Another method for the preparation of alizarin has been patented by Dale and Schorlemmer. It is as follows: 1 part of anthracen is boiled with 4 to 10 parts of strong sulphuric acid, then diluted with water, and the solution neutralised with carbonate of calcium, barium, potassium, or sodium. The resulting sulphates having been removed by nitration or crystallisation, the solution is heated to between 180° and 260° with caustic potash or soda, to which a quantity of potassium nitrate or chlorate has been added, about equal in weight to the anthracen, as long as a blue-violet colour is thereby produced. From this product the alizarin is separated in the usual way by precipitation with an acid. Several other patents have been taken out for the preparation of artificial alizarin.
The specification of Messrs Caro, Graebe, and Liebermann, and dated June 25th, 1869, was the first which was taken out in England. We quote it here because it enters more fully into detail than any of the others.
“Our invention is carried into effect by means of either of the two processes which we will proceed to describe.
“In the one process we proceed as follows—We take about one part by weight of anthraquinone and about three parts by weight of sulphuric acid of about specific gravity of
1·488, and introduce the same into a retort, which may be made of glass, or porcelain, or of any other material not easily acted upon by sulphuric acid, and the contents are then to be heated up to about 260° Centigrade, and the temperature is maintained until the mixture is found no longer to contain any appreciable quantity of unaltered anthraquinone. The completion of this operation may be ascertained or tested by withdrawing a small portion of the product from time to time, and continuing the operation at the high temperature until such product upon being diluted with water is found to form a substantially perfect solution, thereby indicating that the anthraquinone has become either entirely or in greater part converted into the desired product. The products thus obtained are then allowed to cool, and are diluted with water; carbonate of lime is then added in order to neutralise and remove the excess of sulphuric acid contained in the solution; the mixture is then filtered, and to the filtrate carbonate of potash, or carbonate of soda, by preference in solution, is to be added until carbonate of lime is no longer precipitated; the mixture is then filtered, and the clear solution is evaporated to dryness, by which means the potash or soda salts of the sulpho-acids of anthraquinone are obtained, and which are to be treated in the following manner:—We take about one part by weight of this product, and from two to three parts by weight of solid caustic, soda, or potash; water may be added or not, but by preference we add as much water as is necessary to dissolve the alkali after admixture; we heat the whole in a suitable vessel, and the heating operation is continued at a temperature of from about 180° to 260° Centigrade, for about one hour, or until a portion of the mixture is found upon withdrawing and testing it to give a solution in water, which being acidulated with an acid, for example, sulphuric acid, will give a copious precipitate of the colouring matters. The heating operation having been found to have been continued for a sufficient time, the resulting products are then dissolved in water, and we either filter or decant the solution of the same, from which we precipitate the colouring matters or artificial alizarin, by means of a mineral or organic acid, such, for example, as sulphuric or acetic acid. The precipitated colouring matters thus obtained are collected in a filter or otherwise, and after having been washed may be employed for the purpose of dyeing and printing, either in the same way as preparations of madder are now used or otherwise.
“In carrying out our other process we proceed as follows:—We take about one part by weight of anthracene and about four parts by weight of sulphuric acid of specific gravity of about 1·848, and the mixture being contained in a suitable vessel, is heated to a temperature of about 100° Centigrade, and which temperature is to be maintained for the space of about three hours; the temperature is then to be raised to about 150° Centigrade, which temperature is to be maintained for about one hour, or until a small portion of the product when submitted to the two subsequent processes hereinafter described is found to produce the desired colouring matters; we then allow the result obtained by this operation to cool, and dilute it with water, by preference in the proportion of about three times its weight. To the solution thus obtained we add for every part of anthracene by weight which had been employed in the previous operations, from about two to three parts by weight of peroxide of manganese, preferring to employ an excess, and we boil the whole strongly for some time, and in order fully to ensure the desired degree of oxidation the mixture may be subsequently concentrated, and by preference be evaporated to dryness, and the heat be continued until a small portion of the oxidised product, when submitted to the subsequent processes hereinafter described will produce the desired colouring matters. We then neutralise and remove the sulphuric acid contained in this mixture, and at the same time precipitate any oxides of manganese that may be held in solution, by adding an excess of caustic lime, which we use by preference in the form of milk of lime, and we add the same until the mixture has an alkaline reaction. We then filter, and add to the filtrate carbonate of potash or soda, until there is no further precipitation of carbonate of lime. The solution is then filtered and evaporated to dryness, and we thus obtain the potash or soda salts of what we call the sulpho-acids of anthraquinone.
“In effecting the conversion of the oxidised products thus obtained into colouring matters, or into what we call artificial alizarin, we proceed as follows:—We take one part by weight of this product, and from two to three parts by weight of solid caustic soda or potash, and water may be added or not, but by preference we add as much water as may be necessary to dissolve the alkali. After admixture we heat the whole in a suitable vessel, and continue the heating operation at a temperature of about 180° to about 260° Centigrade for about one hour, or until a portion of the mixture is found to give a solution in water, which upon acidulation with an acid, for example, sulphuric acid, is found to give a copious precipitate of the colouring matters. The heating operation having been found to have been continued for a sufficient time, we then dissolve the product in water, and either filter or decant the solution of the same, from which we precipitate the colouring matters or artificial alizarin by means of a mineral or organic acid, such, for example, as sulphuric or acetic acid. The precipitated colouring
matters thus obtained are collected on a filter or otherwise, and after having been washed may be employed for the purpose of dyeing and printing, either in the same way as preparations of madder are now used or otherwise.
“Instead of acting upon anthracen by means of sulphuric acid of the density before mentioned, fuming sulphuric acid may be employed, but we prefer to use the ordinary kind before described.
“In order to effect the process of oxidation, before referred to, other oxidising agents may be used in the place of the oxide of manganese, before mentioned, such, for example, as perioxide of lead, or chromic, nitric, or other acids capable of effecting the desired oxidation may be employed.”