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Buffer capacity is a quantitative measure of the efficiency of a buffer in resisting changes in pH. It may be defined as the “maximum amount of either strong acid or strong base that can be added before a significant change in the pH occurs. In simple terms, a buffer system can resist pH changes. It is indicated by the term buffer index (β).
Conventionally, the buffer capacity is expressed as the amount of strong acid or base, in gram-equivalents, that must be added to one liter of the solution to change its pH by one unit. Mathematically buffer capacity is expressed as:
where ∆B is gram equivalent of strong acid or base added to change pH of 1 liter of buffer solution and ∆pH is the pH change caused by the addition of strong acid or base. Practically it is possible to measure smaller pH changes. The buffer capacity is quantitatively expressed as the ratio of acid or base added to the change in pH produced.
It must be large enough to maintain the product pH for a reasonably long shelf-life. Changes in product pH may be due to the interaction of solution components with one another or with the type of product package, for example, glass, plastic, rubber closures, etc. On the other hand, the buffer capacity of ophthalmic and parenteral products must be low enough to allow rapid readjustment of the product to the physiological pH upon administration. The pH, chemical nature, and volume of the solution to be administered must all be considered. Buffer capacities ranging from 0.01 − 0.1 are usually adequate for most pharmaceutical solutions.
Buffer capacity is always positive. It is expressed as the normal concentration (equivalents per liter) of strong acid or base that changes pH by 1.0. The greater the buffer capacity the smaller is the change in pH upon addition of a given amount of strong acid or base. The buffer index number is generally experimentally determined by titration. For example, when 0.03 mole of sodium hydroxide is added to 0.1 M acetate buffer system the pH increases from 4.76 to 5.03 with a change of 0.27 pH units, Table.1. Therefore, by substituting values in the equation (1) we have;
It is important to remember that buffer capacity is highest when the smallest number of moles of NaOH is added. It is increased by increasing the concentration of the buffer system components. By doubling the total molar concentration of the buffer system will double the capacity of buffer at a given pH. Buffer can also be increased by using equimolar concentrations of the acid (HA) and its conjugate base (A–). The buffer has its greatest capacity, when ratio [salt]/[acid] are equal to 1, i.e. [HA] = [A–]. Therefore, the buffer equation (1) can be written as
pH = pKa …equation (2)
Factors Affecting Buffer Capacity
1. Ratio of [A– ]/[HA]
The buffer capacity depends essentially on the ratio of the salt to the acid or base. The actual concentrations of A– and HA influences the effectiveness of a buffer. The more is the A– and HA molecules available, the less of an effect of the addition of a strong acid or base on the pH of a system. For example, consider the addition of a strong acid such as HCl. Initially, the HCl donates its proton to the weak base (A– ) through the reaction
A– + HCl → HA + Cl–
This changes the pH by lowering the ratio [A–]/[HA], but if there is a lot of A– present, the change in pH will be small. But if we keep adding HCl, the weak base A– will be removed. Once the A– is depleted, any addition of HCl will donate its proton to water as shown in the reaction below.
HCl + H2O → H3O+ + Cl–
This drastic increase in the [H+] leads to a pH drop called “breaking the buffer solution”. The amount of acid a buffer can absorb before it breaks is called the “buffer capacity for addition of strong acid”. A solution with a weaker base, [A–], has a higher buffer capacity for the addition of strong acid. Similarly, a buffer can break when the amount of strong base added is so large that it consumes all the weak acid, through the reaction
HA + OH– → A– + H2O
A solution with more weak acid, [HA], has a higher buffer capacity for the addition of a strong base. The buffer capacity is optimal when the ratio is 1:1; that is when pH = pKa.
2. Total Buffer Concentration:
Buffer capacity depends upon the total buffer concentration. For example, it will take more acid or base to deplete a 0.5 M buffer than a 0.05 M buffer. The relationship between buffer capacity and buffer concentrations is given by the Van Slyke equation:
where C is the total buffer concentration (i.e. the sum of the molar concentrations of acid and salt). A buffer solution containing a weak acid and its salt has a maximum buffer capacity
(βmax) when pH = pKa i.e. [H3O+] = Ka. Therefore, by substituting [H3O+] for Ka in equation (3), we get
Buffers are commercially available with a wide range of pH values, and they come in both premixed liquid form or as convenient dry powder, capsules, or tablets (to be added to distilled water). These solutions contain acids and bases whose equilibrium is dependent on temperature. Thus, the precise pH is also a function of temperature. The buffers whose pH varies with temperature are shown in Table.2. Since the pH values are dependent on temperature, buffers are required to be maintained at a constant temperature. Any change in the temperature of the buffer results in a reduction in the effectiveness of the buffer. Buffer containing base and its salt were found to show greater changes in buffer capacity with temperature.
4. Ionic Strength:
Ionic strength is reduced by dilution. Change in ionic strength changes the pH of buffer solution resulting in decreased buffer capacity. So, whenever the pH of buffer solution is mentioned ionic strength should be specified.
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