The Analysis of Commercial Dyes

The determination of the constitution of an unknown commercial dye is one of the most interesting, but also one of the most difficult, tasks of the dye chemist. In the earlier days when only natural products were used, it was a relatively easy task, in many cases, to determine the origin of a product by examining its outward appearance. Cochineal, indigo, alizarin (madder) have such characteristic appearances that even the inexperienced could distinguish them. The only question in­volved was whether the product was suitable for the particular use in dyeing, or whether it had been either damaged or adulterated. The widely used inorganic colorants, such as Prussian blue, chrome oxide, iron oxide, vermilion,, etc., could also be identified by purely qualita­tive investigation. The situation became quite different with the ap­pearance of commercial organic dyes. Although in the early days of the dye industry, it was still relatively easy to identify, for example, fuchsin, aniline blue, methyl violet, or simple azo dyes, the difficulties increased as more new dyes appeared on the market. To be sure, attempts were made to alleviate these difficulties by compiling tables in which all known dyes were described accurately. So many new dyes were evolved as time went on, however, that it was not possible to keep pace with current developments, and the well-known dye tables of Schultz and Green became of less and less service. The question is no longer: “With which of the dyes described in the tables is this product identical?” but: “What is the composition of a dye which has not been mentioned in any scientific publication?”

The once widely used works, therefore, could no longer be used as the foundation for modem dye analysis, and new methods, independent of the older works, had to be worked out.

It would lead too far afield to try to present all the known facts in this book. On the other hand, it is of interest that the beginner should become acquainted, at least in broad outline, with the principles of modem dye analysis. To this end, references to publications in this

field are given at the end of this chapter, and three examples are given to illustrate the procedures used in determining the constitution of an unknown dye. This subject has been dealt with in some detail in an address given by the author.[74]

Inasmuch as there is a lack of the more exact bases analogous to those on which the science of analytical chemistry is built, methods must sometimes be used which are not encountered in scientific re­search. Basically, however, the methods of modem dye analysis involve the same general principles as the classical analytical procedures. First of all, one must rely upon the available literature to gain a clear opinion as to how to proceed. Today, this literature is not contained, or is contained in only exceptional cases, in the scientific publications which are eventually collected in reference books like those of Beilstein and Gmelin-Kraut. There are, however, other sources which are useful in many cases. These sources are published patents, the trade journals of dye chemistry, and technical communications, all of which may give hints about a product in question.

The product often may be mentioned in a patent and frequently the dye package is labelled “Patented.” The same dye is usually “an­nounced” in the trade journals, and it may be assumed that the date of patenting is not far from the date on which the dye was placed on the market. Furthermore, the manufacturer of the dye is almost always known. Thus, it may be inferred that the dye has been patented by the manufacturer, either by means of an application made by him, or by assignment, and that the patent was granted, or at least applied for, before the first sale of the product. This type of investigation, of course, does not involve the methods of exact science, but, as it was once stated, it is more of the nature of detective work.

Still further importance is attached to the fact that a patent exists. Most patent laws require only that the general procedure be illustrated by several characteristic examples, and not that the disclosure specify the exact compound which is to be manufactured subsequently. The Swiss patent law, on the other hand, states expressly that only those products which are accurately described in the patent shall be given patent protection, and, in addition, that each patent can be drawn to only one product. If it is known, therefore, that a dye is patented in Switzerland, it can be assumed that the unknown dye is accurately described in one particular patent. A Swiss patent, therefore, could be of the greatest use in clearing up the constitution of a dye. In many

cases, these methods fail completely, as when no patent has appeared, or when the dye is manufactured by another firm after the expiration of the patent, or when the name of the dye is changed. Frequently in these cases, the structure can be ascertained by comparison of the properties of the dye with those of a known product.

At one time, the first source to be consulted for information on the nature of a dye was the well-known dye tables of Arthur Green. This work is so far out of date today that it is rarely consulted. The simple color reactions are of little use in modem times, and it is necessary, therefore, to evolve new methods which, unfortunately, are frequently time consuming and still lead to no useful end.

The group to which a dye belongs must first be established. This is relatively easily done, since the different dye classes exhibit different reactions. Thus, a vat dye would be sought in that group which gave the correctly colored vat. Indigo and thioindigo dyes give yellow or color­less vats. They dissolve in concentrated sulfuric acid to give, usually, yellowish green solutions, and they are reprecipitated unchanged from the alkaline reduction mixtures by air oxidation. With anthraquinone vat dyes, it is observed that the vats are usually intensely colored, and this difference permits easy differentiation between indigo and anthra­quinone dyes. Further, many anthraquinone vat dyes yield anthracene or anthracene derivatives on distillation with zinc dust.

