10th Jun 2013
Monday 2-4 pm
Scientific Principles and Enquiry
(Working in Pairs)
Experiment 1 – pH Titrations
In quantitative chemical analysis, acid - base reactions are often used to provide a basis for various titration techniques. The equivalence points of acid - base titrations can be estimated from the colour change of chemical indicators, such as phenolphthalein, methyl red, methyl orange and so on. The choice of an indicator suitable for a particular titration requires a detailed knowledge of the chemical properties of the acid and base. This difficulty can be avoided by using physical methods, which follow the change in some property of the solution as the titration proceeds. Such a property must show a rapid change at the equivalence point, or alternatively, the rate of change must be different before and after the equivalence point. Electrical conductivity and the concentration of hydrogen ions in solution provide two examples of such properties.
In aqueous solutions the concentration of hydrogen ions can vary by many orders of magnitude, and it is therefore convenient to consider it on a logarithmic scale by using the relationship:-
pH = log10 [H+]
Although, strictly, the above relationship is an approximation it is sufficiently accurate for the purpose of this exercise. For the equilibrium:-
the ionic product at 25ºC is:-
Kw = [H+] [OH-] = 1 x 10-14
Thus in neutral solutions:-
[H+] = [-OH ] = 1 x 10-7 mol dm-3
and the pH value is
pH = -log (1 x 10-7) = -(-7) = 7
Acidic and basic solutions can be distinguished in terms of their pH values:
| > 1 x 10-7
| < 7
| 1 x 10-7
| = 7
| < 1 x 10-7
| > 7
In acid-base titrations the pH of the titrated solution changes throughout the titration. A graph of pH as a function of the volume of titrant, the titration curve, is used to determine the equivalence point. The shape of the titration curve and also the pH value at the equivalence point, depend on the strengths of the acid and base.
Strong acid - strong base titrations
When a solution of a strong acid is titrated with a solution of a strong base, the pH titration curve is of the shape shown in Figure 1. It starts with a relatively low value of pH (for ~0.1 mol dm-3 HCl, pH ~ 1). As the base is progressively added the pH increases, slowly at first, but rapidly in the vicinity of the equivalence point; after that it levels off gradually as more base is added.
For the titration shown in Figure 1, where 50cm3 of 0.100 mol dm-3 HCl are titrated with 0.100 mol dm-3 NaOH, the equivalence point occurs when 50cm3 of NaOH are added. At this point all the acid has been converted into a salt that does not hydrolyse (NaCl) and the titrated solution is therefore neutral (pH = 7). Figure 1: The pH curve for titration of 50cm3 of 0.100 mol dm-3 HCl solution with 0.100 mol dm-3 NaOH
Titration of a solution of a strong base with a solution of a strong acid yields a pH curve similar in shape to that shown in Figure 1. The difference, however, is that the pH values are high at the start of the titration and low at its completion.
(b) Weak acid - strong base titrations
In acid - base titrations where one of the reactants is a strong electrolyte and the other is weak, the solutions are not neutral at the equivalence point. The reason for this is hydrolysis of the salt produced by the neutralisation reaction.
The pH curves resulting from titrations of weak acids with strong bases differ from those typical of strong acid - strong base titrations. Differences arise because
the weak acids are not fully dissociated in solution and
the anions of the salts of weak acids and strong...
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