Introduction and Background information
The aims of this experiment were to quantitatively analyse cranberry juice samples using the technique of spectrophotometry and also, to determine the percentage of pure, natural cranberry juice in a selection of cranberry juice drink products. Spectrophotometry is a technique used to measure the level of the chemical constitutes in a solution based on the absorption of light of a certain wavelength (UQ 2017). A spectrophotometer is an instrument that is used to measure the absorbance of light of a sample. Spectrophotometer works by passing a beam of light (photon) of a wavelength through a sample. Certain photons are absorbed by the sample while others pass through the sample. The intensity of transmitted light passing a detector is measured which is known as absorbance (LibreTexts 2019). Spectrophotometry analysis is greatly used in many fields, such as in chemistry, biology, physic, etc. Some experiments include the enzyme-catalysed reactions or the examination of blood and tissue (LibreTexts 2019).
Beer’s Law states that the amount of light of a particular wavelength absorbed by a substance across a constant distance is proportional to the concentration of that substance (UQ 2017). The equation of Beer’s Law is A=εcb. A is absorbance, ε is the molar absorptivity, c is the concentration and b is the path length (UQ 2017). This meant that the more concentrated the sample solution, more light will be absorbed. This relationship between the concentration and absorbance could allow an unknown concentration sample to be measured. The calibration curve is a method that is used to determine the concentration of an unknown sample by comparing to the known concentrations of the standard solutions. In order to calibrate the curve, diluted standard solutions were being prepared based on different amount of concentrations (UQ 2017). Standard solutions were prepared by adding different volumes of the stock solution into a volumetric flask.
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The absorbance values are measured using the spectrophotometer. A calibration curve is drawn by plotting a line graph of absorbance on the y-axis and the concentrations of standard solutions on the x-axis. A best-fit line is then drawn through the points (UQ 2017).
The calibration curve will allow us to find the unknown concentration of a sample by reading the absorbance value of the sample and correspond to the concentration value.
Materials and Methods
Laboratory coat and safety goggles were worn before the experiment starts to ensure safety in the laboratory. In this experiment, the risk of getting injuries was low as the solutions used does not contain any harmful chemicals. However, the equipment used may be fragile and thus, careful handling was required.
The stock solution of cranberry juice is a 20% v/v dilution of filtered, pure cranberry juice. Four calibration standards were prepared using the stock solution. The stock solution will also be analysed.
Preparation of the standard solutions
120 mL of 20% v/v cranberry stock solution was collected in a clean, dry 150 mL beaker. The burette was rinsed and filled up with the 20% v/v cranberry stock solution using a filter funnel. The standard solutions were then prepared by accurately dispensing four different volumes of cranberry stock solution into four volumetric flasks. The standard solutions were added with deionised water to fill up the volumetric flask to the mark on the neck of the flask. The solutions of each flask were mixed thoroughly. For the fifth solution, 20 mL of 20% v/v cranberry stock solution was used and no dilution was required. The data was then recorded as shown in table 1. Each of the solutions’ concentration was calculated using the C1×V1=C2×V2 method.
Results and Discussion
A calibration curve was plotted after the data were recorded. This calibration curve would help to determine the original concentration of the cranberry juice sample. The concentration value taken from the graph represents the diluted sample of the cranberry juice sample. Hence, the formula of C1×V1=C2×V2 was used to determine the original concentration of the cranberry juice sample (Table 6).
Using the calibration curve
6, 23% v/v of cranberry juice sample were calculated. However, the calculated concentration of cranberry juice sample differs from the true concentration of cranberry juice sample which was 22% v/v, as shown in table 7. The percentage error was calculated to be[(22-23)÷22]×100%=4.5%. The difference between the true concentration and calculated concentration could be due to the standard solutions’ concentrations were not well prepared hence they differ from the calculated concentrations. The solutions may not be well prepared due to spillage during the transferring of solutions into the volumetric flask. Another possibility could be due to air bubble that could result in inaccurate volumes of the solution. This would then cause the absorbance value to differ and thus wrongly calibrating the calibration curve. In order to minimise the first error, the burette could be placed directly into the standard flask which could prevent the spillage of the solutions. The second error could be minimised by running the cranberry solution into burette or the pipette to remove the air bubbles or change the equipment used. The true concentration also differs from the published concentration of cranberry juice sample which was 25% v/v (Table 7). The percentage error was calculated to be[(25-22)÷25]×100%=12%. The difference between the true concentration and published concentration could be due to even though the food label claims that the juice contains 25% v/v, the actual concentration may actually be of a lower concentration. This creates a superficial image to consumers that the juice contains a high concentration of cranberry juice. The percentage error of the calculated concentration as compared to the published concentration was also calculated. The percentage error was[(25-23)÷25]×100%=8.0%. One recommendation to this experiment would be having the tutors to do the experiment with the students. This is because the tutors could have used a different batch of solutions from the ones that the students used which could then result in a different concentration. Hence, doing the experiment with the students may ensure a more accurate concentration of the cranberry juice sample.
Calculated concentration of cranberry juice sample/ %v/v True concentration of cranberry juice sample/ %v/v Published concentration of cranberry juice sample/ %v/v 23% 22% 25%
Conclusion
The aims of this experiment were to quantitatively analyse cranberry juice samples using the technique of spectrophotometry and also, to determine the percentage of pure, natural cranberry juice in a selection of cranberry juice drink products. From the experiment, the concentration of the cranberry juice sample was found to be 23% v/v, which differed from the published value which was 25% v/v. The true concentration value of the juice was 22% v/v, which differed from the published value. The difference could be due to even though the food label claimed that the juice contains 25% v/v, the true concentration may actually be of a lower concentration. This creates a superficial image to consumers that the juice contains a high concentration of cranberry juice. Whereas, the difference between the true value and calculated value could be due to that the concentrations of the standard solutions prepared may not be accurately prepared. Hence, causing the absorbance value to differ, thus the calibration curve to be plotted wrongly. This would then affect the calculated concentration of the cranberry juice sample to differ from the true value.
References
- The University of Queensland, 2017, ‘Spectrophotometry’, viewed 23 April 2019, .
- LibreTexts, 2019, ‘Spectrophotometry’, viewed 23 April 2019, .
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