Buffers and Ph

Experiment 1: Design and preparation of buffers effective at different pHs.

Methods

Solution of tris (0.1M) was titrated against HCl (0.2M), as was solution of glycine (0.1M) against NaOH (0.2M) and solution of NaOH (0.2M) against HCl (0.2M) and the pH was measured at regular intervals. This determined the pH ranges over which the different buffers are effective. A acetic acdid/acetate buffer of pH 5.2 was prepared using the Henderson-Hasselbalch equation.

Results

For each titration, the initial pH was measured with a pH meter. (Raw data is presented in Table 1, appendix). The titrant was added to the analyte, the pH was recorded at regular intervals of 1ml until 25ml was added. When there was a big change, the pH was recorded at intervals of 0.5ml.

The curves in Fig 1 show that as the NaOH was added to glycin, the pH increased rapidly up to around pH 8, it then levelled off and stayed constant at around pH 10. Buffering occurred over the pH of 10 – 11. As HCl was added to tris, there was a sharp decrease in pH after around 3ml were added, it then continued to decline slowly until the buffer region was reached over pH ranges of 2.5-6. In contrast, when NaOH was added to HCl, the pH stayed fairly constant until 20ml NaOH were added, pH then jumped from pH 3 to pH 11, the buffer did not form.

For the glycin and NaOH titration, the estimated equivalence point is 11.095 at volume of 13.5. At half the volume 6.75, the pka is 9.75. The accepted pka value of glycine/NaOH is 9.6. This value is close but different due to inaccuracy in measuring the values and other errors.

The equivalence point of 0.1M tris and 0.2M HCl titration is at ph 3.9 at volume 16.75. The half volume is 8.375 so the pka is at around 7.99. The accepted value is 8.0. This value is closer to the expected value.

NaOH and HCL equivalence point is at 6.75 at volume 20.5. At half the volume of 10.25, the pka is 1.5. The shape of this this curve is different as a buffer did not form, there was...