The Bronsted-Lowry theory defines acids and bases by their ability to donate and accept protons. The Henderson-Hasselbalch equation uses these definitions to find the pH or concentrations of a buffer solution. Accurate buffers are useful in titration and industrial applications such as shampoo production.
Bronsted-Lowry theory defines acids and bases by their ability to donate and accept protons of hydrogen. The Henderson-Hasselbalch equation relies on these definitions to create a formula for finding the pH or concentrations of a buffer solution. Accurate buffer solutions can be used in titration or for industrial uses such as shampoo production.
According to the Bronsted-Lowry theory, an acid is any molecule capable of donating a proton of hydrogen, while a base is any molecule capable of accepting the proton. The dissociation reaction of the acid, HA, produces an H+ which is donated and an A-. This A- is then called the conjugate base. The Henderson-Hasselbalch equation is constructed using this reworked dissociation reaction for the acid dissociation constant.
An acid dissociation constant, denoted Ka, is the equilibrium point for the dissociation reaction. It is represented in the formula Ka = ((A-)(H+)) / (HA), where A-, H+ and HA represent their respective concentrations in moles per liter. Since the Henderson-Hasselbalch equation solves for pH, the Ka equation needs to be reworked.
The conversion works around the fact that pH = -log(H+), which means that this should appear in the converted equation. A -log is multiplied on both sides of the Ka equation by -logKa = -log((A-)(H+)) / (HA) and then the log distribution converts it to -logK = -log(H+) + ( -log ((A-) / (HA))). Using the definition of pH, and the fact that pKa = -logK, one can write pKa = pH – log ((A-) / (HA)). In terms of pH, this becomes pH = pK + log ((A-) / (HA)).
The Henderson-Hasselbalch equation can be used by entering the known concentration of the conjugate base in for A- and the known concentration of the weak acid in for HA. An example buffer solution might include 0.10 moles of acetic acid (HC2H3O2) and 0.45 moles of acetate ion (C2H3O2) with a pK value of 1.7 x 10-5. The resulting equation: pH = -log (1.7 x 10-5) + log (0.45 / 0.10) = 5.4.
Solving the Henderson-Hasselbalch equation instead for the concentration of the acid or base can determine how much to use when creating a buffer solution. Buffers are used in titrations to determine the pH level of other acid-base pairs. They also have industrial uses wherever pH needs to be well managed. The shampoo doesn’t become too caustic and the beer doesn’t spoil thanks to the addition of well-timed buffers.
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