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For example, the acid may be acetic acid and the salt may be sodium acetate. The Henderson–Hasselbalch equation relates the pH of a solution containing a mixture of the two components to the acid dissociation constant, K a of the acid, and the concentrations of the species in solution. [6]
The Henderson–Hasselbalch equation, which is derived from the law of mass action, can be modified with respect to the bicarbonate buffer system to yield a simpler equation that provides a quick approximation of the H + or HCO − 3 concentration without the need to calculate logarithms: [7]
The ratio of acid, AH and conjugate base, A −, concentrations varies as the difference between the pH and the pK a varies, in accordance with the Henderson-Hasselbalch equation. The pH of a solution of a monoprotic weak acid can be expressed in terms of the extent of dissociation. After rearranging the expression defining the acid ...
In this case H 0 and H − are equivalent to pH values determined by the buffer equation or Henderson-Hasselbalch equation. However, an H 0 value of −21 (a 25% solution of SbF 5 in HSO 3 F) [5] does not imply a hydrogen ion concentration of 10 21 mol/dm 3: such a "solution" would have a density more than a hundred times greater than a neutron ...
The ratio of concentration of conjugate acid/base to concentration of the acidic/basic indicator determines the pH (or pOH) of the solution and connects the color to the pH (or pOH) value. For pH indicators that are weak electrolytes, the Henderson–Hasselbalch equation can be written as: pH = pK a + log 10 [Ind −] / [HInd]
The Henderson-Hasselbalch equation offers a way to determine the proportion of a substance that is ionized at a given pH. In the stomach, drugs that are weak acids (such as aspirin ) will be present mainly in their non-ionic form, and weak bases will be in their ionic form.
The Henderson–Hasselbalch equation gives the pH of a solution relative to the pK a of the acid–base pair. However the p K a is dependent on ionic strength and temperature, and as it shifts so will the pH of a solution based on that acid–base pair.
This is why weak acids are better absorbed from the stomach and weak bases from intestine where the pH is alkaline. When pH of a solution is equal to pKa of dissolved drug, then 50% of the drug is ionized, another 50% is unionized. This is described by the Henderson-Hasselbalch equation. [citation needed]