Define pH. What is buffer solution? Derive Henderson-Hasselbalch equation for calculating the pH of an acid buffer solution.

Explanation

pH: .The negative value of the logarithm to the base 10, of the hydrogen ion concentration, expressed in moles / lit, in a solution is known as the pH of the solution".
Mathematically `pH=-log_(10)[H^(+)]`
Buffer Solution :
"A buffer solution is that solution which resists any change in its pH value on dilution (or) on addition of a small amount of a strong acid or a strong base" .
Ex acidic buffer : `(CH_(3)COOH+CH_(3)COONa)`
Basic buffer : `(NH_(4)OH+NH_(4)Cl)`
Preparation of buffer solutions :
1) Acid buffer solutions : An acid buffer consists of weak. acid and its salt with strong base.
Ex `: (CH_(3)COOH+CH_(3)COONa)`
Acid buffer solutions are normally prepared by mixing either equal, or different volumes of equimolar solutions of a weak acid and its salt. .
2) Base buffer solutions: A basic buffer solution consists of a mixture of a weak base and its salt with a strong acid.
Ex : `(NH_(4) OH+NH_(4)Cl)`
Base buffer. solutions are prepared generally, by mixing r equal or different volumes of equimolar solutions of a weak base and its salt.
Derivation of Hendersen.s equation for an acid buffer:
i) Consider the acid buffer HA+NaA
`HA hArr h^(+)A-`
`NaA hArr Na^(+)+A^(-)`
Acidic dissociation constant `K_(a)=([A^(+)][A^(-)])/([HA])`
Here due to common ion effect.
`[H^(+)] =K_(a) ([HA])/([A^(-)])=K_(a)(["Acid"])/(["Salt"])`
`pH=-log[H^(+)]`
`=-logk_(a).(["acid"])/(["salt"])`
`=-logK_(a)-log. (["acid"])/(["salt"])`
`=-log k_(a)+log. (["acid"])/(["salt"])`
`:. pK_(a)=-logK_(a)`
`:pH=PK_(a)+log.(["acid"])/(["salt"])`