Question

what is the aim of the experiment for potentiometric titrations of NaOh against a strong , polybasic and weak acid

Answer 1

• Potentiometric titrations are those titrations in which change in EMF(mv) of a analyte can measured by Adding titrant (volume V) into it.
• plotting a graph between Change in EMF with respect to titrant added gives the end point (rapid change in EMF value), which is the neutralization point of the acid base reaction.
• it is generally consist of a reference electrode, indicator electrode and salt bridge.
• This EMF is nothing but the potential difference between the two electrodes.one is the reference electrode whose potential is known and remains constant through out the titration.and other is the indicator electrode whose potential will change while adding titrant into the solution. this EMF can be measured by using potentiometer.
• commonly, reference electrodes used are calomal electrodes, silver-silver chloride electrodes and they will be considered as the left hand electrode in the titration.
• indicator electrodes are membrane electrodes (Glass electrode) and metallic electrodes like platinum, gold.
• salt bridge is used to prevent the mixing of two solutions i.e. analyte where indicator electrode is present and the reference electrode is present.
• Acid- base reactions are known as neutralization reactions.
• in this case NaOH is taken as titrant which is strong base and analytes are strong acid , polybasic acid and weak acid .
• by potentiometric titrations we can know about the type of acid whether it is weak,strong or polybasic by ploting a graph between the change in EMF and the volume of titrant added.
• strong acid could be Hydrochloric acid, sulfuric acid ; weak acids could be acetic acid, oxalic acid; and polybasic acid (having two or more replacable hydrogens) could be Phosphoric acid.
• when titrant is added to the analyte solution the emf changes slowly but at the complete neutralisation of the acid or at the end point the EMF changes rapidly and after that again increases slowly upto some constant value and after that no change in EMF is noted.
• we can also calculate the first derivative and second derivative of the EMF with respect to volume and plot a graph between    and volume (V) of titrant added (first derivative).  
• and Volume (V) is the second derivative graph. it gives the end point more clearly .
• Getting end point from the graphs we can know the acid is strong, weak or polybasic, in polybasic acid two or more end point peaks are visible depending upon the number of replacable hydrogens .

No indicators are used , end point of coloured soutions can be determined, easy method are some of the advantages of potentiometric titrations.