Applied chemistry PPT of Dyes and Pigments for the B.Sc. Students

1. Dyes and Pigments
Dr. Bapu R. Thorat
Government of Maharashtra’s

2. Dyes:
 Introduction
 Qualities of good dye
 colour constituents (chromophore, auxochrome)
 Classification of dyes according to their application
 Synthesis and uses of following dyes
 Nitroso dye - martius yellow
 Azo dye – methyl orange and aniline yellow
 Triphenylmethane dye – crystal violet
 Phthalein dye – phenolphthalein
 Xanthane – fluorescein, anthraquinone alizarin
 Indigo dyes – indigo
Pigments:
 Introduction
 Classification and general properties of pigments
 Inorganic pigments
 Zinc oxide pigments (fundamentals and properties, raw materials, preparation
by direct process (American process), precipitation process),
 Iron oxide (fundamentals and properties, preparation by precipitation process)

3. Dyes
Dyes are the organic compounds that are used to impart colour to textiles, foodstuffs, silk,
wool and other objects.
Dyes are capable of getting fixed to the fabrics/objects permanently and are resistant
to the action of water, soap, light, organic solvents acid, and alkalies.
It must have a suitable colour.

4. Qualities of good dye
 Dye must have suitable Colour
 Solubility in water and/or an organic solvent.
 It must be fixed on the fabric material - absorbed and retained by fibber (substantivity)
or to be chemically combined with it (reactivity).
 Ability to withstand washing (soap, detergents, etc.), washing with chemical solvent
during dry cleaning and exposure to light.

5. Colour constituents (chromophore, auxochrome)
Dye absorbs light in the visible region (400
nm to 750 nm) corresponding to green
colour, then it will appear violet, which is the
complementary colour of green.
Similarly, if a dye absorbs blue colour, it will
appear yellow which is the complementary
colour of blue. Thus, the dyes impart colour
to fabric by absorbing the complementary
colour.
Colour Wavelength
Range (nm)
Frequency
Range (THz)
Red 700-635 430-480
Orange 635-590 480-510
Yellow 590-560 510-540
Green 560-490 540-610
Blue 490-450 610-670
Violet 450-400 670-750

6. Colour constituents (chromophore, auxochrome)
The colour of a compound is due to the
presence of certain groups containing multiple
bonds. These groups which impart colour to a
compound are called chromophores. Some
examples of chromophores are:
– NO2 (Nitro), –N=O (nitroso), –N=N–
(azo), quinonoid structures, etc.
At the same time, there are certain groups which they are not
chromophores themselves but they deepen the colour when
present with coloured compounds. The groups which deepen
the colour of a coloured compound are called auxochromes.
Some examples of common auxochromes are:
–OH, –NH2 , –NHR, –NR2 ,
–Cl, –CO2H, etc.
The combined effect of the two substituents is much more dramatic, especially if the OH group
is converted to the corresponding anion, 4-nitrobenzenolate. Now, λmax is shifted into the visible
region, giving a yellow color, and because ϵ is large, the color is intense.

7. Colour constituents (chromophore, auxochrome)

8. Colour constituents (chromophore, auxochrome)

10. Classification of Dyes

11. Classification of Dyes

13. Nitro dyes
The are polynitro derivatives of phenols and naphthols in which at least one nitro (-NO2)
group or nitroso (-N=O) group is ortho or para to OH/amino group. The colours of
these dyes are not fast and hence they are industrially less important. Example: Naphthol
yellow-S is one of the dyes used for colouring edible materials.

14. These dyes from the largest chemical class of dyestuff. These dyes number in thousands.
They consist of a diazotized amine coupled to an amine or a phenol. They have one or
more azo linkages (-N=N-).
Based on the number of azo groups, we can classify azo dyes as – Monoazo dyes (aniline
yellow, basic orange 2, acid orange 7, etc.), diazo dyes (acid red 73), polyazo dyes (direct
blue 110)

