Introduction Background Indicators are used in acid-base titrations to determine end points as both acid and base are colourless in nature. However, indicators chosen must be suitable according to the type of reactions such as: – Strong acid-weak base titration – Weak acid-strong base titration – Strong acid-strong base titration – Weak acid-weak base titration Only the first two mentioned above will be investigated in this experiment. Aims/Objectives 1. To understand to importance of selecting suitable indicators for detecting the end-points of acid-base titrations. . To obtain titration curves for: a. Strong acid-weak base titration b. Weak acid-strong base titration Learning Outcome 1. Able to choose a suitable indicator for detecting the end points of different types of acid-base titrations. 2. Able to draw and hence determine the end-point on the titration curves. Materials and Methods The experiment is carried out according to the procedures described in Experiment (1): Investigation of the suitability of indicators and instrumental detection of equivalence points in acid-base titration under the General Chemistry module
Results Part 1: Suitability of Indicators A. Strong acid-weak base titration Acid: HCl (aq) Base: NH4OH (aq) Reaction: HC1(aq) + NH4OH(aq) > NH4Cl(aq) + H2O(l) Indicator: Phenolphthalein | |1 |2 | |Final burette reading/ [pic] |29. 90 |29. 70 | |Initial burette reading/ [pic] |0. 00 |0. 0 | |Titre value/ [pic] |29. 90 |29. 70 | Mean titre value = [pic] = 29. 80 [pic] 1 Indicator: Methyl orange | |1 |2 | |Final burette reading/ [pic] |27. 00 |27. 00 | |Initial burette reading/ [pic] |0. 00 |0. 0 | |Titre value/ [pic] |27. 00 |26. 80 | Mean titre value = [pic] = 26. 90 [pic] B. Weak acid-strong base titration Acid: CH3COOH(aq) Base: NaOH(aq) Reaction: CH3COOH(aq) + NaOH(aq) [pic] CH3COONa(aq) + H2O(l) Indicator: Phenolphthalein | |1 |2 | |Final burette reading/ [pic] |25. 50 |25. 0 | |Initial burette reading/ [pic] |0. 00 |0. 20 | |Titre value/ [pic] |25. 50 |25. 60 | Mean titre value = [pic] = 25. 55 [pic] Indicator: Methyl orange | |1 |2 | |Final burette reading/ [pic] |4. 60 |4. 0 | |Initial burette reading/ [pic] |4. 20 |4. 60 | |Titre value/ [pic] |0. 40 |0. 30 | Mean titre value = [pic] = 0. 35 [pic] 2 Discussion 1. The colour change for both phenolphthalein and methyl orange in all titrations done in this experiments are from colourless to pale pink and from red to orange respectively since the base is added from the burette to the acid in the conical flask. 2. Indicator |Colour in acid |Colour in alkaline | |Phenolphthalein |Colourless |Red | |Methyl orange |Red |Yellow | 2. Titration curve of strong acid-weak base – There is a steady increase in pH when 0 – 20. 00[pic] of NH4OH (aq) is added. – There is a sharp increase in pH when 27. 60[pic]of NH4OH (aq) is added. – The end-point for this titration is at pH 5. where the graph is the steepest. Titration curve of weak acid-strong base – There is a steady increase in pH when 0 – 20. 00[pic] of NaOH (aq) is added. – There is a sharp increase in pH when 25. 30[pic]of NaOH (aq) is added. – The end-point for this titration is at pH 9. 5 where the graph is the steepest. Differences between the two curves – Curve of strong acid-weak base starts at pH 1. 11 whereas curve of weak acid-strong base starts at pH 1. 21 – Curve of strong acid-weak base levels off at pH 8. 34 whereas curve of weak acid-strong base levels off at pH 11. 80 – Curve of strong acid-weak base has an end point at pH 5. whereas curve of weak acid-strong base levels off at pH 9. 5 – Curve of strong acid-weak base has a sharp increase when 27. 60[pic]of NH4OH (aq) is added whereas the curve of weak acid-strong base has a sharp increase when 25. 30[pic]of NaOH (aq) is added. 3. Titration of strong acid-weak base The end-point is at 27. 75[pic] of NH4OH (aq) added (obtained from graph). Phenolphthalein only shows colour change when 29. 80[pic] of NH4OH (aq) is added, hence it changes colour at the wrong volume (too late, after end-point volume), thus it would be impossible to find to end-point accurately.
On the other hand, methyl orange changes colour when 26. 90[pic] of NH4OH (aq) is added, hence the colour change coincides more closely with 27. 75[pic]. Titration of weak acid-strong base The end-point is at 25. 75[pic] of NaOH (aq) added (obtained from graph). Methyl orange shows colour change when 0. 35[pic] of NaOH (aq) is added, hence it changes colour at the wrong volume (too early, before end-point volume), thus it would be impossible to find to end-point accurately. On the other hand, phenolphthalein changes colour when 25. 5[pic] of NaOH (aq) is added, hence the colour change coincides much more closely with 25. 75[pic]. 7 4. Titration of strong acid-weak base A suitable indicator for this titration is therefore methyl orange, for the reasons stated in statement (3) above. Titration of weak acid-strong base A suitable indicator for this titration is therefore phenolphthalein, for the reasons stated in statement (3) above. 5. All mean titre values were calculated from titre values that were ±0. 20[pic] of each other. This is known as concordant results. 6. Precautions:
Both Part 1: Suitability of Indicators and Part 2: pH titration curves a. The line of sight is perpendicular to the scale reading on the burette when taking both initial and final reading to avoid parallax error. b. A piece of plain white paper was place behind the burette when taking the readings to avoid error caused by refraction of light when it passes through the curved surface of the burette. c. The reading was taken from the bottom of the meniscus. d. All apparatus were rinsed with distilled water followed by the chemical intended to fill them in with respectively.
Part 1: Suitability of Indicators a. A piece of white tile was placed below the conical flask during the titration to allow for better determination of any colour changes. b. The conical flask was swirled gently so the mixture would be uniform/homogenous of the acid and base added Part 2: pH titration curves a. The magnetic stirrer was turned off and a glass rod was used to stir the other side of the mixture (the side further away from the magnet) and then removed before the reading of the pH was recorded so the mixture would be uniform/homogenous of the acid and base added. . Proper disposal of waste was done to ensure the safety of the environment and as a good hygiene practice. a. Worn latex gloves were thrown into the paddle bin. b. Liquid wastes were drained directed into the sink. c. There were no solid wastes in this experiment. Conclusion Only indicators that changes colour completely over one of two drops at the end-point (determined from pH titration curve) is suitable to be used to determine end-points of titrations Reference No references were used in this report – END OF REPORT – 8 Contents
Introduction ? Background 1 ? Aim1 ? Learning outcome1 Materials and Methods1 Results ? Part 1: Suitability of Indicators ? A. Strong acid-weak base titration1 ? B. Weak acid-strong base titration2 ? Part 2: pH titration Curves ? A. Strong acid-weak base titration ? Table of pH and volume of NH4OH(aq) added3 ? Graph of pH against volume of NH4OH(aq) added4 ? B. Weak acid-strong base titration ? Table of pH and volume of NaOH(aq) added5 ? Graph of pH against volume of NaOH(aq) added6 Discussion7 Conclusion8 Reference8
