Potentiometric Potentiometric titrationstitrations

Lecture notes byLecture notes by::

Ms.M.ShaliniMs.M.Shalini

Lecturer-Chemical EnggLecturer-Chemical Engg,,

Adhiyamaan engineering Adhiyamaan engineering collegecollege..

PrinciplePrinciple

It mesures the change in potential , It mesures the change in potential , can be used for all kinds of titration :can be used for all kinds of titration :

1- acid base1- acid base

2-redox2-redox

3-complexometry.3-complexometry.

When it is usedWhen it is used

It is used when the endpoints are It is used when the endpoints are very difficult to determine , either very difficult to determine , either when:when:

1- very diluted solution.1- very diluted solution.

2-coloured and turbid solution2-coloured and turbid solution

3-absence of a suitable indicator3-absence of a suitable indicator

It is a regular titration but instead of It is a regular titration but instead of the indicator we used the the indicator we used the potentiometerpotentiometer

Electrode will measure the PH of the Electrode will measure the PH of the media media

instrumentinstrument

Combined glass electrode ( double Combined glass electrode ( double function electrodefunction electrode ( (

Potentiometer PH meterPotentiometer PH meter

red ox ( mv)red ox ( mv) Magnetic stirrerMagnetic stirrer

1-hot plate ( use the stirrer and 1-hot plate ( use the stirrer and make sure heat is off).make sure heat is off).

2- magnet capsule 2- magnet capsule

Combined electrodeCombined electrode

internal reference electrode with internal reference electrode with constant potential andconstant potential and

not effected by potential of the not effected by potential of the solution.solution.

reference electrode very sensitive to reference electrode very sensitive to potential of the solution ( Ag / Agcl)potential of the solution ( Ag / Agcl)

Glass combined Glass combined electrodeelectrode

reference electrodereference electrode internal internal reference electrodereference electrode

Ag/AgclAg/Agcl

salt bridgesalt bridge PH sensitive PH sensitive glassglass

( full of ( full of buffer)buffer)

(reserved in a solution of 3 M KCL)(reserved in a solution of 3 M KCL)

objectivesobjectives

Titration of a weak acid ( acetic Titration of a weak acid ( acetic acid ) against a strong base ( NAOH)acid ) against a strong base ( NAOH)

Titration of a weak acid (acetic acid ) Titration of a weak acid (acetic acid ) against a weak base (NH4OH)against a weak base (NH4OH)

Stock solutionStock solution

Unknown acetic acid solutionUnknown acetic acid solution 0.1 N NaOH0.1 N NaOH 0.1 N NH4OH0.1 N NH4OH

ExperimentExperiment

Fill the burette with the standard titrant.Fill the burette with the standard titrant. Pipette 5 mls of acetic acid sample into Pipette 5 mls of acetic acid sample into

100 mls beaker and dilute to 50 by 100 mls beaker and dilute to 50 by distilled water.distilled water.

Put a magnet stirrer and dip in the Put a magnet stirrer and dip in the combined electrode ( make sure that the combined electrode ( make sure that the bulb of the electrode and the junction bulb of the electrode and the junction bridge are dipped completely under the bridge are dipped completely under the water.water.

Switch on the stirrer and allow few Switch on the stirrer and allow few second before reading the PH of the second before reading the PH of the solution.solution.

start the titration by adding 0.5 mls of start the titration by adding 0.5 mls of the titrant, stir well and then read the the titrant, stir well and then read the PH of the solution .PH of the solution .

Continue titration as before until near Continue titration as before until near the end point ( ph change is grater than the end point ( ph change is grater than 0.2 ) , add small portion of the titrant 0.2 ) , add small portion of the titrant ( 0.1mls) , stir well then take the PH ( 0.1mls) , stir well then take the PH reading.reading.

Repeat until about 10 mls of the Repeat until about 10 mls of the standard titration has been added.standard titration has been added.

Plot the potentiometric curve ( PH Plot the potentiometric curve ( PH value against ml of the titrant ) , value against ml of the titrant ) , determine the end point from the determine the end point from the curve and calculate any required curve and calculate any required data.data.

Potentiometric titration curve Potentiometric titration curve of 0.1 n NAOH against ? N of 0.1 n NAOH against ? N

acetic acidacetic acidpH PH at alkaline solutionpH PH at alkaline solution

PH at e.pPH at e.p

PH at acidic solutionPH at acidic solution mls at e.p mls at e.p

mls mls

Data to obtain from the Data to obtain from the graphgraph::

The PH of the acetic acid solution.The PH of the acetic acid solution. The PH of the alkaline solution .The PH of the alkaline solution . The equivalence point of the The equivalence point of the

titration.titration. The PH at the equivalence point.The PH at the equivalence point.

calculationcalculation

The normality of the acetic acid The normality of the acetic acid from the formula:from the formula:

N X V = N-X V-N X V = N-X V- The concentration of acetic acid (in The concentration of acetic acid (in

gm/l) as followgm/l) as follow

a - using the normality :a - using the normality :

c = Eq. wt x normality of acetic c = Eq. wt x normality of acetic acidacid

B- using the equivalent factor B- using the equivalent factor c of acetic acid = ml( at end point )xf- x Fx c of acetic acid = ml( at end point )xf- x Fx

100100 ml of the sampleml of the sampleF= Eq. wt of a.a x N of the titrantF= Eq. wt of a.a x N of the titrant 10001000Eq. wt = M.wt/no. of HEq. wt = M.wt/no. of H CH3COOHCH3COOH