A solution of the pH indicator bromothymol blue (BTB) changes color according to pH. In this interactive, changing the pH changes the color of the BTB solution. For each wavelength of visible light, its absorbance is given to display the full spectrum of BTB. Instructions: Top slider: Drag the slider to the right to increase the wavelength (in nanometers) and see how the BTB absorbance values change. Bottom slider: Drag the slider to the right to increase the pH of the BTB solution and see how the BTB absorbance values change. The Table displays the absorbance values for the selected wavelength for all pH values.