What Is a Buffer? Keeping Acidity Steady When Something Tries to Change It
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A buffer resists changes in acidity, so adding acid or alkali to it shifts the pH far less than the same addition would shift plain water. Blood, seawater and almost every laboratory experiment depend on that resistance, and the mechanism is simple enough to state in a line.
How it works
A buffer contains substantial amounts of both a weak acid and its conjugate base, which is what remains after that acid has given up a proton. The two coexist because a weak acid does not fully dissociate, so a solution containing the acid and a salt of it holds a reservoir of each. When acid is added, the base component reacts with the incoming protons and removes them from solution. When alkali is added, the acid component donates protons to neutralise it. In both directions the added substance is consumed by a reserve that was already present, and the ratio of the two components shifts slightly rather than the free proton concentration changing much. Since pH depends on that ratio logarithmically, a modest change in the proportions produces only a small change in pH, which is the whole effect and the reason the resistance is so effective within its range.
The limits
Buffers are not unlimited and their behaviour is defined by a few quantities:
- •Buffer capacity, the amount of acid or alkali that can be absorbed before the pH moves substantially, which depends on the total concentration of the components
- •The effective range, roughly one pH unit either side of the acid's dissociation constant, outside which the resistance is poor
- •Exhaustion, since once one component is used up the solution behaves like any other and the pH moves sharply
- •Dilution, which reduces capacity though it barely changes pH within the range
- •Temperature, since dissociation constants shift with it and a buffer set at one temperature is not at that pH at another
- •Interference, since some ions in solution interact with the components and change the behaviour
Buffering in the body
Human blood is held between roughly 7.35 and 7.45, and departures of a few tenths in either direction are medical emergencies, because enzymes and proteins depend on a narrow range to function. The main system is carbonic acid and bicarbonate, which is unusual in being an open system, since one component is a dissolved gas that the lungs can remove or retain by changing breathing rate, and the kidneys adjust the other over hours to days. That combination gives short-term and long-term control and is far more powerful than a closed buffer of comparable concentration. Proteins, particularly haemoglobin, contribute additional buffering, and phosphate contributes inside cells. The clinical categories of acidosis and alkalosis, divided into respiratory and metabolic causes, map directly onto which side of the system has failed, which is why blood gas measurements are read as a pair rather than as a single number.
Making one
Preparing a buffer in a laboratory is routine and the decisions involved are worth stating. The first is choosing an acid whose dissociation constant is close to the target pH, since the resistance is strongest there and falls away quickly outside the range, and a list of standard compounds exists covering the useful span. The second is concentration, which sets the capacity and has to be high enough to absorb whatever the experiment will add while low enough not to interfere with what is being studied. The third is composition, since the ions present can bind metals, affect enzymes or precipitate with other components, which is why several buffers exist at the same pH with different chemistry. Preparation is done either by mixing calculated amounts of the acid and its salt or by taking one and titrating to the target pH with a strong acid or alkali, and the second is more reliable because it corrects for the assumptions the calculation makes.
Elsewhere
The same principle operates at very different scales. The ocean is buffered by a carbonate system, which has absorbed a large fraction of the carbon dioxide released by human activity and has become measurably more acidic in the process, and the concern about that acidification is precisely that the buffering, while real, is being consumed and has consequences for organisms building carbonate shells. Soils are buffered by minerals and organic matter, which is why the effects of acid deposition appeared slowly and then sharply in regions with poorly buffered soils. Laboratory work depends on buffers for almost every biological procedure, since enzymes and cells require controlled conditions, and a substantial catalogue of buffer compounds exists covering different pH ranges with different interference properties. Shampoos, foods and pharmaceuticals are buffered for stability and comfort. In each case the purpose is the same, which is to make a system insensitive to disturbances it will certainly receive.
The takeaway
Holding a weak acid and its conjugate base together gives a reserve that absorbs added protons in one direction and donates them in the other, so the ratio shifts while the pH barely moves. The effective range is about one unit either side of the acid's dissociation constant, and beyond capacity the pH moves sharply. Blood uses an open system where lungs and kidneys adjust the two components independently.