15. Phthalein dyes
Phthalein dyes are a class of dyes mainly used as pH indicators, due to their ability to
change colors depending on pH. They are formed by the reaction of phthalic anhydride
with various phenols. They are a subclass of triarylmethane dyes.
Common phthalein dyes include: o-Cresolphthalein, Dixylenolphthalein, Guaiacolphthalein,
α-Naphtholphthalein, Phenolphthalein, Tetrabromophenolphthalein, Thymolphthalein,
Xylenolphthalein

16. Triphenyl methane
Triphenylmethane dyes have triphenylmethyl group. They are basic dyes and are suitable for
wool or silk or cotton.
Basic green 4 (Malachite green) is a typical example and is prepared by condensing
benzaldehyde with N,N-dimethylaniline and oxidising the intermediate leuco base

17. Anthraquinone Dyes
In anthraquinone dyes the chromophore is the carbonyl group, which may be present once or
several times. These dyes are generally vat dyes as exemplified by Mordant red 11
(alizarin). More complex examples are compounds prepared by oxidising anthraquinone
derivatives under basic conditions

18. Xanthene Dyes:
The chromophore of the
aminoxanthene dyes is the
resonance stabilised structure,
where R=H, alkyl or aryl.
The hydroxy xanthenes can be
stabilised by the loss of a proton,
forming an uncharged system in
which the chromophore is a quinoid
structure.
Examples: Acid yellow-73
(Fluorescence), Acid red-87
(Eosin), and Basic violet-10
Phthalocyanines:
The phthalocyanines constitute an
important class of synthetic pigments
and dyes. An important member of
this class is Pigment blue 15 (Copper
phthalocyanine), a brilliant blue
pigment that can be prepared by
heating phthalonitrile with copper
Indigoid Dyes:
Indigoid dyes are also vat
dyes, as represented by
indigo itself. Dibromo
indigo is an example of
this class. In earlier times,
it was used only by rich
people

19. Classification of Dyes
on the basis of their
applications
(viii) Mordant
dyes
(i) Acid dyes
(ii) Basic dyes
(iii) Direct dyes
(iv) Disperse
dyes
(v) Fibre reactive
dyes
(vi) Vat dyes
(vii) Insoluble
azo dyes

20. Classification
of Dyes on the
basis of their
applications
Reactive dyes: These
dyes attach to the fibre
themselves by
irreversible chemical
reactions. These dyes
induce fast colour on
the fibres which is
retained for a longer
time
Disperse Dyes:
These dyes are usually
applied in the form of a
dispersion of finely
divided dye. The dyes
are disperesed in a
soap solution in the
presence of phenol,
cresol or benzoic acid
Ingrain dyes:
Over 60% of the dyes
used are azodyes or
ingrain dyes; fabric to be
dyed is soaked in an
alkaline solution of
phenol or naphthol and is
then treated with a
solution of diazotised
amine
Direct Dyes:
As the name suggest
these dyes can be
directly applied to the
fabrics from their
aqueous solution;
Basic Dyes:
These dyes contain
basic groups like (-NH2)
group or (-NR2) group
therefore these are
called basic dyes
Acid Dyes:
These are azodyes used
in the form of their
sodium salt of
sulphonic acid (–SO3H),
carboxylic acid (–COOH)
or phenol
Applied to wool,
silk and nylon; not
have much affinity
for cotton
dyes attack the
anionic sites present
on the fabrics
Dye modified
nylons, polyester,
wool, cotton,
leather, paper,
etc.
Aniline yellow,
malachite green
and crystal violet
direct dyes attach to
the fibre by means of
H- bonding
dying cotton, wool
and rayon. Martius
yellow and Congo red
used for cotton, silk,
polyster and nylon
colour is not very fast
because the interaction
is only on the surface
used for nylon, polyster
and polyacrylonitrile
celliton fast pink
B and celliton fast
blue B
used to dye fibres like cotton, wool or silk
derivatives of 2,4-dichloro-1,3,5-triazine

