Acid dyes are water-soluble dyes employed mostly in the form of sodium salts of the sulfonic or carboxylic acids. They are anionic which attach strongly to cationic groups in the fibre directly. They can be applicable to all kind of natural fibres like wool, cotton and silk as well as to synthetics like polyesters, acrylic and rayon. But they are not substantive to cellulosic fibres. They are also used in paints, inks, plastics and leather. Orange acid azo dyes produce an orange-pink color. They are used in coloring foods and drugs and as intermediates for making photosensive dyes and drugs. They are used as counter stains in histology and cytology and as components of Mallory's acid fuchsin. Methyl Orange (acid magenta) is a various mixture of sulfonated fuchsins. There are four of these compounds which have three sulfonic groups each. It is used in Andrade's indicator and in various complex stains to demonstrate collagen fibres red and in smooth muscle in contrast to collagen. It is used as a pH indicator. The pH range of Methyl Orange id form pH 3.0 pink-red to pH 4.4 yellow. Methyl Orange is used in oil field industry for alkalinity test of mud filtrate.