In this lab, the measurement wavelength on the colorimeter was set at 470 nm. Th

Question

In this lab, the measurement wavelength on the colorimeter was set at 470 nm. This measurement wavelength was the best choice available on the colorimeter but it was not the best chemical choice for measuring the absorbance of FeSCN2+. a. (5 pts) In terms of instrument sensitivity, explain why the choice of wavelength on the colorimeter is important. b. (5 pts) Since the wavelength setting was not the ideal wavelength (?max) for these absorbance measurements, do you expect the value for your absorbance readings to be too high, too low, or unaffected by the colorimeter setup? Explain. c. (10 pts) What experiment could you do, what data would you collect, and how would you analyze the data to determine the best setting for measuring the absorbance of FeSCN2+? Be sure to fully explain how your experiment and the resulting data would allow you to determine the wavelength of maximum absorbance for the FeSCN2+ complex.

Explanation / Answer

***PLEASE DO NOT COPY WORD FOR WORD***...esp. if you're @CSU FA13 lol

(a) the above 'skinnybrush' sort of left out the answers you are really looking for.

The wavelength choice on the colorimeter is important because this determines the type of light that will be sent through your sample. Your solute will have a specific absorbance that it will be most sensitive to and therefore absorb most of that wavelength of light. So it is important to pick the wavelength that your sample can absorb the most light in. This wavelength will usually be similar to the complementary color of your sample.

For example: in this experiment with FeSCN^2+ the solution was a redish color. The complementary wavelength for reds (higher wavelengths like 650nm+ *[not sure of exact wavelength value for 'red']) would be your blues and almost to UV light (which is at about 400nm). Since the colorimeter has limited selectability for wavelength and so you have to go with the wavelength that is 'beneath' the maximum wavelength sensitivity for your sample. Shown in the answer above we see that FeSCN^2+ has a max wavelength of 450nm. If the colorimeter only has 430nm, 470nm, 565nm and 635nm then you have to go with the 'less intense' or 'under the limit' wavelength, which is 470nm (even though that number is higher; maybe think intensity of light? instead of #values. UV or blacklights have low (~400nm) wavelengths but intense light. Have you ever looked at one?)

(b) The graph above is nice, but it doesn't explain what will happen to the absorbance (A) value. Since the wavelength that is most ideal to use for FeSCN^2+ is 450nm (lower#, higher intensity) and we instead used 470nm, this will give us a light that the sample is less sensitive to. If the sample is less sensitive to the light then it will not absorb the maximum amount of light, so your measurment of (A) will be lower than the accurate (A) measurement. However, doing this consistently gives you precise results (accuracy vs. precision) and hopefully still a nice Beer's law curve.

(c) The graph above comparing wavelengths and absorbance would be similar to a experiment you could run with the appropriate equipment. You would want to start all the way up in the high end of the reds and work your way down and find the maximum (A) or point where (A) starts to decrease again. Then you know you have the best band(s) of light for your sample.

Hope this helps. Sorry it's so long, but I think the extra info will help your understanding instead of just give you the answer.

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