Mohr titration for halide determination in seawater
A Mohr titration estimates chloride in seawater by precipitating silver halides with standardized silver nitrate and using chromate to mark the endpoint. It suits teaching and community laboratories without an autotitrator.
Seawater complicates a fresh-water method. High chloride, coloured dissolved constituents and suspended matter require dilution, controlled pH and quality checks. The result may represent chloride-equivalent halide rather than chloride alone.
The workflow below adapts the visual endpoint for Australian coastal samples, covering collection, reagent preparation, calculation, safety and reporting.
Why seawater changes the chemistry
During the reaction, added Ag⁺ first precipitates Cl⁻ and other silver-reactive halides as AgX. Once their concentrations fall, excess Ag⁺ reacts with chromate to form a brick-red Ag₂CrO₄ endpoint.
Open seawater at about 35 practical salinity units normally contains approximately 19,000 mg/L chloride, or 0.54 mol/L. At the usual 0.1 M titrant concentration, a 10 mL aliquot requires about 54 mL of AgNO₃, making visual detection unnecessarily difficult.
Bromide also consumes silver before the endpoint, although its contribution is only about 0.15% of chloride on a molar basis. Iodide is normally negligible. Label uncorrected results as total silver-reactive halide in chloride equivalents, rather than as chloride alone.
Prepare the sample and reagents
Collect coastal water in a clean HDPE bottle and analyse it promptly. Rinse the container with sample, document temperature and salinity, and collect a field duplicate where the sampling programme permits. Check pH because natural seawater near pH 8 is already suitable for the chromate endpoint.
Use chloride-free deionised water and clean glassware. Do not acidify with hydrochloric acid because it adds the chloride being measured. If adjustment is necessary, use carbonate-free dilute sodium hydroxide or nitric acid and target pH 7–9.
Store silver nitrate protected from light, and prepare or standardize it against an independent chloride standard. Household salt and tap water are unsuitable substitutes for analytical standards.
Choose practical titration conditions
A practical starting design uses 10.00 mL seawater, approximately 90 mL deionised water, 1 mL of 5% w/v potassium chromate and 0.5000 M AgNO₃. Expect about 10.9 mL of titrant for typical seawater.
Increase dilution if suspended material masks the colour, but keep the original aliquot precisely measured because later dilution cannot correct volume errors. Run at least two independent titrations and a matched reagent blank.
| Method | Sample handling | Main advantage | Main limitation |
|---|---|---|---|
| Direct visual Mohr | Small untreated aliquot | Few transfers | Seawater colour and turbidity obscure the endpoint |
| Diluted visual Mohr | Measured aliquot diluted to about 100 mL | Simple, inexpensive and suitable for teaching | Requires careful dilution and operator judgement |
| Potentiometric titration | Measured or conditioned aliquot | Automated endpoint and faster throughput | Electrode fouling, calibration and instrument costs |
The adapted visual method provides the best balance between simplicity and seawater compatibility. It also makes endpoint interpretation easier because dilution reduces the concentration and opacity of the silver halide suspension.
Carry out the visual endpoint
Transfer 10.00 mL of seawater into a 250 mL conical flask using a calibrated volumetric pipette. Add about 90 mL of deionised water and 1 mL of potassium chromate indicator. Prepare a blank containing the same volumes of water and indicator without seawater.
Rinse the burette with the standardized silver nitrate solution, then fill it with 0.5000 M AgNO₃. Add the titrant in small portions while swirling the flask continuously. As chloride and bromide precipitate, the mixture becomes cloudy with white or pale silver halides.
Near the expected endpoint, add titrant dropwise and view the flask against a white tile in neutral daylight. Continue until a faint brick-red or reddish-brown colour persists for approximately 30 seconds. Record both burette readings and repeat the determination using a fresh aliquot.
Calculate and validate the result
Calculate the titrant volume consumed as the final reading minus the initial reading, then subtract the blank volume. With (C) as the AgNO₃ concentration in mol/L and all volumes in mL, halide molarity is (C(V-V_0)/V_s), where (V_s) is the seawater aliquot volume.
Multiplying the resulting molarity by 35.45 gives chloride-equivalent g/L; multiplying by 1,000 converts this to mg/L. If bromide and iodide have been measured independently, subtract their molar concentrations before reporting chloride.
Assess duplicates against a predefined relative-percent-difference criterion, and include a matrix spike and appropriate certified reference material during method establishment. Instrumental checks can confirm that colour or suspended matter has not biased the visual result. For higher throughput, laboratories can compare this approach with potentiometric chloride measurements, although electrode fouling and calibration still require validation in seawater.
Make the workflow fit Australian practice
Australian fieldwork adds context that should appear in the method record. Coastal sites around Sydney Harbour, Melbourne’s Port Phillip Bay, Townsville and Perth experience different rainfall, tidal and runoff patterns. Sampling around beaches, marinas and aquaculture operations often means salinity changes between visits, so record conductivity alongside each titration and collect field duplicates.
The Australian laboratory-supply market commonly carries silver nitrate, potassium chromate and volumetric glassware, making procurement straightforward. The safety burden is less convenient: under Safe Work Australia’s model WHS laws, adopted through state and territory systems, current safety data sheets, training and exposure controls are required for chromate. Collect silver- and chromate-containing rinsate as hazardous waste and check local discharge rules.
For drinking-water context, the Australian Drinking Water Guidelines set 250 mg/L chloride as an aesthetic value, while state and territory authorities apply the guidance through their own regulatory frameworks. Seawater at roughly 19,000 mg/L is far beyond that benchmark and must not be reported as a drinking-water compliance result. The practical takeaway is to use a measured aliquot, validated dilution, blank-corrected duplicates and a recovery check before accepting the visible endpoint as a defensible seawater halide result.