Why do we use a wavelength of 540 nm for KMNO4?

A wavelength of 540 nm is used for potassium permanganate (KMnO4) because it corresponds to the absorption maximum of the permanganate ion (MnO4-) in solution. At this specific wavelength, the absorbance is directly proportional to the concentration of permanganate ions, making it ideal for quantifying the amount of KMnO4 present in a sample.