⚗️ Chlorides in Water
In short
Titrimetric determination of chlorides in water by Mohr method — silver nitrate titration with potassium chromate indicator. The test is performed according to PN-ISO 9297:1994; Measurement range: 5–150 mg Cl⁻/L (without dilution). The procedure comprises 8 steps (about 25–30 min in total); it is used for: Drinking water quality control (limit ≤250 mg Cl⁻/L), Groundwater testing (salinity, seawater intrusion), Road salting impact on water monitoring.
At a glance
- Standard: PN-ISO 9297:1994
- Category: Physicochemistry
- Procedure steps: 8
- Total time of stages: 25 min–30 min
- Measurement range: 5–150 mg Cl⁻/L (without dilution)
- Extended range: up to 400 mg/L (with larger burette or dilution)
- Titrant concentration: AgNO₃ 0.02 mol/L (1 mL = 0.709 mg Cl⁻)
Overview
Chlorides (Cl⁻) are one of the most frequently determined anions in water analysis. They originate from natural sources (rock leaching, seawater), anthropogenic sources (road salting, municipal sewage, chemical industry), and water treatment processes (chlorine disinfection).
Chloride concentration in drinking water should not exceed 250 mg/L (Polish Ministry of Health Regulation) — higher concentrations cause salty taste and may intensify installation corrosion. In natural waters chlorides are salinity indicators — typical concentrations: groundwater 10–100 mg/L, seawater ~19 000 mg/L.
Mohr method (ISO 9297) is classical titrimetric method — simple, rapid, requires no expensive equipment. Used for waters with concentrations 5–150 mg Cl⁻/L (up to 400 mg/L with modifications). Not suitable for highly colored or contaminated waters (interferences).
Method principle
Mohr method is precipitation titration. Chlorides in water sample react with silver nitrate (AgNO₃) forming white silver chloride precipitate (AgCl):
Cl⁻ + Ag⁺ → AgCl↓ (white)
After binding all chlorides, excess Ag⁺ ions react with potassium chromate (K₂CrO₄), which imparts yellow indicator color to solution, forming brick-red silver chromate precipitate:
2Ag⁺ + CrO₄²⁻ → Ag₂CrO₄↓ (brick-red)
Appearance of persistent brick-red color (endpoint) indicates all chlorides have been precipitated. Titration is conducted at pH 6.5–10.5 (optimally pH 8).
Cl⁻ [mg/L] = (V_AgNO₃ × C_AgNO₃ × 35.45 × 1000) / V_sample
Applications
- Drinking water quality control (limit ≤250 mg Cl⁻/L)
- Groundwater testing (salinity, seawater intrusion)
- Road salting impact on water monitoring
- Industrial water quality control (boilers, coolers)
- Surface water testing (salinity)
- Swimming pool water quality control
Key parameters
| Parameter | Value |
|---|---|
| Measurement range | 5–150 mg Cl⁻/L (without dilution) |
| Extended range | up to 400 mg/L (with larger burette or dilution) |
| Titrant concentration | AgNO₃ 0.02 mol/L (1 mL = 0.709 mg Cl⁻) |
| Indicator | K₂CrO₄ — color change: yellow → brick-red |
| Titration pH | 6.5–10.5 (optimally ~8) |
| Precision | ±2% (at concentration >50 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
-
AgNO₃ standardization
Accurately weigh ~0.1170 g NaCl (dried 140°C/2h) into Erlenmeyer flask. Dissolve in 100 mL water. Add 1 mL K₂CrO₄. Titrate with AgNO₃ to persistent brick-red color change.
-
Sample preparation
Measure 100 mL water sample into 250 mL Erlenmeyer flask. Check pH — must be 6.5–10.5. Correct if necessary with NaOH or H₂SO₄. Sample must be clear — filter turbid samples.
-
Indicator addition
Add 1 mL potassium chromate K₂CrO₄ solution (100 g/L). Solution will turn yellow — this is indicator color before endpoint.
-
Titration
Fill burette with AgNO₃ 0.02 mol/L solution. Titrate slowly, mixing (magnetic stirrer or manually). Observe — white AgCl flakes precipitate and dissolve with mixing.
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Endpoint
Continue titration until solution changes color from yellow to persistent brick-red (lasting min. 30 seconds). Record consumed AgNO₃ volume from burette with 0.05 mL accuracy.
-
Blank sample
Perform blank titration: 100 mL deionized water + 1 mL K₂CrO₄. AgNO₃ consumption for blank subtract from result.
