Is eosin acidic or basic
Acid dyes, also called anionic dyes, contain hydroxyl, carboxyl and, above all, sulfonic acid groups and therefore react as Brönsted acids. Acid dyes are mostly used in the form of their sodium salts. The group of acid dyes mainly includes azo compounds, anthraquinone and triphenylmethane derivatives, e.g. Suprablau SES,
Basic dyes are molecules that mainly contain NH2- and NO2-Groups that react as a base due to their lone pair of electrons and take up a proton. Basic dyes are also called cationic dyes because they contain positively charged groups after they have taken up a proton. The group of base dyes includes, for example, the triphenylmethane dyes malachite green and fuchsine.
Basics for coloring with acid or base dyes:
With Acid dyes can be dyed in an acidic solution (pH range from pH 2 - 6) wool, silk, polyamides, base modified polyacrylonitriles (Orlon, Dralon), paper and leather.
The mechanism of dyeing is explained using wool as an example. Wool is made up of amino acids that contain carboxyl and amino groups, but which are largely present as ammonium and carboxylate ions. If you put the wool in an acidic solution, the carboxylate ions take up one proton per group. As a result, the wool fiber is positively charged. The positive charge created by the NH3+- Groups, is initially balanced by acid anions. It can therefore be said that the actual dyeing process is based on the fact that the acid anions are displaced by the dye anions that are created by the acidic reaction of the dye, i.e. the release of a proton. The binding of the dye to the fiber is based on the principle of ionic bonding, as there are electrostatic attractive forces between the positive fiber and the negatively charged dye ion (see Figure 1). In addition to such ionic bonds, the dye is bound to the fiber by hydrogen bonds and dipole forces. The colorations are, however, sensitive to alkalis, since then the --- NH3+-Groups lose their proton and the dye can no longer be bound.
Fig. 1: Binding of the acid dye eosin to wool
One of the first azo acidic dyes, developed in 1876 and still in use today, is naphthol orange (Orange II synthesis
Pararosaaniline and pink aniline are called the colorless carbinol or leuco bases of parafuchsin and fuchsin, respectively. From these famous aniline dyes, which were discovered by Hofmann in 1858 and by Verguin in 1859, blue and violet dyes are also derived from the introduction of alkyl or aryl radicals into the amino groups, which played a decisive role in the rapid development of the paint industry in the 19th century.
Malachite green is a trinitrophenylmethane dye that has been known since 1877.
Chemicals: Eosin; N / A2SO4; concentrated NH3-Solution; Wool and cotton rags
Equipment:600 ml beaker, glass rod
execution: An eosin spatula is dissolved in 100 ml of water. Add 1g of Na2SO4 and 1 ml of NH3Solution. A white wool and cotton sample is immersed in this dye solution and boiled for 15 minutes. The samples are washed out under running water.
Observations: Eosin dyes both materials, wool and cotton, constantly bright red.
Chemicals: Vixen; Glacial acetic acid (concentrated acetic acid); Sodium acetate; Wool and cotton rags
equipment: 600 ml beaker, glass rod
execution: 1 spatula fuchsin is added to 100 ml of hot water in a beaker, about 10 ml of glacial acetic acid and a few spatula tips of sodium acetate are added, the material sample is placed in the dye solution and heated over a low flame for almost 10 minutes. The samples are washed out under running water.
Observations: Here both fabric samples are dyed permanently purple.
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