Methods, Precipitation Titration and Application of Argentometry

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Precipitation Titration:

• Precipitation formation
• Precipitation condition and precipitation purity

Methods in Precipitation titration:

• Mohr method
• Volhard method
• Fajans method

Titrations with precipitating agents are useful for determining certain analytes e.g. Cl⁻can be determined when titrated with AgNO₃.

Methods in Precipitation Titration

Detection of end point:

Chemical

• Precipitation Type - Mohr’s method
• Adsorption – Fajan’s method
• For silver analyses –Volhard method
• Sensors – Potentiometric or amperometric

The chemical types are also classified into:

1. Indicators reacting with titrant forming specific color.   
2. Adsorption indicators.

Mohr Method

• Direct titration
• Basis of endpoint: Formation of a colored secondary precipitate
• Indicator:  Soluble chromate salt (Na₂CrO₄, K₂CrO₄)

Endpoints for Argentometric Titrations

Precipitation Type - Mohr’s method

Ag₂CrO₄ precipitation in neutral pH solution.

Product is coloured

Colour forms just after AgCl or Ag I forms. Small error involved.

Mohr Method

• Has to be performed at a neutral or weak basic solution of pH 7-9 (or 6-10)
• In a lower pH (acid solution)

CrO₄²⁻(aq)  + H⁺(aq) → H₂CrO₄

H₂CrO₄ ↔ 2H⁺(aq)  + CrO₄²⁻(aq)

• In a higher pH (basic solution)

Ag⁺(aq)  + OH⁻(aq) → AgOH(s)

Mohr Method for Cl- determination

• Relies on Ksp differences for two insoluble silver salts

Ag⁺(aq) + Cl⁻(aq) → AgCl(s) (titration rxn)

2Ag⁺(aq)  + CrO₄²⁻(aq) → Ag₂CrO₄(s) (indicator rxn)

• AgCl is less soluble than Ag₂CrO₄ so it will precipitate first
• Ag₂CrO₄ is brick red in color so a color change is observed at the endpoint

Commonly used silver measurement:

• Mohr method = Indictor : K₂CrO₄
• Volhard method = Indictor : (NH₄) Fe (SO₄)²
• Fajans method = Indictor : Adsorption indicator

The problem should be pay attention to:

• The consumption of the indictor: 5×10-3 mol/L. If the concentration of the K₂CrO₄ is too high, the end point is advance, result is low. If the concentration of the K₂CrO₄ is too low, the end point is delay, result is high .K₂CrO₄ exert titration produce positive error, the measured concentration is low, it need a blank test.

Volhard Method

Used as a procedure for titrating Ag⁺, determination of Cl⁻ requires a back-titration:

• First, Cl⁻ is precipitated by excess AgNO₃

Ag⁺(aq) + Cl⁻(aq) → AgCl(s)      

• Excess Ag+ is titrated with KSCN in the presence of Fe³⁺

Ag+(aq) + SCN⁻(aq) → AgSCN(s)

• When Ag⁺ has been consumed, a red complex forms

Fe³⁺(aq) + SCN⁻(aq) → FeSCN²⁺(aq)

Endpoints for Argentometric Titrations. Chemical method for silver analyses. Volhard’s method using thiocyanate , CNS⁻, as titrant.

Iron (III) is the indicator as it forms a red complex ion with thiocyanate, CNS⁻, Fe (CNS)²⁺
The method can be adapted to Chloride analyses.

AgNO₃ is added in  excess. The AgCl precipitate is often filtered off. Then the excess Ag+ backtitrated with thiocyanate , CNS-. Iron (III) acts as the indicator as above.

Endpoints for Argentometric Titrations

Chemical. Adsorption – Fajan’s method A red dye attaches to the silver salt, on the surface of the analyte precipitate particle.

This happens only when the silver ion Ag⁺ is in excess, ie just after the equivalence point.

Fajans Titration

Fajans - adsorption indicator

Adsorption indicator

One kind is the acid dyes, for example fluorescence yellow and its derivative, they are the organic weak acid, dissociates the indicator anion.

Another is the alkalinity dye, like the gentian violet, Luo Danming 6G and so on, dissociates the indicator positive ion.            

                           AgNO3

AgCl）Cl⁻ + FI⁻  === AgCl）Ag·FI⁻

            (yellow green)    Pink

            (fluorescence)

Dichlorofluorescein is green in solution but pink when absorbed on AgCl

Measurement

(1) By adding protective colloid dextrin to prevent sedimentation AgCl excessive pool.

(2) Cl⁻ above 0.005 mol/L; when the concentration of the Br⁻, I⁻, SCN⁻ as low as 0.001 mol/L, also can titrate accurately.

(3) Avoid titrate under strong sunlight.

(4) Acid scope is different, see the table.

(5) The adsorption ability of colloidal particles to the indicator is slightly less than the measured ions .Too big the end is early, too small the end is delay.

the adsorption capacity of AgX :  I⁻ > SCN⁻ > Br⁻ > Eosin > Cl⁻ > fluorescence yellow

Applications

The precipitation titration of mixed ion

In precipitation titration, whether the two ions were able to accurate titration or not, it depends on the size of the solubility product ratio.

Can titrate respectively but I⁻ is adsorpted by AgI has errors. Can not titrate respectively can titrate the volume.

• The determination of the chlorine contained in natural water

Mohr and Volhard Method

• The determination of Silver contained in silver allo: Volhard method
• The determination of halogen contained in organic compounds such as food, organochlorine pesticides, used Volhard method.

C₆H₆Cl₆ + 3OH⁻ = C₆H₆Cl₃ + 3Cl⁻ + 3H₂O

• The determination of sodium chloride

Contained in MSG: Mohr method, no more than 20%

• Glutamate  more than 95% = Grade level
• Glutamate more than 80% = Second level

The Uses of Precipitation Titration

• Mostly used for neutralization reactions when the salt is occured the preciptation can ve observed we use indicators to see the pH range

• Ppt titration is used for such reaction when the titration is not recognised by changing the colours. during the reaction a salt is precipitated as the titration is completed.

• Precipitation titration is an Amperometric titration in which the potential of a suitable indicator electrode is measured during the titration.

• It is used for determination of chloride by Mohr's Method using Silver nitrate.

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