Question

For your indicator, what is the wavelength of maximum absorbance for the pH < 4.00 solution? What is the wavelength of maximum absorbance for the pH > 10.00 solution? What colors correspond to these wavelengths? How do the observed colors of these solutions relate to the colors at their absorbance maxima? This is the procedure: http://www3.chem21labs.com/labfiles/UofC_GL18_Lab.pdf?rf=5204

Answer 1

Bromocresol purple has a change in color from yellow, which is the acid form and purple, which is the basic form:

After you make all of the experiment you should obtain a graph similar to the one given to you in the practice guide. Like this:

To select the wavelength of maximum absorbance for the pH < 4.00 solution pick the line of the graph traced at pH 4. This line has a curvature with a maximum around 450 nm (look more closely in your graph). The color in this case is yellow.

To find the wavelength of maximum absorbance for the pH > 10.00 solution pick now the line traced for the solution at pH 10. The maximum curvature should be around 590 nm. The color for this wavelength is purple.

For the last question: How do the observed colors of these solutions relate to the colors at their absorbance maxima? The colors that we are observing are complementary to the color that we are measuring as the absorption wavelength. This is to say, 590 nm is the wavelength of the red color. Since the red tone is the one absorbed, the color reflected is the one we see, the complement. The same happens with the yellow. Yellow is a complementary color of the wavelength that is being absorbed.