Spectrophotometric Determination of the Concentration of a dye
This lab will demonstrate how the absorbance of a visible wavelength of light can be used to determine the concentration in parts-per-million (ppm) of an unknown solution of dye.
Spectronic 20 spectrophotometer
Set of glass cuvette
Plastic transfer dropper
1. Use the stock solution of dye mixed to 100 ppm to measure and record the absorption at 420 nm wavelength using a calibrated Spectronic 20 Spectrophotometer.
2. Prepare dilutions of the dye starting from 100 ppm at 10 ppm increments. The control is the sample of 0 ppm.
3. Obtain unknown samples (A,B,C) and measure their absorbance.
4. Graph a standard curve using the data and linear trendline from the known concentrations of dye.
5. Determine the unknown samples’ ppm.
Dye dilution data for standard graph – Using Excel complete with all graphing elements.
Absorbance (460 nm)
Standard Graph – Worth 5 points. Must have all elements! ½ point each.
Copy and paste graph here!
Unknown Solutions A, B, C – Draw arrows to find ppm and enter into the table below (3 points)
(derived from Standard Graph)
Discussion: Explain what was done and state your results here. If something looks wrong state what might have caused it. (2 points)
Additional questions – Acts as extra points.
a. Can you use this standard graph to determine the concentration of other dyes?
Explain why or why not in clear sentences that show you know the answer. (2 points)
b. How would you find the optimum wavelength to use on a substance that is mixed in a medium that absorbs/reflects more than one wavelength? (3 points)
For example, a toxic substance in river water.
This is a thinking question and shows you looked at the notes