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Investigation of the Suitability of Indicators and Instrumental Detection of Equivalence Points in Acid-Base Titrations Essays

To understand the importance of choosing suitable indicators for detecting the end points of acid-base titrations. To obtain titration curves some acid-base titrations. Materials and method: As shown in page 2 and 3 in General Chemistry practical manual 2011. Result: Part 1: Suitability of Indicators Strong acid-weak base titration Table I: Volume Titration RoughAccurate 1Accurate 2 Final volume0. 0014. 650. 00 Initial volume35. 2048. 0033. 50 Volume of ammonium hydroxide used (ml)35. 0033. 3533. 50 Average volume of ammonium hydroxide used (ml) (33. 35+33. 50)/2 =33. 43 (2 d. p. ) Table II: Volume Titration
RoughAccurate 1Accurate 2 Final volume0. 0014. 0016. 50 Initial volume33. 2046. 8549. 40 Volume of ammonium hydroxide used (ml)33. 2032. 8532. 90 Average volume of ammonium hydroxide used (ml) (32. 85+32. 90)/2 =32. 88 (2 d. p. ) Weak acid-strong base titration Table III: Volume Titration RoughAccurate 1Accurate 2 Final volume0. 000. 000. 00 Initial volume25. 6025. 1025. 20 Volume of sodium hydroxide used (ml)25. 6025. 1025. 20 Average volume of sodium hydroxide used (ml) (25. 10+25. 20)/2 =25. 15(2 d. p. ) Table IV: Volume Titration RoughAccurate 1Accurate 2 Final volume0. 008. 0016. 00 Initial volume5. 6013. 3021. 20
Volume of sodium hydroxide used (ml)5. 605. 305. 20 Average volume of sodium hydroxide used (ml) (5. 30+5. 20)/2 =5. 25 (2 d. p. ) (d. p. = decimal point) Formula used to calculate the average volume of sodium hydroxide used (ml) Average volume of sodium hydroxide used =(Accurate 1+Accurate 2)/2 Part 2: pH titration curves Strong acid-weak base titration Table V: Volume of ammonium hydroxide (ml)pHVolume of ammonium hydroxide (ml)pH 0. 000. 7320. 501. 73 0. 501. 0321. 001. 76 1. 001. 0421. 501. 79 1. 501. 0522. 001. 83 2. 001. 0622. 501. 87 2. 501. 0723. 001. 91 3. 001. 0823. 501. 95 3. 501. 0924. 001. 99 4. 001. 1124. 502. 05 4. 01. 1225. 002. 10 5. 001. 1325. 502. 17 5. 501. 1526. 002. 25 6. 001. 1626. 502. 33 6. 501. 1727. 002. 44 7. 001. 1927. 502. 60 7. 501. 2028. 002. 80 8. 001. 2228. 503. 16 8. 501. 2329. 004. 39 9. 001. 2529. 506. 69 9. 501. 2730. 007. 86 10. 001. 2930. 508. 13 10. 501. 3031. 008. 25 11. 001. 3231. 508. 34 11. 501. 3432. 008. 42 12. 001. 3532. 508. 48 12. 501. 3733. 008. 54 13. 001. 3933. 508. 59 13. 501. 4134. 008. 64 14. 001. 4334. 508. 68 14. 501. 4535. 008. 73 15. 001. 4735. 508. 76 15. 501. 4936. 008. 80 16. 001. 5136. 508. 82 16. 501. 5337. 008. 85 17. 001. 5537. 508. 88 17. 501. 5738. 008. 90 18. 001. 6038. 508. 91 18. 501. 239. 008. 92 19. 001. 6439. 508. 94 19. 501. 6740. 008. 96 20. 001. 70 Figure 1: Graph of pH value against volume of ammonium hydroxide added Table VI: Volume of sodium hydroxide used (mL)pHVolume of sodium hydroxide used (mL)pH 0. 002. 8918. 005. 07 0. 503. 0318. 505. 11 1. 003. 3319. 005. 16 1. 503. 4519. 505. 20 2. 003. 6120. 005. 27 2. 503. 6820. 505. 33 3. 003. 7721. 005. 40 3. 503. 8521. 505. 47 4. 003. 9122. 005. 57 4. 503. 9722. 505. 66 5. 004. 0423. 005. 77 5. 504. 0923. 505. 93 6. 004. 1224. 006. 23 6. 504. 1724. 506. 62 7. 004. 2325. 0010. 23 7. 504. 0025. 5011. 01 8. 004. 3226. 0011. 27 8. 504. 3626. 5011. 40 9. 004. 927. 0011. 53 9. 504. 4227. 5011. 62 10. 004. 4628. 0011. 68 10. 504. 4928. 5011. 73 11. 004. 5429. 0011. 79 11. 504. 5629. 5011. 82 12. 004. 6130. 0011. 85 12. 504. 6430. 5011. 89 13. 004. 6831. 0011. 91 13. 504. 7131. 5011. 94 14. 004. 7532. 0011. 95 14. 504. 7832. 5011. 98 15. 004. 8233. 0012. 00 15. 504. 8633. 5012. 01 16. 004. 8934. 0012. 04 16. 504. 9334. 5012. 05 17. 004. 9735. 0012. 06 17. 505. 02 Figure 2: Graph of pH value against sodium hydroxide added Discussion: In part 1 A of the experiment, which is the strong acid-weak base titration, phenolphthalein indicator changes colour from colourless to pink at the end point.
