Spectrophotometry

Spectrophotometry is a vary specific type of Spectroscopy which measures how much light is absorbed by measuring the intensity of the light beam that is not absorbed (transmittance).The word "Spectra" means the range of wavelength, "Photo" means light or photons and "Metry" is the measuring how much light a chemical substance absorbs which it calls the absorbance.

But what we do is measure how much light of the original light beam gets through (transmittance). So, those are related to each other absorbance and percent transmittance mathematically.The basic way to works is the incident light which usually contain different kind of wavelength, for example when we see something have a red color that means the object is absorbs all colors' wavelength except red.

It is helpful to know the color wheel because the color wheel will help you to understand or getting the idea of where in the visible spectrum you would except to see the best absorbance. Firstly, we have a light source typically white light contains all wavelengths. We want to collimate the light or make all the wavelengths parallel to one another so, the special collimator or lens can does that, then we pass the light beam through a prism to splits the light into its various wavelength so, for regular white light you get all the colors of the rainbow.

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Spectrophotometer does not just shine all that light at the sample, it shines a vary specific wavelength of light and we can choose that normally by moving a slit in the way of the one wavelength of light or color that we would like to shine through the sample. That particular color of light will then shine through the sample, some of it will be absorbed and some of it will be transmitted. (Io) is the incident light that is the first enters, and (It) is the amount of light that is transmitted through after some has been absorbed.

The remaining light that gets through hits a photocell, photocell is a solid-state detector that picks up how much light, then it prints out on a digital display either absorbance how much was taken away or percent transmittance how much light go through and those two are related. Briefly, you can determine the unknown concentration of the sample by using Beer-Lambert Law which states: there is a linear relationship between the absorbance and the concentration of a sample.

Catch the outflow of solution from the capillary in a beaker until the flow becomes stable. Then quickly switch the outlet tube from the beaker to a volumetric flask and simultaneously start a timer. When It is full, stop the timer and record the time. Return the outlet tube to the beaker. Then carrying out the above flow rate measurement, you should determine the absorbance A of the reaction mixture and record that value together with the distance x from the mixing chamber.

Work quickly to avoid any interference of the reagent solution.3- When both the flow and absorbance measurements are complete, close the outlet stopcock C and then close both stopcock A and B. This is a crucial step in the procedure. If A and B are left open, solution may siphon from one carboy to the other. After a few minutes, determine the absorbance again to obtain the infinite time value. Verify that this value does not change after one more minute.

For the next run, move the capillary support frame so as to line up the second fiducial mark and repeat the first and third steps at this this new distance setting, be careful in moving the capillary support frame. Make two runs at each of the six or seven positions along the capillary tube. Use special care in making the absorbance readings at large values of x.

If time permits, you should also take data at a different driving pressure. Either increase or decrease the gas pressure depending on weather you need more data at low percent reaction or at high, but it may not be safe to exceed about 700 torr overpressures.In this experiment, more of solution A will be used up than solution B if the Fe+3 solution is always used in the first step to make the zero adjustment of the spectrophotometer at each distance setting.

The resulting change in the liquid level for A relative to that for solution B may change the relative flow rates of these solutions. This can be avoided by alternating the use of solution A and B for making the zero adjustments.

References

1. Physical chemistry by Gilbert William Castellan.
2. msu.edu.
3. Wiley online library.
4. UKessay.
5. AliHayek.com

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