21. Acid Dyes:
These are azodyes used
in the form of their
sodium salt of
sulphonic acid (–SO3H),
carboxylic acid (–
COOH) or phenol
Vat dyes:
Vat dyes are the well-known dyes
they are insoluble in water and hence
cannot be used directly for dying.
They are reduced to a colourless
soluble form (leuco) in large wooden
vats with a reducing agent such as an
alkaline solution of sodium
hydrogensulphite. Under these
conditions, the leuco form develops
affinity for the cellulose fibre. Then
the fabric is exposed to air which
oxidises the leuco (colourless) form
to coloured form.
Used to dye cotton fibres. Indigo is
an important example of this type
Mordant Dyes:
These dyes require an additional
substance (generally a metal ion)
for fixing to the fibre. These are
used mainly for dyeing wool. The
method involves the precipitation of
certain mordant material (binding
agent) on the fabrics which then
combines with the dye to forman
insoluble coloured complex called
lake
For acid dyes, metal ions are used as
mordants but for basic dyes, tannic
acid is used as the mordant
Alizarin is a mordant dye. It gives a rose red colour with
Al3+ and a blue colour with Ba2+, a brownish red colour with
chromium (Cr3+) and a black violet with iron mordant
Classification of
Dyes on the basis
of their
applications

22. Synthesis and uses of following dyes
Nitroso dye - martius yellow
Martius yellow - nitro group as the chromophore and hydroxyl or amino group -
auxochrome.
1-Naphthol in sulfonated to produce 1-naphthol-2,4-disulfonic acid, which in turn is subjected
to nitric acid treatment to produce 2,4-dinitro-1-naphthol
 Martius yellow is a yellow dye with a molecular formula C10H6N2O5 that is used as a
biological stain.
 It is used to dye silk and wool, but it is fugitive and does not stand treatment with acid,
alkali and solvents.
 It is also used as a staining agent in microbiology.

23. Azo dye – methyl orange and aniline yellow
Methyl orange is obtained by coupling of N,N-dimethylaniline with diazotized sulphanilic
acid in alkaline medium
 Used as acid dye - wool and silk but its colour fades on exposure to light and washing.
 Used as an indicator in acid-base titrations.
 It is yellow in basic condition (above pH 4.4) while in the acidic medium, it turns to red
(below pH 3.1). The change in colour observed due to change in the structure of the ion. In
basic medium, the ion has azo-chromophore while in the acidic medium, it contains p-
quinonoid as chromophore.

24. p-Aminoazobenzene (aniline yellow)
p-Aminoazobenzene (aniline yellow) can be prepared in a good yield by rearrangement
reaction of diazoaminobenzene with a small quantity of aniline hydrochloride in the
presence of aniline as solvent.
Diazoaminobenzene was treated with aniline hydrochloride under warm condition and then
treated with glacial acetic acid
 Azo dyes containing amino group (basic dye) generates water soluble cations which can
be bind with the anionic sites on the fabric.
 These dyes can be used to colour modifies nylons, wool, cotton, leather and
polyesters.
 It is also used as dye in paper industry

25. Triphenylmethane dye – crystal violet
It is prepared by heating either N,N-dimethylaniline with carbonyl chloride (phosphene) or by
heating Michler’s ketone with N,N-dimethylaniline in the presence of POCl3.
It can be used as indicator – in weakly acidic solution, it is
purple; a strong acidic solution is green and still more strong
acidic solution, it is yellow. The change was observed due to
change in the structure.
weakly acidic strongly acidic More strongly acidic

26.  It can be used in the manufacturing of inks, stamping pads and type writing
ribbons.
 It is also used as an indicator for the quantitative determination of hydrogen ion
concentration in solutions.
 Crystal Violet is used to dye wool and silk directly, but cotton with the help of a
mordant (fixing agent).
 It is also used as a microbiological staining agent, antiseptic and antifungal agent.
Triphenylmethane dye – crystal violet

