Experiment 4 Determination of the acid content of vinegars PDF

Preview

Summary

Lab report. gradeA...

Description

Experiment 4: Determination of the acid content of vinegars Abstract In experiment 4 , visual end point acid base titrations as well as potential metric acid base titration are used. The hypothesis would be to see that the final percentage of weight by volume of acetic acid in vinegar samples would be the same or similar by both methods. Two methods to determine the acid content of vinegar are used, the first method is titration. From experiment 3, it has been noted that titration is a quantitative measure that includes determining the amount of the reagent with known concentration.This is the reagent which is utilized by the analyte. There are three types of quantitative titrimetry and those are volumetric, gravimetric and coulometric. The most commonly used form of titration is volumetric titrimetry and this method involves measuring the amount of a reagent of known concentration. We make one standard solution for acetic acid for both methods. Another prediction of the experiment is that we would get a very low percentage weight by volume of acetic acid given that we use 0.05 m of sodium hydroxide which is a very low concentration. Also because the vinegar was diluted with 500 mL of water approximately. For part 2 of The Experiment the prediction was that there will be a steady increase in pH as increasing volume of sodium hydroxide is added. The significance of doing both methods simultaneously showed us that both these methods could give us a value which is close to each other. For part 1 we found the final percentage of acetic acid to be 2.64 percent and for part 2 we found it to be 2.4%. The difference is not that large which shows us that both part 1 and part 2 or carried out without any mistakes. The importance of the lab is that it enables one to see how different methods can lead to the same conclusion and also which method is more accurate in determining the acetic acid in the sample therefore two quantitative analysis methods were used to find weight by volume percentage of one analytet.

Introduction Two visual representations were used for experiment 4 and the purpose was to determine the final acid concentration in the sample. The goal was to compare the two methods of quantitative analysis which used visual determination and potentiometric determination using pH. The hypothesis was that at the end of the experiment the average moles of sodium hydroxide and acetic acid would be the same; it was also a goal to compare potentiometric acid-base titration with visual end point acid base titrations. Scientists usually do not use potentiometric acid-base titrations or potentiometric analysis unless knowing the pH is absolutely necessary so more often than not visual end point acid base titrations are carried out. However, experiment 4 compared both these techniques of analysis. Acid base titrations take a longer time for completion then potentiometric analysis which saves time. However, pH electrodes are expensive and have a higher maintenance. The indicator used for experiment 4 is phenolphthalein because acetic acid is a weak acid and the pH range was from 8.3 to 10. The indicator is colorless below pH of 8.3 and it's deep shade of pink is almost red above ph10. A very low concentration of acid was expected given that only 50 mL of vinegar was diluted to the volume of 500ml and titrated against only 0.05 M of sodium hydroxide. The concentration of sodium hydroxide was known which allowed us to find the unknown concentration of the analyte. For potentiometric acid base titration we used the same sample of vinegar but 25 aliquots was used for part 2 and 2ml of the sodium hydroxide. The pH was recorded and given that the pH meter stops recording pH above 13 we only added up to 20 ml of sodium hydroxide and calculated pH up to 13.86 after that pH stabilized to 14 until 50ml.

Methods Part 1: Visual endpoint acid base titrations Using a graduated cylinder of 1 L we take 50 mL of commercial vinegar and dilute it to the mark with distilled water. We add 450 mL of distilled water to vinegar to give us a final volume of 500ml. The solution is mixed thoroughly and then 50ml of this solution is pipetted into three 250 ml Erlenmeyer flasks. To the three Erlenmeyer flasks 50 mL of water is added with two drops of phenolphthalein.

