Тема: Буферные растворы и буферные системы
Calculation of the pH of buffer systems
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| Calculation of the pH of buffer systems.
Each of the buffer mixtures is characterized by a certain concentration of hydrogen ions, which the buffer system tends to preserve when an acid or alkali is added to it. Let us consider the example of an acetate buffer mixture, what determines its pH. Acetic acid in an acetate buffer is a weak electrolyte, during the dissociation of which H+ ions are formed, which determine the reaction of the medium: СН3СООН ↔ СН3СОО- + Н+, where the degree of ionization is α << 1. According to the law of mass action, the dissociation constant of acetic acid is (6) From equation (6) we find the concentration of hydrogen ions in the solution: (7) Equality (7) is valid for a solution containing only one acetic acid. The addition of sodium acetate to the acetic acid solution suppresses its dissociation, as a result of which the concentration of non-dissociated acid molecules can be taken equal to the total acid concentration: C (CH3COOH) = Саcids The СН3СООNa salt as a strong electrolyte in an aqueous solution dissociates completely according to the scheme: CH3COONa ↔ CH3СОО- + Na+, where α = 1, then The concentration of СН3СОО- ions in the solution will be determined by СН3СОО- ions formed during the dissociation of the salt in this buffer mixture: C (CH3COO-) = СSalts. Taking these relations into account, we can write that (8) This is the equation of buffer solutions (8), the equation of buffer solutions In practice, the reaction of the solution medium is expressed through pH, taking the logarithm of expression (8) and changing the signs to the opposite, we write: -lgCH+ = -lgK - lgCACIDS + lgCSALTS or, replacing -lgCH+ = pH, -lgK = pK, where pK is the acid constant, we get: (9) Equation (9) is called the buffer system equation and is used to calculate the pH of acidic buffer solutions. For a buffer system formed by a weak base and its salt from a strong acid, the pH can be determined by the equations: рН = 14 – рОН (10) (11) where pK is the index of the base ionization constant. It follows from the above equations that the pH of the buffer solution depends on the value of the dissociation constant of a weak acid or weak base (the nature of the components) and on the ratio of the concentrations of the components of the buffer mixtures. With equal equivalent concentrations of acid and salt, from which the buffer mixture is prepared, when calculating the pH of the buffer, instead of the ratio of the concentrations of the components, the ratio of their volumes can be used: (12) (13) The most important properties of buffer solutions follow from the buffer solution equation: 1) Dilution of the buffer does not affect the pH of the solution, because when diluted, the concentration of the components decreases by the same number of times, and their ratio remains constant. 2) The addition of small amounts of acid and alkali within the buffer tank does not affect the pH of solutions. жүктеу/скачать 160.5 Kb. Достарыңызбен бөлісу:
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