Buffers

1. Buffers

2. Buffers and Henderson-Hasselbalch Equation • Slns of a weak acid and its conjugate base (or a weak base and its conjugate acid) are called buffers. • They are buffers as they keep the pH from fluctuating widely upon the addition of small amounts of a strong acid or base. • So if a buffer is properly made, you can add some HCl or NaOH and only have the pH of the buffer sln change slightly.

3. Buffers Cont. • Buffers are common in nature • Lakes, rivers, and oceans have a carbonate buffer system. • Our blood uses several buffer systems, including a carbonate buffer (which is in equilibrium with CO2) and a phosphate buffer.

4. What’s a Good Buffer? • A good buffer system has fairly high concentrations of the weak acid and base: around 0.010M to 0.10M is considered concentrated. • The concentrations of the weak acid and its conjugate base are ideally roughly equal: [HA] = [A-] • Although this means that the ideal buffer is 50/50, buffers can be used where the ratio is 90/10 to 10/90.

5. Buffer Capacity • We can measure how good a buffer system is by measuring the buffer capacity. • The buffer capacity is simply the amount of strong acid or base which can be added to the buffer without changing the pH greatly. • The more strong acid or base the buffer can absorb (or neutralize), the higher the buffer capacity.

6. Buffer Capacity, Cont. • So the higher the buffer concentration, the higher the buffer capacity. • The more equal the weak acid and conjugate base concentrations are, the higher the buffer capacity.

7. Henderson-Hasselbalch Equation • If we know that we have a buffer system, we can use a simple equation to calculate the weak acid and base concentrations. • pH = pKa + log([base] / [acid]) • This is the Henderson-Hasselbalch eq.

8. Henderson-Hasselbalch Equation • Time for math!

9. Buffer Review • What is a buffer? • A buffer is a solution of a weak acid and its conjugate base (or a weak base and its conjugate acid). • What is the purpose of a buffer? • It is a pH moderator, keeping the pH stable with small additions of H+ or OH-

10. Buffer Review • How does a buffer keep the pH stable? • When you add OH-, the weak acid reacts with it to make the weak conjugate base • When you add H+, the weak base reacts with it to make the weak conjugate acid.

11. Buffer Preparation • How is a buffer made? • There are actually several ways to make a buffer sln, 2 of which are common.

12. Buffers: Preparation Method 1 • You have a desired volume, pH, & concentration: 500.00 mL of a 0.0500M sln of TRIS buffer with a pH of 8.1. • What’s TRIS? • TRIS base is a crystalline solid with a MW = 121.14 amu.

13. Buffers: Preparation Method 1 cont. • You calculate how many g TRIS you need to make the desired volume & M. • You weigh it out, completely transfer to a beaker, add about 75% of the final volume and measure the pH. • Since TRIS is a base, the pH at this point will be high compared to what we want.

14. Buffers: Preparation Method 1 cont. • So we need to add HCl until we get to the desired pH of 8.1. (You are MAKING the conjugate acid of TRIS base!) • Start with 3 or 6M HCl, then when you get close to 8.1, switch to 1M HCl, drop by drop addition.

15. Buffers: Preparation Method 1 cont. • When you have the desired pH, completely transfer to a volumetric flask. • Rinse the beaker thoroughly with DI water. • Dilute to the mark with DI water. • Done!

16. Buffers: Preparation Method 1 cont. • How would you modify the above if you are starting with an acid like ammonium chloride or boric acid?

17. Buffers: Preparation Method 2 • Method 2 is not nearly as common, but it is used. • Let’s say you want a buffer system of piperidine (which is a base) and its conjugate acid piperidine hydrochloride. • Piperidine and its conjugate acid are both crystalline solids.

18. Buffers: Preparation Method 2 cont. • You want 250.00 mL of a 0.100M sln at a given pH. • You will be weighing both the piperidine and the piperidine hydrochloride out so you want to weigh enough piperidine to make 250.00 mL of a 0.050M piperdine sln.

19. Buffers: Preparation Method 2 cont. • You also want to weigh enough piperidine hydrochloride to make 250.00 mL of a 0.050M piperdine sln. • Weigh both solids, add together to a beaker, dilute with about 75% of the desired volume, adjust pH, transfer and dilute to final volume.

20. Buffer Capacity • The buffer capacity is a measure of how well the buffer resists changes in pH. • The more strong acid or base the buffer can absorb (or neutralize), the higher the buffer capacity. • The buffer capacity will be at its highest when pH = pKa or the ratio of A-/HA = 1.

21. Buffer Capacity, Cont. • We also want relatively high buffer concentrations (relative to a typical addition of strong acid or base) or at least as high as is practical. • A 1M buffer would have a higher buffer capacity than the same buffer which is just 0.1 M. • You just have more moles of HA and A-.

22. Buffer Capacity, Cont. • A common way to measure the buffer capacity is by measuringΔpHupon the addition of strong acid or base. • The larger the ΔpH, the weaker the buffer capacity.

23. Picking the Right Buffer • To pick the right buffer, you need to know the desired pH of your system. • Then pick a buffer that has a pKa as close to that pH as possible. • The pKa should be within 1 unit of the desired pH.

24. Buffers & Temperature • As equilibrium constants depend on temperature, a buffer system depends on temperature. • Changing temperature changes both the pH and the pKa.

25. Buffers & Ionic Strength • Using just H-H as given, dilution of a buffer should not change the pH of the buffer. • But this is simplistic, ignoring activities due to ionic strengths.

26. Buffers & Ionic Strength • Diluting a buffer lowers the ionic strength of the buffer, and this in turns changes (raises) the pH. • This is Chapter 12.