Precipitation titration

 

              PRECIPITATION TITRATION (Argentometric titration)

Introduction- It is a type of titration in which precipitate are formed during titration technique.

In this titration titrant are react with analyte and form insoluble substances are called precipitate.

Principle- formation of an insoluble substance by the combination of two ions is known as precipitation.

Method- This titration are mostly used in the titration of silver nitrate with chloride, bromide, iodide, and thiocyanate.

Procedure for Precipitation titration:-       

       I.          1. Mohr’s method

     II.          2. Volhard’s method

   III.          3. Modified volhard’s method

  IV.          4. Fajan’s method

     Mohr’s Method:-

·      ▶ This method are gave by Karl Friedrich Mohr in 1855.

·       ▶ In this method neutral solution of chloride ions is titrated with silver nitrate solution by using in small amount of potassium chromate solution are used as an indicator.

·      ▶ During titration chromate ions combined with silver ions and formed end point, the silver chromate are sparingly soluble

·     ▶   At the end point potassium chromate indicator are produces red coloured with silver chromate (chloride ions completely used)

·       ▶ End point detected after chloride or bromide precipitate are formed.

      Main features:-

Sample	Chloride ions (sodium chloride, potassium chloride, ammonium chloride) bromide ions(sodium bromide, hydrogen bromide, potassium bromide, silver bromide)
Type of titration	Direct titration
Standard solution	Silver nitrate (AgNo3)
Indicator	Potassium chromate
pH	Slightly alkaline
End point colour	Yellowish red

 

Preparation and standardization of 0.1 M silver nitrate:

v   ▶ Take a volumetric flask filled with 250ml water. 

v   ▶ Added 1.2gm NaCl or 1.6 gm of KCl.

v   ▶ Pipetted out 50 ml of solution and transferred into another 250 ml of clean and dry conical flask.

v   ▶ Add 2ml of potassium chromate indicator.

v   ▶ Burette is filled with AgNo₃ solution with the help of funnel note the initial point.

v   ▶ Now the AgNo₃ solution are added into the chloride solution (discus in point no.3).

v   ▶ Shake the solution continuously till red colour develops due to silver chromate (Ag₂CrO₄).

v   ▶ When white curdy AgCl lumps are formed it indicate the end point is near.

v   ▶ At now end point AgNo3 solution is added drop wise till the chromate ions (CrO₄^2-) permanently converted into red colour silver chromate precipitate (ppt).

    Volhard’s Method:-

v ▶ This method are gave by Jacob Volhard.

v ▶ In this method thiocyanate are titrated with silver ions in an acidic medium and as indicator ferric ion are used.

v ▶ Initially precipitate of silver thiocyanate is formed

v ▶ After equivalence point excess thiocyanate added react with ferric ion F+++ and give reddish brown colour ferric thiocyanate.

v ▶ These are mainly used for determination of halide (cl-, Br-, I) with standard silver nitrate solution.

v ▶ Silver chloride precipitate are formed and excess of Ag+ ions are back titrated with standard ammonium or potassium thiocyanate by using Fe+++ indicator.

v ▶ Reddish brown coloured are formed due to thiocyanate is formed at the end point of back titration.  

Main Features:-

Sample	Cl-, Br-, I-
Type of titration	Back titration
Standard solution	AgNO3
pH	In acidic medium
Indicator	Ferric ammonium sulphate/ferric nitrate
End point	Reddish brown

 

Modified Volhard’s Method:-

Principle- Based on indirect volumetric precipitation titration.

Method –

v ▶ In this method nitric acid solution is used to acidify NaCl (or any chloride solution)

v ▶ In the presence of nitrobenzene this solution is titrated with standard solution of silver nitrate.

v ▶ Some moles of silver nitrate are consumed with NaCl and remaining unreacted silver nitrate are determined by titration standard solution of ammonium thiocyanate.

v ▶ Ferric ammonium sulphate (ferric alum) is used as an indicator.

v ▶ Nitrobenzene (C₆H₅NO₂) forms an organic layer around the precipitate of silver chloride particle which are totally insoluble

v ▶ So the nitrobenzene are prevents silver chloride to react with ammonium thiocyanate.

v ▶ Nitric acid are added to prevents ferric ammonium sulphate (indicator) solution to hydrolyse.

v ▶ Permanent brick red colour are indicate end point due to formation of ferric thiocyanate.

NaCl +  AgNO3     ➡            AgCl + NaNO3        +      AgNO3

                                                   Ppt                             Unreacted

AgNO3 + NH4SCN     ➡              AgSCN         +          NH4NO3

Unreacted                                  silver thiocyanate (ppt)

3NH4SCN + Fe3+    ➡                  [Fe (SCN)3]       +    3NH4

                                                    Ferric thiocyanate (brick red colour)

Procedure-

·      ▶  Take accurate weighed 0.1gm of sodium chloride is dissolved in 50ml of water in a glass stoppered flask.

·      ▶  In this solution 50ml of 0.1N silver nitrate, 3 ml of Nitric acid, 5ml of nitrobenzene and 2 ml of ferric ammonium sulphate are added.

·     ▶    Now this mixture is titrated with 0.1N ammonium thiocyanate till the reddish yellow colour is obtained and they denoted the end point.

Application-

This method is used for determination of following ions

Ø  ♦ Chloride

Ø  ♦ Thiocyanate with mercury

Ø  ♦ Bromides

Ø  ♦ Iodides 

Main Features:-

Sample	Cl-, Br-, I-
Type of titration	Indirect titration
Standard solution	AgNO3, NaCl
pH	In acidic medium
Indicator	Ferric ammonium sulphate (ferric alum)
End point	Reddish yellow

 

Fajan’s method:-  

This method are gave on the name of Kazimierz Fajan

When they used the di-chloro-fluorescein as an indicator the result became green coloured and they convert into pink colour so it’s indicate the end point.

Theory

v ▶ Sodium chloride is titrated with silver nitrates

v ▶  Chloride ions are adsorb on the silver chloride precipitate to form uniform layer.

v ▶  During equivalence point silver chloride are adsorbs (Ag+) silver ions (primary adsorb layer).

v ▶  Sodium ions adsorbed (secondary adsorb layer).

v ▶  Primary adsorb layer of chloride ions are holds secondary adsorb layer of sodium ions.

v ▶  If in the solution sodium salt of fluorescein are present then AgCl (silver chloride) are adsorb negative charge of fluorescein ions as secondary layer of sodium ions.

v ▶  During the adsorption pink coloured complex of Ag+ are formed and they denote the end point.

Application-

Ø  ▶  Standardisation of the unknown NaCl solution

Ø  ▶  Standardisation of AgNO3 solution.

Ø  ▶  Determination of complexes.

Main Features:-

Sample	Cl-, Br-, I-, SCN
Type of titration	Direct titration
Standard solution	AgNO3
pH	7-10 (neutral /slight alkaline)
Indicator (adsorption indicator)	Acid indicator- Fluorescein , eosin Basic indicator- Rhodamine 6G, Phenosfranine
End point	Pink precipitate

       I.