Recording an Absorption Spectrum with a Spectronic 20

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Summary

An animation of the hand-plotted absorbance spectrum constructed with points collected using a Spectronic 20 spectrophotometer is shown. The 5 thumbnail images summarize the content of the video. Click an image to see the image gallery.

Narration

Take absorbance measurements starting at 400 nm and then at 20 nm increments. When the absorbance begins to increase, reduce the increments between readings to 2 to 5 nm. After the absorbance peaks, once the reading stabilizes, return to 20 nm increments. Draw a smooth curve through the data points. (lambdamax is the wavelength at the top of the curve.)

Discussion

When measuring the absorbances of several samples that have different concentrations, it is important to identify an appropriate wavelength. This wavelength is the one at which the samples absorb the most light and is called lambdamax.

Lambdamax is determined by plotting absorbance versus wavelength and noting where the sample absorbs the most light. Thus, it is necessary to collect absorbance data at different wavelengths. When absorbance versus wavelength is plotted the resulting plot is referred to as an absorption spectrum.

To record an absorption spectrum, set the wavelength on the Spectronic 20 to 400 nm and calibrate the maximum and zero absorbance using the reference solution. Insert the sample solution and record its absorbance at this wavelength.

Change the wavelength by increasing the wavelength by 20 nm (to 420 nm). Record the absorbance. Remember to recalibrate the maximum and zero absorbance each time the wavelength is changed. Continue collecting data for different wavelengths of light. Initially, change the wavelength in 20 nm increments, but when the absorbance begins to rise rapidly decrease the wavelength increment from 20 nm to 2-5 nm. This smaller increment allows a more accurate determination of the wavelength of maximum absorbance (lambda max). After the absorbance has peaked and again begins to decrease, change the wavelength increment back to 20 nm. Continue recording absorbance data until the upper wavelength is reached.

Keywords

Laboratory Equipment / Apparatus | Qualitative Analysis | Quantitative Analysis | Spectroscopy

These ChemEd DL Resource Groups Include This Video

Spectronic 20 Spectrophotometer
Spectroscopy
Measuring


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