A simple Model for pH Buffers

The understanding of pH buffers is critical in fully understanding pH changes in mash, wort and beer. To most brewers the fundamentals that are associated with pH are not as intuitive as they are for example for temperature. Temperature is something we can feel and we have learned over time, through experience, how it is affected and how changes can be predicted and controlled. This is not the case with pH, which we can neither feel nor see and for which we have developed little intuition. This article tries to illustrate the concept of pH buffers and how they interact with acids and bases using a simple model.

pH and buffers

The chemical fundamentals of pH were discussed in Overview of pH. There we saw that pH refers to the concentration of free hydrogen ions and pH buffers are created by weak acids and bases which donate or absorb hydrogen ions based on their current concentration in the environment, i.e the current pH.

Visializing pH and pH buffers

The ability of a pH buffer to supply hydrogen ions when needed or absorb them when there are too much is best visualized as the water level in a vertical tube which has a bulge that creates a reservoir. The water level represents the pH and the size and location of that reservoir represents the buffer capacity which is a property of the solution and depends on the weak acids and bases present in that solution.

When water is added the water level, as observed in a sight glass, rises. The amount by which is rises depends on two things: the amount of water added and the diameter of the vertical tube at the current water level. The wider the tube the more water is takes to achieve the same water level change. This is very similar to pH in buffered systems. The water level is the pH and the diameter of the vertical tube is the buffer capacity of the system which can change with pH. Adding water is analogous to adding hydrogen ions (i.e. adding acids)

Figure 1: Models for 3 different pH buffer systems. See text for explanation

Figure 1-A shows a system with a weak pH buffer which is buffered around a pH of 8. Once the water level is within the reservoir it takes a moderate amount of water to change the water level. When the level is above or below the reservoir it takes much less water to cause the same water level change. The same is true with pH buffers. Once the buffer is “broken” it's ability to consume or donate hydrogen ions is exhausted and all the hydrogen ions added go directly towards changing the pH. This results in a dramatic fall in pH if a strong acid is added or a dramatic rise if a strong base is added.

Figure 1-B shows a system that is buffered at the same pH as A but with a stronger buffer. The solution may for example contain a higher concentration of the pH buffering substance. At or around the buffered pH it takes the addition of more water (i.e. acid) to move the pH by the same amount as in example A. Figure 1-C has the same buffer strength as A but at a higher pH. At the current pH is is not buffered at all and it takes very little water to change the pH.

Typical pH related experiments

Titratrion

Titration is a process in which known amounts of a strong acid and/or base are added to a sample and the resulting pH change is recorded. That pH is then plotted over the amount of acid equivalents that were added (strong acid) or neutralized (strong base). This reveals where and how strong the sample buffers its pH. In the presented model this is equivalent to adding and draining known amounts of water.

Measuring alkalinity

Alkalinity is the pH buffer capacity of water and it is measured by adding a strong acid until the sample's pH is 4.3. At that point virtually all carbonate and bicarbonate has been converted to carbonic acid and CO2 and the amount of acid that needed to be added is a measure for the water's bicarbonate and carbonate content. In our model this means adding water until water level represents a pH of 4.3. The amount of water added represents the “alkalinity”. From the examples shown in Figure ?? it is clear that the pH of the water does not indicate how well it is buffered. The system A shown in Figure ?? has a current pH of 8.0 but it is comparatively weakly buffered. System B has a lower pH (6.5) but is much stronger buffered and it will take 4 times (the tubes are assumed as being round. This means twice the diameter is 4 times the area of the cross section) as much water (i.e. acid) to lower its pH to 4.3.