Precipitation Titrations: Argentometry

Introduction

Silver ions form sparingly soluble halides, making precipitation a basis for chloride and related analyses. The central reaction for chloride is Ag⁺ + Cl⁻ → AgCl(s), but a precipitate appearing does not automatically show that all chloride has reacted. Argentometric methods use different endpoint signals: a secondary coloured precipitate, back titration of leftover silver, or an adsorption indicator on particle surfaces.

Core explanation

In a direct silver nitrate titration of chloride, each mole Ag⁺ ideally removes one mole Cl⁻ as AgCl. Near equivalence, free Ag⁺ begins to rise because little dissolved chloride remains. The solubility product means small dissolved concentrations of both ions persist; “complete” precipitation is an analytical approximation. Other ions forming insoluble silver salts can interfere, so the matrix and pH matter.

The Mohr method adds chromate indicator to a suitable near-neutral chloride sample. While chloride remains, added silver preferentially forms AgCl. After chloride is essentially consumed, additional Ag⁺ gives red-brown Ag₂CrO₄. The indicator reaction is 2Ag⁺ + CrO₄²⁻ → Ag₂CrO₄(s). The chromate concentration and pH must be controlled: too much or too little indicator and inappropriate acid–base conditions can shift precipitation behaviour.

The Volhard method often works as a back titration. Add a known excess of Ag⁺ to precipitate chloride, then titrate leftover Ag⁺ with standardised thiocyanate: Ag⁺ + SCN⁻ → AgSCN(s). Fe³⁺ can signal the first excess thiocyanate through a coloured complex. The amount of Ag⁺ consumed by chloride is added minus leftover, giving chloride amount. Depending on the procedure, AgCl may be filtered or otherwise controlled because exchange between AgCl and SCN⁻ can affect endpoint accuracy.

The Fajans method uses an adsorption indicator. Near equivalence, the charge character of the AgCl precipitate surface changes as a slight excess of Ag⁺ appears. An anionic dye can adsorb to the positively charged surface and produce a visible colour change. The method depends on precipitate surface behaviour, pH and dye choice, so it should not be treated as merely another soluble indicator colour equilibrium.

All three routes require standardised silver or back-titrant solution, a known aliquot and appropriate blanks. A coloured or turbid matrix may complicate visual endpoints. Direct potentiometric detection with a suitable electrode can sometimes be preferable when visual observation is poor.

Step-by-step reasoning

1. Write Ag⁺ + X⁻ → AgX(s) for the target halide and check competing precipitates. 2. Decide whether direct or back titration suits the sample and endpoint conditions. 3. For Mohr, locate formation of a secondary coloured silver chromate precipitate. 4. For Volhard, subtract thiocyanate-measured residual silver from silver added. 5. For Fajans, interpret the surface adsorption colour change near equivalence.

Visual explanation

Show three small endpoint sketches. In Mohr, white AgCl accumulates then red-brown Ag₂CrO₄ appears. In Volhard, a known Ag⁺ pool splits into AgCl formation and Ag⁺ remainder measured by SCN⁻. In Fajans, the dye attaches to AgCl particle surfaces only after the surface charge changes near equivalence.

Real-world analogy

Counting chloride through silver is like handing each chloride ion one silver token that locks into a solid pair. Mohr watches for the first extra tokens to make a differently coloured pair; Volhard counts unused tokens afterward; Fajans watches the surface of the growing pile change its ability to hold a coloured tag.

Real-world example

A water analyst measuring chloride may use a direct silver nitrate method on a clear sample with appropriate pH. For an opaque or difficult sample, a back titration or instrumental endpoint can reduce visual ambiguity. Any other halides or silver-precipitating ions must be considered because they can consume silver and overstate chloride if unaccounted for.

Why?

Why does Ag₂CrO₄ appear only near the Mohr endpoint under suitable conditions? While appreciable Cl⁻ remains, Ag⁺ is kept low by formation of AgCl. Once chloride is mostly consumed, the rising Ag⁺ concentration can reach the threshold for silver chromate precipitation, giving the coloured signal.

Common misconception

“The first white precipitate is the endpoint” is wrong; AgCl forms throughout much of the titration. Another mistake is equating Volhard thiocyanate moles directly with chloride moles. Thiocyanate counts leftover silver, so subtraction from the amount initially added is essential.

Worked example

A chloride sample receives 0.00500 mol Ag⁺. Excess Ag⁺ requires 15.00 mL of 0.1000 mol L⁻¹ SCN⁻, or 0.001500 mol. Silver consumed by chloride is 0.00500 − 0.001500 = 0.00350 mol. Because Ag⁺ and Cl⁻ react one-to-one, chloride amount is 0.00350 mol, corresponding to about 0.124 g Cl⁻ using 35.45 g mol⁻¹. This assumes no other precipitating ions consume silver.

Quick check

1. What is the observed Mohr endpoint signal in chloride titration with silver nitrate? Answer: A red-brown silver chromate precipitate appears after chloride has been essentially consumed, because a small excess Ag⁺ can then react with chromate indicator.

Exam focus

Balance silver-halide and indicator reactions. Explain which quantity each endpoint measures. For Volhard calculations, explicitly write n(Cl⁻) = n(Ag⁺ added) − n(Ag⁺ left), where leftover silver equals SCN⁻ moles in the one-to-one back reaction. Mention pH and interfering ions where relevant.

Advanced insight

At equivalence, precipitation equilibria produce a curved relationship between free Ag⁺ and added volume. A potential-sensing silver electrode can track this change and locate equivalence without a coloured indicator. The apparent endpoint still depends on equilibrium, mixing and the data-analysis rule, just as visual methods do.

Summary

Argentometry uses insoluble silver halides to relate silver amount to halide amount. Mohr detects a coloured silver chromate precipitate, Volhard back-titrates excess silver with thiocyanate, and Fajans detects dye adsorption on a changed precipitate surface. Reaction stoichiometry is simple, but endpoint chemistry and interferences must be controlled.

Practice questions

1. How many moles Ag⁺ are required to precipitate 0.00200 mol Cl⁻ ideally? Answer: 0.00200 mol Ag⁺, because AgCl formation is one-to-one.

2. In Volhard analysis, why is thiocyanate titre subtracted rather than used as chloride amount? Answer: SCN⁻ measures Ag⁺ left after chloride precipitation. Chloride consumed the difference between initial Ag⁺ and that leftover amount.

3. What physical property gives the Fajans endpoint? Answer: A change in precipitate surface charge and adsorption of a coloured indicator near equivalence, not formation of a second bulk precipitate.