Studying the Visible Spectrophotometer

Studying the Visible Spectrophotometer A. Data table for the chalk part Color observed Purple (faint) Purple Purple blue Dark blue Aqua Blue Green Green Yellow Green Yellow Light orange (Orange yellow) Orange Red orange Red Deeper red Red (faint) wavelength (nm), 404 418 430 458 472 492 510 558 564 572 598 604 618 700 wavelength (nm), 392 448 458 484 500 523 559 572 582 591 604 630 670 710 B. Absorbance spectra from red, blue, and green solutions WavelengthA (red) A (green) 400 0.152 0.748 425 0.17 0.773 450 0.227 0.627 475 0.402 0.319 500 0.556 0.061 525 0.523 0.052 550 0.296 0.119 575 0.055 0.316 600 0.012 0.552 625 0.008 1.196 650 0.007 0.692 675 0.006 0.095 700 0.008 0.016 725 0.005 0.005 750 0.006 0.005 775 0.006 0.003 800 0.007 0.003 400 420 440 460 480 500 520 540 560 580 600 620 640 660 680 700 720 A(blue) 0.12 0.098 0.056 0.065 0.106 0.142 0.172 0.167 0.2 0.333 0.516 1.003 1.011 0.291 0.046 0.011 0.002 C. Questions 1. Sketch of optical path/ Spec 20 is attached w/ parts labeled. •light bulb, provides a near full spectrum of light •selector, attached to a prism that splits light into its differing wavelength lights; when selector moves, prism pivots so that different color light is directed toward slit/ sample compartment •slit, in wall of sample compartment, is usually blocked unless level in sample compartment is tripped to allow light through. It narrows the beam so that only light of the desired wavelength is allowed to hit the sample •detector, detects the intensity of light on other side of the sample compartment (I) so that % transmittance can be calculated. •sample compartment, is where the spec tube with the colored solution is placed to be analyzed •power switch/ 0 % transmittance adjustment—adjusts the intensity of the incoming light to zero •100% transmittance adjustment, narrows beam 2. The wavelength range that could be detected by Mike and me, respectively, was 404-700 nm and 392-710 nm. The slightly larger range for me may be attributable to a difference in gender or maybe age (not as much eye damage). There were no major variations in the colors observed though Mike has some mild color blindness in not being as sensitive to differences in blue and green. Figure 1. Color Wheel for reference for questions 3-8 3. Red solution. a) The wavelength range that exhibits the greatest absorbance is 480 nm-540 nm, which corresponds to the colors blue green, green, and yellow green. b) The wavelengths that aren’t absorbed as strongly are 580-800 nm, which corresponds to the colors transmitted, predominantly orange and red, and the wavelengths from 380-420 nm, which corresponds to a violet color. Therefore, the integrated color seen is red. 4. Blue solution. a) The wavelength range that exhibits the greatest absorbance is 600-670 nm, which corresponds to the colors orange and a little red. b) The wavelengths that aren’t absorbed as strongly are wavelengths less than 580 nm, which corresponds to the colors transmitted, which are violet, blue, green, and yellow green, yellow, and some orange-yellow. Therefore, the integrated color seen is blue. 5. The wavelengths a yellow solution should absorb should correspond to the complementary color on the color wheel, purple (~420 nm) or should average out to 420 nm (for example, absorbing ultraviolet at 240nm and orange at 600nm). 6. Red solution absorbs the higher energy light because red light absorbs predominantly green region and the blue solution absorbs predominantly in the orange-red region. Green light has a lower wavelength than orange-red light, so it has a higher energy (E = hc/ λ). 7. The perceived color of a solution is typically the complementary color to the color(s) of light absorbed. 8. No, the green solution did not appear as expected. I expected it to absorb strongly only in the complementary color region or red light (600-700 nm). Instead, it absorbed strongly in the 600-670 nm region of orange/ red light and in the 400-480 nm region of violet, blue, and greenish blue light. The green color observed is the complementary color to the integrated colors absorbed. 9. n/a 10. On analog models, it is usually easier to measure % transmittance because it is just the ratio of light intensity detected after light has passed through a solution to the initial or blank light intensity (%T = I/ I0). However, higher end digital spectrometers can actually change the %T to absorbance, A, by doing the calculation A= -log (%T). Absorbance is generally used when making spectroscopic measurements so that the numbers are all within one order of magnitude, and because the absorbance (and not the %T) is linearly and directly related to the concentration (A=EcL). 11. Having only a few available wavelengths of light affects you ability to study colored solutions because, though this is rare, if your solution does not absorb light at the given wavelengths, it cannot be studied spectroscopically. If it does absorb the wavelength, but it is not the optimal wavelength of absorption, λmax, the range of concentrations might be limited (your solution may reach 0 absorbance at low concentrations more quickly than if you were using an optimal wavelength). 12. 430 nm = purple- blue K2CrO4 (yellow) KMnO4 (purple/ pink) 470 nm=aqua/ blue green CoCl2 (pink/ red); 565 nm= yellow CuSO4 (blue) 635 nm = red NiSO4 (blue/ green)