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BUFFER SOLUTION

BUFFER SOLUTION. A solution which resists changes in pH when acid or alkali is added to it. Key concepts. Buffer solutions Solutions that contain relatively high concentrations of a weak acid and its conjugate base Designed to hold pH to a constant value

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BUFFER SOLUTION

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  1. BUFFER SOLUTION A solution which resists changes in pH when acid or alkali is added to it.

  2. Key concepts Buffer solutions • Solutions that contain relatively high concentrations of a weak acid and its conjugate base • Designed to hold pH to a constant value • Biological systems (e.g., blood) contain buffers to maintain stability of pH- sensitive chemical reactions

  3. Dilute aqueous acids • Let’s consider a solution containing a week acid, HA, and its salt, NaA. Salts are strong electrolytes, so NaA will completely dissociate in solution:   NaA(aq) Na+(aq) + A-(aq) • The weak acid exists in equilibrium with its ions: HA(aq) + H2O (l) H3O+(aq) + A-(aq)

  4. The ionization constant for the acid is given by : • Ka = [H+][A-] / [HA] • Since we are dealing with weak acids, very little conjugate base (A-) in solution comes from the acid. Further, the presence of the salt in solution reduces the ability of the acid to ionize (common ion effect). The major source of A- is from the salt. Therefore, we can write

  5. Ka = [H+] [salt]o / [acid]o where the subscripts indicate initial concentrations of the salt and the acid. The Henderson-Hasselbalch equation may be derived from this expression: • pH = pKa + log([salt]o/ [acid]o)

  6. What is the use of Buffer? • A buffer is most useful when the concentrations of the acid and base components are similar to one another. Thus, the pH of the solution is close to the pKa value, but can be varied somewhat by adjusting the ratio [salt]/[acid] (you can also think of this ratio as [conj base] / [conj acid]).

  7. Buffers differ not only in their effective pH range, but also in their capacity: the amount of acid or base that can be added before a significant pH change is observed. Buffer capacity is defined as the number of moles of strong acid or base needed to change the pH of 1.0 L of buffer by 1 pH unit. • In this experiment you will prepare buffer solution using two different methods. You will then measure the capacity of your buffers and test the effects of added acid on the pH.

  8. Preparation of the Buffer Solutions • You will be assigned a particular pH of the buffer solutions to be prepared. • One buffer (buffer A) is to be prepared by treating the acid solution with a NaOH solution to partially neutralize the acid and form the conjugate base: HA(aq) + NaOH(aq) NaA(aq) + H2O(l) • The goal is to produce the proper ratio of [A-] / [HA] to produce the desired pH. Note that this ratio is also equal to (# mmols A- / # mmols HA).

  9. 1. Calculate the volume of HA needed to react with 40 mL of the NaOH solution. • a. The number of millimoles of A- produced by reaction will equal the number of millimoles of NaOH added and will also equal the number of millimoles of HA used up in the reaction. • b. The volume of HA used must contain the number of millimoles of HA present in the final buffer plus the number of millimoles to be converted to A-. • c. Knowing the total number of millimoles of HA needed to make the buffer, calculate the volume of acid required. Do your calculations on the handout.

  10. 2. When you have completed the calculation, mix that volume of acid with 40.0 mL of the NaOH solution and measure the pH of your buffer. Label the buffer with the measured pH and the letter “A” and set it aside. To prepare the second buffer (buffer B), you will need to calculate the mass of the solid salt (conjugate base) of your assigned acid needed to be added to 20.00 mL of your acid. Again, you can use the ratio # mmols A- / # mmols HA to determine the mmols of conjugate base needed.

  11. Effect of Added Acid • If a strong acid such as HCl is added to the buffer solution, the conjugate base A- will react with added acid: H+(aq) + A-(aq) HA(aq) • On your handout, calculate the pH expected if 1.0 mL of the supplied strong acid solution is added to 20.0 mL of buffer solution A (question #5). Prepare this solution by mixing 20.0 mL of buffer A and 1.0 mL of strong acid. Measure and record the pH.

  12. Effect of Added Base • If a strong base such as NaOH is added to the buffer solution, the acid HA will react with the added base: OH-(aq) + HA(aq) A-(aq) + H2O(l) • On your worksheet, calculate the pH expected if 1.0 mL of the supplied NaOH solution is added to 20.0 mL of buffer A (question #7). Prepare this mixture and measure and record the pH.

  13. Buffer Capacity • We will now test the capacity of both of your buffers by titrating each of them with the standardized NaOH solution. Begin with 20.0 mL of buffer A or B in a beaker. Adjust the pH meter so it can continually monitor the pH of your solution. Record the initial pH of the solution and the initial volume of NaOH in your buret. Now, begin adding the standardized NaOH solution SLOWLY. Once the pH of the solution has changed by 1.0 unit, stop the titration and record the volume of NaOH delivered. Repeat this experiment with your other buffer. You will use this data to calculate the buffer capacity.

  14. The End …… Thank You ……

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