Experiment 11: Quantitative Determination of Copper (II) Concentration by Spectrophotometry C. Grefaldia1, C. Hernia2 College of Home Economics, University of the Philippines, Diliman, Quezon City 1101 National Institute of Geological Sciences, College of Science, University of the Philippines, Diliman, Quezon City 1101
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Performed 5 July 2017; Submitted 7 July 2017
Answers to Questions
1. What is the significance of the addition of ammonia to Cu (II) solutions? This is performed to forward the reaction between copper and ammonia to form a copper-ammonia blue complex in a reaction as follows: Cu + 4NH3 [Cu(NH3)4]2+ [Cu(NH3)4]2+ complex gives a more intense colour than a standard copper solution which allows the spectrophotometer to examine it more effectively. [1] 2. Why is Beer-Lambert Law expressed in terms of absorbance instead of transmittance? The Beer-Lambert’s Beer-Lambert’s law gives a convenient linear relationship between the absorbed energy with the concentration of the absorbing species which makes the calculation easy. A = abc [2] where a is the absorptivity, b is the path length and c is the concentration of the component. Since the absorptivity of the solution and the path length are kept constant throughout the experiment, we can establish that absorbance is directly proportional to the concentration of the analyte while transmittance is proportional to the intensity of the sunshine that has entered the pattern. A = log 1/T [1]
3. What are the limitations of the
Beer’s Law?
One limitation to Beer’s Law is the assumption that the radiation that reaches the sample is of a single wavelength – that is, that the radiation is purely monochromatic. Beer’s Law is not applicable to polychromatic light since it causes negative deviation to Beer’s Law. Another limitation is that at high concentrations, solute molecules can cause different charge distribution on their neighboring species in the solution. Since UV-visible absorption is an electronic phenomenon, high concentrations would possibly result in a shift in the absorption wavelength of the analyte. At times, even electrolyte concentrations (such as those present in buffers) play an important role in altering the charge distributions and affecting UV-visible absorbance.
Moreover, limitation is due to the presence of stray radiation. The radiation exiting from a monochromator is often contaminated with minute quantities of stray radiation (radiation from the instrument outside the wavelength of band selected). This kind of radiation is usually due to reflection and scattering by the surfaces of lenses, mirrors and windows. [3] 4. Why is it significant to scan over a wavelength range? Why is the analytical wavelength used in the determination of the absorbance of the standard and sample solutions? It is important to scan over a wavelength range in order to maintain precision over measurement readings. Errors in the wavelength scale of a spectrophotometer can have a significant impact on photometric measurements even when wavelength is not called out specifically as an acceptance criterion for the analytical result. 5. Why do we have to measure absorbance reading against reagent blank solutions? This is to ensure that the readings are correct. Spectrophotometer is set at a particular wavelength and this wavelength is where the component being measured absorbs the most. But, sometime the component being measured contains other components beside the component being measured. So, in order to minimize this error in reading, a blank blank solution that doesn’t contain the component being measured is measured and this provides a base reading. Therefore, when the solution being measured is put in the instrument, the reading will be only due to the component. [1] 6. What is the significance of the y-intercept of your calibration curve? Discuss its deviation from the theoretical value. Utilizing the equation y= mx + b, absorbance could be controlled by comparing it to y. this is determined by the spectrophotometer. X is equal to the concentration of the solution, m is equal to the absorptivity and b is the yintercept. In instances where there is no y-int , it signifies that there is no deviation from the theoretical value. In the y-int, there is no concentration of Cu(II) in light of the fact that x = zero. Hypothetically, the absorbance now would be zero on the grounds that there is no coloured solutuon because of the absence of copper which implies that no li ght ought to be absorbed because of the solution being vapid or colourless. The presence of a y-int even in the wake of discrediting the affecting of the cell and the alkali solution on the estimations of absorbance means that something else is retaining light because of the presence of an absorbance value even at zero ppm concentration. For this situation, a y-int would deviate from the theoretical value by expanding the estimations of absorbance. [1]
7. Cite other spectrophotometry.
analytical
applications
of
Spectrophotometry is applied in different fields. In microbiology, it is used in order to quantify the number of microbes in a specimen and detect impurities. For instance, benzene, an impurity found in cyclohexane, can be distinguished at 255nm. [1] The dissociation constants of acids and bases can likewise be determined spectrophotometrically from graphing absorbance and wavelength at various pH values. Drugs can be quantitatively analyzed by making a solution of drug and measuring the absorbance at a particular wavelength. Chemical kinetics can also be examined through spectrophotometry. This is performed by measuring absorbance changes based on UV radiation passing a reaction cell. [4] 8. What are the possible sources if errors and their effect on the calculated parameters? Rationalize. Possible sources of errors for this experiment would be random, systematic and gross. Random errors are often unavoidable. They result from the use of glassware and may influence the precision of the data. Systematic errors could happen from method or instrument. The absorbance value gotten from the spectrophotometer may increase if the reaction cell was held improperly. This is a direct result of the fingerprints might be left in the cell. These fingerprints may absorb light from the spectrophotometer which builds absorbance value. Since a similar cell was utilized for all tests in this trial, it is essential that it is washed with the solution going to be tested thoroughly. This is done to guarantee that the right concentration of the Cu(II) is being analyzed or the absorbance value may increase or diminish relying upon the contaminated concentration. The reagent blank test must be performed keeping in mi nd the end goal to discredit the impacts of the cell and the ammonia solution on absorbance or the absorbance value will increase. Instrumental errors may occur in this experiment from the spectrophotometer. Any stray light in the spectrophotometer will decrease the absorbance value. This is due to the additional presence of unabsorbed light. The cell/cuvette used for analysis must be standardized because a wider cell results in a longer path length. The spectrophotometer should likewise be assessed before use to guarantee it is as yet viable for examination. Gross mistakes can come about because of contaminated glassware particularly amid preparation of the solutions. Undesirable reagents may enter the prepared stock solutions and affect the concentration of copper which will influence the absorbance value. This will influence the development of the calibration curve which will affect the estimations for the concentration of the unknown sample. All concentrations must be uncontaminated before testing to keep such mistakes from happening. References [1] Skoog,, Fundamentals of Analytical Chemistry, 8th ed.; Brooks/Cole-Thomson Learning: California, 2004. Print [2] UPD Chem 26.1 Analytical Chemistry Laboratory Manual, Institute of Chemistry, University of the Philippines, 2017 edition. Print. [3] Tissue, Brian, Beer-Lambert Law, 2000 http://www.tissuegroup.chem.vt.edu/chemed/spec/beerslaw.html Retrieved 6 July 2017
[4] Skoog et al, “Applications of Absorption Spectroscopy.” Pharmatutor.org. n.d. 2010 Retrived 6 July 2017