27. Phthalein dye – phenolphthalein
Phenolphthalein is synthesized by
electrophilic aromatic substitution
of phthalic anhydride and 2
equivalents of phenol in the
presence of concentrated
methanesulfonic acid at 900C to
yield the product, phenolphthalein
1. Used as a laboratory reagent and pH indicator in acid–base titrations, it turns colorless in acidic
solutions and pink in basic solutions.
2. It exerts laxative effects by stimulating the intestinal mucosa and constricting smooth muscles.
However, it is no longer used as a laxative due to the suspected carcinogenicity of this compound.
3. It belongs to class of dyes known as phthalein dyes.
4. Phenolphthalein is slightly soluble in water and usually is dissolved in alcohols for use in
experiments.
Phenolphthalein adopts four different states in aqueous solution: Under very strongly acidic conditions, it
exists in protonated form (HIn+), providing an orange coloration. Between strongly acidic and slightly
basic conditions, the lactone form (HIn) is colorless. The doubly deprotonated (In2-) phenolate form (the
anion form of phenol) gives the familiar pink color. In strongly basic solutions, phenolphthalein is
converted to its In(OH)3− form, and its pink color undergoes a rather slow fading reaction and becomes
completely colorless above 13.0 pH

28. Xanthane – Fluorescein
Fluorescein is a xanthen derivative. The preparation and properties of it are closely related to
phthalein dyes. It is prepared by heating phthalic anhydride with two moles of resorcinol with
anhydrous oxalic acid at higher temperature 383-3930C
It is red powder insoluble
in water. Since it is
coloured, therefore non-
quinonoid uncharged
structure (I) is not
satisfactory.
To obtain the red colour, two quinonoid structures are possible with different conjugation pattern,
one having the p-quinonoid structure (II) while the other the o-quinonoid structure (III).
When fluorescein dissolve in alkalies to give a reddish-brown solution which on dilution,
gives a strong yellowish green fluorescence due to formation new ion (IV)

29. Xanthane – Fluorescein
 The sodium salt of fluorescein is known as Uranine.
 The colour are however fugitive.
 Fluorescein gives an yellow-green fluorescence in dilute alkaline solutions and is
used as a dye for wool and silk.
 It is also used as a tracer for detecting water leakages in pipelines, as a staining
agent in microscopy, as a purgative and antiseptic

30. Anthraquinone - alizarin
The compound alizarin comes from the root of the madder plant (extraction).
It is used as a dye to color fabrics.
It was used by the British Army to make their ''red coats'' red during the American Revolution.
Alizarin can also be used as a pH indicator.
Alizarin is obtained when
catechol is condensed with
the phthalic anhydride in the
presence of the anhydrous
Aluminium chloride or conc.
sulphuric acid at 140-1500C
It can also prepared from anthraquinone via sequence of reaction such as sulfonation,
neutralisation, desulfonation and finally protonation

31. Uses:
1. pH indicator - 0-6.4 yellow and 6.4-12.0 red.
2. Biochemical assay (stain).
3. Can be used to identify calcium in tissue sections.
4. Calcium sites covered & surrounded by heavy orange-red precipitate.
5. Alizarin is a classic mordant dye.
6. It provides different colours depending on the metal ion used. For example, with Al3+,
alizarin gives a rose red color; with Ba2+, a blue color; with Fe3+, a violet color and
with Cr3+, a brownish red color.
7. Alizarin was used widely for dyeing wool, cotton etc.
Anthraquinone - alizarin

32. Indigo dyes – indigo
India is the birthplace of indigo, its official name being indigotin. It is the oldest vat dye
known. It occurs as indican which is the glucoside of indoxyl in Indian plant, Indigofera
tinotoria and the European plant, isatis tinctoria.
The indican on hydrolysis with either HCl or enzymes gives indoxyl which upon aerial
oxidation gives indigotin.

33. Indigo dyes – indigo
Laboratory synthesis involves
an aldol condensation of o-
nitrobenzaldehyde with
acetone, followed by
cyclization and oxidative
dimerization to indigo.
Aniline gets condensed with chloroacetic acid at high temperature forming N-phenylglycine
which is fused with NaOH and NaNH2 at 573K gives indoxyl. It gets oxidized by air to
indigotin. In another synthesis, anthranilic acid gets condensed with chloroacetic acid at high
temperature forming N-phenylglycine-o-carboxylic acid. This when fused with KOH-NaNH2
produces indoxylic acid which subsequently undergoes decarboxylation to form Indoxyl

34. Indigo dyes – indigo
Application of Indigotin Dye:
1. The primary use for indigo is as a dye for cotton yarn, which is mainly for the
production of denim cloth for blue jeans.
2. On average, a pair of blue jean pants requires 3–12 g of indigo.
3. Small amounts are used for dyeing wool and silk.
4. Indigo carmine, or indigo, is an indigo derivative which is also used as a colorant.

