# Online Calculator - pH of Buffer Solution

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

1. Select the acid / base combination of buffer solution
2. Fill concentrations of each acid and base
3. pKa value will be filled automatically, when you select the acid / base combination
4. If, entered concentrations are not matched to a buffer solution conditions, it will be noted. Otherwise, calculated pH will be given.

## 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.

1. Check that solution is buffer or not?
2. 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

#### pH = pKaNH4+ + log10([NH3]/[NH4+])

• pH = 9.25 + log10(0.06/0.02)
• pH = 9.25 + log10(3)
• pH = 9.25 + 0.4771
• pH = 9.72

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