Precipitation Titration Notes

Precipitation Titration Notes

Precipitation Titration

A precipitation titration reaction occurs when solutions of two different ionic compounds are mixed and an insoluble solid separates out of the solution that insoluble solid is known as a precipitate.
eg.,

where AgCl = precipitate

where AgCl =precipitate

In precipitation cation comes from one solution and anion from another hence precipitation is itself ionic.

Precipitation Titration

MOHRS METHOD

The fall under the category of determining the endpoint by the formation of coloured precipitate and is used for the determination of chlorides and bromides is a neutral solution.
TITRANT:- AGNO3 ( SECONDARY STANDARD SOLUTION)
INDICATORS:- 5% POTASSIUM CHROMATE K2CRO4)
which forms a reddish-brown precipitate.

Determination substance:- chloride, bromide
PH range — 6.5 to 10.3
iodides cant be determined by mohrs method because I- will also produce coloured precipitate with Ag+

PRINCIPLE


the reaction is carried out in a neutral medium for the determination of Cl- & Br –

METHODOLOGY:-

Titration of 0.1 M NaCl by using 0.1M AgNO3 in the presence of K2CrO4
-AgNO3 (0.1M standard ) will be added from the burette into the solution containing NaCl & indicator ( K2CrO4)
AgCl appears 1st because of concentration of Cl- ion is much higher as compared to
At the endpoint all Cl- ions react with AgNO3 & form AgCl, after that addition of extra deep of AgNO3 leads to reacting with ion ( from indicator ) & it will form reddish-brown ppt of AgCrO4.
At the endpoint, reading is taken & cl ion concentration is determined.

LIMITATION

It is impossible to use titration in acidic solution otherwise it will produce silver hydroxide
it is impossible to use titration in the presence of ammonia ions ( due to ligand formation)
it is impossible to use titration in the presence of reducing which reduces chromate ions
to ions
it is impossible to use titration in the presence of many anions ( etc ) which gives the painted precipitate of silver ions.

VOLHARD METHOD AND MODIFIED VOLHARD METHOD

In this method formation of a soluble colour compared at the endpoint.
it is used to determine halogenides, thiocyanates, cyamides, sulphides, chromate, oxalates, carbamates.
it is a back titration method, so two titrants is used.
type 1 – standardized AgNO3
type 2 – KSCN OR NH4SCN

INDICATOR; Iron alum/ ferric ammonium sulphates

PRINCIPLE

STEP 1; Standardization of AgNO3 by 0.1N NaCl

STEP 2; determination of analyte i.e. KCl

where, AgCl=precipitate, AgNO3=excess

In this reaction, AGNO3 is used in excess amount because of that reason the excess of AGNO3 remains in a reaction as it is after the formation of a precipitate (AGCH)
that excess AGNO3 used in next reaction

i.e

(0.1N)
OR

Then remain SCN i.e excess SCN react with

(REDDISH COLOUR ENDPOINT)

METHODOLOGY

(wikipedia)

1.take 0.1N NaCl standard solution in a conical flask, add 1 ml K2CRO4(5% SOLUTION) and titrate with AgNO3 solution.

2.at the endpoint reddish brown/brick red colour ppt will form. reading will be taken and calculation will be done.

3.take sample solution (i.e. 10ml KCl ) & add an excess of standardized 0.1 N AgNO3 solution then add few drops of iron alum indicator solution.

4.titrate the above solution by using a standard solution of KSCN/NH4SCN (0.1N)

5.at the endpoint reddish colour precipitate of Fe(SCN)2+ will occurs due to completion of the reaction.

6.calculate the amount of KCl

LIMITATION

volhards method is carried out in acidic medium only because in basic medium Fe3+
may give Fe(oh)2

MODIFIED VOLHARDS METHOD

step 1 is a common same as volhard method.
chloroform /nitrobenzene or other wetting agents is added after addition of AgNO3 in step 2.
these agents will form a layer surrounding the precipitate of AgCl prevents solubility of AgCl.

FAJONS METHOD

K FAJONS introduced this method.
K fajon introduce a useful type of indicator for precipitation titration.
such indicators are adsorbed on the surface of the precipitation at the equivalence point and this absorption is accompanied by a colour change.

Types of indicators are as follows.

1.acid dyes eg, fluorescein, eosin etc,
2.basic dyes eg, rhodamine series
titrant – AgNO3 ( SECONDARY STANDARD SOLUTION ) so standardization a primary standard solution of NaCl (by measured volume)
medium :- PH- 6.5 -10.3 (FOR CHLORIDE)
PH- 2.0-10.3 (FOR BROMIDE AND IODIDE)

INDICATORS – dichlorofluorescein ( for chloride)

Eosine ( for bromide & iodide)
mechanism of indicator action
The property of colloidal precipitate to absorb its own ions which are in excess is made use of the case.
when a NaCl solution is titrated with AgNo3 the AgCl precipitate will absorb chloride ions which are initially in excess.
this the primary adsorbed layer will be formed by chloride ions, which is taken will hold the secondary adsorbed layer of oppositely charged Na+
at the equivalence point, Ag+ ion in excess & hence Agcl ions adsorb Ag+ ion as primary adsorb layer & No3 as secondary adsorb layer.
now if the Na+ salt of fluorescein is also present in the solution then the -ve charged fluorescein ions would be adsorbed instead of NO3- as secondary layer & this adsorption occurs along with a change to pink coloured complex of Ag & fluorescein ions.

ESTIMATION OF NaCl

principle:-


(precipitation)

(INDICATOR) (REDDISH-BROWN / COLOURED PPT)
PREPARATION OF COLOURED REAGENTS

1.preparation of 0.1N AgNo3
16.99 gm AgNo3= dissolved in 1000ml of distilled water.

2.preparation of 0.1N NaCl
5.84 gm of NaCl = dissolved in 1000ml of distilled water.

3.preparation of 5% K2CrO4 indicator
5g of K2CrO4 =dissolved in 100ml of distilled water

METHODOLOGY

STEP-1 Preparation & standardization of 0.1 N AgNo3
-take 10ml of 0.1 N NaCl, add 1ml of K2CrO4 indicator & titrate by using AgNo3 till the appearance of the reddish-brown colour precipitate.
-titration is done three or more times until we will not get precise reading (two or more reading should be the same)
consider the reading are as follows

 

N1V1=N2V2
N1
STEP 2. ESTIMATION OF NaCl
take 0.25g of NaCl & dissolve in 50ml of distilled water, add 1 ml of K2cro4 indicator & titrate against standardization AgNO3 solution till the appearance of reddish brown ppt.
let us consider
burette reading = 45ml

Sem 1 b pharm books of set

to know more about precipitation titration visit Wikipedia


Conductometry