LSM2106 Chap.2 pH, Acid Chemistry and Buffer Behaviour
pH, Acid Chemistry and Buffer Behaviour
Why one equation governs so much of this subject
Nearly every organic compound in a cell carries a group that can gain or lose a proton, and its behaviour depends on which state that group is in. Hydrogen ion concentration is therefore a control variable rather than a background condition.
The second lecture introduces acid chemistry and pH, and the first practical gives the topic a whole laboratory session: calibrating a meter, finding the buffering region of a solution, understanding the properties of buffers, and calculating with the Henderson-Hasselbalch equation.
A weak acid dissociates only partly, and the position of that equilibrium is its dissociation constant.
Taking negative logarithms of both the hydrogen ion concentration and the constant turns the equilibrium into a working form in which pH equals the pK plus the logarithm of the ratio of conjugate base to undissociated acid. The essential feature is that the logarithmic term holds a ratio.
It is insensitive to how much material is present and sensitive only to the proportion between the two forms, which is why pH equals pK when the two are equal, and why diluting a buffer changes its capacity without moving its pH.
Measurement, choice and capacity
A meter does not count protons.
It measures a potential across a glass membrane that lets protons through and holds other cations back, with a solution of known composition trapped inside. That potential depends explicitly on temperature, and real electrodes deviate from the theoretical response by amounts that vary across the scale.
Both facts are the reason calibration is done with standard buffers at more than one point, and the reason a reading taken on a cold solution against a meter set for a warm one is not a measurement of anything.
Choosing a buffer is choosing a pK. Two buffers adjusted to the same pH behave differently when acid or base is added, because what they hold in reserve depends on how far the working pH sits from their own pK.
A phosphate buffer with a pK of 6.8 and a Tris buffer with a pK of 8.1, both set to pH 7.0, make this visible: one holds most of its material as conjugate base and resists added acid well, the other holds almost all of it in the protonated form and resists added base.
The practical rule that follows is to select a buffer whose pK lies within about one unit of the working pH, subject to the buffer being chemically compatible with whatever is being studied.
Temperature is the final complication.
The dissociation constant of the buffer itself shifts with temperature, so a solution adjusted on the bench is not necessarily at that pH in a cold room, which matters for any procedure carried out on ice.
What this chapter covers
- 01
Weak acid dissociation and the meaning of the dissociation constant
- 02
The Henderson-Hasselbalch form and why it contains a ratio
- 03
Buffering as interconversion between a weak acid and its conjugate base
- 04
The glass electrode, its temperature dependence and multi-point calibration
- 05
Buffer selection by pK, and the compatibility limit on that rule
- 06
Capacity as distinct from pH, and what dilution changes
- 07
Temperature effects on buffer pH during cold-room work
Preparing a buffer to a specified pH
- 3Convert the difference between pH and pK into a ratio of the two forms.
- 3Turn that ratio into amounts within the stated total.
- 2Separate the effect of dilution on pH from its effect on capacity.
Key terms
- Dissociation constant
- The equilibrium constant for the loss of a proton from a weak acid, expressed as the product of conjugate base and hydrogen ion concentrations divided by the concentration of the undissociated acid.
- pK
- The negative logarithm of the dissociation constant. It is the pH at which the protonated and deprotonated forms of a group are present in equal amounts.
- Buffer
- A mixture of a weak acid and its conjugate base, which resists change in pH by converting one form into the other when acid or base is added.
- Buffering region
- The span of pH, roughly one unit either side of the pK, over which a buffer holds a useful reserve of both forms and therefore resists change effectively.
- Buffer capacity
- The amount of acid or base a buffer can absorb for a given change in pH. It depends on the absolute concentrations present, so dilution reduces it without moving the pH.
- Glass electrode
- A membrane that lets protons pass while holding other cations back, across which a potential develops that depends on the difference in hydrogen ion concentration on either side.
- Equivalence point
- The point in a titration at which added base exactly matches the acid originally present, seen on a titration curve as the steep region between two plateaux.
pH, Acid Chemistry and Buffer Behaviour FAQ
What is the difference between buffer pH and buffer capacity?
The pH is set by the proportion of conjugate base to protonated acid, so it survives dilution unchanged. The capacity is set by how much of each form is actually present, so it falls in direct proportion when the solution is diluted. A buffer at the right pH with almost nothing dissolved in it will hold that pH against almost nothing.
How do I pick between two buffers that are both at the right pH?
Compare each pK with the working pH and ask which direction the challenge will come from. A buffer whose pK sits below the working pH holds more conjugate base and absorbs acid better; one whose pK sits above holds more protonated form and absorbs base better. Then check chemical compatibility, because a buffer that chelates a required metal ion or acts as a substrate is unusable however well its pK matches.
Why calibrate a meter at three standard buffers rather than one?
One standard fixes only the offset. Electrodes also deviate in slope, and that deviation varies across the range, so a meter standardised at a single point can still read correctly there and badly two units away. Standardising at pH 4, then 7, then 10 constrains both offset and slope across the working range.
Does a buffer keep the same pH when I move it to an ice bath?
Not necessarily. Temperature changes the dissociation constant of the buffer, and the size and even the direction of the shift depend on which buffer it is. This matters for any protein work done in the cold, because the sample experiences the pH the solution actually has at that temperature rather than the one written on the bottle.
Exam move
Practise until the ratio calculation is reflexive in both directions: given pH and pK, produce the ratio; given the amounts, produce the pH. Then sketch a titration curve from memory and mark the pK, the buffering region and the equivalence point on it. Most short-answer marks in this topic are for saying which of those three a described situation is sitting at.
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