Analysing Iron Tablets
FC024 10th Jun 2013
Monday 2-4 pm
Scientific Principles and Enquiry
(Working in Pairs)
Experiment 1 – pH Titrations Introduction
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: | [H+] | pH | Acidic solution | > 1 x 10-7 | < 7 | Neutral solution | 1 x 10-7 | = 7 | Basic solution | < 1 x 10-7 | > 7 |
In acid-base titrations the pH of the
Monday 2-4 pm
Scientific Principles and Enquiry
(Working in Pairs)
Experiment 1 – pH Titrations Introduction
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: | [H+] | pH | Acidic solution | > 1 x 10-7 | < 7 | Neutral solution | 1 x 10-7 | = 7 | Basic solution | < 1 x 10-7 | > 7 |
In acid-base titrations the pH of the