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Absorbance spectroscopy A tutorial By Dr. Lara Baxley Cal Poly, SLO.

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Presentation on theme: "Absorbance spectroscopy A tutorial By Dr. Lara Baxley Cal Poly, SLO."— Presentation transcript:

1 Absorbance spectroscopy A tutorial By Dr. Lara Baxley Cal Poly, SLO


3 Question: What makes a solution appear a certain color? Answer: a solution will appear a certain color if it absorbs the complementary color (the opposite color in the color wheel). 3

4 For example: If a solution appears red, this means that it is primarily absorbing green. White light containing all colors shines on the sample Sample absorbs green, but transmits all other colors Eye sees the remaining combination of colors as red The Color Wheel 4

5 A Spectrophotometer A spectrophotometer measures the amount of light absorbed by a sample. Here’s how it works: 1. A light bulb emits white light 2. Light passes through a slit to form a narrow beam 3. A diffraction grating separates the colors of light 4. Another slit allows just one color to pass 5. Light passes through the sample 6. A detector measures the final amount of light 5



8 max The wavelength of maximum absorbance is called max. For best accuracy, when measuring the absorbance of several solutions, it is best to measure as close to max as possible. 8

9 A Colorimeter The Vernier Colorimeters used at Cuesta College are different from spectrophotometers. 9 A colorimeter uses LED lights that emit specific wavelengths. This means that a colorimeter can only measure absorbance at these specific wavelengths. 430 nm 470 nm 565 nm 635 nm With a colorimeter, the wavelength closest to max is usually selected.

10 Using a Colorimeter A colorimeter contains the LEDs, a sample holder, and a detector. 10 detectorLEDs sample holder The sample goes in the sample holder. Select the best wavelength. For this sample, the best wavelength is 470 nm. 430 nm 470 nm 565 nm 635 nm

11 Using a Colorimeter Question: What would happen if the wrong wavelength were selected? 11 detectorLEDs sample holder If the wrong wavelength is selected there will be little to no absorbance. All four wavelengths could be tested (one at a time) to determine the maximum absorbance. 430 nm 470 nm 565 nm 635 nm


13 Which of these two solutions will have a higher absorbance at max ? Did you answer solution A again? That’s right! The higher the concentration, the greater the absorbance. Concentration and Absorbance 13

14 Beer’s Law The mathematical relationship between concentration and absorbance is called Beer’s Law. It looks like this: A =  bc The parts of the equation are: A = absorbance  = molar absorptivity (constant for a given solute at a given wavelength) b = width of the tube holding the sample (1.00 cm in our lab) c = molar concentration (mol/L) b = 1.00 cm 14

15 Beer’s Law In the above equation, which parts are constant, and which are variable?  and b are constants (under the correct conditions), A and c are variables. This equation is rarely used in this form. Instead, the data is typically graphed and the data fit to a best-fit line. The next few slides will show how this works. A =  bc 15

16 Beer’s Law Imagine that you tested the absorbance of the 5 solutions shown below: A =  bc What trend do you predict for their relative absorbance readings at max ? 16

17 Beer’s Law The actual absorbance readings are shown here: A =  bc Look at this data and look at Beer’s law. What variables would you graph in order to make the data fit a straight line (y = mx + b)? y = mx + b 17

18 Beer’s Law Graph y = m x + b A =  bc Ac If you were to graph this data, what variable would you graph on the y-axis? Absorbance, because it’s the dependent variable (it’s also on the left of the equals sign). What would you graph on the x-axis? Concentration, because it’s the independent variable (it’s also on the right of the equals sign). Let’s see how this works… 18

19 Beer’s Law Graph y = m x + b A =  bc Ac 19


21 Beer’s Law Graph Using the graph, calculate the molar absorptivity of this substance, including units. Hint: Remember that b = 1.00 cm Looking at the equation above, m =  b, therefore, A =  bc y = m x + b  = m b = M cm = M -1 cm -1 21

22 Beer’s Law Graph How do you figure out the units of the slope? Slope = rise/run or  y/  x Therefore, the units of the slope are the (y-axis units)/(x-axis units) It is important to remember that A does not have units. A =  bc y = m x + b 22



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