Investigation of the suitability of indicators and instrumental detection

Objectives
1. To understand the importance of choosing suitable indicators for detecting the end points of acid-base titrations.
2. To obtain titration curves for some acid-base titrations. Introduction
Indicator is usually a weak organic acid or base that has distinctly different colours in its protonated and deprotonated forms. There are 4 types of acid-base titrations such as strong acid-weak base titration, weak acid-strong base titration, strong acid-strong base titration and weak acid-weak base titration. Both acid and base are mostly colourless in nature. In order to determine their equivalence points and end points, indicators are used in the acid-base titration. However, indicators chosen must be suitable according to the type of reactions. But in this experiment, there are only two types of reactions which is strong acid-weak base titration and weak acid-strong base titration will be investigated. Materials

Chemicals
0.1M Hydrochloric acid, 0.1M acetic acid, 0.1M sodium hydroxide, 0.1M ammonium hydroxide, phenolphthalein, screened methyl orange, methyl orange Apparatus
100mL Beakers, 250mL Conical flasks, 50mL Burettes, 25mL Pipettes, Pipette fillers, Funnels, pH meters Procedure
The experiments are carried out according to the procedures that stated in Experiment (1): Investigation of the suitability of indicators and instrumental detection of equivalence points in acid-base titration under the General Chemistry module.

Discussion
1. Observation for the colour changes using phenolphthalein and screened methyl orange (or methyl orange) in the titration. For the Part I (A), initially few drops of phenolphthalein are added into conical flask containing hydrochloric acid, and the mixture is colourless. The mixture is then titrated with ammonium hydroxide. The mixture changes from colourless to pink when the end-point is reached. The indicator is then replaced by screened methyl orange. In the acidic state, the indicator shows red colour. When it is titrating, grey colour is formed when it is in neutral, and then turn to green when it reaches the end-point which is in the alkaline state. For the Part I (B), phenolphthalein is added to the conical flask containing acetic acid and the mixture is colourless. When titrating with sodium hydroxide, the solution will turn from colourless to pink when the end-point is reached. By replacing phenolphthalein with screened methyl orange, the solution will show red at first. When the acid is titrated by the sodium hydroxide, the solution will turn to grey, then to green.

2. The characteristics of each titration curve and the difference between the two titration curves. For the Part II (A), hydrochloric acid is titrated by ammonium hydroxide. It is a strong acid-weak base titration, the pH of hydrochloric acid is measured at first. As it is a strong acid, so it will have the lower pH value. The titration starts at lower pH. When the titration start, the pH increase slowly. The curve is less steep, as the buffer solution is being set up when the base is added into the acid which can resist large increase in pH. It is then rise rapidly and steeply near the equivalence point.

From the graph, the equivalence point can be determined, which is 5.1. For the Part II (B), the reaction is occurred in between acetic acid and sodium hydroxide, weak acid-strong base titration. The titration is began at the higher pH value as compared with the Part II (A) because acetic acid is a weak acid. As it has been titrated, the pH increase slowly at first, then rapidly near the equivalence point. So there is a short pH range in which the rapid change occurs. From the graph, the equivalence point for the titration is gained with the pH value of 8.6. It is considered as alkaline. Form both observations, we can notice that the equivalence point of strong acid-weak base titration is less than pH 7, whereas the equivalence point for weak acid-strong base titration is more than pH 7.

3. Comparison of the results obtained using the acid-base indicators and titration curves. Explain any discrepancies in your finding. For the strong acid-weak base titration, the titration is occurred between ammonium acid and hydrochloric acid. From the graph plotted in part II (A), the volume of base added at the equivalence point is 29 mL. By comparing this value with the values obtained at Part I (A), the average volume of base added into the mixture of acid and phenolphthalein is 31.67 mL, while the average volume of base added into the solution of acid with screened methyl orange is 29.50 mL. Screened methyl orange will be the more suitable indicator for this titration, as the value obtained is closer to the 29.00 mL, which is only 0.5 mL different with the value determined from the titration curve. For the weak acid-strong base titration, it is occurred in between sodium hydroxide and acetic acid.

From the graph plotted, the volume of base added at the equivalence point is 25.20 mL. The average volume of base added into the mixture of acid with phenolphthalein is 24.73 mL, whereas 4.70 mL for the mixture of acetic acid with screened methyl orange. By comparing those values, the volume of base added into the solution containing phenolphthalein is closer to the 25.20 with the difference of 0.47 mL only. Hence, phenolphthalein has been chosen as the suitable indicators for weak acid-strong base titration.

By observing the results obtained, the values from the acid-base indicators and titration curves are not exactly same although the titration has been repeated for few times to get the average results because there may be some errors occur. Firstly, when the acid is pipetted into the conical flask, there might be some acid stick at the wall of the apparatus, it will causes a small change in the volume of the solution. Secondly, when washing each of the apparatus, some small droplets of water might stick at the wall of the apparatus. When the chemical is filled into the apparatus, the concentration of the chemicals will be affected. On the other hand, when the chemical is poured into the burette, some air bubbles might trapped in the burette, it will lead to the reading value become higher than the actual value. Besides, when the experiment is carried out, parallax error might occurs when taking the reading from scale at the burette. The reading must be taken when the eye located at the same level as the liquid level. If it is either higher or lower than the liquid level, parallax error occurred.

4. The suitability of the indicator in each titration.
The graph of strong acid-weak base titration shows that screened methylorange will be the more suitable indicator for this titration. From  the graph plotted at Part II (A), the volume of base added at the equivalence point is 29.0 mL. The average reading for bases added into the solution with the presence of phenolphthalein is 31.67 mL, whereas 29.50 mL for screened methyl orange. The difference between the value of 29.0 mL and 29.50 mL is only 0.5 mL. Hence, screened methyl orange is more suitable as the indicator for titration of ammonium hydroxide and hydrochloric acid.

On the other hand, the graph of weak acid-strong base indicates that phenolphthalein is the more suitable indicator. Based on the graph plotted at Part II (B), the volume of base added at the equivalence point is 25.20 mL. According to the results that obtained from Part I (B), the average volume of sodium hydroxide added to the acetic acid that contain phenolphthalein is 24.73 mL, whereas 4.70 mL for the screened methyl orange. By comparing both values with 25.20 mL, 24.73 mL has the smaller difference which is only 0.47 mL. Therefore, phenolphthalein will be chosen as the suitable indicator for the titration between sodium hydroxide and acetic acid. Conclusion

The suitability of indicators are determined by the strength of the acid-base titrations. It can be used to determine the equivalence points and the end points of acid-base titrations. In strong acid-weak base titration, screened methyl orange is the more suitable indicator to determine the equivalence point. In the titration of hydrochloric acid and ammonium hydroxide, the equivalence point is pH 5.1 which is less than 7, so screened methyl orange is chosen as the indicator. Whereas in weak acid-strong base titration, like the titration between acetic acid and sodium hydroxide, phenolphthalein is chosen as the suitable indicator to determine the equivalence point which is pH 8.6. As the phenolphthalein always suitable used to determine the equivalence point which is above pH 7 like weak acid-strong base.

Reference
1. http://www.chemguide.co.uk/physical/acidbaseeqia/phcurves.html 2. http://www.chemguide.co.uk/physical/acidbaseeqia/indicators.html