35. Pigments
 Pigments are various organic and inorganic insoluble substances, which are widely
used as surface coatings.
 They are also employed in the ink, plastic, rubber, ceramic, paper and linoleum
industries to impart colour.
 The pigment industry is usually regarded as associated with paints, but in fact it is a
separate industry.
 A large number of pigments are mined or manufactured for the commercial preparation
of paints.
 About 45 year back, white lead [2PbCO3 + Pb(OH)2], Zinc oxide (ZnO) and lithopone
(ZnS + BaSO4) were the principal white pigments in use while the coloured pigments
consisted of Prussian blue, lead chromates, various iron oxides and a few lake colours

36. Pigments
Classification
The important pigments used for making paints are:
(i) White : White lead, titanium dioxide, zinc oxide.
(ii) Red : Red lead, Iron oxides, cadmium reds
(iii) Blue : Cobalt blue, Iron blues, etc.
(iv) Green : Chromium oxide, chrome green
(v) Black : Carbon black, lamp black, furnace black, etc.
(vi) Metallics : Copper powder, zinc dust, aluminium, etc.
(vii) Metal Protective pigments: Red lead, blue lead, zinc and basic lead, etc.
Pigments are broadly classified into two types:
1. White Pigments
2. Coloured Pigments

37. Pigments
General properties of pigments:
1. Pigment should have minimum particle sizes (0.2-0.4).
2. Pigment should have maximum covering power.
3. Pigment should have freely mixing power.
4. Pigment should be chemically inert.
5. Pigment should have good resistance to chemicals.
6. Pigment should be resistance to solvent.
7. Pigment should have acceptable brilliance, hardness and stability on dyed and
printed goods.
8. Pigment should have good wet, light, and abrasion resistance.
Pigment should have good characteristics for excellent dispersion including:
 Particle size and distribution
 Electrical charge
 Specific gravity
 Purity and crystalline structure
 Condition of precipitation
 Should be applied to all fibber
 Should be cheap

38. Pigments
Zinc oxide pigments
Zinc Oxide is an inorganic compound which is also known as Calamine or Zinc White. It is
naturally found as a mineral zincite. It is mostly produced synthetically.
ZnO has attracted much interest as one of the multifunctional inorganic nanoparticles due to
its unique combination of superior physical, chemical, biological, electrical, optical, long-
term environmental stability, biocompatibility, low cost and non-toxic properties.
nano-ZnO can potentially be applied to gas sensors, photocatalyst for degradation of waste
water pollutants, catalysts, semiconductors, varistors, piezoelectric devices, field-emission
displays, ultraviolet (UV) photodiodes, surface acoustic wave (SAW) devices, UV-shielding
materials, rubber, medical and dental materials, pigments and coatings, ceramic, concrete,
antibacterial and bactericide, and composites. It has wide applications in bandages, ointments,
pastes, and dental cement.
Preparation: precipitation and coprecipitation method, sol-gel processing method, hydrothermal method,
solvo thermal method, pyrolysis method, and microwave irradiation method
Direct (American) process: The starting material for this
process are either zinc ores or industrial byproducts. In
the first step, the zinc-containing material is reduced by
a carbon-containing agent such as anthracite or coal at
high temperatures (1000-12000C). The zinc vapour and
the CO gas are then oxidized to zinc oxide and carbon
dioxide above the reaction bed or at the furnace exit.
Wet process: Soluble zinc salts are treated
with a solution of sodium carbonate or
sodium hydroxide. The resulting
precipitation of zinc carbonate or hydroxide
is then calcinated at about 800 °C to form
zinc oxide

