Spectrophotometric Assay of Paracetamol
Write a lab report based on results obtained from this experiment. In lab report describe and discuss the spectrophotometric assay of paracetamol.
The collected data must be presented, analysed, and interpreted. The lab report is followed by a few calculation questions which you need to answer and provide an explanation for your solution.
Practical details:
The maximum absorbance wavelength (λmax) of paracetamol is 243 nm. In order to assess the content of paracetamol in an unknown sample, we need first to prepare a calibration curve which displays the relationship between absorbance and concentration
You will have the following:
Paracetamol stock solution (10 mg/litre) in 5% ethanol/water.
Dilution solution 5% ethanol/water
Samples containing paracetamol
Glassware, volumetric flasks, tubes to prepare your solutions, micropipettes, and UV
cuvettes.
Preparation of standard solutions
Prepare standard solutions of paracetamol starting from the stock solutions with the
following concentrations. The final volume of each solution should be 5 ml.
Calculate the needed volumes of both the stock solution and the dilution solution to
prepare your standards 1–5.
Table 1: preparation of standard solutions
Standard # 1 2 3 4 5 Blank
Volume of stock
solution (ml)
Volume of dilution
solution (ml)
Final volume (ml) 5 5 5 5 5 5
Final concentration
mg/litre 1 2 4 6 8 0
Measuring absorbance of standard solutions
Use UV cuvettes for this experiment.
Prepare the spectrophotometer by using the blank solution to give zero absorbance at 243 nm.
You can use the same cuvette to measure the absorbance of standard solutions, starting from the lowest to the highest concentration.
Return the solution to its tube after each measurement and tap the cuvette on a clean
tissue paper to remove the remaining droplets.
Record the absorption values in the following table:
Table 2: Absorbance readings for standard solutions
Concentration
mg/litre 1 2 4 6 8
Absorbance
reading 1
Absorbance
reading 2
Absorbance
reading 3
average
Use the absorbance readings to draw the calibration curve of absorbance vs concentration and find out the regression equation using Excel.
Preparation of sample solution
You will be provided with a sample containing unknown amount of paracetamol.
Make note of your sample code.
Preparation of solution A: Weigh out 125 mg of your unknown sample and transfer
it carefully into 100ml volumetric flask. Add around 50 ml of the dilution solution (5% ethanol in water) and swirl gently until the powder is completely dissolved. Make the volume up to 100 ml and mix well.
Preparation of solution B: take 5 ml of solution A using a micropipette and transfer
it into a 50 ml volumetric flask. Make the volume up to 50 ml using the dilution
solution.
Preparation of reading solutions C: using the micropipette, measure 500 μl of
solution B and make it up to 10 ml with dilution solution. Prepare 3 repeats.
Use a clean cuvette to measure the absorbance of solution C at 243 nm.
– Record the absorbance values in the following table:
Table 3: Absorbance readings for solution C
Solution C 1 2 3 Average Standard
deviation
Absorbance
Use the calibration curve and the regression equation to calculate the percentage of
paracetamol in your sample.
Discuss the reliability and precision of your measurements. Comment on the r–
squared value of your calibration curve.
Discuss the sources of errors in the experiment.
Additional calculation questions:
1– A sample of an infusion was diluted 10 ml to 250 ml and then 10 ml to 200 ml. It was then analysed and was found to contain sodium at 0.789 mg/100 ml. Calculate the concentration of sodium in the original sample in %w/v. The sample was composed of a mixture of sodium lactate and sodium carbonate in equimolar amounts. Calculate the amount of sodium lactate and sodium carbonate in mg/10 ml of the sample (Na = 23, lactate = 89, carbonate = 60) (10 marks)
2– Convert the following concentrations into the required expression (5 marks)
Concentration given Expression required
0.5% NaCl molarity
1 mM of KBr mg/ml
5 mg/100 ml paracetamol molarity
0.15 N H2SO4 g/litre
0.5% ethanol/water (w/v) ppm
3– 0.641 g of a semi–synthetic alkaloid was dissolved in 25 ml of 1% w/v acetic acid and was analysed directly by HPLC. The solution was found to contain 1.42 mg/100 ml of an impurity.
What is the level of impurity in % w/w and ppm? (5 marks)
4– Calculate the pH of a buffer system made by dissolving 1.2 g of acetic acid and 0.82 g of sodium acetate in 500 ml of distilled water (pKa of acetic acid = 4.7) (5 marks)
5– Explain how to prepare standard solutions (5 ml each) of NaCl with the following
concentrations: 10, 20, 30, 50, 70 ppm. The stock solution has a concentration of 0.05% w/v. (5 marks)
6– The following tables shows the results of an IQ test for two groups of students
Group A 1 2 3 4 5 6 7 8 9 10 11 12 13
IQ 105 108 103 106 106 114 107 104 114 110 111 112 112
Group B 1 2 3 4 5 6 7 8 9 10 11 12 13
IQ 118 118 109 120 109 112 127 106 110 116 115 106 116
Use T–test to examine if there is a significant difference between the average IQs of the two groups at a:
a– 0.05 level of significance
b– 0.001 level of significance
Show your calculations and explain the meaning of results
(10 marks)
