Titration Report - Titrating Acetic Acid and NaOH PDF

Preview

Summary

This was a recent titration report...I'm uploading because I need premium access okay...

Description

1

Acid-Base Titration Experiment Anekeaini Cheok Year 12 INTRODUCTION: Vinegar is a common household acid, derived both naturally and synthetically from the oxidation of ethanol (C₂H₆O) into acetic acid1 (CH ₃COOH) solution. While the acetic acid content of vinegar may vary, its concentration generally ranges from 4 to 8% v/v2. The precise concentration of acetic acid in a vinegar sample can be determined through methods of volumetric analysis, specifically titration.

Figure 1.1 - The two step oxidation of ethanol into ethanoic acid Titration is a form of quantitative chemical analysis employing the measurement of volumes to determine the concentration of a substance in solution. There are two components of a titration: the titrant, a solution of known concentration, and an analyte solution of unknown concentration. Using a burette, the two are gradually combined until the reaction end-point is reached. As most titrations involve acid-base reactions, where either species could behave as the titrant or analyte, the end-point occurs at the moment of neutralization when solution composition changes suddenly from excess H+ to OH- ions (or vice versa, depending on the analyte species). A pH indicator is added, typically to the analyte, to signal this end-point through a visible colour change of solution. The end-point volume is not necessarily - but ideally identical to the equivalence point volume, a calculated value at which the moles of titrant are equal to the moles of analyte, resulting in neutralization. In titrations, the titrant is a standard solution of precisely known concentration, to ensure accuracy when calculating the unknown analyte concentration. A standard solution may be of primary or secondary nature, depending on the properties of the solute involved. A highly pure and stable solute, with large molar mass, can be used for the direct preparation of a primary standard solution. However, if a solute cannot 1 Also known an ethanoic acid 2 Stubbings, Janice. "Acetic Acid In Vinegar By Direct Titration Chemistry Tutorial". Ausetute.Com.Au, 2021, https://www.ausetute.com.au/titratevinegar.html. Accessed 29 May 2021.

2

be obtained in a pure-enough form for use as a primary standard - for instance, it has high atmospheric reactivity - it is classified as a secondary standard, and used to prepare a solution at approximately the desired concentration. This solution is then standardised against a primary standard solution, to determine exact molarity. This experiment involves the neutralization of acetic acid (CH ₃COOH) by sodium hydroxide (NaOH) - a weak acid and strong base, respectively - to quantitatively determine the molar concentration of CH ₃COOH in a sample of vinegar. This is seen in Figure 1.2:

Figure 1.2 - Chemical equation for the reaction of acetic acid and NaOH As acetic acid and NaOH are, respectively, a weak acid and strong base, the reaction equivalence point is shown by the following titration curve to be approximately pH=9 (Figure 1.3). Hence, this experiment utilises phenolphthalein indicator, which operates in the basic range of 8-10 (Figure 1.4).

Figure 1.3 - Titration curve indicating the equivalence point of a titration between a weak acid and strong base as being ~pH=9

3

Figure 1.4 - Graph showing the basic pH range at which phenolphthalein indicator works Due to its reactive and hygroscopic nature, the NaOH solution is standardised against potassium hydrogen phthalate (KHP/C8H5KO4) before use as a secondary standard. KHP is a non-hygroscopic solid behaving as a monoprotic acid; this enables its exact number of moles to be determined. The reaction between KHP and NaOH is represented by Figure 1.5:

Figure 1.5 - Chemical equation for the standardisation of NaOH using KHP AIM: To quantitatively determine the concentration of acetic acid (CH ₃COOH) in a sample of Fehlbergs Double Concentrated vinegar. HYPOTHESIS: The concentration of acetic acid (CH ₃COOH) in Fehlbergs Double Concentrated vinegar is 8% v/v, or 1.33 mol/L. MATERIALS: (A) Standardisation of NaOH: ❏ 1x 25 mL pipette (with filler) ❏ 1x 50 mL burette (with stopcock) ❏ 1x 250 mL volumetric flask ❏ 4x 250 mL conical flask ❏ Retort stand ❏ Burette clamp