39. Pigments
Properties:
1. Zinc oxide crystallizes in three forms: hexagonal wurtzite, cubic zincblende, and the rarely
observed cubic rock salt).
2. Zinc oxide - white powder that has excellent hiding power. This means that it can cover a surface
effectively, and the color of the underlying material does not show through.
3. Zinc oxide is also known for its high refractive index, which means that it reflects light very
efficiently. This property contributes to its whiteness and brightness.
4. When zinc oxide is added to a material, it imparts a white color to it. This is because zinc oxide
particles scatter and reflect light, making the material appear white.
5. It is a semiconductor material and is used in the production of electronic components such as diodes
and transistors.
6. Zinc oxide is also a UV absorber and is commonly used in sunscreens and other cosmetic products
to protect the skin from harmful UV radiation.
7. It has antibacterial properties and is used in the production of textiles, plastics, and paints to prevent
the growth of bacteria and fungi.
8. ZnO is a relatively soft material with approximate hardness of 4.5 on the Mohs scale.
9. The high heat capacity and heat conductivity, low thermal expansion and high melting temperature
of ZnO are beneficial for ceramics.
10. ZnO has a relatively large direct band gap of ~3.3 eV at room temperature; therefore, pure ZnO is
colorless and transparent. Advantages associated with a large band gap include higher breakdown
voltages, ability to sustain large electric fields, lower electronic noise, and high-temperature and
high-power operation.

40. iron oxide
Iron oxide is a compound made from iron and oxygen. There are 16 known iron oxides and
oxyhydroxides, the most famous of which is rust, a type of ferric oxide.
Iron oxides and oxyhydroxides are widespread and play important roles in many geological
and biological processes. They are used in iron ore, pigments, catalysts, thermites and are
contained in hemoglobin. Iron oxide is a cheap and permanent pigment found in paints,
coatings, and colored concrete.
The trivalent state is more common and is found in Fe2O3, hematite, which is red, and in
hydrated ferric oxide, Fe2O3·H2O, limonite or goethite, which is yellow. The divalent state
occurs along with the trivalent state in the magnetic oxide, magnetite, Fe3O4, also written as
FeO-Fe2O3, which is black.
Preparation:
As the most conventional method, the co-precipitation method consists of mixing ferric and
ferrous ions in a 1:2 molar ratio in very basic solutions at room temperature or at elevated
temperature. The reaction mechanism can be simplified as:
Fe2+ + 2Fe3+ + 8OH− ⇆ Fe(OH)2 + 2Fe(OH)3 → Fe3O4↓ + 4H2O
The nucleation of the Fe3O4 nucleus is easier when the solution pH is lower than 11, while
the growth of the Fe3O4 nucleus is easier when the solution pH is higher than 11
It is widely used in building materials, coatings, inks, rubber, plastics, ceramics, glass products,
hardware glass polishing, theater oil paint, painting, cosmetics, pharmaceutical coloring, photocopying
materials, catalysts, electronics industry and magnetic recording materials for recording and video
recording

41. Properties: Iron oxides such as magnetite, hematite and goethite are commonly used as pigments for black, red,
brown and yellow colors respectively
1. These pigment types are strong absorbers of ultraviolet radiation and mostly used in automotive
paints, wood finishes, construction paints, anticorrosive coatings, plastic industry, nylon, rubber and
print ink.
2. The excellent weather fastness, UV absorption properties, high transparency and increase color shades
when combined with organic pigments and dyes.
3. Alkali resistance: It is very stable to any concentration of alkalis and other types of alkaline
substances, especially cement and lime mortar commonly used in construction, and it does not
pulverize cement construction components and does not affect its strength.
4. Acid resistance: It has certain resistance to general weak acids and dilute acids, but can also gradually
dissolve in strong acids, especially under warming and thicker conditions.
5. Lightfastness: The color does not change under strong sunlight.
6. Heat resistance: It is stable within a certain temperature limit, and the color and luster begin to change
when it exceeds the temperature limit. As the temperature increases, the degree of change becomes
more and more significant.
7. Weather-resistant: Not affected by the climatic conditions such as cold, hot, dry and wet atmosphere.
8. Fouling-resistant air: It is very stable in any fouling gas, such as hydrogen sulfide, carbon oxide,
sulfur oxide, hydrogen chloride, nitrogen oxide and other gases.
9. Water resistance, oil resistance, solvent resistance: insoluble in water, various mineral oils, vegetable
oils and ethers, esters, ketones and other organic solvents, and there is no penetration phenomenon.