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 CO 2 ) 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: mL of a M sln of TRIS buffer with a pH of 8.1. What’s TRIS? TRIS base is a crystalline solid with a MW = 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 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 mL of a 0.050M piperdine sln.

19 Buffers: Preparation Method 2 cont. You also want to weigh enough piperidine hydrochloride to make 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 = pK a 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 measuring Δ pH upon the addition of strong acid or base. The larger the Δ pH, 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 pK a as close to that pH as possible. The pK a 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 pK a.

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.