Lesson 5 · 25 min

Precipitation titration: argentometry

Mohr, Volhard and Fajans: three quite different endpoints for one job — chloride in seawater. The pAg curves come from solubility products.

In this lesson

Compute argentometric pAg–V titration curves from solubility products
Explain the Mohr, Volhard and Fajans endpoints and their conditions (pH, indicator concentration, filtration)
Determine chloride in seawater, including back-titration calculations
Recognise error sources such as the indicator blank, conversion of AgCl to AgSCN, and photodecomposition

For more than a century oceanographers reported the “salinity” of seawater from a single silver nitrate titration: measure the chlorinity, then multiply by a conversion factor. Conductivity took over only in 1978. The roughly 0.56 mol/L of chloride in seawater is precipitated by Ag⁺ as AgCl, and the endpoint is a colour that appears just as precipitation is complete — by three quite different tricks.

Predict first

25.00 mL of diluted seawater containing K₂CrO₄ indicator (about 0.005 M) receives AgNO₃ drop by drop. Which precipitate forms first?

Pick an answer and lock it in before you look. A wrong guess teaches more than no guess.

Ag₂CrO₄, because its Ksp (1.2 × 10⁻¹²) is smaller than that of AgCl (1.8 × 10⁻¹⁰) AgCl, because it needs a far smaller [Ag⁺] Both at once Neither until the equivalence point

The pAg curve

Plot pAg = −lg[Ag⁺] against the volume of AgNO₃. Before equivalence chloride is in excess and $[\ce{Ag+}] = K_\text{sp}/[\ce{Cl-}]$ ; at equivalence $[\ce{Ag+}] = [\ce{Cl-}] = \sqrt{K_\text{sp}}$ , pAg = 4.87; after it the excess Ag⁺ sets pAg. The less soluble the precipitate, the bigger the jump: at the same concentration the break for AgI is far taller than for AgCl — which is why mixed halides can be titrated one after another.

K_\text{sp} = [\ce{Ag+}][\ce{Cl-}] = 1.8\times10^{-10},\qquad \mathrm{pAg}_\text{eq} = \tfrac{1}{2}\,\mathrm{p}K_\text{sp} = 4.87

Mohr: for Ag₂CrO₄ to appear exactly at equivalence, [CrO₄²⁻] = K_sp(Ag₂CrO₄)/K_sp(AgCl) ≈ 0.0066 M; about 0.005 M is used in practice so the yellow of chromate is not too deep.

$K_\text{sp}$: solubility product of AgCl (25 °C)
$\mathrm{pAg}$: −lg[Ag⁺]

Three endpoints

Mohr (direct titration, K₂CrO₄ indicator): excess Ag⁺ forms brick-red Ag₂CrO₄. The solution must be at about pH 7–10: in acid, CrO₄²⁻ turns into HCrO₄⁻/Cr₂O₇²⁻; in base, Ag⁺ forms Ag₂O. The eye needs a little excess Ag⁺ to see the red, so an indicator blank is run on a chloride-free CaCO₃ suspension and subtracted.
Volhard (back titration, Fe³⁺ indicator): in nitric acid a known excess of AgNO₃ is added and the Ag⁺ left over is titrated with KSCN, precipitating AgSCN; once Ag⁺ is gone, the next SCN⁻ forms red FeSCN²⁺ with Fe³⁺. The acid medium keeps carbonate, oxalate and phosphate from interfering. But AgCl is more soluble than AgSCN, so the AgCl must be filtered off (or coated with nitrobenzene); otherwise $\ce{AgCl(s)} + \ce{SCN-} \mathrel{\ce{->}} \ce{AgSCN(s)} + \ce{Cl-}$ keeps consuming SCN⁻, the red fades and the endpoint comes late. For Br⁻ and I⁻ no filtration is needed.
Fajans (adsorption indicator): a precipitate adsorbs its own lattice ion that is in excess. Before equivalence Cl⁻ is in excess and the AgCl colloid is negative; after it Ag⁺ is in excess, the surface turns positive and dichlorofluoresceinate anions are adsorbed onto it, changing from yellow-green to pink. Keep the precipitate colloidal (dextrin helps) and out of strong light.

10.00 mL of seawater diluted to 100.0 mL · 25.00 mL aliquot · 0.1000 M AgNO₃

Argentometry: chloride in seawater

Added = 10.00 mL Filter off the AgCl before back-titrating

Red FeSCN²⁺ visible (6.4 × 10⁻⁶ M): 6.36 mL; equivalence 6.02 mL

Chloride from the endpoint = 1.364 mmol (true 1.398 mmol, error −2.36 %)

AgCl is more soluble than AgSCN: left in the flask, SCN⁻ converts AgCl to AgSCN and eats extra titrant, so the endpoint fades and comes late.

Volhard: without filtering, red FeSCN²⁺ appears about 0.3 mL after equivalence and the chloride comes out about 2 % low; tick “Filter off the AgCl” and the endpoint returns to equivalence. Then switch to Fajans and drag the volume past equivalence to see the surface charge flip.

Worked example

Chloride in seawater (Mohr)

10.00 mL of seawater is diluted to 100.0 mL; a 25.00 mL aliquot with 1 mL of 5 % K₂CrO₄ is titrated with 0.1000 M AgNO₃ to brick red, using 14.03 mL; the indicator blank is 0.05 mL. Find the chloride concentration of the seawater in mol/L and g/L.

Concept check

In a Volhard chloride determination, if the AgCl is not filtered off before the KSCN back titration, the reported chloride will be:

too high too low unaffected impossible to predict

Try the calculation

For brick-red Ag₂CrO₄ to appear exactly at the AgCl equivalence point, what chromate concentration is needed? (Ksp: AgCl 1.82 × 10⁻¹⁰, Ag₂CrO₄ 1.20 × 10⁻¹²)

Key takeaways

pAg curve: before equivalence [Ag⁺] = Ksp/[Cl⁻], at equivalence pAg = ½pKsp (4.87 for AgCl), afterwards set by excess Ag⁺.
Mohr: chromate indicator, pH 7–10, subtract an indicator blank.
Volhard: back-titrate excess Ag⁺ with SCN⁻ in acid, Fe³⁺ indicator; filter off AgCl when determining chloride.
Fajans: after equivalence the precipitate surface turns positive and adsorbs dichlorofluorescein, turning pink.

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Data and references

D. C. Harris, Quantitative Chemical Analysis, 9th ed. (2016), ch. 7 and 26 (precipitation titrations; Volhard and Fajans titrations; Ksp(AgSCN) = 1.1 × 10⁻¹²).
D. A. Skoog, D. M. West, F. J. Holler, S. R. Crouch, Fundamentals of Analytical Chemistry, 9th ed. (2014), ch. 17 (argentometry, visual threshold of FeSCN²⁺).
A. E. Martell, R. M. Smith, Critical Stability Constants, Vol. 4 (1976): Ksp of AgCl and Ag₂CrO₄, as tabulated in D. Harvey, Analytical Chemistry 2.1.
F. J. Millero et al., “The composition of Standard Seawater and the definition of the Reference-Composition Salinity Scale”, Deep-Sea Res. I 55, 50–72 (2008) (chloride 19.35 g/kg at S = 35).