Each flask is then titrated with 0.05 molarity of sodium hydroxide until the first permanent pink color is seen which would indicate the endpoint of the titration. 3 samples are taken for part 1 of the procedure

Part Two: potentiometric acid base titrations A two-point calibration of the pH meter is done using a pH 7 buffer and a pH 10 buffer. From the previously made sample solution in part 1 we pipetted again 25 ml aliquots into a 250 ml Erlenmeyer flask. We only use one flask. The buret is placed above the sample and the pH meter with millivolt scale is immersed in the sample; a stir bar is also used to mix the solution when the titrant is added. The pH meter must not be moved around as it can easily break. 2 ml of the 0.05 molarity of sodium hydroxide is added from the buret and the pH of the solution is recorded as we add 2 ml of sodium hydroxide until 50 ml but after 20 ml is added the pH is stabilized which would indicate the end of titration.

Results and Calculations part 1 determination of the acid content of vinegar using visual end point acid base titration flask number one Initial buret reading equals 0 final buret reading equals 45.5 ml titrant volume equals 45.5ml number of moles of 0.05 molarity sodium hydroxide solution equals 2.275 X 10 ^ -3mol Flask number 2 Initial buret reading equals 1.7 ml final buret reading equals 44.8 ml titrant volume equals 43.1 ml number of moles of 0.05 molarity sodium hydroxide solution equals 2.155 x 10 ^ -3mol

Flask number 3 Initial buret reading equals 0 final buret reading equals 43.5 ml titrant volume 43.5 ml number of moles of 0.05 molarity sodium hydroxide solution equals 2.175 x 10 ^ -3mol Average number of moles of sodium hydroxide equals 2.202 x 10^-3mol.

The number of moles of acetic acid equals the number of moles of sodium hydroxide due to the one-to-one ratio. therefore number of moles of acetic acid equals 2.202 x 10 ^ -3 mol molar mass of acetic acid equals 60.052 grams therefore mass of acetic acid equals 2.202 x 10 ^ -3 x 60.052 which equals 0.132 grams therefore (0.132/ 50)x 500= 1.32 grams (1.32/50)*100= 2.64% acetic acid in vinegar, this is a percentage weight by volume.

Part 2 determination of the acid content of vinegar potentiometrically using a pH meter.

Volume of sodium hydroxide added in milliliters

pH

2

3.80

4

4.06

6

4.34

8

4.46

10

4.57

12

4.71

14

4.88

16

5.05

18

6.20

20

13.86

22

14

24

14

26

14

28

14

30

14

32

14

34

14

36

14

40

14

42

14

44

14

46

14

48

14

50

14

the neutral volume is given at 20 ml so the volume used is 0.02L. the number of moles of sodium hydroxide equals 0.02 x 0.05 = 1 x 10 ^ -3 mol. The number of moles of sodium hydroxide equals the number of moles of acetic acid due to the one-to-one ratio. Therefore the number of moles of acetic acid equals 1 x 10 ^ -3 mol. mass of acetic acid equals 1 x 10 ^ -3 x 60.052= 0.060 052 grams. therefore percentage weight by volume acetic acid in sample equals 0.060 052/50* 500= 0.60052 0.60052/ 25* 100= 2.4% Therefore given the small difference in the results we can conclude the both methods provide us with an accurate amount of acetic acid in the sample of vinegar.

Conclusion From the visual end point detection the titrant volume is observed to be above 40 ml whereas for potentiometric acid base titration the titrant volume is around 20 ml. More than double of the titrant volume was used in the visual endpoint detection method and it could be due to a number of reasons for example for the visual end point acid base titration more of human error is involved for example the experimenter could miss the endpoint and add more than the required titrant volume, improper mixing of the solution or inadequate addition of the indicator could also lead to a higher titrant volume. These errors are not present in the potentiometric acid base titration method because only one instrument is used and the only error comes from error in calibration which is not a significant error however we see that both methods give us a similar percentage of acetic acid in sample so it can be said both are adequate methods on finding the concentration of an analyte.

Reference CHEM 3251 Experiment 4 Determination of the acid content of vinegars Lab manual 2021, Mechref ,Yehia pp 1-5...