PHARMACEUTICAL ANALYSIS

SHAMBADITYA GOSWAMI | ASST PROF | 2018

PRECIPITATION TITRATION

PRECIPITATION REACTION

Precipitation refers to a chemical reaction that occurs in aqueous solution when two ions bond together to form an insoluble salt, which is known as the precipitate.

A precipitation reaction can occur when two solutions containing different salts are mixed, and a cation/anion pair in the resulting combined solution forms an insoluble salt; this salt then precipitates out of solution.

The following is a common laboratory example of a precipitation reaction. Aqueous silver nitrate (AgNO3) is added to a solution containing potassium chloride (KCl), and the precipitation of a white solid, silver chloride (AgCl), is observed:

AgNO3 (aq) + KCl (aq) → AgCl (s) + KNO3(aq)

This reaction can be also be written in terms of the individual dissociated ions in the combined solution. This is known as the completeionic equation:

Ag+ (aq) + NO3

−(aq) + K+

(aq) + Cl−(aq) → AgCl(s) + K+ (aq) + NO3

−(aq)

PRECIPITATION TITRATION

In a precipitation titration, the stoichiometric reaction is a reaction which produces in solution a slightly soluble salt that precipitates out. To determine the concentration of chloride ion in a particular solution, one could titrate this solution with a solution of a silver salt, say silver nitrate, whose concentration is known. The chemical reaction occurring is

Ag+(aq) + Cl-(aq) AgCl(s).

A white precipitate of AgClis deposited on the bottom of the flask during the course of the titration.

Since the chemical reaction is one silver ion to one chloride ion, we know that the amount of silver ion used to the equivalence point equals the amount of chloride ion originally present.

Since by the definition of molarity n = cV, the number of moles of either silver ion or chloride ion can be calculated from the number of moles of the other, and the molar concentration or the volume of added solution can be calculated for either ion if the other is known.

ARGENTOMETRIC

TITRATION

In precipitation titration, silver nitrate or silver ions is

involved. As the Latin name of Silver is Argentum, the

precipitation titration is often termed as Argentometric

titration.

It has been broadly classified into following methods-

1. Mohr’s Method

2. Volhard’s Method

3. Fajan’s Method

4. Gay Lussac Method

MOHR’S METHOD

Theory:

In the Mohr titration, the given chloride solution (eg. NaCl) is titrated with a standard AgNO3 solution when Cl-ions are precipitated as AgCl.

AgNO3(aq) + NaCl(aq) AgCl(s) + NaNO3

A small amount of K2CrO4 (Potassium Chromate) is added to the chloride solution which acts as an indicator.

When AgNO3 is gradually added, the end point is confirmed by the first permanent appearance of brick red coloured precipitate of Ag2CrO4.

Ag+(aq) + Cl-(aq) ↔AgCl(s)

2Ag++ CrO42-↔Ag2CrO4

Limitations: Ag2CrO4is soluble in acid so, the solution should be always

neutral.

If the solution is acidic, first neutralize with CaCO3 to make solution neutral, then start the titration process.

If the solution is basic, AgNO3is precipitated as Silver hydroxide. So, first neutralize with HNO3 and then start the titration.

The titration is not suitable for iodides. For chloride and bromide the method is applicable and suitable.

If BaCl2/BaBr2is used in the process as a sample, slight excess of K2SO4 is added, otherwise BaSO4 will be precipitated.

Procedure: Determination of Normality of the

given chloride solution by Mohr’s Method

Preparation of 0.1N AgNO3 solution:

Dissolve 4.248g AgNO3 in 250ml water.

Titration:

1. Standard Solution: 0.1N AgNO3 in burette.

2. Sample Solution: 25 ml NaCl in clean conical flask.

3. Indicator: 1 ml K2CrO4 (Potassium Chromate) in conical flask

4. End Point: Brick Red Precipitate

5. Blank Titration: Perform the blank titration taking 25 ml

water in place of sample.

Organic Indicators used in Mohr’s Method:

O

O

O

O

O

O

Na

Na

Sodium Rhodizonate

OH

O-

O

O

O-

OH

Tetra hydroxy quinonine

VOLHARD’S METHOD

Theory

The method is used for the determination of silver.

The titration of Ag+ ions in acidic medium is done with a standard

Potassum thiocyanate solution (KSCN).

Ferric ion is used as indicator.

When all silver ions have been precipitated, the extra drop of

thiocyanate combines with Fe3+ ion to form a blood-red coloured

complex and it marks the end point.

Reaction

Ag+

+ SCN- AgCNS

White ppt

Fe3+

+ 3SCN- Fe(CNS)3

Blood Red/ Brown ppt

Procedure:

Titration:

Standard Solution: KSCN (11g in 1 lit) solution (0.1M) in

burette.

Sample Solution: 25 ml 0.1N AgNO3 in clean conical flask.

Acidic Medium: Add 5ml 6N HNO3 in conical flask.

Indicator: 1 ml Ferric ammonium sulphate (40%) in conical

flask

End Point: During titration at first white colour precipitate of

AgSCN is formed (the solution appears milky white). At the

end point blood red/ brown colour appears.

Modified Volhard’s Method

In volhard’s method, if chloride is present, then it can react

with SCN-.

To prevent this-

Either filter AgCl before titration with thiocyanate

Or, add some Nitrobenzene before titration with thiocyanate.

Nitrobenzene forms an oily coating on the AgCl surface to

prevent the reaction with SCN-.

AgCl+SCN- AgSCN+ Cl-

FAJAN’S METHOD

Theory:

This method involves the titration of AgNO3 with any halide

solution in the presence of Adsorption indicator.

Standard solution of AgNO3is added into unknown

concentration of halide (egNaCl) and a white precipitate of

AgCl is formed.

AgNO3(aq) + NaCl(aq) AgCl(s) + NaNO3

1st Stage: At this stage Cl- ions will make a primary

adsorption layer over the precipitate of AgCl and Na+ will

make a secondary adsorption layer.

Primary Adsorption Layer

Secondary Adsorption Layer

2nd Stage: Addition of AgNO3 make reaction mixture fuel

with Ag+ and NO3- ion.

Now, Ag+ ion will make primary adsorption layerand NO3-

ion will make a secondary adsorption layer.

Primary

Adsorption Layer

Secondary Adsorption Layer

3rd Stage: Fluoescein (Adsorption indicator) indicator forms

F- (Fluoreceinate ion) and replace NO3- ion over surface of

AgCl.

This replacement changes the colour mixture into pink.

Primary Adsorption Layer

Secondary Adsorption Layer

Adsorption Indicators:

Indicators which are used in Fajan’s Method are dyes such as

Fluorescein, Dichloro-fluorescein, Eosin etc..

These indicators are weak acidic in nature.

Procedure:

Titration:

Standard Solution: 0.1N AgNO3 in burette.

Sample Solution: 25 ml halide solution in clean conical flask.

Indicator: 8-10 drops Fluorescein in conical flask

End Point: During titration at first white colour precipitate is

formed. At the end pointpinkcolour appears.

Assay of Sodium Chloride I.P. 1996

Volhard’s Method

Take 0.1g of NaCl

Dissolve in 50 ml of water

Add 50 ml0.1M Silver Nitrate, 5ml of 2M nitric acid,

2ml of dibutyl phthalate

Shake well and add 2 ml of ferric ammonium

sulphate as indicator, shake vigorously

Titrate against 0.1M ammonium thiocyanate until

the colour becomes yellow.