Textile wet processing employs a huge amount of water, dyes, chemicals and auxiliaries for processes such as dyeing, printing and finishing. Chemical analysis always involves the use of different chemicals. In order to assure accurate analysis results, the chemicals used need to be standardized, the procedures must be followed exactly and the data obtained have to be analyzed statistically. If an instrument is used, it should be maintained and calibrated properly.
In a chemical analysis, especially involving quantitative analysis, the amount of chemical used is critical and can be determined by the measurement of concentration if it is a solution, or by weight, if it is a solid. Sometimes, the concentration of a solution can be easily determined by using another known solution through titration. For acids and bases, if the concentration is sufficiently low, the pH concept is generally used to represent the concentration of the acid or base in the aqueous solution. For the analysis of common chemicals, such as caustic soda, acetic acid, soda ash, sodium dithionite, hydrogen peroxide, and so on, titrimetric analysis and gravimetric analysis are widely used. For the analysis of surfactants and other chemicals, qualitative spot tests and specialized instruments should be utilized.
Before the analysis of chemicals in textile wet processing we should to know about concentration, titration, weighing, pH etc. Now a short identity of these are given below.
The concentration of a solute is usually expressed as the amount of a solute in a unit volume of a solution. The amount of a solute can be in grams (g), kilograms (kg), moles (mol), or normals (n). The unit volume of a solution is always in litres (l).
Titration is a method by which the concentration of an unknown solution can be determined using a standardized solution with a known concentration through a stoichiometric reaction. The end point of the chemical reaction is indicated by the colour change of an indicator or an instrumental reading. The standard solution of a known reagent is the titrant and the unknown solution is the titrand.
Weighing is an important operation in gravimetric analysis. Usually it involves the use of an electronic balance with a minimum readability of 0.1 mg. In order to ensure reproducible results, sample handling is very critical especially when hygroscopic materials are weighed.
pH is a scale between 0 and 14 used to express the concentration of hydronium (H3O+, or H+) ions in a solution. It is defined by Equation .
pH = – log [H+]
List of Major Chemicals and Auxiliaries Used in Textile Wet Processing:
Acids, bases, salts, surfactants, oxidising agents and reducing agents are the major chemicals those are widely used in textile wet processing industry.
An acid (from the Latin acidus/acēre meaning sour) is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and reacting with bases such as sodium carbonate. Aqueous acids have a pH under 7, with acidity increasing the lower the pH. Chemicals or substances having the property of an acid are said to be acidic. The following standard solutions are used in the acid analysis. They are usually prepared in advance and consumed within a certain period of time.
H2SO4, 0.1 N, 0.25N, 0.5 N and 1 N;
HCl, 0.1N, 0.25 N, 0.5 N and 1 N;
HNO3, 0.1 N;
There are two types of acid:
- Inorganic acid
- Organic acid
1. Inorganic Acid:
Inorganic acid are Sulphuric acid (H2SO4), Hydrochloric acid (HCl), Nitric acid (HNO3), Phosphoric acid (H3PO4), etc.
Sulphuric Acid (H2SO4):
The concentration of sulphuric acid (H2SO4) can be determined by using Baume’s (ºBé) hydrometer. The titration of sulphuric acid is carried out using sodium hydroxide in the presence of phenolphthalein as an indicator. The end point is reached when a faint pink color is persistent.
The concentration of hydrochloric acid (HCl) can be determined using a hydrometer, in a very similar manner to the determination of sulphuric acid concentration. Hydrochloric acid is a volatile acid at high concentration.
The concentration of nitric acid (HNO3) can be determined using a hydrometer. If titration is used to determine the concentration, phenolphthalein is the indicator.
The concentration of phosphoric acid (H3PO4) can be determined in a similar manner to that discussed for H2SO4, HCl and HNO3.
2. Organic Acids:
Organic acids are HCOOH (formic acid), Acetic acid etc.
HCOOH (formic acid) is the simplest organic acid in terms of its organic structure. Concentrated HCOOH is usually 88% in strength. Since formic acid is a volatile acid, precautions should be taken to prevent loss of strength in the sample preparation stage. The concentration of formic acid can be determined by acid– base titration as well as by redox titration owing to the reduction power of formic acid.
Acetic acid is a weak acid. It is available at different concentrations. Highly concentrated acetic acid at 98% and above is called glacial acetic acid because its freezing point range is between 13.3ºC (98%) and 16.7ºC (100%). Glacial acetic acid is flammable. The concentration of acetic acid can easily be determined using acid–base titration with phenolphthalein as an indicator. The water used should be free from CO2, prepared by boiling before use.
