UV-VS Spectroscopy - Lab Report Example

UV-VS spectroscopy is a concept referring to absorption spectroscopy, usually through the help of light from the visible and adjacent ranges. A qualitative analysis on the performance characteristics of a UV/VIS absorption spectrometer is important in accessing how this apparatus helps in determining the concentration of unknown solutions. In this experiment, the beer lambert law was considered to help in determining the concentration of the unknown solutions. Introduction UV-VS spectroscopy is a concept referring to absorption spectroscopy, usually through the help of light from the visible and adjacent ranges. In this experiment, there was need to introduce the students to how the UV/VIS applies in the laboratory. It involved studying how the adjacent and visible lights can help in determining the concentration of given ions in solutions. A qualitative analysis on the performance characteristics of a UV/VIS absorption spectrometer was also an important objective of this experiment. The analysis would help in providing more information about the mechanism of the concept in determining the concentration of ions. The third objective of the experiment was to apply the significant characteristics of UV/VIS absorption spectroscopy in real scenario. This involved determining concentrations of the CO2+ and Cr3- in a sample of solution through the help of this concept. In order to achieve the above objectives, there was need to apply in Perkin Elmer Lambda 35 UV/VIS spectrometer in combination with other apparatus to collect data. Perfoming a qualitative analysis involved acquiring and analyzing spectra of various substances under different conditions. Different instrumental parameters were also used as a modification for the experiment of measuring spectra of different optical materials. In the second part involving simul analysis of a two-component mixture using the spectroscopy method, two light-absorbing ions (Cr2+ and Cr3+) were used. A solution containing both of the ions was presented for analysis. The preference of these ions resulted because of they cannot interfere with the light-absorbing properties of each other. This means that they allow for the summation of their absorbance properties. This experiment worked under the principle: the total absorbance at any given wavelength is equal to the sum of the absorbance that the two components would show if measured individually under the same conditions. Further, the beer lambert law was also considered to help in determining the concentration of the unknown solutions. in this experiment. A = ebc (where e is a constant of proportionality, called the absorbtivity, b is path length, c is the concentration, and A the absorbance) Experiment Preparation In part one of the experiment, solutions of benzene, iodine, hexane, ethanol and deonized were presented, besides the UV/VIS spectrometer. In part 2, 5 solutions of CoCl2 of 0.20 M were prepare. Solutions of CrCL3 of 0.050 M concentration were also prepared. In order to ensure accuracy of the concentration used, analytical balances were used to measure the amounts of stock solutions in each volumetric flask before diluting them with water. Another inherent part in obtaining the correct concentration was the assumption that the density for each solution was 1.00 g/Ml. Apparatus used Perkin Elmer Lambda 35 UV/VIS Spectrometer Quartz, glass, and plastic 1-cm cuvettes Chemicals used Stock solution of CoCl2 (0.20 M), Stock solution of CrCl3 (0.050 M), Deionized water and an “Unknown” solution containing Co2+ and Cr3+ ions in unknown concentrations, Benzene, Iodine, Hexane, Ethanol. Procedure In part 1, the cover of the Ultraviolet / Visible (UV/VIS) spectrometer was opened through the assistance of the instructor to help in examining its design. The main components including the lights sources, choppers and mirrors were identified. The optical diagram of the apparatus was then drawn, with proper labeling of the relative positions for the main components. The cover of the apparatus was closed and then turned on followed by opening The Lambda 35 program. Instrumental parameters were set up to allow for scanning of a spectrum. A spectrum of a 1-cm quarts cuvette was recorded while holding it with an empty cuvette holder. The measurement was then repeated for a glass and a plastic cuvette. In 350 – 800 nm wavelength range, a spectrum of standard 1-cm plastic cuvette was recorded with an empty cuvette actin as a reference. Spectrum of benzene vapor was also measured but under different slit width. Hexane, enough to dissolve the benzene drop, was added in the cuvette followed by recording the spectrum of the solution under wavelength of 190 – 300 nm. This experiment was repeated for iodine in hexane and iodine in ethanol and the spectrums recorded. In part 2, the spectra of CoCl2 and CrCl3 solutions were measured under 425 – 625 nm range and the slit width of 1.0 nm. Two wavelengths, at which the absorptivities of the two ions differ most, were selected. In this process, the fixed wavelength mode of operation in the program was used. The absorbances of the two solutions, with different concentration, were measured and recorded. Further, the absorbances the solution containing both ions in unknown concentration was measured and recorded. The spectrum scanning mode was set back to help in determining the spectrum of the unkown solution. Results The following are the tables showing results obtained from the experiment COCL2 Mass of cocl2 (g) Actual M .20 46.4417 .1856 35.9144 .1437 23.3343 .0933 11.608 .04643 Crcl3 Mass G Actual M 0.05 22.6704 .04534 17.837 .03567 4.0253 .01919 6.4226 .01284 Co 2+ Actual M A (510) A (600) .20 1.0387 .1357 .1856 .9554 .1301 .1437 .7169 .0973 .0933 .9893 .0782 .04643 .2328 .0295 Cr 3+ Actual M A(510) A(600) 0.05 .2086 .7357 .04534 .1853 .6650 .03567 .1596 .5247 .01919 .0775 .2687 .01284 .0529 .1808 Unknown solution concentration (m) A(510) A(600) spectra .6775 .3123 From the table of CO 2+ Cr3+, the relationship of concentration and absorbance at 510 and 600 was determined as follows in the graph: Graph for known CO 2+ In this equation y = 5.1816x – 0.0067, the slope is 5.1816 Slope is the molar absorptivity Hence molar absorptivity = 5.1816 Graph for unknown CO 2+ Slope = molar absorptivity = 0.6655 Graph for known CO3+ Slope = molar absorptivity =4.2955 Graph for known CO3+ Slope = molar absorptivity = 15.35 The relationship between the two, absorbance and concentration, gives a straight line from the origin. The relationship is the same when using absorbance at 510 and at 600. The same, straight line from the origin phenomena happens when using Cr3+. The following is the calculation of the ions in the unkown solutions. The absorbance of the solution for both ions is equal hence: 15.34x – 0.0194 = 0.6655x + 0.0051 15.35x – 0.6655x = 0.0051 + 0.0194 14.68x = 0.0245 A = 0.00167 Hence the concentration of the ions in unknown solution Cr3+ A = ebc Where eb = 15.35 C = A/eb 0.00167/15.35 1.088 x 10-4 Co2+ 0.00167/0.6655 2.5 x 10-3 Discussion As shown in the relationship between the two, absorbance, and concentration, there is a linear relationship between the two. This case happens for both Co2+ and Cr3+. These cases obey the beer lambert law, which asserts that the absorbance of a particular solution is directly proportional to the path length and the concentration of the absorbing solution. Even though for the calibration curve is straight from the origin, there are some outliers. This may have happened because of possible mistakes encountered when taking readings from the analytical balance. Further, reading from the volumetric flask may have also resulted to the slight error. Rounding up and truncating the readings may have posed as a source error while doing the calculations. The external interference, such as temperature and moisture in the atmosphere, on the readings may have also acted as a source of error. Moisture settling on the solutions, would have acted in slightly increasing the expected weight The huge differences between the predicted and the recorded values for the spectra in known and unknown solution, shows that there was third light-absorbing component. The unknown solutions may have exhibited these properties because of the external interference, which usually affects the absorbance activity. An effective way of reducing the percentage error exhibited in the is to perform multiple trials. Multiple trials are an efficient way of improving the accuracy of the calculated average. The more trials help in offsetting any mistake, which may arise in the case of carrying out a single experiment. Read More