Questions: Precipitation Titration: Argentometry and Related Methods

Question 1

A student uses the Volhard back-titration method to determine chloride content but skips the filtration step (leaving AgCl in solution during the back-titration with KSCN). What error does this introduce?

• A. A negative error — extra AgCl consumes the silver titrant
• B. A positive error — AgCl dissolves during back-titration, consuming extra thiocyanate
• C. No error — AgCl is insoluble and does not participate in the back-titration
• D. A negative error — AgCl adsorbs the ferric indicator, masking the endpoint

Answer: (B)

AgCl (Ksp ≈ 1.8 × 10⁻¹⁰) is more soluble than AgSCN (Ksp ≈ 1.1 × 10⁻¹²). As KSCN is added during the back-titration, it drives the dissolution of AgCl by consuming Ag⁺ and shifting the equilibrium. This dissolved Ag⁺ reacts with additional thiocyanate, causing the endpoint to come later than it should — a positive error in the chloride determination. This is why filtering or adding nitrobenzene to coat the AgCl is essential before back-titrating.

Question 2

Why cannot the Mohr method (chromate indicator) be used in strongly acidic solution?

• A. AgCl becomes more soluble in acid, preventing precipitation
• B. Silver nitrate reacts with the acid instead of the halide
• C. Chromate converts to dichromate in acid, making the indicator ineffective
• D. The endpoint color change is masked by the acid's own color

Answer: (C)

Chromate (CrO₄²⁻) is in equilibrium with dichromate (Cr₂O₇²⁻) at lower pH. In acidic conditions, the equilibrium shifts toward dichromate, dramatically reducing the free [CrO₄²⁻]. Ag₂CrO₄ therefore requires much more excess Ag⁺ before it precipitates, making the endpoint come far too late and producing a large positive error. The Mohr method requires near-neutral to slightly basic conditions (pH 6.5–10).

Question 3

In the Fajans method, the adsorption indicator changes color because the precipitate surface charge reverses at the equivalence point.

• T. True
• F. False

Answer: True

This is precisely the mechanism. Before the equivalence point, excess Cl⁻ ions adsorb on the AgCl precipitate surface, giving it a net negative charge that repels the anionic indicator dye. After the equivalence point, excess Ag⁺ adsorbs instead, making the surface positive. The positively charged surface now attracts the anionic indicator (e.g., fluorescein anion), and the adsorbed indicator changes color (yellow-green to pink). The color change is not due to a soluble complex forming in solution but to adsorption on the solid surface.

Question 4

The Volhard method cannot be used in acidic solutions because the indicator decomposes under acidic conditions.

• T. True
• F. False

Answer: False

This is the opposite of the truth — the Volhard method works specifically *because* it operates in acidic solution. It uses ferric ion (Fe³⁺) as the indicator, which is stable in acid, and the back-titration with KSCN proceeds cleanly. This acid compatibility is actually the Volhard method's key advantage over the Mohr method (which requires neutral pH). Acidic conditions also prevent the precipitation of Fe(OH)₃ and allow acid digestion of complex matrices before titration.

Question 5

Why must the AgCl precipitate be filtered off before the thiocyanate back-titration in the Volhard determination of chloride, but this step is not necessary for iodide or bromide?

Model answer: AgCl is more soluble than AgSCN (Ksp of AgCl > Ksp of AgSCN), so AgCl slowly dissolves as KSCN is added, consuming extra thiocyanate and causing a positive error. AgBr and AgI are less soluble than AgSCN, so they don't dissolve during the back-titration — they are stable to thiocyanate addition and the back-titration proceeds without interference.

The key is comparing Ksp values: AgI < AgBr < AgSCN < AgCl. For AgCl determination, AgCl dissolves in the presence of thiocyanate because AgSCN is less soluble — the more stable solid forms at the expense of the less stable one. For AgBr and AgI, the precipitate is already more stable than AgSCN, so no dissolution occurs. This solubility ordering is fundamental to understanding when the filtration step is required.