PH Buffer Solution

How to make Buffer Solution

Buffer solution is as below:
a. Mixed of weak acid with salt from that weak acid substance

Example:
- CH₃COOH mixed CH₄COONa
- H₃PO₄  mixed NaH₂PO₄
b. Mixed of weak base with salt from that weak base substance.

Example
NH₄OH mixed NH₄ Cl
The properties of buffer solution:
- pH solution will not change if dissolved.
- pH solution will not changed if added a little of acid or base solution..

How To Calculate Buffer Solution pH

The way to calculate buffer solution PH:
To calculate pH of buffer solution of mixed weak acid with their salt (the solution will have pH < 7) using the formula as follows:
 [H+] = Ka. Ca/Cg
pH = pKa - log Ca/Cg = - log Ka + log Cg/Ca

where::
Ca = concentration of weak acid
Cg = concentration of the salt Ka = weak acid ionization constant

Example:
1. Calculate the pH solution for mixed of acetic acid 0.01 mol with solution sodium acetate 0.1 mol in 1 liter of water. Known Ka for sodium acetate = 10^-5 answer:

Ca = 0.01 mol/liter = 10^-2 M
Cg = 0.10 mol/liter = 10^-1 M
pH= pKa + log Cg/Ca = -log 10^-5 + log10^-1/log10^-2 = 5 + 1 = 6

 2. To calculate of buffer solution contain of mixed weak base with thier base salt (the solution will have pH  > 7), use the formula as follows:

[OH^-] = Kb . Cb/Cg
pOH = pKb + log Cg/Cb

where:
Cb = weak base concentration
Cg = salt concentration
Kb = constant of weak base ionization

Example:
Calculate mixed solution of 1 liter contain of NH₄OH 0.2 mol with HCl 0.1 mol (Kb= 10^-5)
answer:
NH₄OH(aq) + HCl(aq) <====> NH ₄Cl(aq) + H ₂ O(l)
mol NH₄OH reacted = mol HCl available = 0.1 mol
mol NH₄OH rest = 0.2 - 0.1 = 0.1 mol
mol NH₄Cl formed = mol NH ₄OH reacted = 0.1 mol

Because weak acid rest and mixed with salt (NH₄Cl), the mixture will formed buffer solution:
Cb (rest) = 0.1 mol/liter = 10^-1 M
Cg (formed) = 0.1 mol/liter = 10^-1 M
pOH = pKb + log Cg/Cb = -log 10 ^-5  + log (10 ^-1 /10 ^-1 ) = 5 + log 1 = 5
pH = 14 - pOH = 14 - 5 = 9