4

❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏

Analytical scale Watch Glass Glass funnel NaOH (solid) Potassium hydrogen phthalate (KHP) (solid) Phenolphthalein indicator (in dropper) Deionised/distilled water in wash bottle White A4 paper (optional) Labels (optional)

(B) Titration of acetic acid with sodium hydroxide: ❏ 1x 25 mL pipette (with filler) ❏ 1x 50 mL burette (with stopcock) ❏ 1x 250 mL volumetric flask ❏ 4x 250 mL conical flask ❏ Retort stand ❏ Burette clamp ❏ Glass funnel ❏ 25 mL Fehlbergs Double Strength Vinegar - 8% acetic acid context ❏ Standardised 0.1 M NaOH from (A) ❏ Deionised/distilled water in wash bottle ❏ Phenolphthalein indicator (in dropper) ❏ Labels (optional) ❏ White A4 paper (optional) RISK ASSESSMENT: See attached document METHOD: (A) Standardisation of 0.1 M of sodium hydroxide with the primary standard potassium hydrogen phthalate (KHP) 1. Set up equipment as indicated in diagram (A). 2. Measure and dissolve 1.0g of NaOH flakes in 250 mL of distilled water, in a volumetric flask. Stopper and invert until homogenous. Label as ‘~0.1M NaOH’. 3. Weigh out 4 samples of KHP solid, each between 0.3 to 0.4g. Place into separate 250 mL conical flasks. Pipette 50 mL of distilled water into each flask, and swirl until the KHP is fully dissolved. 4. Add 3 drops of phenolphthalein indicator to each KHP sample. 5. Rinse the burette: first with distilled water, then with the ~0.1M NaOH solution, ensuring that internal surfaces are thoroughly coated. Drain liquid

5

through the stopcock. 6. Mount the burette and fill with ~0.1M NaOH solution, until the meniscus is above the 0.00 mL mark. Open the stopcock to remove air and excess solution. 7. Adjust meniscus to 0.00 mL. Record the initial reading of the NaOH solution in a table. 8. Titrate the first KHP sample by adding NaOH solution, and combining until the endpoint is reached. This will be indicated by a faint pink colour that persists, despite swirling. 9. Record the NaOH burette reading at the endpoint, to the nearest 0.01 mL. Calculate the rough titre, which should be used as a guide for the following titrations. Dispose of the titrated solution. 10. Refill the burette with a suitable quantity of NaOH. Repeat steps 6 to 9 for the remaining KHP samples, or until 3 values within a range of 0.02 mL have been obtained. This may involve preparation of more KHP samples for further trials. 11. Stopper the remaining NaOH solution for the following experiment. Rinse all other glassware with distilled water. 12. Calculate the accurate molarity of the standardised NaOH, using titre values. Diagram(A):

Retort stand with burett e clamp

Containing NaOH solution (to be standardis ed) Stopcock Containing KHP solution (primary standard)

(B) Titration of vinegar (acetic acid) with standardised sodium hydroxide 1. Set up equipment as indicated in diagram (B). 2. Pipette 25.00 mL of vinegar into a 250.00 mL volumetric flask. Dilute by a factor of 10 by making the solution up to the graduation mark with distilled water. Stopper and invert flask to ensure homogeneity. Label the solution as

6

3. 4.

5.

6.

7.

8.

9.

‘dilute vinegar’. Pipette 25 mL of the dilute vinegar solution, and add 3 drops of phenolphthalein indicator to each sample. Rinse burette thoroughly with distilled water, then with the standardised NaOH solution from experiment (A). Ensure internal surfaces are thoroughly coated. Drain liquid through the stopcock. Fill the burette with standardised NaOH solution to a level slightly above the 0.00 mL mark. Open the stopcock to remove excess solution and air, and adjust to the 0.00 mL mark. Record this initial volume in a table. Titrate the vinegar solution by slowly adding NaOH solution, until the endpoint is reached. This will be indicated by a faint pink colour that persists, despite swirling. Record the final burette reading to the nearest 0.02 mL. Calculate the rough titre, which should be used as a guide for the following trials, and dispose of the titrated solution. Refill the burette to an appropriate volume, and repeat steps 3-8 for the remaining vinegar solutions, or until you acquire three consistent titre values that are within a range of 0.02 mL. This may take more than 3 trials. Calculate the molarity of acetic acid in the vinegar sample, using titre values.

