This document discusses the classification and properties of dyes. It outlines seven main classifications of dyes based on their chemical structure: 1) nitro and nitroso dyes, 2) azo dyes, 3) triphenyl methane dyes, 4) phthalein dyes, 5) indigoid and thioindigoid dyes, 6) antraquinone dyes, and 7) miscellaneous dyes. It then provides examples of azo dyes, including methyl orange. Methyl orange is described as an acidic azo dye containing a sulphonic acid group that makes it water soluble and allows it to be used as a dye and pH indicator that is orange in alkaline solutions and red

1. Shri Shivaji Education Society, Amravat
J.D.Patil Sanguldkar Mahavidyalay,
Daryapur
Dr. Aparana. S. Dighade
Department of Chemistry
Topic – Dyes II

2. Classification of dyes
classification of dyes on the basis of structure
Depending upon the type of chromophore present in their structures the dyes are classified
into following main types.
i. Nitro and Nitroso dyes
ii. Azo dyes
iii. Triphenyl methane dyes
iv. Phthalein dyes
v. Indigoid and thioindigoid dyes
vi. Antraquinone dyes
vii. Miscellaneous dyes. (Azine dyes, Sulphur dyes, Xanthene dyes, phthalocyanine
dyes)

3. Nitro and Nitroso dyes
These dyes contains nitro and nitroso groups as the chromophore and OH as a auxochrome.
These are polynitro derivatives of phenols and naphthols in which at least one nitro(NO2) or nitroso(N=O)
group is ortho or para to the –OH group. Naphthol yellow-s is one of the dyes used for colouring edible
materials.

4. Azo dyes
Azo dye is a large class of synthetic organic dye that contains nitrogen as the azo gp –N=N- as
primary chromophore their molecular structure. More than half the commercial dyes belong to this
class.
These dyes contain one or more azo groups as chromophores. The auxochromes present in these
dyes are –OH, -SO3H, -NH2, -NR2 etc. these dyes possess a wide range of colours ranging from
yellow to orange and red. These are prepared In folloeing stages.
Eg. Methyl orange, congo red,para red,bismark brown.
diazotization
Aromatic amine diazo salt
diazotization
diazo salt +(aromatic phenol or amine)
azo dye

5. Methyl orange
It is a azo dye containing one azo(-N=N-) gps. It contains sulphonic acid gp. And hence it is a acidic azo
dye. This SO3H gp makes the dye more soluble and is also used as reactive point for fixing the dye. This SO3H
gp. acts as auxochrome
Methyl orange – dimethyl amino azobenzene sulphonic acid as follow.

7. Properties and uses
 Water soluble because it contains acidic –SO3H group..
 It is not sufficiently fast to light and soap. Hence it is used as a dye.
 It is generally isolated as the sodium salt, which is used as dyeing.
 It imparts orange
 It is used as an indicator in acid alkali titration. It is orange in alkaline
solution an red in acid solution.