The end point was achieved when 33. 43mL (average reading) of 0. 1M ammonium hydroxide added into 25mL of 0. 1M hydrochloric acid. In the other hand, methyl orange indicator changes colour from orange to yellow at the end point. The end point for the indicator was achieved when 32. 88mL (average reading) 0. 1M ammonium hydroxide added into 25mL of 0. 1 M hydrochloric acid. In part 1 B of the experiment, which is the weak acid-strong base titration, phenolphthalein indicator changes colour from colourless to pink when 25. 15mL (average reading) of 0. 1M sodium hydroxide was added in 25mL of 0. M acetic acid. However, the methyl orange indicator changes colour from orange to yellow when 5. 25mL (average) of 0. 1M sodium hydroxide added into 25mL of 0. 1M acetic acid. pH range and colour changes of the indicators can be refer at table VII. For the graph of pH value against volume of ammonium hydroxide added (figure1), pH increased in sudden at the starting point which was pH 0. 7 until pH 1. Then, the pH increased slowly to form a smooth curve until pH3. At equivalent point, the pH value increased drastically until pH 8. Beyond the equivalent point, pH increased slowly again with addition of base.
The equivalent point for the weak acid-strong base titration in this experiment is pH 5. 6. For the graph of pH value against volume of sodium hydroxide added (figure 2), pH increased slowly at the starting point which was pH 2. 9. However at the point (7. 50, 4. 00), the pH is slightly decreased and this caused the titration curve slightly out of shape. This may be due to the pH reading was took before the pH meter stabilized. Then, the pH increased again until pH 6. 60. At equivalent point, the pH value increased critically. Beyond equivalent point, pH increased slowly again with addition of base.
The equivalent point for this strong acid-weak base titration is pH 8. 70. The result obtained using acid-base indicators in strong acid-weak base titration (part 1A) showed that the end point obtained when using methyl orange is pH 8. 50 and the end point obtained when using phenolphthalein is pH 8. 62. Compare with the result obtained from the titration curve (figure 1), the end point for the both indicators were way too far to the equivalent point which is pH 5. 60. For the weak acid-strong base titration (part 1B), the end point obtained when using phenolphthalein is pH 9. 0 and the end point obtained when using methyl orange is pH 4. 10. Comparing the results obtained to the titration curve (figure 2), the end point of phenolphthalein indicator is closer to the equivalent point compare to the methyl orange indicator. These results are not accurate compare to the theory of the titration. This may be due to systematic error and random error occurred throughout the experiment. One of the possible error is parallax error where we incorrectly positioning of the eye when reading the measurement at the burette.
Besides, the burette and the pipette may be containing small amount of the distill water causing the acid or base diluted. The mixture of acid and base is not equally stirred when the pH reading taken also one of the probability error. Every different indicators change colours at different pH range. While the equivalent point of the titration is where both acid and base solution are mixed in exactly equation proportions. Therefore, we must choose an indicator which the colour changes near to the equivalent point.
When hydrochloric acid titrate with ammonium hydroxide (strong acid-weak base titration), the end point of methyl orange is closer to the equivalent point compare to the phenolphthalein. Although the end points of both indicators are far from equivalent point, the nearest end point is chosen. Theoretically, methyl orange is more suitable to use during the strong acid-weak base titration. (Refer to table VIII) When acetic acid titrate with sodium hydroxide (weak acid-strong base titration), the end point of phenolphthalein is closer to the equivalent point compare to methyl orange.
Therefore phenolphthalein is chosen for the acetic acid titrates with sodium hydroxide. Theoretically, phenolphthalein is more suitable to use in a weak acid-strong base titration. (Refer to table VIII) Table VII: IndicatorpHHInpH range Colour change acidalkali Methyl orange3. 73. 2- 4. 2redyellow Methyl red5. 14. 2-6. 3redyellow Bromophenol blue7. 16. 0-7. 6yellowblue Phenolphtalein9. 38. 2-10. 0colourlesspink Table VIII: Type of titrationChanges in pH at end pointSuitable indicator Strong acid-strong base3. 5 to 10. 5any indicator Strong acid-weak base3. 0 to 7. 0methyl orange Weak acid-strong base6. 5 to 10. 5phenolpthalein

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