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# Obtaining pH curves for acid/alkali titrations.

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Introduction

Obtaining pH curves for acid/alkali titrations Obtaining pH curves for acid/alkali titrations Planning In order to obtain a pH curve, I must first devise a way in which to measure both the pH and the volume of alkali being added to the weak acid in order to draw a graph from my results and then from this graph find Ka. Method I will set up the equipment as shown in the diagram below: I will use a pipette and pipette filer to take 25cm3 exactly of the weak acid (0.1M ethanoic acid, CH3COOH) and place it in a beaker. I will then take a burette and place in it 50cm3 of 0.1M sodium hydroxide (NaOH). I will then measure the exact amount of NaOH added to the acid whilst using an accurate pH meter rather than indicator to measure the change in pH. Before using the pH meter, I will first dip it into two buffer solutions of pH 4 and pH 7 and calibrate it so that it reads accurately. I will record the values for the pH and the volume of NaOH added. ...read more.

Middle

This excess of dissociated OH- ions leaves a strongly alkaline solution as the solution has the composition of NaOH with Na+ and OH- ions in solution. Although my equivalence point should be at exactly 25cm3, this may not work out experimentally due to variations in the concentrations of the acid and alkali and inaccuracies associated with the use of a pH meter which may well be wrong by the same amount every time, thus creating a set of results either higher or lower than they should be. Results Volume (cm3) +- 0.1 pH (1.d.p) +- 0.1 pH (1.d.p) +- 0.1 Average pH (2.d.p) +- 0.10 0 2.7 2.7 2.70 2 3.3 3.4 3.35 4 3.7 3.7 3.70 6 3.9 4.0 3.95 8 4.1 4.1 4.10 10 4.3 4.3 4.30 12 4.4 4.5 4.45 14 4.6 4.6 4.60 16 4.7 4.8 4.75 18 4.9 4.9 4.90 20 5.0 5.1 5.05 22 5.2 5.3 5.25 23 - 5.4 5.40 24 5.6 5.5 5.55 25 - 5.7 5.70 26 5.8 6.0 5.90 27 - 6.5 6.50 28 7.7 10.0 8.85 29 - 11.3 11.30 30 11.7 11.8 11.75 31 - 12.0 12.00 32 12.1 12.1 12.10 ...read more.

Conclusion

Evaluation Although I have obtained reasonable results, I feel there are several ways in which my results could be improved. The pH meter I used, despite taking quick readings was not the most accurate possible. If I wanted to improve the overall accuracy of my results, I could use a more accurate pH meter that remained accurate after calibration, a problem I encountered with my pH meter. Furthermore, although I can blame my pH meter for inaccuracy in my pH readings, it is more likely that the inaccuracy in the equivalence point is due to slight differences in the concentration of the acid and alkali. This is a problem that is difficult to remedy as I was provided with the solutions of 0.1M sodium hydroxide and 0.1M ethanoic acid. However, if I made up these solutions accurately myself, I would have more control over the accuracy of the concentration. If I eliminated these errors, I think my experiment would be near perfect. I could use phenolphthalein to show the exact point at which the graph goes vertical using the colour change. This would be beneficial as it would back up my pH readings and enable me to calculate the equivalence point more easily. ...read more.

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