pH of buffer solution can be calculated by Henderson-Hasselbalch equation. This equation needs three values to
calculate pH. In this calculator, you can select different combinations of buffer solutions to determine pH value.
Definition of Buffer Solution
A buffer solution is defined as an aqueous solution which contains a weak acid and its conjugate base or a weak base with conjugate acid. pH of buffer
solution slightly changes when a small amount of strong acid or strong base is added.
Buffer Solution - pH Calculator
Please follow following steps to determine pH of buffer solution.
Select the acid / base combination of buffer solution
Fill concentrations of each acid and base
pKa value will be filled automatically, when you select the acid / base combination
If, entered concentrations are not matched to a buffer solution conditions, it will be noted. Otherwise, calculated pH will be given.
Select the combination of weak acid / weak base
Enter concentrations of acids and bases
Example: Calculating pH of buffer solution
Here we are going to calculate pH of acetic acid and sodium acetate solution.
Calculate pH of 0.05 M acetic acid and 0.02 M sodium acetate solution
To show buffer characteristics, there should be enough acid and base concentration. First we should check the ratio of concentrations acetic acid
and acetate ion (which gives basic property). pKa value of acetic acid is 4.75
Sodium acetate is a strong electrolyte and completely dissociates to sodium ion and acetate ion as below.
CH3COO-Na+ → CH3COO- + Na+
Because sodium acetate is dissociated completely, acetate ion concentration is 0.02 M.
Acetic acid (CH3COOH) concentration: 0.05 M
Acetate ion (CH3COO-) concentration: 0.02 M
[CH3COO-] / [CH3COOH] = 0.02 / 0.05
[CH3COO-] / [CH3COOH] = 0.4
Because ration is between 0.1 and 10, solution shows buffer properties. Therefore, we can continue to calculate pH of solution.
Apply Henderson-Hasselbalch equation for acetic acid / acetate ion mixture
pH = pKaCH3COOH + log10([CH3COO-]/[CH3COOH])
pH = 4.75 + log10(0.02/0.05)
pH = 4.75 + log10(0.4)
pH = 4.750 + (-0.398)
pH = 4.352
Example 2: Preparing Buffer solution with ammonia and hydrochloric acid
You were given 40 cm3 of 0.1 M ammonia solution and you have added 10 cm3 of 0.1 M HCl solution.
Check that solution is buffer or not?
If solution is a buffer solution, calculate pH value.
Ammonia and hydrochloric acid reacts with each other and form ammonium chloride. Ammonium ion shows acidic characteristics. Therefore,
if both ammonia and ammonium chloride exist in a considerable concentration after the reaction, final solution can be a buffer solution.
NH3 + HCl → NH4Cl
Then, we should calculate how much ammonium chloride is formed and how much ammonia is still remaining in the final reaction.
Initial amount of ammonia = 0.1 M * 0.040 dm3
Initial amount of ammonia = 0.004 mol
Added amount of HCl = 0.1 M * 0.01 dm3
Added amount of HCl = 0.001 mol
Formed amount of ammonium chloride = 0.001 mol
Because, initial ammonia amount is higher than added amount of HCl, ammonia remains in the final solution with ammonium chloride (HCl is the
limiting reagent). Let's calculate
concentrations.
Final concentration of ammonia = 0.003 mol / 0.05 dm3
Final concentration of ammonia = 0.06 M
Ammonium chloride is a strong electrolyte in the water and dissociates as below.
NH4Cl → NH4+ + Cl-
Final concentration of ammonium ion = 0.001 mol / 0.05 dm3
Final concentration of ammonium ion = 0.02 M
Now, we can check the ratio of [base]/[acid] to get confirmed the solution is buffer or not.
[ammonia]/[ammonium ion] = 0.06 / 0.02
[ammonia]/[ammonium ion] = 3
Because ratio is located between 0.1 to 10, this solution is a buffer solution.
Apply Henderson-Hasselbalch equation for ammonia / ammonium ion solution