⚗️ Chlorides in Water
In short
Titrimetric determination of dissolved chloride in water by Mohr's method — silver nitrate titration with potassium chromate as indicator. The test is performed according to PN-ISO 9297:1994; Range: 5–150 mg/l; up to 400 mg/l with a burette of larger capacity or after dilution. The procedure comprises 7 steps; it is used for: Drinking water — chloride 250 mg/l (Journal of Laws 2026, item 748, Annex 1 Part C), Ground and surface waters (1.2), Waste water — PN-ISO 9297 is the reference method for chloride (Journal of Laws 2019, item 1311, Annex 12); the precision data of the standard include municipal waste water (Table 2).
At a glance
- Standard: PN-ISO 9297:1994
- Category: Physicochemistry
- Procedure steps: 7
- Range: 5–150 mg/l; up to 400 mg/l with a burette of larger capacity or after dilution
- Standard volumetric solution: AgNO₃ 0.02 mol/l (1 ml corresponds to 0.709 mg Cl⁻)
- Indicator: potassium chromate 100 g/l, 1 ml; end point — red-brown colour
Overview
STATUS (as of 28.10.2026). PN-ISO 9297:1994 — Polish version, published 22.12.1994, implements ISO 9297:1989 [IDT]; it replaced PN-C-04617-02:1975. The standard is current — no withdrawal note.
SCOPE (clause 1). Titrimetric determination of dissolved chloride in water in the range 5–150 mg/l; the range can be extended to 400 mg/l by using a burette of larger capacity or by diluting the sample. Because of many interferences, the method is not applied to heavily polluted waters of low chloride content. Normal concentrations of the common constituents of ground water, surface water and drinking water do not interfere.
INTERFERENCES (1.2, Table 1). Interfering are substances forming insoluble silver compounds (bromides, iodides, sulfides, cyanides, hexacyanoferrates(II) and (III)), compounds forming complexes with silver (ammonium and thiosulfate ions) and compounds that reduce chromate (including iron and sulfite) — all of them lead to high results; if necessary, bromide and iodide are determined separately and the result is corrected. Highly coloured or turbid solutions may obscure the end point. Table 1 gives the concentrations that increase the result by about 2 % in the presence of 70 mg/l of chloride, e.g. bromide 3 mg/l, iodide 5 mg/l, ammonium 100 mg/l, phosphate 25 mg/l.
PRECISION (Table 2). Interlaboratory trial (Germany, 1983): drinking water with a mean of 12.75 mg/l — repeatability variation coefficient 1.7 %, reproducibility 4.5 %; drinking water with added chloride, 64.20 mg/l — 0.6 % and 1.2 %; municipal waste water, 106.6 mg/l — 0.6 % and 1.2 %.
LEGAL REQUIREMENTS. Regulation of the Minister of Health of 22 May 2026 (Journal of Laws 2026, item 748): chloride 250 mg/l; the water should not be corrosive (Annex 1 Part C Table 2 item 2); measurement uncertainty — 15 % of the parametric value (Annex 4 Part B). The Regulation of the Minister of Maritime Economy and Inland Navigation of 12 July 2019 (Journal of Laws 2019, item 1311) lists PN-ISO 9297 as the reference method (volumetric method) for chloride in waste water, alongside PN-EN ISO 10304-2 and PN-EN ISO 15682 (Annex 12, item 17).
CHLORIDE IN WATER (textbook “Chemia wody”, Wrocław University of Science and Technology, 2024). Chloride comes, among others, from rock weathering, runoff from agricultural land, industrial waste water and mine waters. Sodium chloride at concentrations above 250 mg/l gives water a salty taste, and chloride — like sulfate — accelerates the corrosion of water pipes.
Method principle
Chloride reacts with added silver ions to form insoluble silver chloride, which precipitates quantitatively. A small excess of silver ions forms red-brown silver chromate with the chromate ions added as an indicator — this reaction indicates the end point. Throughout the titration the pH is maintained in the range 5 to 9.5 to allow precipitation.
ρCl = (Vs − Vb) · c · f / Va [mg/l], where Vs and Vb are the volumes of silver nitrate used for the sample and for the blank (ml), c is the actual concentration of the silver nitrate (mol/l), f = 35 453 mg/mol, Va is the volume of the test sample (ml, at most 100 ml, dilutions taken into account).
Applications
- Drinking water — chloride 250 mg/l (Journal of Laws 2026, item 748, Annex 1 Part C)
- Ground and surface waters (1.2)
- Waste water — PN-ISO 9297 is the reference method for chloride (Journal of Laws 2019, item 1311, Annex 12); the precision data of the standard include municipal waste water (Table 2)
- Concentrations of 5–150 mg/l, and with a larger burette or after dilution — up to 400 mg/l
Key parameters
| Parameter | Value |
|---|---|
| Range | 5–150 mg/l; up to 400 mg/l with a burette of larger capacity or after dilution |
| Standard volumetric solution | AgNO₃ 0.02 mol/l (1 ml corresponds to 0.709 mg Cl⁻) |
| Indicator | potassium chromate 100 g/l, 1 ml; end point — red-brown colour |
| Titration pH | 5–9.5; with ammonium above 10 mg/l — 6.5–7 |
| Blank | not more than 0.2 ml AgNO₃ per 100 ml of water |
| Precision (Table 2) | CVr 0.6–1.7 %, CVR 1.2–4.5 % (12.75–106.6 mg/l) |
Standard
- Standard number
- PN-ISO 9297:1994
- Title (PL)
- Jakość wody — Oznaczanie chlorków — Metoda miareczkowania azotanem srebra w obecności chromianu jako wskaźnika (Metoda Mohra)
- Title (EN)
- Water quality — Determination of chloride — Silver nitrate titration with chromate indicator (Mohr's method)
Step-by-step procedure
-
Sample
Sampling and preservation according to ISO 5667-1, -2 and -3. Pipette a 100 ml test portion (or a smaller volume diluted to 100 ml) into a white porcelain basin, or into a conical flask or beaker held against a white background (6.1).
