Precipitation Titrations: Mohr's Method

How much salt is in a packet of crisps? How salty is seawater, or the water from a well near the coast? How much chloride has leaked into a river from a road-gritting depot? One of the oldest and most reliable ways to answer these questions is to titrate chloride ions with silver nitrate, a family of techniques called precipitation titrations or argentometric titrations (from argentum, Latin for silver).

The principle

Silver ions react with halide ions to form very insoluble precipitates:

Ag⁺(aq) + Cl⁻(aq) → AgCl(s) (white) Ag⁺(aq) + Br⁻(aq) → AgBr(s) (cream) Ag⁺(aq) + I⁻(aq) → AgI(s) (yellow)

The reaction is fast and has a simple 1 : 1 ratio. Adding silver nitrate from a burette until all the chloride has precipitated tells you how much chloride was present.

The challenge is seeing the end point. The flask is already full of white precipitate, so the reaction itself doesn’t give an obvious signal. Three classic methods solve this in different ways.

1. Mohr’s method (chromate indicator)

Developed by Karl Friedrich Mohr in 1856, this is the most commonly taught method.

Indicator: a small amount of potassium chromate, K₂CrO₄, added to the chloride solution.

How it works: silver chromate, Ag₂CrO₄, is red-brown and is more soluble than silver chloride. As silver nitrate is added:

1. Silver ions react with chloride first, forming white AgCl, because AgCl is less soluble.
2. Once virtually all the chloride is gone, the next drops of silver nitrate have nothing to react with except chromate.
3. Red-brown silver chromate starts to form: 2Ag⁺ + CrO₄²⁻ → Ag₂CrO₄(s)
4. The end point is the first permanent reddish-brown tinge in the yellow suspension.

Conditions matter:

• The solution must be neutral or slightly alkaline, about pH 6.5–10.
  • In acid, chromate is converted into dichromate (CrO₄²⁻ + H⁺ ⇌ HCrO₄⁻, then Cr₂O₇²⁻), which doesn’t form the silver chromate precipitate properly, so the end point is late.
  • In strongly alkaline solution, silver ions precipitate as brown silver oxide.
• Samples can be neutralised with a little sodium hydrogencarbonate or calcium carbonate if necessary.
• A blank titration is often done (indicator and a chloride-free suspension such as calcium carbonate in water), to measure the small extra volume of silver nitrate needed to produce a visible colour. This is subtracted from each titre.

Worked example: chloride in tap water

25.00 cm³ of water is titrated with 0.0500 mol/dm³ silver nitrate using potassium chromate indicator. The titre is 14.20 cm³ (after blank correction). Calculate the chloride concentration in mol/dm³ and mg/dm³.

• n(Ag⁺) = 0.0500 × 0.01420 = 7.10 × 10⁻⁴ mol
• n(Cl⁻) = 7.10 × 10⁻⁴ mol (1 : 1)
• [Cl⁻] = 7.10 × 10⁻⁴ ÷ 0.02500 = 0.0284 mol/dm³
• In mg/dm³: 0.0284 × 35.45 × 1000 = 1007 mg/dm³

That’s much higher than typical drinking water (often under 250 mg/dm³), suggesting the sample is brackish or contaminated with salt.

2. Volhard’s method (back titration with thiocyanate)

Developed by Jacob Volhard in 1874. It works in acidic solution, which is its big advantage over Mohr’s method.

How it works:

1. Add a known excess of standard silver nitrate to the chloride solution. All the chloride precipitates as AgCl, and some Ag⁺ is left over.
2. Add iron(III) ions (usually as iron(III) ammonium sulfate) as the indicator, in nitric acid.
3. Back-titrate the excess Ag⁺ with standard potassium thiocyanate, KSCN: Ag⁺ + SCN⁻ → AgSCN(s) (white)
4. When all the excess silver has reacted, the next drop of thiocyanate reacts with Fe³⁺ to form the deep red complex: Fe³⁺ + SCN⁻ → [Fe(SCN)]²⁺
5. The end point is the first permanent red-brown colour.

