UV (direct or indirect absorption) or amperometric — the measurement of the oxidation current of the chlorite (the oxidation voltage depends on the pH of the eluent; carbon electrodes prove suitable). The concentrations are established from a calibration of the whole procedure; in particular cases — by the standard addition method; control experiments on the validity of the calibration are necessary, and sometimes repetitions. An example eluent: a concentrate of 19.1 g of Na₂CO₃ and 14.3 g of NaHCO₃ in 1000 ml (0.18 and 0.17 mol/l; stable for several months at 2–6 °C), 50 ml of which is diluted to 5000 ml (0.0018 mol/l Na₂CO₃, 0.0017 mol/l NaHCO₃), stored in dark glass and replaced every 3 days. Water: resistivity ≥ 18 MΩ·cm (25 °C), free of particles. Stock solutions of 1000 mg/l: NaClO₃ 1.2753 g (dried only in a desiccator; in glass, 1 month at 2–6 °C), NaCl 1.6484 g (dried at 105 °C; in polyethylene, 3 months), NaClO₂ ≈ 1.7 g (80 %; in glass, a week in the dark; the chlorite concentration is determined iodometrically before use in accordance with ISO 10530) — each with 1 ml of NaOH in about 800 ml of degassed water, made up to 1000 ml. Sodium chlorite may contain up to 20 % of NaCl, and therefore the chlorite standards (10 mg/l) are prepared separately from the mixed chlorate and chloride standard (10 mg/l), with 0.1 ml of NaOH per 100 ml, on the day of use; 5 to 10 calibration solutions (e.g. 0.1–1.0 mg/l from 1–10 ml of the standard made up to 100 ml with 0.1 ml of NaOH) are spread evenly over the range; the blank is water with 0.1 ml of NaOH per 100 ml. Apparatus: an eluent reservoir, an HPLC pump, an injector with a loop (e.g. 50 µl) and a column meeting the quality requirements of clause 8.

Applications

• Drinking water disinfected with chlorine dioxide or hypochlorite — chlorite and chlorate as disinfection by-products
• Raw water and swimming-pool water — control of ClO₂⁻/ClO₃⁻ and of chloride
• Waterworks laboratories — a single IC instrument for the anions (part 1) and for the disinfection by-products (part 4), compatible with EPA 300.0/300.1
• Control of chlorine dioxide generators and of hypochlorite solutions (chlorate as a decomposition product)
• Determination of chlorite at a low level (0.01 mg/l) with amperometric detection

Key parameters

Standard

Standard number

PN-EN ISO 10304-4:2022-08 (ISO 10304-4:2022)

Title (PL)

Jakość wody — Oznaczanie rozpuszczonych anionów metodą chromatografii jonowej — Część 4: Oznaczanie chloranów(V), chlorków i chloranów(III) w wodach o niskim stopniu zanieczyszczenia

Title (EN)

Water quality — Determination of dissolved anions by liquid chromatography of ions — Part 4: Determination of chlorate, chloride and chlorite in water with low contamination

Step-by-step procedure

1. Sample

   Remove solid particles and organic compounds (oils, detergents, humic acids) before the analysis; where the chloride and bromide are elevated, remove them on special exchangers; dilute to the working range if necessary.

2. Eluent

   Prepare a degassed eluent (e.g. 50 ml of the carbonate/hydrogen carbonate concentrate made up to 5000 ml) or as recommended by the column supplier; store it in dark glass, replace it every 3 days and prevent it from becoming saturated with air again.

3. Standards

   From the stock solutions of 1000 mg/l (the chlorite standardised iodometrically) prepare a mixed ClO₃⁻/Cl⁻ standard of 10 mg/l and a separate ClO₂⁻ standard of 10 mg/l (0.1 ml of NaOH per 100 ml); prepare 5 to 10 calibration solutions spread evenly over the range (e.g. 0.1–1.0 mg/l) and a blank — on the day of use.

4. Calibration and control

   Establish the calibration function of the whole procedure for the chosen detector (CD/UV/AD); check its validity by control experiments; with difficult matrices use the standard addition method.

5. Analysis

   Inject the samples (loop of e.g. 50 µl); record the CD signal (with or without a suppressor), the UV signal at 207–220 nm or the AD signal at 0.4–1.0 V; check that the quality requirements for the column are met (clause 8) — dilute the sample if necessary; repeat the measurement when required.

6. Result

   Read the concentrations of ClO₃⁻, Cl⁻ and ClO₂⁻ in mg/l from the calibration; state the detector used, the preparation of the sample and any deviations.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Sodium chlorite and sodium chlorate are strong oxidants — keep them away from combustible substances and acids (release of ClO₂)
• Sodium hydroxide — goggles, gloves
• Helium/degassing and the HPLC system under pressure — check that it is tight

Frequently asked questions