💧 Free and Total Chlorine in Water

Physicochemistry PN-EN ISO 7393-2

Overview

STATUS (as of 28.10.2026). PN-EN ISO 7393-2:2018-04 — English version, published 24.04.2018, implements EN ISO 7393-2:2018 and ISO 7393-2:2017 [IDT]; it replaced PN-EN ISO 7393-2:2011. The standard is current — no withdrawal note.

SCOPE (clause 1). The method determines free chlorine and total chlorine in water, in the laboratory and in the field. The absorbance of the red colour complex is measured with a photometer, or the colour is compared visually with a scale of standards that is regularly checked. The method is suitable for drinking water and other waters in which other halogens (bromine, iodine) and other oxidizing agents are present in almost negligible amounts; seawater and waters containing bromides and iodides require special procedures. Practical range: 0.0004 to 0.07 mmol/l, i.e. 0.03 to 5 mg/l of total chlorine as Cl₂; at higher concentrations the test portion is diluted. Informative Annex A describes the differentiation of combined chlorine of the monochloramine, dichloramine and nitrogen trichloride type, and Annex C — disposable reagent-filled cuvettes for drinking water and other low-polluted waters.

TERMS (clause 3). Free chlorine is chlorine present as hypochlorous acid, hypochlorite ion or dissolved elemental chlorine. Combined chlorine is the part of total chlorine present as chloramines — monochloramine NH₂Cl, dichloramine NHCl₂, nitrogen trichloride NCl₃ — and organic chloramines. Total chlorine is the sum of free and combined chlorine.

PRINCIPLE (clause 4). Free chlorine reacts directly with DPD at pH 6.2–6.5, forming a red colour complex. Total chlorine is determined by the same reaction in the presence of an excess of potassium iodide. Ready-to-use kits (tablets, powders, solutions) must contain a suitable buffer system, acid, EDTA and DPD; a kit with a different pH range must be validated by the user for their samples. The sample pH before the test should be between 4 and 8 — if necessary it is adjusted with sodium hydroxide or sulfuric acid.

MEASUREMENT (7.1–7.2). A spectrometer, photometer or colorimeter for use at (510 ± 20) nm or (550 ± 20) nm, with cells of an optical path length of at least 10 mm — or a comparator with a scale of permanent colour standards (glass, plastic) for the DPD technique. Field photometers with ready-to-use reagents are calibrated by the manufacturer and its instructions must be followed.

INTERFERENCES (clause 5). Chlorine dioxide is measured as total chlorine (this can be corrected by a separate determination of chlorine dioxide). The reaction is also affected, depending on concentration, by other oxidizing agents: bromine, iodine, bromamines, iodamines, ozone, hydrogen peroxide, chromate, oxidized manganese, nitrite, iron(III) ions, peracetic acid and copper ions. EDTA in the buffer and in the DPD reagent suppresses the interference of copper(II) below 8 mg/l and iron below 20 mg/l, chromate is eliminated with an excess of barium chloride, and the effect of oxidized manganese is assessed by an additional determination on a portion of the sample treated with sodium arsenite or thioacetamide. In waste waters and cooling waters the amount of interfering substances may be large — the user has to demonstrate how they deal with them. Turbid samples, or samples in which a precipitate forms after adding the buffer, are filtered with chlorine-demand-free equipment, but filtration may lower the free chlorine result.

DRINKING WATER AND SWIMMING POOLS — LEGAL REQUIREMENTS. Regulation of the Minister of Health of 22 May 2026 on the quality of water intended for human consumption (Journal of Laws 2026, item 748), Annex 1 Part D: free chlorine up to 0.3 mg/l and chloramines up to 0.5 mg/l at the consumer's tap, if the water is disinfected with chlorine or its compounds; in water tanks of land, air or water transport vehicles free chlorine 0.3–0.5 mg/l. Annex 4 Part B: measurement uncertainty for free chlorine and chloramines up to 30 % of the parametric value, limit of quantification not greater than 30 % of the parametric value. For swimming pool water (Regulation of the Minister of Health of 9 November 2015, consolidated text Journal of Laws 2022, item 1230): in the pool basin free chlorine 0.3–0.6 mg/l, in basins with equipment producing a water-air aerosol 0.7–1.0 mg/l, in basins for teaching infants and children up to 3 years 0.3–0.4 mg/l; combined chlorine in the basin up to 0.3 mg/l.

WHAT WE DO NOT GIVE HERE. Precision data from Annex B or the procedures of Annexes A and C — we only know their titles. The course of the determination of free and total chlorine and the calculation formulae (9.4–10) are given according to the previous edition of the standard (ISO 7393-2:1985); in the current, technically revised edition, clause 9.4 begins with the same words.

