Precipitation Titrations: Argentometry Methods & Uses

Argentometry is the family of precipitation titrations that uses silver nitrate (AgNO3) as the titrant. A solution containing halide ions (Cl-, Br- or I-) is titrated with AgNO3, and the silver ions combine with the halides to form an insoluble silver halide precipitate. The volume of AgNO3 used at the end point tells exactly how much halide was in the sample.

Chloride and bromide levels are measured everywhere: drinking water quality, salt in food, pharmaceutical purity and environmental monitoring. Argentometry gives a fast, cheap result accurate to about 1% using simple glassware, which is why it is still taught and used more than a century after it was developed.

Key Concepts

• Titrant and precipitate: AgNO3 is the standard titrant; Ag+ + X- gives AgX(s) in a strict 1:1 mole ratio. AgCl is white, AgBr pale yellow, AgI deeper yellow.
• Solubility product (Ksp): precipitation begins when [Ag+][X-] exceeds Ksp. At 25 C: AgCl = 1.8 x 10^-10, AgBr = 5.0 x 10^-13, AgI = 8.3 x 10^-17. AgI precipitates first from a mixture.
• Mohr method: potassium chromate (K2CrO4) indicator; after all chloride precipitates as white AgCl, the next drop of Ag+ forms brick-red Ag2CrO4. Works only at pH 6.5 to 10.
• Volhard method: back titration for Cl-, Br-, I-. Excess AgNO3 is added, and leftover Ag+ is titrated with thiocyanate (SCN-) using Fe3+ indicator; the end point is the first permanent blood-red [Fe(SCN)]2+ colour.
• Fajans method: adsorption indicators (fluorescein, eosin) change colour when adsorbed on the silver halide surface at the equivalence point; works at lower concentrations.

Formulas

• Stoichiometry: n(Ag+) = n(X-), so c(AgNO3) x V(AgNO3) = c(X-) x V(sample).
• Percent chloride: %Cl = (V x c x 35.45 / m_sample) x 100, with V in litres and m in grams.
• Volhard back titration: n(X-) = (c_Ag x V_Ag) - (c_SCN x V_SCN).
• Units: volume in litres, concentration in mol/L; include the dilution factor if the sample was diluted.
• Assumptions: complete precipitation, no competing ions (sulfide, phosphate, cyanide also precipitate with Ag+), and pH inside the indicator's working range.

Worked Example

A 25.00 mL well-water sample is titrated by the Mohr method with 0.0100 M AgNO3; 14.80 mL is needed to reach the brick-red end point.

1. Moles of Ag+ = 0.0100 mol/L x 0.01480 L = 1.480 x 10^-4 mol.
2. Moles of Cl- = the same = 1.480 x 10^-4 mol (1:1 reaction).
3. Mass of Cl- = 1.480 x 10^-4 x 35.45 = 5.247 x 10^-3 g = 5.247 mg.
4. Concentration = 5.247 mg / 0.02500 L = 209.9 mg/L, about 210 mg/L. The WHO guideline value is 250 mg/L, so this water passes.

Common Misconceptions

• The end point equals the equivalence point. It does not; the colour change always comes slightly after the true stoichiometric volume, and the Mohr method corrects this with a blank titration.
• Mohr's method works in acid. Below pH 6.5, chromate converts to dichromate, which gives no red precipitate with silver, so the end point never appears.
• Any halide suits any argentometric method. Volhard covers Cl-, Br-, I- but not fluoride; Fajans titration of chloride with fluorescein needs pH 7 to 10.

Real-World Applications

• Water laboratories: daily chloride checks; above about 250 mg/L water tastes salty and corrodes pipes.
• Food industry: salt (NaCl) content of snacks, cheese and processed foods.
• Pharmaceuticals: chloride and bromide impurity assays for quality control.
• Agriculture: chloride in soil extracts guides irrigation and fertiliser use.

Argentometry turns an invisible ion concentration into a visible colour change using the very low solubility of silver halides. Master the Mohr, Volhard and Fajans methods, control the pH and run a blank correction.