Preparing Buffers and Buffer Capacity

Name: Espera, Leza Andrea A. Date Performed: June 25, 2013
Lab partner: Catalan, Christian A. Date Due: July 5, 2013 Date Submitted: July 5, 2013

EXPERIMENT 1
PREPARING BUFFERS AND BUFFER CAPACITY

INTRODUCTION
A buffer solution is one in which the pH of the solution is "resistant" to small additions of either a strong acid or strong base. Buffers usually consist of a weak acid and its conjugate base, in relatively equal and "large" quantities. A buffer system can be made by mixing a soluble compound that contains the conjugate base with a solution of the acid such as sodium acetate with acetic acid or ammonia with ammonium chloride.
The buffer capacity refers to the maximum amount of either strong acid or strong base that can be added before a significant change in the pH will occur. This is simply a matter of stoichiometry. The maximum amount of strong acid that can be added is equal to the amount of conjugate base present in the buffer. The maximum amount of base that can be added is equal to the amount of weak acid present in the buffer.

DATA

Concentration of Buffer Trial 1 Trial 2
0.1 M ml NaOH pH 0 4.57 0 4.64 0.5 4.75 0.1 4.66 1.2 4.95 0.5 4.74 2.4 5.28 0.7 4.8 3 5.56 1 4.85 3.3 5.75 1.5 5 1.8 5.06 2 5.1 2.2 5.18 2.5 5.28 2.8 5.4 3.1 5.6 3.5 5.93
0.3 M 0 4.75 0 4.89 0.4 4.78 1.8 4.96 2.5 4.82 2.9 5.04 2.9 4.85 4.1 5.15 3.5 4.9 5.1 5.28 3.9 4.94 6.1 5.42 4.6 5 7.1 5.59 5.2 5.05 8.1 5.85 5.5 5.06 9.1 6.41 6 5.11 6.1 5.12 6.6 5.16 7.1 5.22 7.3 5.25 7.6 5.27 7.9 5.3 8.8 5.45 9.5 5.55 9.9 5.63 10.5 5.94
0.5 M 0 4.8 0 4.7 0.6 4.82 1 4.74 1 4.82 5 5.03 1.5 4.83 7 5.12 2 4.88 10 5.3 2.5 4.9 13 5.56 3 4.95 15 5.81 3.5 4.96 4 4.98 5 5.03 6 5.06 7 5.14 8 5.2 9 5.26 10 5.33 11 5.42 13 5.62 15 5.95

RESULTS AND DISCUSSION
Buffer capacity quantifies the ability of a solution to resist changes in pH by either absorbing or desorbing H+ and OH- ions. When an acid or base is added to a buffer system, the effect on pH change can be large or small, depending on both the initial pH and the capacity of the buffer to resist change in pH. Buffer capacity (β) is defined as the moles of an acid or base necessary to change the pH of a solution by 1, divided by the pH change and the volume of buffer in liters; it is a unitless number. A buffer resists changes in pH due to the addition of an acid or base though consumption of the buffer. As long as the buffer has not been completely reacted, the pH will not change drastically. The pH change will increase (or decrease) more drastically as the buffer is depleted: it becomes less resistant to change.
Buffer capacity is determined through a titration, a technique in which a known volume and concentration of a base or acid is added to the analyte of unknown concentration. In the preparation of the acetate buffer in the experiment, the Henderson-Hasselbach equation was used to calculate the concentration of the weak acid and its salt needed to prepare a 100 ml buffer at pH 5. The Henderson-Hasselbach equation is: (for a weak acid and its salt) (for a weak base and its salt) where [salt], [acid] and [base] are the molar concentrations of salt, acid and base.
In a laboratory environment, a buffer solution can be created by mixing a weak acid with its conjugate base. The ions naturally present in rivers are buffering components that allow the pH of the water to remain stable over time. Buffer capacity of river water is very important, usually necessitating narrow pH ranges that are critical to the survival of most organisms. If the buffer capacity of river water is too small or the pH of the water is outside its buffer range, it can be lethal to the river’s ecosystem. According to Van Vooren, buffer capacity can be used in the analysis of water samples in order to determine the water quality (2001). Table 1. Summary of the data from titrations for buffer capacity determination 0.1 M 0.3 M 0.5 M Trial 1 Trial 2 Trial 1 Trial 2 Trial 1 Trial 2 mL NaOH 3.50 3.30 10.50 9.10 15.00 15.00
L of NaOH 0.0035 0.0033 0.0105 0.0091 0.015 0.015 moles of NaOH 3.50E-04 3.30E-04 1.05E-03 9.10E-04 1.50E-03 1.50E-03 pH change 1.29 1.18 1.19 1.52 1.15 1.11 volume of buffer 0.01 L buffer capacity 0.0271 0.0280 0.0882 0.0599 0.1304 0.1351
Buffer capacity average 0.0276 0.0741 0.1328

Basing from the results, one can conclude that the Buffering Capacity increases as the molar concentration (molarity) of the buffer salt/acid solution increases. Also, the closer the buffered pH is to the pKa, the greater the Buffering Capacity.

SAMPLE CALCULATIONS

β= (moles of NaOH)/(change in pH x volume of buffer)

For 0.1 M (trial 1) β= (0.10 M NaOH x 0.0035 L NaOH)/((5.93-4.64) x 0.01 L)

= 0.0271

REFERENCES Harris, Daniel C. Quantitative Chemical Analysis. (7 ed.). W. H. Freeman and Company. 2007. Harris, Justin. Preparation of Buffers and Buffer Capacity Measurement. Carmen Wiki. The Ohio State University. 11/18/11. Retrieved from https://carmenwiki.osu.edu/download/ attachments/26518655/Buffer+Lab.pdf?version=1&modificationDate=1301198124224. Vooren, L. Van, Steene, LM. Van De, Ottoy, J.-P., and Vanrolleghem, P.A. (2001). Automatic Buffer Capacity Model Building for the Purpose of Water Quality Monitoring. 11/18/11. Retrieved from .

References: Harris, Daniel C. Quantitative Chemical Analysis. (7 ed.). W. H. Freeman and Company. 2007. Harris, Justin. Preparation of Buffers and Buffer Capacity Measurement. Carmen Wiki. The Ohio State University. 11/18/11. Retrieved from https://carmenwiki.osu.edu/download/ attachments/26518655/Buffer+Lab.pdf?version=1&modificationDate=1301198124224. Vooren, L. Van, Steene, LM. Van De, Ottoy, J.-P., and Vanrolleghem, P.A. (2001). Automatic Buffer Capacity Model Building for the Purpose of Water Quality Monitoring. 11/18/11. Retrieved from .