Artificial Alizarine - the Coloring Matter of Madder.

Scientific American 20, 16.11.1861

The folloring is the substance of a paper from the Comptes-Rentus, which was read by M. Dumas for its author - M. Z. Raussin - at a late meeting of the Academy of Sciences in Paris: -

Binitronaphthaline is a fruitful source of coloring products. The action of sulphides and protosalts of tin, dissolved in caustic potash, cyanide of potassium &c., yields with this substance very rich red violet, and blue derivatives. When the reducing agents are acids, as, for instance, when a mixture of zinc and weak sulphuric acid is employed, or iron filings and ecetic acid, minute grains of tin and hydrochloric acid, &c., the binitronaphthaline undergoes no alteration.

By making concentrated sulpuhic acid react on crystallizes binitronaphthaline, no reaction is produced. When the temperature of the liquid is raised to 250°., binitronaphthaline dissolves completely, as soon as the liquid becomes amber color. Only after long boiling will concentrated sulphuric acid begin to react on this substance. If powdered madder root is treated by concentrated sulphuric acid at 100° C., all its organic materials are carbonized. Only one among them can resist this violet treatment, and that is the coloring matter of the root itself - namely, alizarine. Now, all chemists know that the formula of the latter substance, as well as its principal properties, denotes that probably it belongs to the naphthalic series.

The formula of alizarine is generally represented by C₂₀H₆O
₆; that of binitronaphthaline by C₂₀H₆(NO₄)₂. An opportune reducing agent, which, by carrying off two molecules of oxygen, and making nitrogen pass to the state of ammonia, would probably convert binitronaphthaline into alizarine. Experience has confirmed this idea. By the following process, artificial alizarine may be prepared: -

Introduce a mixture of binitronaphhaline and concentrated sulphuric acid into a large porcelain capsule, heated in a sand bath. When the temperature is raised, binitronaphthaline dissolves completely in sulphuric acid. When the mixture reaches 200°C., throw in minute grains of zinx, and in a few instants sulphurous acid is disengaged. The operation takes about half an hour. Then, if a drop of the acid mixture is made to fall into cold water, it develops a beautiful red-violet color, owing to the formation of alizarine. Sometimes the reaction becomes energetic, if a large mass is operated upon, if there is too much zinc, and if the temperature is not carefully attended to. In such a case the sulphuric acid boils rapidly; abundant white vaports are disengaged with extraordinary noise and violence. It must be added that the latter inconvenience is easily avoided by adding only small wuantities of granulated zinc, and by watching the temperature. When this accident does happen, the proportion of alizarine is greatly diminished, but still a considerable quantity remains in the residue.

The reaction over, dilute the liquid with eight or ten times its volume of water, and then boil it. In a few minutes, throw the liquid on a filter. Cooling causes it to deposit alizarine, in the form of a red jelly, sometimes adhering to the vessels, sometimes suspended in the liquid. In either case, this jelly appears, under the microscope, to be composed of a massa of very distinct, pointed crystals. The mother waters are colored dark red, and hold in solution large quantities of alizarine. They can be immediately used for dyeing after dilution with water and proper saturation. They contain large quantities of sulphate of ammonia. Some undissolved alizarine remains on the filter, which is easily carried off by caustic alkalies or carbonates, and precipitated anew by acids.

In the preceding reaction, zinc can be replaced by various substances; for example, tin, iron, mercury, sulphur, charcoal, &c. - in a word, by all bodies simple or compound, organic or inorganic, which react upon and reduce sulphuric acid at a high temperature.