-
pH adjustment
If the pH is outside 5–9.5, adjust it with nitric acid or sodium hydroxide and note the volume used; with ammonium above 10 mg/l adjust the pH to 6.5–7. The pH is adjusted in one aliquot, and the same amounts of acid or hydroxide are added to the next one without measuring the pH. If the pH is below 5, calcium carbonate or sodium hydrogen carbonate is preferable (6.1, note 2).
-
Indicator
Add 1 ml of potassium chromate solution (6.1).
-
Titration
Titrate by dropwise addition of silver nitrate until the colour of the solution just changes to red-brown (Vs). After adding one drop of sodium chloride solution the colour should disappear; the sample titrated in this way is kept for comparison with the next titrations (6.1).
-
Large titrant volume
If the volume used exceeds 25 ml, the determination is repeated with a burette of larger capacity or a smaller test portion (6.1).
-
Blank test
100 ml of water instead of the sample; the volume used should not exceed 0.2 ml of silver nitrate — otherwise the purity of the water must be checked (6.2).
-
Calculation
ρCl = (Vs − Vb) · c · f / Va, f = 35 453 mg/mol; the result is reported to the nearest 1 mg/l, giving three significant figures only (clause 7).
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| 25 ml burette | complying with ISO 385-1; if the volume used exceeds 25 ml — a burette of larger capacity or a smaller test portion (5.1, 6.1) | — |
| Titration vessel | a white porcelain basin, or a conical flask or beaker against a white background (6.1) | — |
| Pipette | for a 100 ml test portion or a smaller volume diluted to 100 ml (6.1) | — |
| pH measurement | to check whether the pH of the sample is within 5–9.5 (6.1) | — |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Silver nitrate — standard volumetric solution 0.02 mol/l | 7761-88-8 | dissolve 3.3974 g of AgNO₃ dried at 105 °C in water and dilute to 1000 ml; stable for several months if stored in the dark in a brown glass bottle with glass stoppers; standardized against 10 ml of sodium chloride solution diluted to 100 ml, without pH adjustment (4.1) |
| Potassium chromate — indicator, 100 g/l | 7789-00-6 | dissolve 10 g of K₂CrO₄ in water and dilute to 100 ml (4.2) |
| Sodium chloride — standard reference solution 0.02 mol/l | 7647-14-5 | dissolve 1.1688 g of NaCl dried at 105 °C in water and dilute to 1000 ml (4.3) |
| Nitric acid about 0.1 mol/l and sodium hydroxide about 0.1 mol/l | — | for adjusting the pH of the sample (4.4–4.5) |
| Calcium carbonate or sodium hydrogen carbonate powder | — | for adjusting a pH below 5 and improving the buffer capacity — in such an amount that a carbonate residue remains after titration (4.6, note 2) |
Health and safety (OHS)
- All silver compounds and solutions are sensitive to light; silver salts temporarily stain the skin brown (note 1 to clause 4)
- Potassium chromate is a chromium(VI) compound — handle it according to the safety data sheet
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-ISO 9297:1994 — “Water quality — Determination of chloride — Silver nitrate titration with chromate indicator (Mohr's method)”.
How does this method work?
Chloride reacts with added silver ions to form insoluble silver chloride, which precipitates quantitatively. A small excess of silver ions forms red-brown silver chromate with the chromate ions added as an indicator — this reaction indicates the end point.
What is the measuring range and accuracy?
Range: 5–150 mg/l; up to 400 mg/l with a burette of larger capacity or after dilution; Standard volumetric solution: AgNO₃ 0.02 mol/l (1 ml corresponds to 0.709 mg Cl⁻); Indicator: potassium chromate 100 g/l, 1 ml; end point — red-brown colour; Titration pH: 5–9.5; with ammonium above 10 mg/l — 6.5–7.
How long does the test take?
The procedure comprises 7 steps. The standard does not give a duration for every stage — the laboratory's own procedure decides.
What equipment is required?
25 ml burette, Titration vessel, Pipette, pH measurement.
Where is this test used?
Drinking water — chloride 250 mg/l (Journal of Laws 2026, item 748, Annex 1 Part C); Ground and surface waters (1.2); Waste water — PN-ISO 9297 is the reference method for chloride (Journal of Laws 2019, item 1311, Annex 12); the precision data of the standard include municipal waste water (Table 2); Concentrations of 5–150 mg/l, and with a larger burette or after dilution — up to 400 mg/l.
What safety precautions apply?
All silver compounds and solutions are sensitive to light; silver salts temporarily stain the skin brown (note 1 to clause 4); Potassium chromate is a chromium(VI) compound — handle it according to the safety data sheet.
Which laboratory can perform this test?
The test is performed by laboratories accredited to ISO/IEC 17025. On LabCoda you can find them by the standard number PN-ISO 9297.