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Description

Acid dyes are generally divided into three classes which depend on fastness requirements, level dyeing properties and economy. The classes overlap and generally depend on type of fiber to be colored as well as the process used.

Acid dyes affix to fibers by hydrogen bonding, Van der Waals forces and ionic bonding. They are normally sold as Sodium salt, therefore they are in solution anionic. The reaction in water is as such:

Na₂D + 2H₂O ⇌ H₂D + 2NaOH

The aniline dye anion bounded to the sodium gets dissolved to form the active amine . This reaction is endothermic and thus, adding heat will push equilibrium to the right, causing the dissolution of the dye. Whilst this experiment shows the dissolution of acid dyes in water, many choose to activate dyes in acid dye-baths instead. According to the Brønsted–Lowry acid–base theory, an acid is a molecule or ion capable of donating a proton, and this is determined by the acid dissociation constant. Compared to most acids, water has a much higher pKa value, meaning that it dissociates to give H⁺ with more difficulty. In this context, if an acid is used instead of water, then the hydrogen ion (H⁺) is more easily able to dissociate in order to react with the aniline dye anion, allowing the dye to dissolve.

Animal protein fibers and synthetic fiber nylon contain many cationic sites. Therefore, this creates attraction of the anionic dye molecule to a cationic site on the fiber. The strength (fastness) of this bond is related to the tendency of the dye to remain dissolved in water over fixation to the fiber.

Uses

Fibers

In the laboratory, home, or art studio, the acid used in the dye-bath is often vinegar (acetic acid) or citric acid. The uptake rate of the dye is controlled with the use of sodium chloride. In textiles, acid dyes are effective on protein fibers, i.e. animal hair fibers like wool, alpaca and mohair. They are also effective on silk. They are effective in dyeing the synthetic fiber nylon, but of minimum interest in dyeing any other synthetic fibers.

Lee's stain used in gallbladder cells.

PTAH stain used in Human squamous epithelial cells.

Medicine

In staining during microscopic examination for diagnosis or research, acid dyes are used to color basic tissue proteins. In contrast, basic dyes are used to stain cell nuclei and some other acidic components of tissues. Regarding cellular structures, acid dyes will stain acidophillic structures that have a net positive charge due to the fact that they have a negatively charged chromophore. Acidophillic structures include the cytoplasm, collagen and mitochondria. The two have an affinity for each other due to the conflicting charges. Examples of acid dyes used in medicine include:

• Lee's stain (stains reddish-pink).
• Phosphotungstic Acid Hematoxylin (PTAH) stain (stains blue).
• Eosin stain (stains pinkish-orange).

Food Industry

Acid dyes can also be used as food colouring, helping to increase the attractiveness of certain foods, and thus becoming more appealing to customers. Some examples include erythrosine, tartrazine, sunset yellow and allura red, to name a few, many of which are azo dyes. These dyes can be used in frosting, cookies, bread, condiments or drinks. In order to prevent health hazards, a dye must be approved for consumption before it can be marked as edible. Some separation methods that can be used to identify unapproved dyes include the solid phase extraction process, the overpressured thin layer chromatography process, and the use of reversed-phase plates.

Classes of acid dyes

There are three classifications of acidic dyes, which are all classified by their dyeing behaviour. This includes their wet fastness, migration ability, and dyeing pH.:

• Leveling acid dyes: These dyes have the lowest molecular weight of all three, and as a result will migrate more rapidly before fixation occurs. Leveling acid dyes have simple and small molecules, and will exhibit low wet fastness due to their high mobility. Thus, they are not normally suited for use as apparel fabric. They require an acidic dyebath to be applied, often using Sulphuric acid and Sodium sulphate mixtures (pH2-4), together with leveling agents such as ethoxylated fatty amines.

• Milling dyes: These dyes have a higher molecule weight than Levelling dyes due to their larger molecules, meaning that they will migrate slower in comparison to Levelling dyes. Their low mobility causes them to exhibit a higher wet fastness, which is useful for dyeing wool materials. Milling acid dyes are sometimes called 'Neutral acid dyes' as they don't require an acidic dyebath, and will commonly be applied using Acetic acid (pH4-7).

• Metal complex acid dyes: These dyes are composed of large acid dye molecules complexed with a metal ion, which will usually be chromium or cobalt. Metal complex acid dyes have the highest molecular weight of all the dyes, giving them low mobility and the highest wet fastness. Once fixed to the fiber, they will not migrate. Due to this, they are commonly used on nylon and synthetic poly-amide fibers. These dyes are very economical, however they will produce duller shades compared to the other classes. Metal complex acid dyes take a larger range of pH in the dyebath (pH2-7).

Health hazards

Some dyes are mutagenic and carcinogenic, meaning that it can cause mutations in the genome or lead to cancer. This can be tested through the Ames test, which determines the carcinogenicity of certain dyes based on their ability to mutate auxotrophs. Some acidic dyes that are carcinogenic include methyl orange, acid red 26, and trypan blue. The exposure to chemicals used in the dye generating process can also result in a series of skin and respiratory diseases for the workers who tend to the process, such as allergies, irritation, and Musculoskeletal disorders.