Introduction
An acid (HA) is defined as that substance that in an aqueous solution that dissociates to yield up H3O+ ion and a conjugate A-, that is;
1
An Equilibrium constant Ka is used to measure the strength of the acid. The constant can be represented as;
2
A strong acid has a very large Ka; its equilibrium moves towards the right-hand side of equation (1). Concentration of dissociated acid [HA] is very small as well. A weak acid has a small Ka (less than one). It is slightly dissociated where H3O+ ion and a conjugate A- concentration is smaller than that of the acid.
An acid/base titration is monitored with an indicator or a pH meter. The aim of a typical titration process is to determine the point of equivalence; the point at which enough titrant is added to the analyte to exactly neutralize it. Purposely, this experiment is aimed at studying the changes of the pH aqueous solution when a strong base is titrated against it. It is as well aimed at using measurements on the acids to find the pKa value. pKa is the quantity that is used to measure the strength of an acid. Strong acids have low pKa values whereas weak ones have high pKa values.
When a strong base, for instance sodium hydroxide, is titrated against a weak acid, the following reaction results.
3
In the reaction, the amount of A- ions corresponds to the amount of the base added. After complete neutralization, the amount of sodium hydroxide added is equal to the amount of acid present prior the experiment.
Procedure
Titration of sodium hydroxide (NaOH) against hydrochloric acid (HCl)
A 100ml beaker was placed on a magnetic stirrer base and a stirrer bar placed in the beaker. Approximately 40ml of distilled water measured with a measuring cylinder was added. 5ml of HCl were carefully pipetted into the beaker. A suitable amount of acid was placed in a small beaker and a sample taken from it. The stirrer was turned on and pH electrode immersed into the acid. The burette was then filled with a standard 0.2M NaOH. The burette was clamped above the titration beaker with HCl. The pH value of the acid was measured before any NaOH was added and the values recorded in table 1.
0.3ml increments of NaOH were added whereas measurements were recorded in the table until 1.8ml of NaOH was attained. Afterwards, 0.1ml increments of NaOH were again added and the pppH recorded. This was done until a pH of 9 was attained. The 0.3ml increments were reverted until the pH exceeded 10.5 value. The electrodes were removed and rinsed with distilled water. Calibrations were checked against pppH7 buffer solution and any drift in the calibration recorded prior setting the meter. Finally, the stir bar was removed and the contents of the beaker discarded.
b) Titration of NaOH with propanoic acid (C2H5COOH)
A 100ml beaker was placed on a magnetic stirrer base and a stirrer bar placed in the beaker. Approximately 40ml of distilled water measured with a measuring cylinder was added. 5ml of C2H5COOH were carefully pipetted into the beaker. A suitable amount of acid was placed in a small beaker and a sample taken from it. The stirrer was turned on and pH electrode immersed into the acid. The burette was then filled with a standard 0.2M NaOH. The burette was clamped above the titration beaker with C2H5COOH. The pH value of the acid was measured before any NaOH was added and the values recorded in table 1.
0.3ml increments of NaOH were added whereas measurements were recorded in the table until 1.8ml of NaOH was attained. Afterwards, 0.1ml increments of NaOH were again added and the pH recorded. This was done until a pH of 9 was attained. The 0.3ml increments were reverted until the pH exceeded 10.5 value. The electrodes were removed and rinsed with distilled water. Calibrations were checked against pppH7 buffer solution and any drift in the calibration recorded prior setting the meter. Finally, the stir bar was removed and the contents of the beaker discarded.
Results
Part a
Part 2
Using Microsoft excel or MATLAB, the above data was translated into graphical representation. The value of NaOH in the burette were plotted against the values of the weak acid in the beaker.
Figure 1 a
Figure 1 b
Discussion
An indicator used in the titration changes the color when the endpoint is reached. A properly selected indicator gives the endpoint as the equivalence point. The pH versus volume the titrant is referred to as the titration curve. The equivalence point occurs at the very point where just a small addition of the titrant cause rapid rise in the pH. In the graph representation, it is that point where the slope changes from positive to negative. It is referred to as the point of inflection. Figure 3 below shows the above explanation.
Figure 2. Buffer region
Figure 3. Titration of sodium hydroxide with hydrochloric acid.
In the experiment, two weak acids are titrated against a strong base. The weal acid dissociates into a hydrogen ion and a conjugate base that alters the appearance of the titration curve.
Figure 4
The addition of strong base sodium hydroxide to weak acid propanoic and hydrochloric acid creates the buildup of salt of the acid producing a buffering effect. The buffering effect causes resistance to altering in pH. The pH of the Veq corresponds to pH of the conjugate base which hydrolyzes in water.
Plotting the above reaction in a titration curve enables determination of the equivalence point. At exactly one-half the volume of the equivalent point, the pH measured is approximately equal to pKa.
Conclusion
The objective of this experiment was to study the varying pH of a weak acid and the pKa value when titrated with a strong base. As per the collected data and the figures are shown in the graphs, purpose of the exercise was achieved.
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