To determine the dissociation constant Pka of a weak acid (salicylic acid) by hendrson hassel balch equation.

 Experiment

Aim: 

To determine the dissociation constant Pka of a weak acid (salicylic acid) by hendrson hassel balch equation.

Principle:

Weak acid and weak base do not ionizes completely in aqueous solution like strong acid and bases. 

The degree of ionisation and dissociation constant (pka) which express covalantely in the term of pka. It is the negative logarithm of acid dissociation constant (pka) is the same way as pH is used to represently negative logarithm of hydronium ion concentration. 

It’s dissociation constant value (pka) for weak acid (or) base can be determined by using hendrson hassal balch equation. 

pH = pka + log X (Ionised acid)/(unionized acid)

pH = pka + log X (salt)/ (acid)

At 50% neutralization point that is when (ionized acid)=(unionized acid) the value of Pka-pH.

The value of pka can be derived by recording the pH at 50% neutralized point.

Materials required:

• Salicylic acid

• pH meter

• Burette

• Standard NaoH

• Conical flask

• Indicator

Procedure:

• 10ml of 0.5% w/v of salicylic acid in methanol is pipette out to be a conical flask.

• This is then titrated against 0.5% N NaoH solution using of methyl red is an indicator to complete neutralization.

• Burette reading is noted similarly 10ml of 0.5% W/V of salicylic acid is taked into a 

conical flask and titrated against standard 0.5N NaoH to 50% neutralisatiion point 

before the pka value of salicylic acid at 30 ̊C.

Calculation:

Let the volume of 0.5 N NaoH consumed in titration with 10ml of 0.5% salicylic acid solution. 

It complete neutralization = Xml.

Volume of 0.5N NaoH required for half neutralization = X/2ml

 

pH of the half neutralization solution=

pH= pka + log X salt/acid

Result:

The Pka value of salicylic acid 30 ̊C is _____