-
Calculations
Cl⁻ [mg/L] = [(V₁ - V₀) × c × 35.45 × 1000] / V_sample, where V₁ = AgNO₃ volume for sample, V₀ = blank, c = AgNO₃ concentration [mol/L], V_sample in mL.
-
Quality control
Titrate duplicate — difference max 2%. NaCl control standard — deviation max ±5%. Standardize AgNO₃ with each new batch.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Glass burette 25 or 50 mL | Class AS burette from borosilicate glass, 0.05 mL graduation | 150–400 PLN |
| Digital burette (optional) | Brand Titrette, Eppendorf Top Buret — higher accuracy | 1 500–3 500 PLN |
| Erlenmeyer flasks 250 mL | Borosilicate glass, wide neck | 10–25 PLN/pc |
| Volumetric pipettes / automatic | Volumetric pipette 100 mL class A or automatic pipette | 100–800 PLN |
| Magnetic stirrer | For uniform mixing during titration | 1 500–4 000 PLN |
| pH meter (for pH correction) | For checking and correcting sample pH to range 6.5–10.5 | 3 000–15 000 PLN |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Silver nitrate AgNO₃ | 7761-88-8 | Titrant 0.02 mol/L (3.3974 g/L) or 0.1 mol/L. Standardization vs NaCl. Store in dark bottle (photosensitive!). |
| Potassium chromate K₂CrO₄ | 7789-00-6 | Indicator: solution 100 g/L. Add 1 mL per 100 mL titrated solution. Yellow color = indicator, brick-red = endpoint. |
| Sodium chloride NaCl | 7647-14-5 | Standard for AgNO₃ standardization. Dried at 140°C/2h. Analytical purity. |
| NaOH / H₂SO₄ diluted | — | For sample pH correction to range 6.5–10.5. |
| Deionized water | — | For solution preparation and blank. Checked for chloride absence. |
Health and safety (OHS)
- Silver nitrate AgNO₃ — corrosive, causes permanent skin stains (black). Use nitrile gloves!
- Potassium chromate K₂CrO₄ — toxic, carcinogenic (Cr⁶⁺, cat. 1B), mutagenic. Minimize exposure, work carefully.
- Silver and chromium-containing waste — collect separately as hazardous waste.
- Safety goggles, nitrile gloves, and laboratory coat mandatory.
- Do not pour post-titration waste into sink — collect in waste container.
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?
Mohr method is precipitation titration. Chlorides in water sample react with silver nitrate (AgNO₃) forming white silver chloride precipitate (AgCl): Cl⁻ + Ag⁺ → AgCl↓ (white) After binding all chlorides, excess Ag⁺ ions react with potassium chromate (K₂CrO₄), which imparts yellow indicator color to solution, forming brick-red silver chromate precipitate: 2Ag⁺ + CrO₄²⁻ → Ag₂CrO₄↓ (brick-red) Appearance of persistent brick-red color (endpoint) indicates all chlorides have been precipitated.
What is the measuring range and accuracy?
Measurement range: 5–150 mg Cl⁻/L (without dilution); Extended range: up to 400 mg/L (with larger burette or dilution); Titrant concentration: AgNO₃ 0.02 mol/L (1 mL = 0.709 mg Cl⁻); Indicator: K₂CrO₄ — color change: yellow → brick-red.
How long does the test take?
The times given for the individual stages add up to approximately 25 min–30 min. The procedure comprises 8 steps.
What equipment is required?
Glass burette 25 or 50 mL, Digital burette (optional), Erlenmeyer flasks 250 mL, Volumetric pipettes / automatic, Magnetic stirrer, pH meter (for pH correction). Indicative cost of the core instrument (Glass burette 25 or 50 mL): 150–400 PLN.
Where is this test used?
Drinking water quality control (limit ≤250 mg Cl⁻/L); Groundwater testing (salinity, seawater intrusion); Road salting impact on water monitoring; Industrial water quality control (boilers, coolers); Surface water testing (salinity); Swimming pool water quality control.
What safety precautions apply?
Silver nitrate AgNO₃ — corrosive, causes permanent skin stains (black). Use nitrile gloves!; Potassium chromate K₂CrO₄ — toxic, carcinogenic (Cr⁶⁺, cat. 1B), mutagenic. Minimize exposure, work carefully.; Silver and chromium-containing waste — collect separately as hazardous waste.; Safety goggles, nitrile gloves, and laboratory coat mandatory.; Do not pour post-titration waste into sink — collect in waste container.
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-EN ISO 9297.