Acid base indicators

In the strong acid-weak base titration, 0. 1 M ammonium hydroxide ( NH4OH ) was titrated with 0. 1 M hydrochloric acid ( HCl ). 2 drops of phenolphthalein were added as indicator into the conical flask which contains 25mL of hydrochloric acid, the mixture in the conical flask was colourless. Ammonium hydroxide was then titrated slowly into the conical flask and the conical flask was swirled gently to ensure complete reaction between the strong acid and weak base.

The titration stops when the colour of the titrand changed from colourless to pale pink. The average titrant (29. 55 mL) of 0. 1 M of ammonium hydroxide was added into the conical flask. Phenolphthalein was then replaced by screened methyl orange as an indicator, the initial colour of the mixture in the conical flask was red. The average titre of (26. 45 mL) of 0. 1 M of ammonium hydroxide was added to change the colour of the titrand from red to grey in conical flask. In weak acid-strong base titration, 0. 1 M sodium hydroxide ( NaOH ) was titrated with 0. 1 M acetic acid (CH3COOH). 2 drops of phenolphthalein indicator were added into the conical flask which contains 25mL of acetic acid.

Sodium hydroxide was then titrated slowly into conical flask and the conical flask was swirled gently to ensure the reaction is reacted completely. The titration stops when the colour of the titrand changed from colourless to pale pink. The average titrant (28. 60 mL) of 0. 1 M of sodium hydroxide was added into conical flask. By using screened methyl orange as indicator, the average titrant (2. 20 mL) of 0. 1 M of sodium hydroxide was added to change the colour of titrand from red to grey in the conical flask.

In experiment part II (A), pH titration curve in the strong acid-weak base titration indicates that the initial pH value of the titrand is pH 1. 33. The low initial pH value is due to the strong acid, 0. 1 M hydrochloric acid ( HCl ). During the titration, the pH value increases steadily but not significant as the buffer solution is being built up, when extra base was added into the acid which can resist pH change. It increases to pH 2. 94 after 25. 00 mL of ammonium hydroxide was added. However, the pH value increases greatly from 3. 59 to 5. 99 when added extra 0. 50mL of ammonium hydroxide as shown in the Graph 1.1. The pH values increase slowly as extra ammonium hydroxide was added. The equivalence point, which is the steepest point of graph, is pH 4. 79.

In experiment part II (B), pH titration curve in the weak acid-strong base titration illustrates that the initial pH value of the titrand is pH 2. 32, the pH is higher due to the weak acid, 0. 1 M acetic acid. The pH value rises progressively due to the presence of buffer solution. The changes in pH values are small even when extra base was added. It increases to pH 7. 00 after 26. 00 mL of sodium hydroxide was added.

The pH rises from 9. 03 to 10. 08 as another 0.5mL of sodium hydroxide was added. Based on the Graph 1. 2, the equivalence point, which is the steepest point of graph, is pH 9. 915. The differences between the two titration curves The starting value of pH in strong acid-weak base titration is lower than that of the weak acid-strong base titration. The pH value of the equivalence point in strong acid-weak base titration is below 7. 00 whereas the pH value in weak acid-strong base titration is above 7. 00. For the strong acid-weak base titration, which phenolphthalein was used as an indicator, the average titre to cause colour change in the indicator is 29.55 mL.

While for screened methyl orange, the average titre to change the colour of the indicator from red to grey is 26. 45mL. Based on Graph 1. 1, the volume of ammonium hydroxide added at equivalence point is 25. 70mL. By comparison, the value obtained when screened methyl orange was used as the indicator has only a slight difference of 0. 75mL from the value obtained from the titration curve. Hence, screened methyl orange is a more suitable indicator for strong acid-weak base titration.

