# 7 Investigation of Buffer Systems

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2 However, a buffer will cease to retard the change of when all the reacting component is consumed by the OH - or H 3 O + added. Further addition of strong acid or base will cause a rapid change in. Buffer capacity is a quantitative measure of the resistance of a buffer solution to change in on addition of strong acid or base. Normally it can be defined as the millimoles of strong acid or base required to change the of 1 liter of buffer by ±1.0 unit. Buffer capacity is related to the concentrations of base and acid components. Higher concentration of base component in a buffer can consume more H 3 O + added resulting in a higher buffer capacity for the addition of a strong acid. In a same fashion, a buffer consisting of higher concentration of acid component has a higher buffer capacity for the addition of a strong base. The of a buffer solution can be calculated by using Henderson-Hasselbalch equation = pk a + log [A- ] [HA] where K a is the acid ionization constant, [HA] is the concentration of acid component and [A - ] is the concentration of base component Procedure Part I-A: Preparing Buffers 1. Transfer M CH 3 COOH with a graduated cylinder into a 100 ml-beaker. 2. To the same beaker, add M NaOH and DI water. 3. Stir well. 4. The amount of solutions transferred for buffers are listed in the following table Solution Volume transferred, ml Buffer a Buffer b M CH 3 COOH M NaOH DI water Part I-B: Calculating of Buffers (K a CH 3 COOH = 1.8 x 10-5 ) Buffer a Buffer b Calculations James Madison University and JMU Board of Visitors

4 Part II-C: Reporting Data and Determining Buffer Capacity For each buffer, make a plot of versus volume of HCl and. All three plots should be done on the same graph. Use different symbols or different colors for data points. Plot the of each buffer at a volume of zero midway on the x-axis. The volumes of are positive and the volume of are negative The scale for the y-axis will range from the lowest value measured to the highest measured for the three buffers. The scale for the x-axis will range from the largest amount of acid and base added. Make your graph large enough so that the three plots are not crowded James Madison University and JMU Board of Visitors

5 Final Report Your final report should compare the buffer capacity of the three buffer solutions used in this experiment. Answer the questions: Which of the three solutions had the highest buffer capacity for the addition of a strong acid? Which of the three solutions had the highest buffer capacity for the addition of a strong base? Which of the three solutions had the smallest buffer capacity for the addition of both acid and base? What experimental evidence do you have to support the result? James Madison University and JMU Board of Visitors

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