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Experiment to obtain curves which show how the pH changes during an acid-base titration and;To use these curves to choose suitable indicators for different combinations of acid and alkali.

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Introduction

Experiment 15 23-3-99 Aim: * To obtain curves which show how the pH changes during an acid-base titration and; * To use these curves to choose suitable indicators for different combinations of acid and alkali. Procedure: 1. 25.0cm3 of 0.100 M ethanoic acid was pipetted into a 100cm3 beaker, a small stirring paddle was added, and the beaker was stood on a magnetic stirrer. 2. The electrode of a pH meter was carefully clamped so that the bulb is completely immersed in the acid and is clear of the stirring paddle. If a magnetic stirrer (or mechanical stirrer) is not available, it is better to swirl the beaker by hand. If a pH meter is not available, it is better to use a conical flask rather than a beaker. 3. A burette with 0.100M ammonia solution was filled and was clamped over the beaker (or flask) containing the acid. 4. A value of the pH of the acid was obtained, either by reading the pH meter (which must, of course, have been calibrated) ...read more.

Middle

25.5 25.6 25.65 25.7 25.8 25.9 26.1 26.3 27.0 pH 2.6 2.7 2.7 2.7 2.7 2.7 2.7 2.8 3.0 Volume /cm3 27.2 27.35 27.5 27.7 27.8 27.9 27.95 28 28.1 pH 3.1 3.2 3.3 3.5 3.8 4.2 4.7 6 6.8 Volume /cm3 28.15 28.2 28.3 28.5 28.7 28.9 30.0 35.0 40.0 pH 7 7.2 7.5 7.7 7.9 8.0 8.3 8.8 9.0 0.100M NaOH added to 25.0cm3 of 0.100M HCl Volume /cm3 0.0 5.0 10.0 15.0 20.0 24.0 24.2 24.4 24.6 24.8 pH 2.15 2.15 2.15 2.25 2.45 2.75 2.80 2.85 2.85 2.90 Volume /cm3 24.9 24.95 25.0 25.05 25.1 25.2 25.4 25.6 25.8 26.0 pH 2.95 2.95 2.95 2.95 3.00 3.00 3.05 3.15 3.25 3.40 Volume /cm3 26.2 26.5 26.8 27.0 27.1 27.5 28.0 30.0 35.0 pH 3.60 4.1 7.3 9.5 9.90 10.30 10.55 10.85 11.15 Calculation * A graph of four curves are plotted. The pH is shown on the vertical scale, while the volume of added alkali is shown on the horizontal scale. ...read more.

Conclusion

0.100M NH3 added to 25.0cm3 of 0.100M HCl Methyl red D. 0.100M NaOH added to 25.0cm3 of 0.100M HCl Bromothymol blue Discussion: 1. The reaction equation for the reaction occurring in the curves is noted: Curve Equation A NH3 + CH3COOH --> CH3COONH4+ B NaOH + CH3COOH --> CH3COONa+ + H2O C NH3 + HCl --> NH4Cl D NaOH + HCl --> NaCl + H2O 2. It is better to initialize the reading of the burette to zero by adding more alkali so that the volume of alkali added can be directly read from the burette and calculation is not needed. 3. The end point of the titration is not, in fact, 25cm3. The end point is a little bit higher than or lower than 25cm3 depend on which combination of acid and base is used. This is the strong conjugate acid or base from weak base or acid can react with H2O and from base or acid, which make the pH value of the solution much higher or lower. 4. It is better to rinse the bulb of the pH meter before next titration, rather than dry the bulb with tissue paper. ...read more.

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