Dye Identification

I was looking for simple methods to identify class of dyes on fabric. Broadly I work with cotton, linen, silk and union fabrics of silk and viscose. The dyes that I am looking for to identify are direct, reactive, acid ( on silk), sulphur, vat, aniline black and napthols.

Here are some methods to test these dyes. I got the idea from this document. However I have not tested them so far.

1. Direct Dyes:

                   This test of direct dye holds valid for both cotton and silks.

A 100-300 mg portion of the dyed sample is placed in a 35 ml test tube, 5-10 ml of water and 0.5 to 1 ml of conc. ammonia are added and the mixture is boiled in order to bleed off a sufficient amount of dye for redyeing a piece of white cotton cloth.

When a sufficient amount of dye has bled from the sample, the sample is removed, a piece of white cotton cloth weighing 10-30 mg is placed in the test tube and 5-30 mg of common salt is added. After boiling gently for 40-80 seconds and cooling to room temperature, the cotton is removed, rinsed and examined.

Redyeing on cotton in an ammonical solution in the presence of salt to shade and strength comparable to the shade and strength of the original sample is a direct evidence of direct dye.

2. Acid Dyes:

                     For wool and Silk

A 100-300 mg portion of the dyed sample is placed in a 35 ml test tube, 5-10 ml of water and 0.5 to 1 ml of conc. ammonia are added and the mixture is boiled in order to bleed off a sufficient amount of dye for redyeing a piece of white cotton cloth.

The above part is same as acid dyes. However, if the sample in the direct dyes bled but left the test cotton white or only slightly stained, the colored extract is neutralized with 1 ml of 10% sulphuric acid solution and a few drops of acid are added in excess. A 20-30 mg of wool is added and the mixture is boiled for 1 to 2 min. The wool is rinsed and examined.

Redyeing of wool from an acid bath indicates the presence of acid dyes provided the presence of direct dyes is not shown.

3. Sulphur Dyes:

                   For Cotton

A 100-300 mg of dyed sample is placed in a 35 ml test tube and to it are added 2 to 3 ml of water, 1 to 2 ml of 10% sodium carbonate solution and 200-400 mg of sodium sulfide chips.

The mixture is raised to a boil and boiled for 1 to 2 minutes. The sample is removed and to the test tube are added 25-30 mg of white cotton and 10-20 mg of common salt. After boiling for 1 to 2 minutes, the cotton sample is removed and placed on filter paper and allowed to reoxidize.

Under these conditions, sulfur dyes redye cotton in a shade which differs from the original only in strength. A few easily reducable vat dyes will color the white cotton but in a shade markedly different from the original dyeing.

4. Vat Dyes:

    For Both Cotton and Protein ( For protein also follow the portion in Red font)

Vat dyes are to be tested after the sulphur dye test has come negative.

A 100-300 mg dyed sample is placed in a 35 ml test tube to which are added 2 to 3 ml of water and 0.5 to 1 ml of 10% caustic soda solution (The mixture is boiled till all the fiber is dissolved). After being brought to boil a 10-20 mg portion of sodium hydrosulphite  is added and boiling continued for another 0.5 to 1 minute. The sample is removed( Not Needed) and 25-50 mg of white cotton cloth and 10-20 mg of salt is added. Boiling is continued for 40-80 seconds, followed by cooling to room temperature. The cotton is removed and placed on filter paper to oxidize (then in a bath containing sodium nitrite and acetic acid).

Redyeing of cotton to a shade differing only in strength from the original dyeing indicates the presence of vat colors.

5. Napthol and Insoluble Azo Dyes:

The most characteristic property of this class is bleeding in pyridine. A 10-50 mg dyed sample is placed in a 10-15 ml test tube, 1-2 ml of pyridine added and the sample boiled. All napthol bleed to certain extent.

6. Reactive Dyes:

Boil in water with a temperature more than 60 deg in Sodium hydrosulphite and caustic soda as in vat dyes. First the color will come out and then that color will decolorize.

Posted in: Wet process