For the weak acid-strong base titration, the equivalence point determined from the titration curve, Graph 1. 2 is 27.30mL. As phenolphthalein was used average titre of 28. 60 mL sodium hydroxide is required to change the colour of the indicator from colourless to pale pink. Whereas for screened methyl orange, the average titre obtained is 2. 20mL. Comparatively, the volume of sodium hydroxide added into the acetic acid containing phenolphthalein is closer to the equivalence point, 27. 30mL. There is only a slight difference of 1. 30mL, whereas for when screened methyl orange is used, the value differs for 25. 10mL. Therefore, for weak acid-strong base titration, phenolphthalein is a more suitable indicator to be used.

By looking at the results acquired, the values from the titration curves and acid-base indicators have small difference. Despite repeating the titration for several times to obtain the average results, some errors may occur during the titration. The causes of the uncertainties in the results between acid-base indicators and titration curves are system errors and random errors. Firstly, the Eye level and the meniscus level are not perpendicular to the burette when the reading was taken. This leads to the system error. Moreover, the changes in colour of the titrand were difficult to be determined.

Thus, this causes the difference in the results. Besides, overshot might happen during the titration which causes inconsistency in the reading and results. Furthermore, tiny water droplets remaining in the burette and conical flask after washing will reduce the concentrations of both the titrant and titrand which eventually will lead to inaccuracy in the reading and results.

(1) The colour change and pH range of the indicators: (2) (3) Indicators Colour change pH range Phenolphthalein Colourless to light pink 8. 30 – 10. 00 Screened methyl orange Red to grey-green 2.90 – 4. 40 Phenolphthalein is not a suitable indicator in the strong acid-weak base titration because its range of colour change is between pH 8. 30 and 10. 00. However, based on Graph 1. 1, the equivalence point of the titration is pH4. 79. Excess ammonium hydroxide was added as the equivalence point is off from the pH range. By replacing phenolphthalein with screened methyl orange for strong acid-weak base titration, which its pH range of colour change is from 2. 90 to 4. 40 it is more suitable to be used as the indicator.

The range of screened methyl orange is closer to the equivalence point of strong acid-weak base titration compared to that of phenolphthalein. Thus, screened methyl orange is more suitable to be used as indicator in strong acid-weak base titrations compared to phenolphthalein. On the other hand, screened methyl orange is not a suitable indicator for weak acid-strong base titration. Its pH range of colour change is from pH 2. 90 to 4. 40. The equivalence point of the titration according to the Graph 1. 2 is pH 9. 915. The colour of screened methyl orange will change even before the equivalence point of titration is reached.

If phenolphthalein is used for weak acid-strong base titration, the pH range when the colour changes from colourless to pale pink is between pH 7. 97 and pH 10. 80. The equivalence point, pH 9. 385 falls in the pH range of phenolphthalein. Hence, phenolphthalein is chosen as the more suitable indicator compared to screened methyl orange. Conclusion: With regards to this experiment, there are two ways to determine the volume of base required to react in the acid-base titration which are using the suitable indicators and obtaining titration curves.

Suitable indicators allow determination of the accurate equivalence points of the titration. Moreover, a pH meter can be used to detect the pH values of the titrand at every 0. 5mL interval of base added. A titration curve can be plotted from recorded pH values. To conclude, screened methyl orange is a more suitable indicator than phenolphthalein for strong acid-weak base titration. Contrastingly, phenolphthalein is more suitable to be used as an indicator for weak acid-strong base titration.

Bibliography

1. Bla Bla writing. [Online]. [cited 2017 October 20. Available from: https://blablawriting. com/investigation-of-the-suitability-of-indicators-and-instrumental-detection-essay.

2. Science. uwaterloo. ca. [Online]. [cited 2017 October 20. Available from: http://www. science. uwaterloo. ca/~cchieh/cact/c123/indicator. html. 3. Cheminit-online. co. za. [Online]. [cited 2017 October 20. Available from: https://www. cheminit-online. co. za/useful_data/2.

3. htm. 4. Chemguide. co. uk. [Online]. [cited 2017 October 20. Available from: http://www. chemguide. co. uk/physical/acidbaseeqia/indicators. html.