Chemistry 11–12 · Year 12

Precipitation titration of chloride in seawater with silver nitrate and a chromate indicator (Mohr method)

Module 8: Applying Chemical Ideas

Practical, model not builtMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.

The idea

Silver ions precipitate chloride quantitatively, and the first excess of silver forms red-brown silver chromate, marking the end point so the chloride concentration can be found by titration.

Safety card

Medium riskLearners carry it out, with a teacher supervising

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

silver nitrate is corrosive and stains skin and fabric
potassium chromate is a carcinogen, mutagen and skin sensitiser
silver and chromate wastes are hazardous

Controls

chromate indicator prepared and dispensed by the teacher, 1 mL per flask
gloves and eye protection
all silver and chromate wastes to a labelled container for silver recovery or collection, never the sink
spills rinsed with water at once

Note

NSW Department of Education Chemical Safety in Schools (CSIS) package, 2021 Technical Update: Section 1.7 (risk assessment) and Volume 2 Appendix D (generic assessment advice and DoE chemical categories); record a RiskAssess (riskassess.com.au) risk assessment before the lesson and check the school's hazardous chemical register (CSIS Section 1.9) for local restrictions. CSIS Volume 2 Appendix F says chromium(VI) salts are believed to be carcinogenic and that great care must be taken in handling them, that their use in dilute aqueous solution near neutral pH is relatively safe, and that chromate precipitates are never isolated and are prepared in the smallest quantities that still allow easy observation; the silver chromate formed at the end point here is left in the titration flask and goes to the waste container. Where a school does not hold potassium chromate, the gravimetric method covers the same syllabus point.

What you need

Seawater sample, filtered, 20 mL (or a 0.500 mol/L sodium chloride check solution)
0.100 mol/L silver nitrate standard (about 4.25 g AgNO3 in 250 mL, stored in a brown bottle), 100 mL, in a burette
Potassium chromate indicator about 0.25 mol/L (1 g K2CrO4 in 20 mL water), 1 mL per titration, teacher-dispensed
20 mL and 10 mL pipettes, 100 mL volumetric flask, 250 mL conical flasks, 100 mL measuring cylinder, white tile
Labelled silver and chromate waste container

How to do it

Pipette 20 mL of seawater into the 100 mL volumetric flask and make up to the mark with deionised water.
Pipette a 10 mL aliquot of the diluted seawater into a conical flask and add about 50 mL of deionised water and 1 mL of chromate indicator (a lemon-yellow colour).
Titrate with 0.100 mol/L silver nitrate, swirling; the white silver chloride makes the liquid cloudy, and the end point is the first permanent red-brown tinge of silver chromate against the white tile. Run a rough titration first.
Repeat until three titres agree within 0.1 mL; average the concordant titres.
Calculate the moles of silver nitrate, hence moles of chloride in the aliquot, the chloride concentration of the diluted and of the original seawater in mol/L and g/L, and the equivalent sodium chloride concentration.
Check that the sample pH lies between 6.5 and 10 (the range the University of Canterbury method sheet gives) and explain, using Ksp values, why silver chloride precipitates first and silver chromate only at the end point.
In the simulation vary the dilution and the silver nitrate concentration and watch the titre and the silver-ion concentration at the end point change.

What you should see

Open-ocean seawater holds 19.35 g of chloride per kilogram (0.5459 mol/kg) at the reference salinity; at a density of about 1.024 kg/L that is 0.559 mol/L, so after the fivefold dilution a 10 mL aliquot needs 11.2 mL of 0.100 mol/L silver nitrate. The 0.500 mol/L check solution needs 10.0 mL. Coastal or estuary water reads lower where fresh water mixes in. The precipitate is white and curdy throughout and the yellow mixture turns a persistent red-brown at the end point; a titre well above the others means the end point was overshot.

What changes

What you change: volume of silver nitrate added
What you measure: appearance of red-brown silver chromate at the end point, hence the titre and the chloride concentration
What you keep the same: aliquot volume
indicator volume
pH between 6.5 and 10
lighting
same reader

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

The red colour appears when all the silver has been used (it appears when the first excess of silver remains after all the chloride is used).
Any indicator works for a precipitation titration (the indicator must form a coloured precipitate that is slightly more soluble than the analyte's).
Seawater is mostly sodium chloride so the titration measures salt (it measures chloride, plus any bromide; sodium is inferred only if the other cations are known).

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Chemistry Stage 6 Syllabus (2017), NESA; the current syllabus, taught in 2026 (codes read from the syllabus document)CH12-15 CH11/12-2 CH11/12-3 CH11/12-4
Chemistry Stage 6 Syllabus (2017), NESA; the current syllabus, taught in 2026 (codes read from the syllabus document); a Working Scientifically outcome not among those the syllabus targets in Module 8, which it allows in any moduleCH11/12-6
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

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