An acid dye is a dye that is typically applied to a textile at low pH. They are mainly used to dye wool, not cotton fabrics.
Acid dyes are anionic, soluble in water and are essentially applied from acidic bath. They possess acidic groups, such as SO3H and COOH and are applied on wool, silk and nylon when an ionic bond is established between a protonated –NH2 group of the fibre and acid group of the dye. Overall wash colour fastness is poor, although lightfastness is quite good. As dye and fibre contain opposite electrical natures, strike rate and uptake of acid dye on these fibres is faster; electrolyte at higher concentration is added to retard dye uptake and to form levelled shades. Acid generates cations on fibre and temperature helps to substitute the negative part of the acid with anionic dye molecules.[A K Roy Choudhary, “Textile Preparation and Dyeing”, Science Publishers, USA (2006)]
Description
Acid dyes are generally divided into three classes according to their fastness requirements, migration ability, and dyeing pH.
Acid dyes affix to fibers by hydrogen bonding, Van der Waals forces
Animal protein fibers and the synthetic fiber nylon contain many cationic sites that bind anionic dye. The strength (fastness) of this bond reflects the strength of this ionic interaction.
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 fiber fibers.
Histology
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
Cell nucleus and some other acidic components of tissues.
Regarding cellular structures, acid dyes will stain acidophilic structures that have a net positive charge due to the fact that they have a negatively charged
chromophore. Acidophilic 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).
- * 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 (red-pink),
tartrazine (yellow),
sunset yellow, and allura red, to name a few, many of which are
Azo dye.
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.
Structures
The chemistry of acid dyes is complex and diverse. Most acid dyes are related in basic structure to the following:
-
Anthraquinone type: Many acid dyes are synthesized from chemical intermediates that form anthraquinone-like structures as their final state. Many blue dyes have this structure as their basic shape. The structure predominates in the leveling class of acid dye.
-
: The structure of azo dyes contains the azo group (R-N=N−R. Most azo dyes are not acid dyes, but many acid dyes are azo dyes. Many acid dyes of the azo type are red in color.
-
Triarylmethane dye: These predominate in the milling class of dye. There are many yellow and green dyes commercially applied to fibers that are related to triphenylmethane.
Acid Black 1.svg|C.I. Acid Black 1
Acid Yellow 36.svg|C.I. Acid Yellow 36
Acid Blue 117.svg|C.I. Acid Blue 117
Acid Orange 19.svg|C.I. Acid Orange 19
Acid Blue 25 (2).svg|C.I. Acid Blue 25
C.I. Acid Orange 3.svg|C.I. Acid Orange 3
Classes of acid dyes
Acidic dyes can be classified according to their dyeing behavior. This includes their wet fastness, migration ability, and dyeing pH:
-
Leveling acid dyes: These dyes have relatively low molecular weights. Consequently, they migrate more readily before fixation and they exhibit low wet fastness. They are not normally suited for use as apparel fabric. They require an acidic dye bath, often using sulfuric acid and sodium sulfate mixtures (pH2-4),
-
Milling dyes: These dyes are high molecule weight, with the result that migrates slowly. Consequently, they exhibit wet fastness, which is useful for dyeing wool materials. Milling acid dyes are sometimes called 'Neutral acid dyes' as they do not require an acidic dye bath. They are commonly applied using Acetic acid (pH4-7).
-
Metal complex acid dyes: These dyes are composed of acid dye molecules complexed with a metal ion, which will usually be chromium or cobalt. Metal complex acid dyes have high molecular weights, giving them low mobility and high wet fastness. Due to this, they are commonly used on nylon and synthetic polyamide fibers. Metal complex acid dyes are economical. However, they produce relatively dull shades. Metal complex acid dyes take a larger range of pH in the dyebath (pH2-7).
Safety
Some dyes are mutagenic and carcinogenic, including
methyl orange, acid red 26, and
trypan blue.
