Reference no: EM133151001

SMA0020 Practical Determination of the relative proportions of caffeine and aspirin by GC-MS

The purpose of this experiment is to determine the concentration of caffeine and ethyl-2- acetoxybenzoate (aspirin derivative) contained in the solution provided. A calibration curve will be constructed using standards, then the unknown sample will be measured and its caffeine and ethyl-2-acetoxybenzoate content calculated.

Your graphs (drawn in Excel or a similar program) and your chromatograms must be attached to your report.

Part 1 - Calibration curve
Run each standard solution once, starting with the lowest concentration. Record your measurements in the table below and plot separate calibration graphs for peak area and peak height vs. concentration for each standard (four graphs in total).
Caffeine:

Ethyl-2-acetoxybenzoate:

Part 2 - Analysis of unknown solution
Record the peak areas and peak heights for the unknown sample in triplicate.
Caffeine:

Ethyl-2-acetoxybenzoate:

Calculate the concentration of caffeine and ethyl-2-acetoxybenzoate in the unknown solution, including an error value:

Questions (continue over the page if necessary)
1. Why are calibration standards always analysed in increasing order of concentration?

2. Comment on the linearity of your calibration graphs

3. Comment on the magnitude of the errors in your measurements of the unknown concentration.

Experiment - pH titrations, acid base titrations and the determination of acid dissociation constants.

Quality of data
Concordant burette readings and accuracy of determined values.
Appropriately labelled graphs of pH vs vol. NaOH (both acids) and ΔpH/Δvol. vs vol. (weak acid)
Results and data analysis
The concentration of the approximate 0.1 M HCl
calculated to four significant figures using data
obtained from the pH titration and from the acid/base
titration using an indicator with 95% confidence
interval.
The concentration of the approximate 0.1 M ethanoic
acid using data obtained from the pH titration and
from the acid/base titration using an indicator with
95% confidence interval.
The acid dissociation constant pKa of ethanoic acid
found and compared with literature values. Ka and pH
of the weak acid determined
Answers to specific questions on this practical experiment.
Conclusion
Shows that the student understands the aims of the practical and can write a short conclusion on the work which has been carried out.

Exact concentration of standard NaOH provided (to 4s.f.):

Part A (Strong Acid)

Plot graph overleaf as you are completing the titration and adjust step size as needed

For online submission replace the hand drawn graph overleaf with a fully labelled computer- generated graph.

Equivalence point
Vol. NaOH added at equivalence point (X): pH:

11: a) Steepest part of curve (pH range):
b) Indicator chosen:

Insert your computer drawn graph of pH vs volume NaOH for the strong acid (HCl) on this page

Part B (Weak Acid)

Plot graph overleaf as you are completing the titration and adjust step size as needed

For submission replace the hand drawn graph overleaf with two fully labelled computer- generated graphs, pH vs vol and ΔpH/Δvol vs vol.

Equivalence point
Volume NaOH added at equivalence point: pH: Volume NaOH according to ΔpH/Δvol. graph:
Volume NaOH at half-equivalence: pH = pKa:

14: a) Steepest part of curve (pH range):
b) Indicator chosen:

Insert your computer drawn graph of pH vs volume NaOH for the weak acid (CH3COOH) on this page

Also insert your computer drawn graph of ΔpH/Δvolume vs volume NaOH for the weak acid (CH3COOH) here

Analysis (show your working)
Calculation of precise concentration of HCl from pH titration value:

HCl conc:

Calculation of precise concentration of HCl from indicator titration with 95% CI:

HCl conc: ±

Calculation of precise concentration of CH3COOH from pH titration value:

CH3COOH conc:

Calculation of precise concentration of CH3COOH from indicator titration with 95% CI:

CH3COOH conc: ±

Compare your measured pKa value to a literature value and comment on any differences:

Using your pKa value calculate the pH for ~10 cm3 of added NaOH and compare to the graph value:

Using you pKa value calculate Ka:
Hence calculate the expected pH of the weak acid solution and compare to your initial pH reading (when no base added) accounting for any discrepancies:

Questions

Q1 What is a pH electrode? How is this device able to detect changes in acidity?

Q2 Why is it that the solvent interferes with the determination of the the equilibrium constant for strong acids and indeed strong bases?

Q3 Which graphs best identify the endpoint of the titration (pH against v) or (ΔpH/Δv against v). Why is this the case?

Conclusions (How could the method or your results be improved?)

Attachment:- Practical Determination.rar