Lab Description: Acid-Base Titration
In this lab exercise we will evaluate the effectiveness of several indicators for the determination of the point of completion of a specific acid-base neutralization reaction. We will also determine the unknown concentration of the strong base NaOH by its reaction with a known amount of the weak acid, potassium acid phtalate (HKC8H4O4, abbreviated KHP). This will be accomplished using the titration method. The KHP solution will be created and its volume and concentration recorded. The KHP solution will be poured in a flask along with a few drops of one of three indicators we will be evaluating. The NaOH solution will be poured into a buret (with volume markers) and will be used as the titrant. The strong base will be added slowly to the acidic solution, gradually neutralizing the acid. The volume of base added can be determined by the difference in the initial and final volume marks on the buret.
At a certain volume of added NaOH, all the KHP acid will be neutralized due to the large equilibrium dissociation constant (Kb) of the base. This point of titration is referred to as the equivalence point. Considering the 1:1 stoichiometry of this acid-base reaction
NaOH(aq) + C6H4(COOH)(COOK) (aq) C6H4(COONa)(COOK)(aq) + H2O(l)
the point of equivalence is the point of titration when the number of moles of NaOH (Na) added is equal to the number of moles of KHP (Nb) in the solution. The number of moles of KHP in the solution can be calculated very simply by dividing the known mass of the sample in the solution by its molecular mass. The unknown concentration of the NaOH can then be calculated in the following manner:
At the point of equivalence of a reaction of 1:1 stoichiometric ratio, Na = Nb.
The number of moles of a solute is the concentration times the volume (N = Vc ).
Thus Vaca = Vbcb.
Knowing all other variables we can solve for cb by restructuring the previous equation as cb = caVb/Va.
However, in order to determine the equivalence point the dissociation of the indicators being used must coincide with the pH at the equivalence point. The indicator, a weak organic acid, will dissociate at a certain pH. The dissociation of an indicator is concurrent with a color change or some other physical change which informs the observer of the solution's approximate pH. A decreased amount of H3O+ (a product of acid dissociation) makes it more probable for the dissociation reaction of the indicator to occur since equilibrium must be maintained. Depending on the specific dissociation constant of each indicator a different H3O+ concentration (and thus pH) will trigger the dissociation of each indicator.
Since we do not know the dissociation equilibrium of each indicator, we cannot calculate the exact range of pH at which a color change will appear. Thus we will must repeat the titration experiment with a pH-meter and record the pH of the acid-base solution per milliliter of NaOH added. The results of this part of the experiment will be used as the correct reference in order to determine which indicators change color at a pH range that coincides with the approximate pH at the equivalence point of the given titration. The calculations of the concentration of NaOH must thus exclude the unsuitable indicator(s).
Method and Explanations
·Acid Base Titration with Different Indicators
We first created a solution of NaOH by adding 10ml of 6M NaOH to 500 ml of distilled water. This solution was poured into a plastic bottle with a lid and was shook vigorously for a few minutes. It is essential that the solution be homogeneous for the titration experiment to be successful for in order to investigate and calculate the NaOH concentration it must be constant throughout the solution. We then rinsed and dried four clean beakers and labeled them from one to four. We weighed precisely 0.50 g of KHP in each...