CHEMISTRY IN MY BRAIN

BUFFER SOLUTION 
 

figure1 buffer solution

 What is buffer solution?
solution that has ability to maintain its pH when small amount of strong acid or strong base is added to the solution.

*Calculating buffer pH

Monoprotic acids

First write down the equilibrium expression.

HA A⁻ + H⁺

This shows that when the acid dissociates equal amounts of hydrogen ion and anion are produced. The equilibrium concentrations of these three components can be calculated in an ICE table.

ICE table for a monoprotic acid

	[HA]	[A−]	[H+]
I	C0	0	y
C	-x	x	x
E	C0-x	x	x+y

The first row, labelled I, lists the initial conditions: the concentration of acid is C₀, initially undissociated, so the concentrations of A⁻ and H⁺ would be zero; y is the initial concentration of added strong acid, such as hydrochloric acid. If strong alkali, such as sodium hydroxide, is added y will have a negative sign because alkali removes hydrogen ions from the solution. The second row, labelled C for change, specifies the changes that occur when the acid dissociates. The acid concentration decreases by an amount -x and the concentrations of A⁻ and H⁺ both increase by an amount +x. This follows from the equilibrium expression. The third row, labelled E for equilibrium concentrations, adds together the first two rows and shows the concentrations at equilibrium.
To find x, use the formula for the equilibrium constant in terms of concentrations:

Substitute the concentrations with the values found in the last row of the ICE table:

Simplify to:

With specific values for C₀, K_a and y this equation can be solved for x. Assuming that pH = -log₁₀[H⁺] the pH can be calculated as pH = -log₁₀(x+y).

source: https://en.wikipedia.org/wiki/Buffer_solution#Calculating_buffer_pHhttps://en.wikipedia.org/wiki/Buffer_solution#Calculating_buffer_pH