This is known as a beer's law plot, the expression for which is a = e*c. a is the absorbance, c is the concentration, and e* is the effective molar absorptivity which is the slope of a beer's law plot

This is known as a beer's law plot, the expression for which is a = e*c. a is the absorbance, c is the concentration, and e* is the effective molar absorptivity which is the slope of a beer's law plot. the slope must be accurately determined because it will enable you to solve for the concentration of your bromothymol blue solution of unknown concentration.