A base in chemistry is a substance that can accept hydrogen cations (protons) or more generally, donate a pair of valence electrons. A soluble base is referred to as an alkali if it contains and releases hydroxide ions (OH−) quantitatively.
Bases are two types:
- Inorganic and
- Organic bases
1. Inorganic Bases:
Inorganic bases are Sodium hydroxide (NaOH), Sodium carbonate (Na2CO3), Ammonium hydroxide (NH4OH) etc.
Sodium hydroxide (NaOH) is also called caustic soda. It is available in solution at different concentrations or in solid form. Commercial NaOH often contains a little sodium carbonate (Na2CO3) as a by-product of the manufacturing process. This small amount of Na2CO3 will usually not influence its use in textile wet processes.
Owing to its strong alkalinity, NaOH can react with CO2 in air easily. It can also absorb water very quickly.
Sodium carbonate (Na2CO3) is also called soda ash. In textile wet processes, it is often available in anhydrous form. Its purity can be > 99% Na2CO3 (58% Na2O).
If the concentration of a Na2CO3 solution needs to be determined, a titrimetric method identical to the ones listed for NaOH in this section can be used. If the existence of bicarbonate is a concern (very rarely in textile wet processes) the following method can be used to determine the content of bicarbonate in sodium carbonate.
Ammonium hydroxide (NH4OH) is a water solution of ammonia gas (NH3). It can also be called aqua ammonia or ammonia water. The concentration determination can be done using either a hydrometer or an acid–base titration. Since ammonia is volatile, the concentration determination should be done with care to avoid any loss of strength. If a hydrometer is used, the sample and the hydrometer should be cooled to 5–10ºC. Acid–base titration can also be used to determine the concentration of NH4OH.
2. Organic Bases:
Organic bases are Triethanolamine, N(CH2CH2OH)3, Ethylenediamine (H2NCH2)2 etc.
Triethanolamine, N(CH2CH2OH)3, is a strong organic base miscible with water, methanol and acetone. The pH of its 0.1N aqueous solution is 10.5. Analytical grade N(CH2CH2OH)3 is a highly hygroscopic and viscous liquid with a pale yellow or no colour. Its melting point is between 18 and 21ºC. Its density is about 1.12.
Ethylenediamine, (H2NCH2)2, is a strong organic base miscible with water and alcohol. It is a colourless and viscous liquid with a density of 0.898 and a melting point of 8ºC. The pH of a 25% aqueous solution is 11.5. Like triethanolamine, it is an aliphatic amine soluble in water and, therefore, can be determined by the acid–base titration with methyl orange as an indicator.
Salts are the products of the acid-base neutralisation reaction. The salts used most in textile wet processes are common salt (NaCl, sodium chloride) and Glauber’s salt (Na2SO4, sodium sulphate). The content analysis of salts is usually conducted by using a precipitation titration method which may be followed by filtering and weighing procedures to obtain the final results.
Industrial grade NaCl has a content of 92–98%. The precipitation titration can be conducted using 0.1 N AgNO3 as the titrant and 5% K2CrO4 as the indicator (the Mohr method). The sample chloride solution should be buffered with calcium carbonate to a pH between 6.3 and 7.2 in order to avoid any interference from other ions present in the solution.
Na2SO4 is available in two types, anhydrate and decahydrate. Its content analysis can be conducted based on the precipitation method using barium chloride (BaCl2).
An excess amount of barium chloride is added into the sample solution which has been filtered beforehand to form BaSO4 precipitate as indicated by the following reaction:
Na2SO4 + BaCl2 →2NaCl + BaSO4↓
Surfactants are widely used in textile wet processes for the purpose of wetting, dispersing, emulsifying and cleaning. The molecular structures of surfactants have a distinctive hydrophilic moiety and a distinctive hydrophobic moiety. When they are used at a sufficient concentration, the surface/interface tension of the solution is lowered and micelles are formed, which give the solution extra properties.
According to their ionic properties in aqueous solution, traditional surfactants can be divided into four categories: anionic, cationic, amphoteric and non-ionic.
Surfactants are four types:
- Anionic surfactants,
- Cationic surfactants,
- Non-ionic surfactants and
- Amphoteric surfactants
Amphoteric surfactants contain both anions and cations. They should show positive results when tested using either the basic methylene blue test for anionic surfactants or the alternative bromophenol blue test for cationic surfactants.