Method principle

Free chlorine reacts directly with N,N-dialkyl-1,4-phenylenediamine (DPD) at pH 6.2–6.5, giving a red colour complex; its intensity is measured photometrically at (510 ± 20) nm or (550 ± 20) nm or compared visually with a scale of colour standards. Total chlorine is determined by the same reaction in the presence of an excess of potassium iodide.

The measurement is made immediately after sampling — preferably on site, within 5 minutes. Free chlorine is measured immediately after mixing the sample with the buffer and the DPD reagent, and total chlorine — 2 minutes after adding potassium iodide. Combined chlorine is the difference between total and free chlorine. Calibration is based on potassium iodate standard solutions, which release iodine equivalent to chlorine in acid medium.

Applications

• Water intended for human consumption — free chlorine (up to 0.3 mg/l) and chloramines (up to 0.5 mg/l) at the consumer's tap when the water is disinfected with chlorine or its compounds (Journal of Laws 2026, item 748)
• Swimming pool water — free and combined chlorine in pool basins (requirements: Journal of Laws 2022, item 1230)
• Control of disinfection in the water supply network and in water tanks of transport vehicles (free chlorine 0.3–0.5 mg/l)
• Field measurements with portable photometers and ready-to-use reagents — when the reagents meet the requirements of the standard
• Waste waters and cooling waters — only after the user has demonstrated how the effect of interfering substances is eliminated

Key parameters

Standard

Standard number

PN-EN ISO 7393-2:2018-04

Title (PL)

Jakość wody — Oznaczanie chloru wolnego i chloru ogólnego — Część 2: Metoda kolorymetryczna z N,N-dialkilo-1,4-fenylenodiaminą, do rutynowych celów kontrolnych

Title (EN)

Water quality — Determination of free chlorine and total chlorine — Part 2: Colorimetric method using N,N-dialkyl-1,4-phenylenediamine, for routine control purposes

Step-by-step procedure

1. Sampling

   Preferably analyse on site within 5 minutes of sampling. If this is not possible — chlorine-demand-free dark glass bottles filled completely, analysis immediately after arrival; protect from bright light, agitation and heat. The time between sampling and analysis and the reason for the laboratory measurement are stated in the test report (clause 8).

2. Test portions

   Two portions of 100.0 ml. When total chlorine exceeds 5 mg/l (70 µmol/l), a smaller volume of sample is diluted with water to 100.0 ml. Turbid samples — preferably with blank value matching; filter only when unavoidable, with chlorine-demand-free equipment (9.1–9.2).

3. Calibration

   Into 100 ml volumetric flasks measure 0.3 to 50 ml of the potassium iodate standard solution (0.03–5 mg/l Cl₂), add 1.0 ml of sulfuric acid, after 1 min 1.0 ml of sodium hydroxide, make up to 100 ml with water. Transfer each solution to a flask containing 5 ml of buffer and 5 ml of DPD (added less than a minute before), mix and measure within 2 min. Calibration according to ISO 8466-1 for each new DPD solution, daily check of one point (9.3).

4. Free chlorine

   Transfer the first portion without rinsing to a 250 ml conical flask with 5 ml of buffer and 5 ml of DPD solution, mix, fill the cell and measure immediately under the calibration conditions — reading c1. With very acid, very alkaline or saline water check that the buffer has brought the pH to 6.2–6.5; if not, add more buffer.

5. Total chlorine

   Transfer the second portion without rinsing to a flask with 5 ml of buffer and 5 ml of DPD, add about 1 g of potassium iodide, mix, fill the cell and measure after 2 minutes — reading c2.

6. Oxidized manganese (when it may be present)

   Additional portion: in a 250 ml flask add 1 ml of sodium arsenite or thioacetamide solution, mix, then 5 ml of buffer and 5 ml of DPD; measure immediately — reading c3 corresponds to oxidized manganese and is subtracted from the results (5.4).

7. Result

   Free chlorine: c = (c1 − c3) · V0 / V1, total chlorine: c = (c2 − c3) · V0 / V1, in mmol/l (V0 = 100 ml, V1 — volume of sample in the test portion; without oxidized manganese c3 = 0). Conversion to mg/l Cl₂ — factor 70.91. Combined chlorine = total chlorine − free chlorine.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• The standard states that it does not address all safety problems — practices are established by the user, and the test is to be carried out by suitably qualified staff
• Solutions containing mercury(II) chloride (buffer additive) must be disposed of safely (6.2)
• Sodium arsenite for the manganese correction (6.10) is an arsenic compound — handle it according to its safety data sheet or use thioacetamide instead, which the standard allows
• Sulfuric acid (ρ = 1.84 g/ml) and sodium hydroxide are corrosive — add the acid to water cautiously, with continuous stirring (6.5)

Frequently asked questions