Experiment 2: pH and Buffers

Question 1

Water dissociates according to what eqn?

Answer 1

H2O <—> [H+][OH-]

Question 2

Kw is what under 25 degrees?

Answer 2

10^-14 M^2

Question 3

pH formula?

Answer 3

pH= -log[H+]

Question 4

How does a pH electrode work?

Answer 4

Solutions containing ions are capable of carrying an electrical currents. This fact is exploited to measure pH. A common pH meter has a thin walled bulb of glass contain 0.1M HCL that is in contact with a metal wire electrode. When this is placed in solution of unknown [H+], a potential is set up across the thin glass wall.

Question 5

The optimal behaviour of most biological molecules takes place where?

Answer 5

w/n a narrow range of pH values

Question 6

What are buffers?

Answer 6

In natural biological systems, pH is maintained by the activity of many other molecules that are present, and one of the names for these molecules regulating pH is buffers.
Buffers are classes of molecules whose presence in a system minimizes the pH change when acid/base is introduced into the system.

Question 7

Buffers are usually what?

Answer 7

weak acids

Question 8

Weak acid equilibria?

Answer 8

HA <—-> H+ + A-

Question 9

A truly functional buffer requires the presence of what?

Answer 9

BOTH the weak acid and its conjugate base for it to maintain a solution at a near constant pH.

Question 10

What does the Henderson-Hasselbach eqn imply? (there are 4 implications)

Answer 10

1. the pH of the buffer is determine by the ratio of concentrations, not the concentrations alone.
2. if HA (WA) and A (CB) are present in equal concentration; the ph of the solution will be equal to the pKa.
3. it is very important to be aware that the Henderson-Hasselbach eqn implies that buffers do not prevent change, but rather they simply function to minimize change.
4. Henderson-Hasselbach eqn also implies that the ability of a buffer to resist changes in pH upon addition of an acid/base diminishes precipitously (rapidly) at pH values +/- 1 unit of the pKa. This does not mean that there is no buffering when you are greater than 1 pH unit from the pKa, it just means that there us no longer effective buffering bc there is relatively little of either of the WA or the CB.

what this point is saying is that the buffer stops working well when the change of the pKa is +/- 1 when adding an acid/base.

Question 11

Buffer capacity is defined as what?

Answer 11

the number of moles of [H+] that is required to lower 1L of the buffer 1 pH unit.

Question 12

How do you select a buffer?

Answer 12

First and most obvious selection criteria for choosing a buffer is its pKa. One should close a buffer that has the ability to buffer at t he desired experimental pH.

Question 13

Good’s Buffers (try to remember at least 4 out of the 8)

Answer 13

1. pKa between 6 and 8 - since the vast majority of biochemical reactions take place w/n this range.
2. high solubility in water - most biological reactions occur in aq solutions.
3. Exclusion or minimal transport by biological membranes.
4. Minimal salt effects - this is referring to the preference that the buffer components themselves do not affect the ions that are important to biological processes.
5. Minimal effect on buffer dissociation by temperature, other ions, or buffer concentration.
6. Chemically stable - you do not want your buffer degrading into other chemical species.
7. Minimal light absorption in the UV and visible portion of the electromagnetic spectrum.
8. Readily available

Question 14

Why is TRIS not a great buffer?

Answer 14

Has sensitivity of pKa to temperature. It’s pKa also varies with the concentration to which it is prepared, and some pH meter electrodes are not compatible with it.