Henderson-Hasselbalch Analysis Lab
A practical was carried out to show how the Henderson-Hasselbalch equation works and to apply and hone the skills of pipetting, buffer-making, pH-meter use and graphing. The primary goal of the practical was to determine the pKa value of a weak acid.
The Ka is an acid dissociation constant, it is a quantitative measure of the strength of an acid in solution. The pKa is defined as the negative log of the Ka, pKa= -log Ka. The lower the value for the pKa the more acidic the acid (greater concentration of H+ ions). The pKa of a compound relates to how likely, at a given pH, the compound will be ionized.
The Henderson-Hasselbalch equation is used to quickly …show more content…
This is achieved through the existence of the equilibrium HA (aq) ⇄ H+ (aq) + A- (aq). When a strong acid or alkali is added the equilibrium shifts to the right or the left adhering to Le Châtelier 's principle, which states that whenever an equilibrium is subject to change of concentration, temperature, volume, or pressure, the system will readjust itself to some extent, to counteract the effect of the change. This means that if a strong acid is introduced to a buffer, the hydrogen ion concentration would increase by less than the amount expected. This is because adding H+ ions lowers the pH which shifts the equilibrium to the left, thus lowering the H+ ion concentration raising the …show more content…
The equivalence point can be shown by pH indicators, these are substances that change colour in response to a chemical change; common examples are methyl red, bromothymol blue and phenolphthalein. Sometimes the equivalence point can also be estimated with conductance
Titrations are frequently recorded as graphs called titration curves, the volume of the titrant is generally the independent variable and the pH of the solution the dependant variable. These graphs should frequently form double-sigmoidal lines. The equivalence point can be found by calculating the second derivative of the titration curve.
In this particular practical acetic acid was titrated with sodium hydroxide. In this reaction the proton (H+ ion) is transferred to the water molecule creating a hydronium ion. Acetic acid is a weak acid, this is because it doesn 't ionise fully when it is dissolved in water. Another way of saying this is the “back” reaction is more successful than the forward one. Below is shown the equation for the dissociation of acetic acid into acetate.