A saturated bromine aqueous solution can also be used to determine the type of amphoteric surfactant. Add 5 ml of 1% sample solution to 1.5 ml saturated bromine aqueous solution. Observe the colour of the precipitate. Heat the mixture and observe the change in the precipitate. If the precipitate is a yellow to yellow-orange colour and is dissolved to form a yellow solution after heating, the sample is an imidazoline or alanine type of amphoteric surfactant. If the precipitate is a white to yellow colour and insoluble after heating, the sample is the other type of amphoteric surfactant.
Oxidising agents and reducing agents
Oxidising agents are mainly used for bleaching and reducing agents are mainly used for vat dyeing in textile wet processes. These agents are often strong chemicals and need to be handled with care. The assay of these agents is almost always based on the redox titration. In a redox reaction, an oxidising agent (oxidant) is reduced (it gains electrons) and a reducing agent (reductant) is oxidised (it loses electrons). The redox reaction can be written as two half reactions shown below:
Oxidation reaction: reducing agent → oxidized form + n e–
Reduction reaction: oxidising agent + n e– → reduced form
The net reaction is: reducing agent + oxidising agent → oxidised form + reduced form
e. Oxidising Agents:
Hydrogen peroxide (H2O2) can be titrated with potassium permanganate (KMnO4) in an acid medium. H2O2 is the reducing agent and KMnO4 is the oxidising agent.
In hypochlorite bleaching of textiles, active chlorine is the species measured for the control of the bleaching process. Iodometry is the method used to determine the content of active chlorine.
Either sodium permanganate or potassium iodide can be used to titrate the sodium perborate (NaBO3•4H2O). Dissolve 0.2 g of sample in 200 ml distilled water, add 40 ml 6 N H2SO4, titrate with 0.1 N sodium permanganate until a pink colour appears.
f. Reducing Agents:
Sodium hydrosulphite (Na2S2O4)
It is the Dilute of 10 ml 40% formaldehyde with 50 ml distilled water.
Glucose (C6H12O6) can be used as a reducing agent in vat and sulphur dye applications. It can be analyzed by iodometry. Accurately prepare a 0.5% glucose solution.
Sodium thiosulphate (Na2S2O3•5H2O) can be titrated easily by iodometry. Accurately weigh a 5 g sample and dissolve it in 500 ml distilled water to make a 1% sample solution.
g. Miscellaneous Chemicals
The specific gravity of ethanol (C2H5OH) is directly related to its content. The specific gravity of ethanol at 15ºC.
Ethylene glycol and glycerol
ASTM method D161518 may be used to estimate the concentration of ethylene glycol and glycerol in an aqueous medium.
Urea is tested for the content of nitrogen using H2SO4 and formaldehyde. The indicator used is a mixed indicator containing 0.5 g phenolphthalein and 0.5 g thymol phthalein dissolved in 100 ml ethanol. A 25% formaldehyde solution used should be neutralized before use. The procedures of the method are briefly described below.
- Dissolve 1 g fully dried sample in a small amount of water; add 3 ml concentrated H2SO4; mix well and heat on a hot plate.
- Heat until the release of CO2 (bubbling) has stopped and dense white smoke (SO3) is emitted; leave to cool down.
- Add 50 ml distilled water and 2 drops of methyl red indicator.
- Neutralise the acidity of the solution with 6 N NaOH added dropwise until the red colour changes to a pink colour; add 0.5 N NaOH slowly to change the solution colour to a faint pink.
- Add 40 ml 25% neutralised formaldehyde solution and 5 drops of the mixed indicator; stand for a few minutes.
Fluorescent whitening agents
Fluorescent whitening agents (FWA) are a special type of chemical that can significantly increase the apparent whiteness of treated fabrics. They absorb UV radiation and re-emit the absorbed energy in the blue visible light range which makes the treated fabrics appear whiter. The easiest test for the effect of FWAs is simply a visual examination of the whiteness of treated fabrics. Manufacturer’s recommendations should be followed in order to achieve the best whitening effect.
Ethylenediamine tetraacetate (EDTA)
Ethylenediamine tetraacetate (EDTA) can form a few different water soluble salts with calcium, potassium and sodium, for example, calcium disodium, trisodium and tetrasodium salts. EDTA tetrasodium salt is used most widely in many industrial applications as a powerful chelating agent. Its 1% solution has a pH of 11.3. It can chelate with many divalent and trivalent metal ions to form water soluble metal complexes.
- Chemical testing of textiles Edited by Qinguo Fan
- Dyeing and Chemical Technology of Textile Fibers by E.R Trotman, 6th Edition
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Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. He is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.