Passage V

A solution containing neutral red (an acid-base indicator) is yellow if the pH is greater than 8.0 or red if the pH is less than 6.8. In solution, a neutral red molecule is either uncharged (NR) or charged (NRH⁺). The color of a neutral red solution depends on the NR and NRH⁺ concentrations (see Figure 1).

A colored component of a solution will strongly absorb visible light over a certain range of wavelengths (measured in nanometers, nm). At any wavelength in that range, the concentration of the component will be directly proportional to the solution's absorbance. Absorbance is measured by using a spectrophotometer. In a spectrophotometer, a certain wavelength of light is directed through a sample of the solution onto a detector that determines what fraction of the light is absorbed by the solution.

Experiment 1

Two 1. 00 L buffer solutions (solutions that maintain a constant pH) that were pH = 3.0 and pH = 9.0, respectively, were prepared. Then, 15.00 mg of neutral red was added to each solution. At pH = 3.0, virtually all neutral red molecules are charged. At pH = 9.0, virtually all neutral red molecules are uncharged. Next, 5.00 mL of the pH = 3.0 solution was added to a cuvette (a transparent container). The cuvette was placed in the spectrophotometer. The absorbance over the visible spectrum was measured. The procedure was repeated with the pH = 9. 0 buffer solution (see Figure 2) .

Experiment 2

The procedure in Experiment 1 was used to study neutral red at 5 other pHs (see Figure 3).

(Note: Without neutral red present, the buffer solutions used in Experiments 1 and 2 do not measurably absorb visible light.)

Figures 2 and 3 adapted from Krystin P. Alter, John L. Molloy, and Emily D. Niemeyer, "Spectrophotometric Determination of the Dissociation Constant of an Acid-Base Indicator Using a Mathematical Deconvolution Technique." ©2005 by Division of Chemical Education, Inc., American Chemical Society.