How Do Buffers Control PH?
Buffers are chemicals that, when added to water, tend to maintain a certain pH. This is due to the buffer's ability to either accept or donate a proton (hydrogen ion, H+) or hydroxyl (OH-) to keep the pH in a certain range. Buffers are different than strong acids and bases because buffers do not donate all of their protons or hydroxyls within their buffering range.
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Background
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The measure of pH gives the concentration of hydrogen ions in a solution. Pure water has a pH of 7.0 and has an equal balance of hydrogen ions and hydroxyl ions. The pH will be lower if an acid is added and higher if a base is added. For various purposes, it is useful to add a buffer so that the pH remains constant even if an acid or base is added to the solution. Buffers are used to maintain constant pH.
Buffer Equilibrium
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Different buffers work at different pHs. A buffer is most efficient at maintaining a constant pH when the pH equals its acid dissociation constant, its pKa. At this pH, the buffer consists of an equal solution of protonated and de-protonated conjugate base. This often denoted by a theoretical buffer molecule, AH, dissociating into a proton, H+, and the conjugate base, A-. The pKa is determined by the pH where the concentration of AH equals the concentration of A-.
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Buffering Acids
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At its pKa, a buffer can maintain a constant pH by accepting free protons. Adding an acid to the solution, such as HCl, will cause free protons to enter the solution. The conjugate base will accept the free protons, causing the pH to be unchanged. The solution will remain at the same pH as long as there is enough conjugate base to accept the additional protons. The reaction can be written as the dissolution of the acid: HCL--H+ + Cl- leading to free H+ in the solution. The further reaction with the buffer occurs: H+ + A- -- HA. This removes the free protons and constant pH is maintained.
Buffering Bases
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Adding a base such as sodium hydroxide to a solution will cause an increase in the hydroxyl concentration. In a buffered solution, the protons attached to the undissociated buffer are donated to the solution, forming H2O with the free hydroxyls. This counters the effect of the base and maintains the pH of the solution as long as there is undissociated buffer available in the solution. In this case, the reaction can be written as the dissolution of the base: NaOH-- Na+ + OH- leading to the second reaction of OH- + HA -- H2O + A-.
Different Buffers
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Different buffers have different pKas and can be used to buffer solutions at a wide range of pHs. Chemicals used as buffers often have the ability to donate or accept multiple protons or bases. These buffers hence have two or more different pKas corresponding to how many protons or hydroxyls per molecule they can donate or accept. Different buffer molecules can be combined to form customized buffer ranges. For a list of biological buffers, see Resources.
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References
Resources
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