shade should be classified, whether, for instance, a reddish-blue is to be regarded as a blue or a violet. In such cases the tables referring to both shades should be employed; since the same general scheme is adopted throughout the tables, this can scarcely give rise to confusion. The same remark applies to possible mixtures; thus in examining a certain green shade, both the yellow and the blue tables may require to be used. To distinguish between individual dyestuffs of the same group, the reactions towards concentrated sulphuric acid, caustic soda, etc., may be employed, as published in the tables of Lange, Gnehm, Herrmann and others. For additional confirmation it is well to compare the sample both as to shade and reactions with a dyed pattern of the coloring matter or coloring matters to which it is believed to correspond. It is obvious that in many instances the analytical procedure may be greatly simplified by exclusion of dyestuffs, the presence of which is rendered improbable or impossible by the special circumstances of the case, e.g., by the material, shade, or mode of application of the color. STRIPPING TEST FOR ACID COLORS. A few salt dyestuffs are partially stripped by weak ammonia, and may thus give rise to the impression that they are acid colors. To avoid this error it is advisable to add a small piece of white cotton when carrying out the test. If the dyestuff is an acid one, the cotton is either not tinted, or becomes white on boiling a second time with weak ammonia. TRANSFERENCE OF BASIC COLOR TO WOOL. The tannin mordant is first removed, as in testing for a basic color, by boiling the pattern for half a minute with saline caustic soda. It is then well washed to remove all alkali, and is boiled with a piece of white wool (half the size of the cotton or less) in a little plain water for one or two minutes. In most cases the dye base will leave the cotton almost entirely, and dye the wool a full shade. If the color does not develop on the wool one or two drops of weak formic acid (1: 100) may be added. In the case of a few dyestuffs which are more difficult to strip (e.g., basic greys) it is necessary to extract the color with dilute hydrochloric acid (1:20), carefully neutralizing the extract with ammonia before adding the wool. TRANSFERENCE OF ACID COLORS TO WOOL. The cotton is simply boiled with a small piece of wool and weak formic acid (1: 100). TANNIN TEST FOR BASIC COLORS. Add a few drops of Tannin solution to the formic acid extract. Shake well, and if the precipitate does not form at once allow to stand a few minutes. Some coloring matters, such as the Rhodamines, Gallocyanines and Chrome colors of the Rosaniline series (which contain carboxyl or hydroxyl groups in addition to basic groups) only precipitate slowly, while the precipitate being more finely divided is sometimes difficult to see. BLEEDING TEST FOR SALT DYESTUFFS. In testing for salt dyestuffs by the bleeding test, the sample is placed in a test tube together with a smaller piece of white mercerized cotton cloth, and boiled with soap solution for about a minute. The soap solution may also be replaced by a 5 per cent. solution of sodium carbonate. LEAD ACETATE TEST FOR SULPHIDE COLORS. The sample is just covered with acid stannous chloride solution. The mouth of the test tube is closed by a cap of filter paper closely wrapped round it, into the centre of which is placed by means of a glass rod one drop of lead acetate solution. The contents of the test tube are slowly heated to the. boiling point, when a blackish-brown stain of lead sulphide appears if a sulphide color is present. The brown spot will again disappear on boiling the solution longer, owing to the decomposition of the lead sulphide by the hydrochloric acid evolved. In order to avoid the possibility of error through extraneous sulphur present on the cotton, the pattern may be previously boiled with 10 per cent. caustic soda. It must, however, be borne in mind that the indications of the sulphide dyestuffs are rendered less sharp by this latter treatment. It is important to pay special attention to the cleanliness of the test tubes employed for this test, as the author has found that tubes which have been previously used for hydrosulphite reductions acquire a thin deposit of sulphur upon their walls, which on boiling with stannous chloride gives rise to hydric sulphide and thus may lead to error. REDUCTION AND REOXIDATION TESTS. The reduction with Hydrosulphite X is carried out by boiling the sample with the reagent for from half to two minutes. The azines, thiazines, oxazines, etc., and most of the azo dyestuffs are fully reduced in about half a minute, but the insoluble azo colors and some salt dyestuffs require from one to two minutes to complete their reduction. In testing the reoxidizability by air, the reduced sample should be exposed to the fumes from an ammonia bottle, which in many cases accelerates oxidation. EXAMINATION OF YELLOW OR ORANGE SHADES. TABLE I. Boil with weak ammonia (1:100). A. Much color is stripped. Boil with acidified water and small piece of white wool. X. Color is not transferred to the wool. Sn is present in ash. Persian Berries on tin mordant. Y. Color is transferred to the wool. Acid dyestuff. Not decolorized. Pyrone or Quinoline group: Quino- Permanently decolorized. Azo group. Indian Yellow, B. Color is not stripped. Boil half minute with saline caustic soda, rinse and boil twice with weak formic acid. X. The color is completely destroyed, both alkaline and acid solutions and the fibres being colorless. Treat original fibre with cold ammonium sulphide. Fibre blackened. Cr present in ash. Chrome Yellow or Chrome Orange. Fibre not blackened. Boil with hydrosulphite X. Decolorized, Flavinduline. Test for A1 in ash. Al present, Alizarine Yellow A. Al absent, Thioflavine T. Y. Color is completely or largely stripped, giving a colored acid extract, which is precipitated with Tannin solution: Basic dyestuff. Transfer to wool and boil with hydrosulphite B. Wool is permanently decolorized: Azo group. Chrysoidine, Tannin Orange, Janus Yellow, etc. Wool is not decolorized. Boil cotton with hydrochloric acid (1:20). 4 Not decolorized. The conc. H2SO, and alcoholic solutions are fluorescent: Acridine group. Phosphine, Benzoflavin, Acridine Yellow, Acridine Orange, etc. Z. The color is stripped or the acid extract is not precipitated by tannin solution. Reduce with hydrosulphite X. Decolorized and color not restored by air or per- Boil with soap solution and white mercerized cotton. Test ash for Cr and Cu. No Cr or Cu present: Salt Azo dyestuff untreated. or chromed. White cotton is not stained. Boil with pyridine. The color is stripped: Insoluble colors Metanitraniline Orange or Nitrotoluidine Orange (formed on fibre). Color is not stripped. Cr present in ash: Azo Apply lead acetate test. H2S is evolved. After reduction with Hydrosulphite Boil with soap solution and white mercerized cotton. |