EXPERIMENT 1 : THE VISIBLE SPECTRA OF SOFT DRINKS
OBJECTIVES: i. ii. iii.
To determine the origin of the colour of a soft drink sample from its absorption spectrum. To determine the wavelength at maximum absorbance (λmax) for a soft drink sample. To determine the unknown concentration of a soft drink from the calibration curve.
INTRODUCTION: The word spectroscopy is used to refer to the broad area of science dealing with the absorption, emission, or scaterring of electromagnetic radiation by molecules, ions, atoms, or nuclei. Different regions of the electromagnetic spectrum such as infrared, visible, ultraviolet, or X-ray radiation can be used to interact with matter. White light is visible light that contains all the colors of the rainbow. If white light hits a colored solution, the electrons in the solution will absorb some of that light, while the rest will be transmitted through the solution. The Spectronic 20 instrument used can correctly be called colorimeter, because it measures the absorption of light in the visible spectrum that we perceive as color, and the technique used is said to be colorimetric. Although the instruments that are used to measure the interaction of various regions of electromagnetic radiation with matter differ a great deal in design and operation, they all contain the same basic components.
APPARATUS: Beaker Pipet Glass rod Volumetric flask, 50 mL Dropper
Retort stand
CHEMICALS: A can of soft drink (blue, red, green, yellow or orange in colour).
PROCEDURE: A. Preparation of ‘Standard’ Solutions of Soft Drink (Known Concentration) 1. Soft drink was poured into a beaker and was stirred to remove the carbonation. 2. 5.00mL of the soft drink was pipette into a 50.0 mL volumetric flask and was diluted to the mark with distilled water. The solution was covered and shook to make a homogenous solution. The solution was stored in a small beaker. 3. Step 2 was repeated using 10 mL, 15 mL, 20 mL and 25 mL of soft drink. B. Operation of the Spectronic 20 and Determination of λmax Instrument : Thermo Spectronic/Genesys 20 Operating instructions 1. The Spectronic 20 was turned on and waited for the instrument to warm up (minimum 15 minutes). 2. The wavelength was set to 600 nm 3. The transmittance (%T) (Adjusting dark current – nothing should be in the sample compartment) was adjusted 0%. 4. A cuvette was obtained. The cuvette may look alike an ordinary test tube, but it is made of special high quality glass and is much more expensive. The tube was cleaned and rinsed it with distilled water, the tube was filled with full of ‘blank’ solution (the blank is distilled water in this experiment). Any solution and fingerprints from the outside of the tube was carefully wiped away using a Kimwipe. 5. 0 absorbance and 100% transmittance was adjusted with the cuvette containing the blank in the sample holder. The cuvette was removed and without emptying the distilled water was set it aside. 6. Another cuvette was cleaned and it was rinsed with a small amount of the standard soft drink whose absorbance is to be measured. Then it was filled with ¾ full with solution, it was wiped with a Kimwipe and it was placed in the sample holder with hash marks aligned. The absorbance was read and recorded. 7. The cuvette was removed, the top was closed and the wavelength was changed to a setting which is 20 nm lower.
8. 0% transmittance was reset if it has changed (sample compartment must be empty). 9. The cuvette of distilled water was inserted and it was reset the 100% T. the cuvette was removed. 10. The cuvette containing the same soft drink solution used in step (6) was inserted. 11. The absorbance was read and recorded the readings in Table 1.2. 12. Steps 8 were repeated through 11 until 360 nm, taking absorbance readings at each 20 nm interval. 13. The absorption spectrum of soft drink was plotted using graph paper and λmax was determined. C. Preparation of ‘Unknown’ Soft Drink Sample 1. Some of the soft drink was poured into a beaker and stirred to remove the carbonation. 2. The soft drink was poured without measuring the volume into a 50 mL volumetric flask and diluted to the mark with distilled water. The flask was stopper and shook to homogenize the solution. The colour of unknown solution prepared was make sured not darker than the most concentrated standard solution. 3. The ‘unknown sample’ was put into a cuvette until it was about ¾ full. D. Quantitative Analysis of The Soft Drink Solution 1. 2. 3. 4.
Spectronic 20 was set to the wavelength maximum (λmax) obtained. 0 and 100% T was set. The absorbance of each 5 ‘standard’ soft drink solution was measured and recorded. The absorbance of the ‘unknown’ soft drink solution was measured and recorded.
E. Cleaning Up 1. The waste can be poured down the drain. 2. All cuvettes was cleaned and dried.
DATA:
CALCULATIONS: i.
Solution1 x 100 = 10%
ii.
Solution 2 x 100 = 20%
iii.
Solution 3 x 100 = 30%
iv.
Solution 4
v.
x 100 = 40% Solution 5 x 100 = 50%
vi.
Concentration of Solution 2,
y = 0.044x – 0.06 0.898 = 0.044x – 0.06 0.898 + 0.06 = 0.044x 0.958 = 0.044x
0.218 mol/L
QUESTIONS: 1. Based on Beer’s Law, when the concentration of an analyte increases, how will the
following be affected (increase, decreased, no change) ? i.
Absorbance: the absorbance will increase with an increase in concentration of analyte.
ii.
Transmittance: the transmittance will decreased with an increase in concentration.
2. Why it is important to first obtain the absorption spectrum of the soft drink before making a calibration curve? It important because we need to plot graph absorbance versus concentration. The plotted graph used to determine the concentration of unknown soft drink solution. We can find the concentration of unknown by determining where its absorbance cross the calibration line in our graph. We also can obtained the concentration in the equation y= mx+c. 3. What is the purpose of using the ‘blank’ solution? A blank solution is a solution that does not contain a detectable amount of the analyte of interest. This solution is typically used to calibrate instrument such as the colorimeter, which is used to measured the absorbance of light, λ by a special solution. Also,blank solution trace sources of artificially introduced contamination. 4. What colour is/are being absorbed in your soft drink? Blue. DISCUSSIONS:
Based on the experiment that had carried out to determine the origin of the colour of a soft drink sample from its absorption spectrum, to determine the wavelength at maximum absorbance (λmax) for a soft drink sample and to determine the unknown concentration of a soft drink from the calibration curve. Spectronic 20 instrument also can called a calorimeter because it measures the absorption of light in the visible spectrum that we perceive as colour and the technique used is said to be calorimetric. Although the instruments that are used to measure the interaction of various regions of electromagnetic radiation with matter differ a great deal in design and operation, they all contain the same basic components. We have been set the wavelength from 600 nm until 380 nm. Then, we got highest absorbance which is 1.249 A at 480 nm. It is also called λmax. From the graph, we calculate the concentration of unknown solution and we got 0.218 mol/L. Some errors can be occurred while preparing the soft drink sample such that the eyes is not parallel to the pipet and the wrong measurement when took the readings. Because of that, we obtained the inaccurate result. CONCLUSIONS: The origin of the colour of a soft drink sample from its absorption spectrum is blue. The wavelength at maximum absorbance (λmax) for a soft drink sample 480 nm. The unknown concentration of a soft drink from the calibration curve is 0.218 mol/L. REFERENCES: 1. Basic Instrumental Analysis Laboratory Experiments for An Introductory Course In Instrumental Analysis. 2. http://en.wikipedia.org/wiki/Blank_%28solution%29