Many heterocyclic dyes behave analogously to ordinary indigo in that they are decolorized when treated with reducing agents and are regenerated on reoxidization. This applies to the azines, thiazines, ox- azines, and other similar dyes. In contrast to these, the triphenylmethane dyes are decolorized easily by reduction, but their reduction products are usually considerably more difficult to reoxidize by air, i. e., the leuco compounds in this series are relatively stable. However, they can be very easily and quantitatively oxidized by chloranil (see page 146).

It is possible in some cases to obtain analytically pure dyes. This is most successful with the vat dyes, which can frequently be crystal­lized from a high boiling solvent such as chlorobenzene or nitrobenzene, or from glacial acetic acid or pyridine. Tetrabromoindigo (Ciba blue 2B), for example, can easily be obtained analytically pure from dichloro­benzene, as can other vat dyes of the type of indanthrene blue. With these dyes, quantitative chemical analysis is often of great value. Other special methods of analysis can also be employed, such as the Zeisel determination of alkoxyl groups.

Consideration must also be given to the spectroscopic method of Formanek which depends on the determination of the absorption maxi­mum. Formanek showed that many dye groups had characteristic ab­sorption spectra, and his method is often of use. The tables compiled by Formanek, however, are out of date and are helpful only in recogniz­ing the dyes which are listed. They are of as little use as the Schultz and Green tables with respect to dyes which have not been described.

One method which is very useful in the azo dye field is the so-called reductive-splitting reaction. Most azo dyes can be split at the —N—N— group to produce two amines which can be separated and studied. Any nitro groups which are present are simultaneously reduced, of course. The reaction is effected by various reducing agents: hydrosulfite, stan­nous chloride, zinc dust, and many others. No reducing agent can be applied universally/ In certain cases, hydrosulfite may bring about not only fission, but also the introduction of a sulfo group into one of the fission products. Stannous chloride, on the other hand, may cause a rearrangement (benzidine or semidine type) of the first-formed hydra — zo compound. For example, orange I, the dye from diazotized sul — fanilic acid and a-naphthol, on reduction with stannous chloride in hy­drochloric acid solution, does not yield 1,4-aminonaphthol and sulfanilic acid, but instead it forms the semidine by rearrangement and no splitting occurs. In other cases, hydrosulfite carries the reduction only as far as the hydrazo stage and no splitting ocurs. These cases, however, are the exceptions.

Since it is not within the scope of this book to go into great detail, we shall consider only the general principles of azo dye analysis.

It is first established whether the product is a single compound. This is done by dusting a small sample of the dye on moistened filter paper. A mixture can frequently be recognized from spots of different color. (Mixtures of water-insoluble dyes can often be detected by sprinkling a small sample onto’concentrated sulfuric acid, advantageously placed in a depression in a white porcelain plate for the purpose.) Attempts are then made to purify the dye by reprecipitation until a pure material is obtained. Every effort is made to obtain the purest sample possible. This is a general rule for all dye analyses.

Reductive splitting is carried out, after the best conditions for the reaction have been established by experiment, and the resulting solu­tion is investigated. Occasionally, part of the reduction products separ­ate directly from the warm solution in a more or less pure condition. These are filtered off, recrystallized, and, if possible, analyzed quantita-

tively. The reduction product may already be described and the identity can be established by carrying out any necessary reactions. The small book by Brunner is useful in this connection.

Solutions prepared by stannous chloride reduction can advantage­ously be detinned electrolytically. This is an easy process to carry out and it has the advantage over HoS precipitation that no foreign substances are added to the solution.

Mixtures of various dyes, as well as other compounds, can sometimes be separated by the Tswett chromotographic adsorption method. This elegant method is not widely applicable, however, succeeding only with simple dye mixtures.[75]

•If a pure reduction product is obtained, it is analyzed quantitatively and then it is established whether the product found can be related to an example in a patent. A quantitative analysis is not necessary in those cases where the product is a well known compound (aniline, sulfanilic acid, H acid, etc) or where its identity can be established by reference to tables. Thus, 1-amino-y acid and 7-amino-H acid are recog­nized immediately, without further work, on the basis of accurately described color reactions. These color reactions are carried out by placing a dilute solution of the substance being investigated on filter paper and spotting it with various reagents such as metal salt solutions, acids, alkalis, oxidizing agents such as ferric chloride, hydrogen perox­ide, etc. The colorations produced lead, in very many cases, to im­mediate recognition of the fission product, thus establishing part of the structure of the dye. When enough of the fission products have been identified, and the probable structure of the dye is established, perhaps by reference to a patent, the synthesis of the probable structure is undertaken. When the dye has been synthesized successfully, the task is complete.

Three examples will now be given of dyes which are not recognized from their appearance. These examples should show how one proceeds, in any given case, to identify a dye product. The dyes in question are: (1) Polar brilliant red 3B (Geigy). This dye has not been described in a German patent; (2) Benzo light grey BL (By). A patent could be found by searching the literature; and (3) Brilliant sulfo flavine (I. G.). Here, the inventor and the German patent number are identified in a communication which is available to anyone, and an analysis of the dye therefore appears unnecessary.

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