ANALYTICAL CHEMISTRY(Precipitation Titrations)

Dr.S.SURESH

Assistant Professor

Email:avitsureshindia@gmail.com

Precipitation titration

• Titrations with precipitating agents are useful for determining certain analyte. Example: Cl– can be determined when titrated with AgNO3

Determination of chloride

Principle• Chlorides are present in all types of water resources at

a varying concentration depending on the geo-chemical conditions in the form of CaCl2, MgCl2 and NaCl.

• Chlorides are introduced into the water resources from the discharge of effluents from chemical industries, sewage disposal and seawater intrusion in coastal region.

• The concentration of chloride ions more than 250 ppm is not desirable for drinking purpose. The total chloride ions can be determined by argentometric method. (Mohr’s Method)

Estimation of chloride (by Mohr’s method)

• In this method Cl‒ ion solution is directly titrated against AgNO3 using potassium chromate (K2CrO4 ) as the indicator.

AgNO3 + Cl‒ AgCl ↓ + NO3

‒

(in water) (White precipitate) • At the end point, when all the chloride ions are removed.

The yellow colour of chromate changes into reddish brown due to the following reaction.

2AgNO3 + K2CrO4 Ag2CrO4 ↓ + 2KNO3

(yellow) (Reddish brown)

Estimation of chloride

Titration

Estimation of Chloride ion: 20 ml of the given water sample is pipette out into a clean conical flask. 1 ml of freshly prepared potassium chromate solution is added as an indicator and titrated against standard AgNO3 solution taken in the burette. The end point is the change of colour from yellow to reddish brown colour. Repeat the titration for concordant values.

Calculation:

Volume of AgNO3 used V1 = -------- ml (Burette Reading)

Normality of AgNO3 N1 = -------- N (From Standardisation)

Volume of water sample V2 = 20 ml

Normality of water sample N2 = ?

N2 = V1 x N1

V2

Normality of water sample(chloride ion) N2 = ------ N

Amount of Cl– ions present in 1 litre = Normality (N1) X Equivalent weight

Amount of Cl– ions present in 100 ml = Normality X 35.46 X the given water sample

= ----------- gms.

1000

100

Estimation of chloride (Volhard’s Method)

• Volhard’s method is used in the estimation of chloride ions with standard solution of AgNO3

• It is an example of titration in which indicator forms a coloured complex ion with the titrant(i.e) silver nitrate.

Determination of chloride by Volhard Method

This is an indirect method for chloride determination where an excess amount of standard Ag+ is added to the chloride solution containing Fe3+ as an indicator. The excess Ag+ is then titrated with standard SCN- solution until a reddish brown color is obtained which results from the reaction:

NaCl + AgNO3 → AgCl + NaNO3 + excess Ag+

Excess Ag+ + SCN- → AgSCN ↓

Fe3+ (Yellow) + SCN- = Fe(SCN)2+ (Reddish brown)

Determination of Chloride ion

Preparation of standard ammonium thiocyanate• By weighing appropriate quantity of ammonium

thiocyanate, its 250ml of 0.1M solution is prepared in distilled water.

Determination of unknown chloride ions

Given chloride solution is diluted to 250ml using distilled water. From this 20ml solution is pipetted out into a conical flask to which 20ml of 0.1M AgNO3 is added.

After shaking the solution,2ml of nitrobenzene and 5ml of 30% HNO3 and ferric alum are added as indicator.

It is then titrated with standard NH4SCN till reddish brown colour is obtained to supernatant solution. Suppose the reading is ‘x’ ml, Then (20-x)ml will be the amount of NH4SCN required to precipitate Cl– ions.

Then the amount of Cl– ions in given solution can be calculated.