There are three common types of precipitation titrations - 1. Mohr Method - CrO

There are three common types of precipitation titrations -

1. Mohr Method - CrO ₄ ^2- indicator

2. Volhard Method - back titration of excess Ag ⁺ with SCN ^- , Fe ³⁺ indicator

3. Fajans Method - adsorption indicator =

dichlorofluorescein anion

Titration of a halide (Cl ^- , Br ^- , I ^- ) with AgNO ₃ to form a precipitate using K ₂ CrO ₄ as the indicator.

e.g. Ag ⁺ + Cl ^- → AgCl

At eq. pt. color change to red

Mohr Metodu

The indicator combines with excess Ag ⁺ at the eq. pt. Before the eq.

pt., the solubility of the Ag-halide precipitate must be less than the Ag-indicator, otherwise the latter would precipitate out during the titration!

2 Ag ⁺ + CrO ₄ ^2- → Ag ₂ CrO ₄ (s) red

1.Excess AgNO ₃ is added to precipitate out all of the halide.

e.g. Ag ⁺ + Cl ^- → AgCl(s)

2.The excess Ag ⁺ is back-titrated with KSCN using Fe ³⁺ as the indicator

then Ag ⁺ + SCN ^- → AgSCN(s) excess titrant

Volhard Metodu

after the endpoint -

Fe

+ SCN

→ FeSCN

[Cl

] = total Ag

- amount backtitrated

6

Example

A 10 mL of a chloride sample was treated with 15 mL of 0.1182 M AgNO ₃ . The excess Ag ⁺ was titrated with 0.101 M SCN ^-

requiring 2.38 mL to reach the red Fe(SCN) ²⁺ end point. Find the concentration of chloride in g/L.

mmol Ag ⁺ reacted = mmol Ag ⁺ taken - mmol Ag ⁺ back-titrated

mmol Ag ⁺ reacted = mmol Cl ^-

mmolAg ⁺ back-titrated = mmol SCN ^-

mmol Cl ^- = 0.1182x15 - 0.101x2.38 = 1.53 M _Cl ^- = 1.53/10 = 0.153 M

g/L Cl ^- = 0.153 x 35.5 = 5.44g

Fajans Method Fajans Method

The indicator adsorbs onto the colloid particles after the eq. pt.

and changes color to pink.

Adsorption Indicator - Dichlorofluorescein