What Are Buffers & Why Are They Important?
Nov 04 2014
Control over the variables in an experiment or analysis is an important element to make sure the analysis is accurate and repeatable. There are many variables that you might have to consider, both chemical and physical, during the analysis. For example, a variable like temperature can be controlled using an oven or water bath.
When adding small amounts of a sample to a solvent, you can cause chemical changes such as a change in pH. This can lead to further changes such as changes in solubility and sample precipitation. Whilst this can be annoying in a beaker — imagine if it happens in a chromatography column. So how can we control a variable like pH?
What is a Buffer?
A buffer is simply something that resists changes — and a buffer solution is a solution that resists changes to a solution’s pH when small quantities of acid or alkali are added. So in HPLC for example, if adding a sample to a mobile phase can shift the pH, a buffer can be added to the mobile phase to resist the pH changes — giving the analyst control over the pH.
How do they work?
The ions in solution that cause a change in pH are hydrogen ions (H+) or hydroxide ions (OH-). Addition of H+ increases the concentration of hydrogen ions, so reducing pH; OH- combine with H+ in solution forming water, by removing hydrogen ions, and thus increase the pH. A buffer solution has to resist these changes.
There are two basic forms of buffer solution, acidic buffer and alkali buffer. Although the different buffers work in slightly different ways, the process is very similar. An acidic buffer contains a weak acid (HA) and its conjugate base (A-), an alkali buffer the opposite. In solution the two species are in equilibrium giving:
HA + H2O ↔ H3O+ + A-
If an acid (H+ or H3O+) is added to the solution, the equilibrium moves to the left to remove the extra hydrogen ions. If an alkali is added the opposite happens. In both cases the added reagent is effectively removed as the equilibrium changes and the pH changes very little. This is an example of Le Chatelier’s principle in action.
Chromatography and pH
There are several ways that the pH of a mobile phase can change. Perhaps the most obvious is when an injection is made and the sample and mobile phase mix — especially if the sample’s pH is significantly different to the mobile phase pH. A mobile phase can also absorb carbon dioxide on standing, this makes it more acidic, similar to ocean absorbing CO2 from the atmosphere.
pH is important in chromatography because the retention time of the sample’s components can be pH dependent. If the pH changes, the retention time will change making the analysis much more difficult. For further information in this area, an article highlighting the importance of buffers is: Controlling Selectivity on Zwitterionic HILIC Columns by Adjusting pH and Buffer.
Image Source: Trains Coupling
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