A practical complication: silver chloride is slightly more soluble than silver thiocyanate, so AgCl can slowly react with the thiocyanate and cause a fading end point. The AgCl is therefore usually filtered off, or coated with a little nitrobenzene or another organic liquid, before back-titrating. (Bromide and iodide don’t have this problem.)

Worked example: Volhard back titration

50.00 cm³ of 0.1000 mol/dm³ AgNO₃ is added to 25.00 cm³ of a sodium chloride solution. The excess Ag⁺ needs 18.75 cm³ of 0.0800 mol/dm³ KSCN. Calculate the NaCl concentration.

• Ag⁺ added = 0.1000 × 0.05000 = 5.000 × 10⁻³ mol
• Excess Ag⁺ = SCN⁻ used = 0.0800 × 0.01875 = 1.500 × 10⁻³ mol
• Ag⁺ that reacted with Cl⁻ = 5.000 × 10⁻³ − 1.500 × 10⁻³ = 3.500 × 10⁻³ mol
• [Cl⁻] = 3.500 × 10⁻³ ÷ 0.02500 = 0.1400 mol/dm³

The logic is exactly that of any back titration: reagent added − reagent left over = reagent used by the analyte.

3. Fajans’ method (adsorption indicators)

Developed by Kazimierz Fajans in the 1920s, this uses adsorption indicators, organic dyes such as dichlorofluorescein or fluorescein.

How it works:

• Before the end point, the AgCl particles are surrounded by excess Cl⁻ ions and carry a negative surface charge. The negatively charged indicator ions aren’t attracted to them.
• After the end point, there’s a slight excess of Ag⁺, and the particles become positively charged. The indicator anions are now adsorbed onto the surface, where they change colour, typically from yellow-green to pink.

The colour change happens on the surface of the precipitate. Fajans’ method needs a finely divided precipitate (sometimes stabilised with dextrin) and a suitable pH.

Comparing the methods

	Mohr	Volhard	Fajans
Type	direct	back titration	direct
Indicator	potassium chromate	iron(III) (forms red FeSCN²⁺)	adsorption dye (e.g. dichlorofluorescein)
End point	first red-brown Ag₂CrO₄	first red FeSCN²⁺	pink colour on precipitate
pH	about 6.5–10	acidic (nitric acid)	depends on indicator; often near neutral
Main advantage	simple	works in acid; avoids interference by ions that precipitate in neutral solution	sharp, clear end point
Main limitation	not in acid; chromate is toxic	AgCl must be removed or coated	sensitive to conditions

Sources of error

• Wrong pH in Mohr’s method (late end point in acid; Ag₂O in alkali).
• Light: silver halides darken on exposure to light (the basis of old photography), which can obscure the end point. Titrate promptly and avoid bright sunlight.
• Adsorption of chloride onto AgCl can make the end point slightly early; vigorous swirling helps.
• Indicator error: a little extra silver nitrate is needed to form a visible amount of Ag₂CrO₄; a blank correction fixes this.
• Interfering ions: bromide, iodide, sulfide, carbonate and phosphate also react with silver and can give falsely high results.

Safety and cost

• Silver nitrate stains skin black (the stain wears off as skin renews) and is an irritant; wear gloves and eye protection.
• Potassium chromate is toxic, a skin sensitiser and a possible carcinogen; handle small quantities carefully and dispose of residues as heavy metal waste.
• Silver is expensive, so silver residues are often collected and recovered.

Key takeaways

• Precipitation titrations measure halides using silver nitrate: Ag⁺ + Cl⁻ → AgCl, a 1 : 1 reaction.
• Mohr’s method uses chromate indicator; red-brown Ag₂CrO₄ forms once all chloride has precipitated. It needs pH 6.5–10.
• Volhard’s method is a back titration with thiocyanate and Fe³⁺ indicator, and works in acidic solution.
• Fajans’ method uses adsorption indicators that change colour on the precipitate’s surface.
• Silver nitrate and chromate need careful handling; silver residues are usually recovered.