Potentiometric titrations

Principle It mesures the change in potential, can be used for all kinds of titration : It mesures the change in potential, can be used for all kinds of titration : 1- acid base 1- acid base 2-redox 2-redox 3-complexometry. 3-complexometry.

When it is used It is used when the endpoints are very difficult to determine, either when: It is used when the endpoints are very difficult to determine, either when: 1- very diluted solution. 1- very diluted solution. 2-coloured and turbid solution 2-coloured and turbid solution 3-absence of a suitable indicator 3-absence of a suitable indicator

It is a regular titration but instead of the indicator we used the potentiometer It is a regular titration but instead of the indicator we used the potentiometer Electrode wil masure the PH of the media Electrode wil masure the PH of the media

instrument Combined glass electrode ( double function electrode ( Combined glass electrode ( double function electrode ( Potentiometer PH meter Potentiometer PH meter red ox ( mv) red ox ( mv) Magnetic stirrer Magnetic stirrer 1-hot plate ( use the stirrer and make sure heat is off). 1-hot plate ( use the stirrer and make sure heat is off). 2- magnet capsule 2- magnet capsule

Combined electrode internal reference electrode with constant potential and internal reference electrode with constant potential and not effected by potential of the solution. not effected by potential of the solution. reference electrode very sensitive to potential of the solution ( Ag / Agcl) reference electrode very sensitive to potential of the solution ( Ag / Agcl)

Glass combined electrode reference electrode internal reference electrode reference electrode internal reference electrode Ag/Agcl Ag/Agcl salt bridge PH sensitive glass salt bridge PH sensitive glass ( full of buffer) ( full of buffer) (reserved in a solution of 3 M KCL) (reserved in a solution of 3 M KCL)

objectives Titration of a weak acid ( acetic acid ) against a strong base ( NAOH) Titration of a weak acid ( acetic acid ) against a strong base ( NAOH) Titration of a weak acid (acetic acid ) against a weak base (NH4OH) Titration of a weak acid (acetic acid ) against a weak base (NH4OH)

Stock solution Unknown acetic acid solution Unknown acetic acid solution 0.1 N NaOH 0.1 N NaOH 0.1 N NH4OH 0.1 N NH4OH

Experiment Fill the burette with the standard titrant. Fill the burette with the standard titrant. Pipette 5 mls of acetic acid sample into 100 mls beaker and dilute to 50 by distilled water. Pipette 5 mls of acetic acid sample into 100 mls beaker and dilute to 50 by distilled water. Put a magnet stirrer and dip in the combined electrode ( make sure that the bulb of the electrode and the junction bridge are dipped completely under the water. Put a magnet stirrer and dip in the combined electrode ( make sure that the bulb of the electrode and the junction bridge are dipped completely under the water.

Switch on the stir and allow few second before reading the PH of the solution. Switch on the stir and allow few second before reading the PH of the solution. start the titration by adding 0.5 mls of the titrant, stir well and then read the PH of the solution. start the titration by adding 0.5 mls of the titrant, stir well and then read the PH of the solution. Continue titration as before until near the end point ( ph change is grater than 0.2 ), add small portion of the titrant ( 0.1mls), stir well then take the PH reading. Continue titration as before until near the end point ( ph change is grater than 0.2 ), add small portion of the titrant ( 0.1mls), stir well then take the PH reading.

Repeat until about 10 mls of the standard titration has been added. Repeat until about 10 mls of the standard titration has been added. Plot the potentiometric curve ( PH value against ml of the titrant ), determine the end point from the curve and calculate any required data. Plot the potentiometric curve ( PH value against ml of the titrant ), determine the end point from the curve and calculate any required data.

Potentiometric titration curve of 0.1 n NAOH against ? N acetic acid pH PH at alkaline solution PH at e.p PH at acidic solution mls at e.p mls mls at e.p mls

Data to obtain from the graph: 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 titration. The equivalence point of the titration. The PH at the equivalence point. The PH at the equivalence point.

calculation The normality of the acetic acid from the formula: The normality of the acetic acid from the formula: N X V = N-X V- N X V = N-X V- The concentration of acetic acid (in gm/l) as follow The concentration of acetic acid (in gm/l) as follow a - using the normality : a - using the normality : c = Eq. wt x normality of acetic acid c = Eq. wt x normality of acetic acid

B- using the equivalent factor c of acetic acid = ml( at end point )xf- x Fx 100 c of acetic acid = ml( at end point )xf- x Fx 100 ml of the sample ml of the sample F= Eq. wt of a.a x N of the titrant Eq. wt = M.wt/no. of H CH3COOH CH3COOH