Diagram (B):

Retort stand with burett e clamp

Containing NaOH solution (the standard solution) Stopcock Containing a solution of dilute vinegar (acetic acid) and drops of phenolphthalein indicator

7

RESULTS: Expected (calculated) concentration of acetic acid:

Standardisation of NaOH Titration number

Rough titration (mL)

1

2

3

Initial burette reading (mL)

0.00

15.9

0.00

17.4

Final burette reading (mL)

15.9

33.3

17.4

34.75

Titre (mL)

15.9

17.4

17.4

17.35

Mean titre excluding rough tire (mL)

17.38

Concentration of KHP:

8

Standardised concentration of NaOH:

Titration of Acetic Acid with NaOH Titration number

Rough titration

1

2

3

Initial burette reading (mL)

0.00

0.00

0.00

0.00

Final burette reading (mL)

32.90

34.00

33.9

33.9

Titre (mL)

32.90

34.00

33.9

33.9

Mean titre excluding rough tire (mL)

33.93

Volume of vinegar: 0.025 L NaOH titre: 0.03393 L

9

NaOH concentration: 0.0988 M Acetic acid concentration: unknown 1:1 Mole ratio

~ 8% v/v Note that the standardised concentration of NaOH (0.0988 M) has been used. DISCUSSION: Titration of acetic acid and standardized 0.1M NaOH solution was used to determine the percentage concentration of acetic acid in Fehlbergs Double Concentrated Vinegar. The expected concentration of acetic acid was calculated to be 8% v/v, or 1.33 M, with an experimentally determined concentration of 8% v/v and 1.34 M. Resultantly, the experimental results supported the hypothesis, and could be considered highly accurate. The accuracy of results ultimately originated from the standardization of the prepared NaOH solution with KHP. Knowing the exact molarity and moles of NaOH enabled the experimenter to determine the concentration of acetic acid, given 1:1 stoichiometry. Despite having an expected molarity of 0.1M, the standardised concentration of NaOH was found to be 0.0988 M, with the proximity of the two

10

values suggesting significant accuracy in solution preparation. The accuracy of the acid-base titration between acetic acid and NaOH was increased through the implementation of highly precise equipment and techniques. This involved use of a 25 mL volumetric pipette and graduated burette for the measurement of volumes (accurate to ±0.5% and ±0.1%, respectively); an analytical balance to weigh out quantities of KHP and NaOH solids (accurate to ±0.1%); and volumetric flask for the making of solutions (accurate to ±0.2%). The use of precise equipment and techniques enabled the experimenter to attain highly reproducible results - evidenced by the consistency of values obtained in both the standardisation and titration processes. For the standardisation of NaOH, an average titre of 17.38 mL was calculated from trials of 17.4 mL, 17.4 mL and 17.35 mL. As each of these values were within 0.05 mL, it is shown that the method could be repeated to produce results of a highly similar, if not identical standard. The titration of acetic acid with NaOH also generated precise results, with an average titre of 33.93 mL calculated from trials of 34.0 mL, 33.9 mL and 33.9 mL. The precision of these values is indicated by their proximity, within a range of 0.01 mL, demonstrating their reproducible nature. The repetition of trials, until three concurrent values were obtained, ensured experimental reliability as a dependable average titre was obtained for use in calculation. The experimental design further utilised reliable, academic sources to maximise the accuracy and relevance of the method. However, greater confirmation of reliability could be obtained by having others conduct the experiment and attempt to attain similar results, or to repeat the method using acetic acid samples from different bottles of Fehlbergs vinegar to determine product consistency. Accuracy may have been affected by a variety of factors, which account for differences in titre values, and between the expected and experimentally determined concentration of acetic acid. Parallax error during the measurement of volumes may have resulted in the experimenter’s improper reading of the meniscus; this may have occurred with use of the volumetric pipette or flask, if measurements were not read at eye-level, or with the graduated burette, due to tilting of the retort stand. Parallax error is thus the most likely cause of inconsistencies between the expected and experimental values - however, accuracy could also have been affected by measuring KHP solid within a range of 0.3-0.4g. As the amount of KHP for each trial varied from 0.32g to 0.36g, this may have impacted individual and overall titres. Furthermore, the sustained use of NaOH base in the burette may have worn away at the glass and graduation marks, leading to possible inaccuracy in measurements. A margin of error also exists with the use of phenolphthalein, as a small quantity of indicator reacts with the titrant, leading to minute inaccuracies in burette reading. The

