The plot on the left shows you a calculated (hypothetical) titration curve (a plot of pH vs. vol. base added) on a scale of pH=0 to 14 from the beginning of the titration to just after the equivalence point.

The composition of the mixture in your beaker can be adjusted with the sliders labeled "nA-" and "nHA", which makes it possible to access intermediate points in a titration. The strength of the acid is given by the first slider, which adjusts the pKa value of the acid.

The initial values are set to give you a solution which is 0.5 M in both the acid and its conjugate base, a 1:1 buffer mixture. If you move the pKa slider slightly, a bar graph pops up on the right, displaying the concentrations of acid and conjugate base separately.

Finally, marked on the titration curve with a circle is that point in a hypothetical titration of the pure acid, which would have the same concentration of acid and conjugate base as the mixture that you prepared with your sliders.

1. Strength of the Acid
Vary the pKa value of the acid HA. Note what happens to the composition of the mixture (the bar graphs) and its pH (the marked point). No surprises so far. Note that the ratio of acid to conj. base starts to deviate from 1:1 for very low pKa. Do you have an explanation for this behavior?

2. Buffer Composition
Change the composition of the buffer mixture by varying the amount of conjugate base (A-) and acid (HA). Note how the corresponding bar graphs (=concentration of HA and A-) go up or down. Also observe how the marker on the curve indicates the change in the pH of the solution and the change in the amount of base that you would have had to add to the pure acid to achieve the same composition.

3. Buffer Action
Move the sliders back to a 1:1 mixture of a weak acid with its conjugate base (about 0.5 M each).

a. Test the buffer by adding some base to it (move the slider labeled "nb" (for moles of base added) to the right). Note how now both concentrations (HA and A-) change: If you add base, which one goes up and which down? Why?

b. The marker moves to the right. Explain.

c. Estimate the pH change for a small addition of base. (The total plot window covers pH=0 to pH=14 vertically)

d. Move the added base slider (nb) back and add some acid (na). How do the concentrations of HA and A- react now? Explain.

e. Why does the marker move the other way now?

f. Estimate the pH change for a small addition of acid.

4. Big Deal?
Prepare a simple salt (as in NaCl) solution: Move pKa to 0.0 (for HCl), add 0.5 M Cl- (A-) and no acid (HA).
Note: Briefly ignore the bar graphs, the equations used for their display do not work beyond the equivalence point.

Add a little acid, estimate the pH change, and compare to your results from 3.

5. Best Buffer
Compare two buffers: one mixed 1:1 and one 4:1, but both with the same total of acid and conjugate base (HA + A-) to be fair.

a. How do the pH values of the buffers compare?

b. How much acid and how much base can each buffer "handle" respectively (in separate tests) before the pH goes through the roof (or south)?

6. Buffer pH
Given your findings from above (1. and 5.):

a. Name two methods to adjust a buffer pH

b. Which method is preferable in terms of the buffering ability?

7. Buffer Capacity
Mix two 1:1 buffers with very different total concentrations of acid and conjugate base.

a. How do the pH values for those two buffers compare?

b. How much acid and how much base can each buffer "handle" respectively (in separate tests) before the pH goes through the roof (or south)?

Note: This difference is called "buffer capacity"

8. Buffers in Nature
Think about the importance of buffers in biology (your blood stream, soil...) and what species might be involved in these buffers