How to Calculate pH of a Buffer
A buffer solution is a solution that is designed to maintain a fairly constant pH, even in the presence of other acids or bases. Buffer solutions are prepared by taking a specific acid or base in solution, and adding a certain amount of the corresponding conjugate (the conjugate in HCl, for example, is Cl-) in a different solution. Such a solution resists large changes in pH because of the free conjugate ions that can bond with incoming H+ or OH- ions.
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Instructions
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Write the chemical reaction for the dissociation of your buffer solution.
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Determine the dissociation constant for the relevant acid or base. The dissociation constant is the ratio of dissociated ions to initial compound present at equilibrium, and is represented as Ka for an acid or Kb for a base. See Resources for lists of common dissociation constants.
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Take the negative log (base 10) of your dissociation constant. If the solution is at equilibrium, this is the pH of your buffer solution. For example, hypobromous acid has a dissociation constant of 2 x 10^-9. log(2 x 10^-9) ~= -8.699, so the pH of a hypobromous acid buffer would be approximately 8.699.
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Tips & Warnings
If your buffer solution has had a strong acid or base added to it (in other words, if it is not at equilibrium), the pH will change slightly. In order to account for this change, calculate the ratio of the concentration of the conjugate base to the concentration of un-dissociated molecules. Take the log of this value and add that to the pH. For example, if the ratio of the concentration of conjugate base to un-dissociated molecules in hypobromous acid is 1/2, add log(1/2) to 8.699. log(1/2) ~= -0.301, so the pH is 8.699 - 0.301 = 8.398.
References
Resources
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