The pH of a solution can change due to dissolution of atmospheric carbon dioxide, leaching of alkali from glass container, and/ or a chemical reaction that occurs in the solution. However, buffers would resist such pH change and maintain the solution at some acceptable pH level. In addition, there are some factors that could affect the pH of a buffer which include:
1. Ionic strength of solution.
Addition of neutral salts such as sodium chloride affect pH of a buffer solution by altering ionic strength. Ionic strength affects ionization constants, activity of ionized species (salt form) of the buffer, and activity of hydronium ion which alters pH of solution. On the other hand, addition of water (dilution) to a buffer solution can alter its pH due to reduction in ionic strength. The following buffer equation for an acid (HA) and its conjugate base (A-) accounts for the ionic strength by incorporating activity coefficient term, γA- :
activity coefficient is usually less than 1. Accordingly, logγA- is negative. Thus, it is expected to have lower pH for acid- conjugate base pair when ionic strength is accounted for.
2. Temperature.
Temperature change affects Ka, Kb, and Kw. Therefore, pH of buffers is influenced by temperature. Each buffer is influenced differently. Some buffers decrease in pH (e.g. boric acid- sodium borate) while others increase in pH (e.g. acetic acid- sodium acetate) as temperature increase. Moreover, the magnitude of change is relatively small for acidic buffers when compared to basic buffers.
1. Ionic strength of solution.
Addition of neutral salts such as sodium chloride affect pH of a buffer solution by altering ionic strength. Ionic strength affects ionization constants, activity of ionized species (salt form) of the buffer, and activity of hydronium ion which alters pH of solution. On the other hand, addition of water (dilution) to a buffer solution can alter its pH due to reduction in ionic strength. The following buffer equation for an acid (HA) and its conjugate base (A-) accounts for the ionic strength by incorporating activity coefficient term, γA- :
2. Temperature.
Temperature change affects Ka, Kb, and Kw. Therefore, pH of buffers is influenced by temperature. Each buffer is influenced differently. Some buffers decrease in pH (e.g. boric acid- sodium borate) while others increase in pH (e.g. acetic acid- sodium acetate) as temperature increase. Moreover, the magnitude of change is relatively small for acidic buffers when compared to basic buffers.
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