11

indicator may not change colour at the exact volume of equivalence due to the presence of unreacted NaOH inside the conical flask, and the experimenter could have perceived the moment of endpoint differently between trials. This may have led to the slight ‘overshooting’ of the true endpoint volume. Furthermore, accuracy was compromised by having to make a second 250 mL solution of NaOH, which was not standardised and unlikely to be of the exact molarity of the original solution. This could have impacted the accuracy of its titration with acetic acid, as the rough and first trials used the standardised solution, while the latter two used the unstandardised NaOH. This would have caused inconsistencies in experimentation. To maximise accuracy, the experimental process should be altered. This includes measuring an exact amount of KHP solid (i.e 0.35g), rather than an amount between a range of 0.3-0.4g, to exercise greater control of the method and increase precision of titres. A larger (500 mL) solution of NaOH should be prepared and standardised prior to the acid-base titration, to ensure that all trials are conducted against a consistent source of NaOH. It would also be beneficial to use distilled water to rinse the inside of the conical flasks while titrating, as this ensures that all the NaOH reacts with the KHP/acetic acid, and promotes the accuracy of titres. The experimenter did not do so in this experiment, which could justify the difference in expected and actual results. The experimenter should further rinse conical flasks prior to experimentation, with both distilled water and the relevant solution (i.e KHO or dilute vinegar), to prevent contamination with chemical residue and maintain the desired solution concentration. Additionally, if available, a Diji burette should also be used in place of a traditional burette to ensure greater precision and accuracy during titrations. It would also be experimentally beneficial to avoid continued use of base in the burette, to maintain the instrument’s precision and accuracy. This acid-base titration was a valid experiment, as the method was designed to respond to the aim and hypothesis, and determine whether the acetic acid content in Fehlbergs Double Concentrated Vinegar was 8% v/v (as calculated). Despite the absence of an independent variable, the experimenter maintained validity by exercising control over all other major variables in the standardisation and titration processes. For the standardisation of NaOH, the dependent variable was the quantity of NaOH titre (mL) to achieve the endpoint; with controlled variables including quantities of chemicals used, use of distilled water when making solutions, the method of titration and equipment used across trials. In turn, the dependent variable in the acid-base titration was the NaOH titre (mL) needed to reach the endpoint; with controlled variables being the brand of vinegar being sampled (Fehlbergs Double Concentrated), the amount of distilled water and vinegar used in each trial, method of titration and equipment used across trials. Validity was further ensured with use of an

12

appropriate pH indicator (phenolphthalein), which enabled the experimenter to accurately identify the moment of endpoint by generating a distinct colour change at the equivalence point. These factors contributed to the internal and external validity of the titration, as the experimenter adhered to the scientific method and ensured fair experimentation. CONCLUSION: This experiment sought to quantitatively determine the unknown concentration of acetic acid in a sample of Fehlbergs Double Concentrated Vinegar, by titrating acetic acid (in vinegar) with a standardised 0.1M NaOH solution. The concentration of acetic acid was accurately found to be 8% v/v or 1.34 mol/L, through experimentation, thus supporting the hypothesised value of 8% v/v and 1.33 mol/L. It can be determined that Fehlbergs Double Concentrated Vinegar does indeed contain 8% v/v acetic acid content, as calculated. ACKNOWLEDGEMENTS: Thank you to Ms Parsons and Mr Lui for their assistance in completing this experiment. BIBLIOGRAPHY: See attached document...