The use of an azo dye as an indicator - methyl orange

The reaction with naphthalen-2-ol Naphthalen-2-ol is also known as 2-naphthol or beta-naphthol. It contains an -OH group  attached to a naphthalene molecule rather than to a simple benzene ring. Naphthalene  has two benzene rings fused together. The reaction is done under exactly the same conditions as with phenol.  The naphthalen-2-ol is dissolved in sodium hydroxide solution to produce an ion  just like the phenol one. This solution is cooled and mixed with the benzenediazonium  chloride solution. An intense orange-red precipitate is formed - another azo compound. The reaction with phenylamine (aniline) Some liquid phenylamine is added to a cold solution of benzenediazonium chloride,  and the mixture is shaken vigorously. A yellow solid is produced. These strongly coloured azo compounds are frequently used as dyes known as azo dyes.  The one made from phenylamine (aniline) is known as "aniline yellow" (amongst many The use of an azo dye as an indicator - methyl orange Azo compounds contain a highly delocalised system of electrons which takes in both benzene rings and the two nitrogen atoms bridging the rings. The delocalisation can also extend to things attached to the benzene rings as well. If white light falls on one of these molecules, some wavelengths are absorbed by these delocalised electrons. The colour you see is the result of the non-absorbed wavelengths. The groups which contribute to the delocalisation (and so to the absorption of light) are known as a chromophore.
Modifying the groups present in the molecule can have an effect on the light absorbed, and so on the colour you see. You can take advantage of this in indicators. Methyl orange is an azo dye which exists in two forms depending on the pH:
As the hydrogen ion is lost or gained there is a shift in the exact nature of the delocalisation in the molecule, and that causes a shift in the wavelength of light absorbed. Obviously that means that you see a different colour. When you add acid to methyl orange, a hydrogen ion attaches to give the red form. Methyl orange is red in acidic solutions (in fact solutions of pH less than 3.1). If you add an alkali, hydrogen ions are removed and you get the yellow form. Methyl orange is yellow at pH's greater than 4.4. In between, at some point there will be equal amounts of the red and yellow forms and so methyl orange looks orange.