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Taxonomy of chemical reagents and solvents

Er. Mandeep Singh
Thursday, December 31, 2020, 08:00 Hrs  [IST]

A chemical compound is a naturally occurring pure substance formed by some suitable means such as exchange or sharing of electrons among elements. Atoms of elements combine with each other in a definite proportion of mass according to the laws of chemical combination, which results in creation of a molecule such as molecule of water (H2O) is formed by combining one part of oxygen atom (O) with two parts of hydrogenatoms (H) in a definite ratio of mass of O:H atoms as 8:1 respectively.

The natural diversity of chemical compounds is responsible for a variety of substances present in the universe and properties they exhibit. A general classification scheme for chemical compounds is illustrated in figure 1 that helps us to understand the diverse nature of chemical substances.

A “chemical reagent” is a molecular substance that participates in a chemical reaction through one of the following schemes:

Reactant – A reactant is a general name for reagent that is fully or partially consumed in a chemical reaction that results in formation of new substances called products. The reactant that is fully consumed in a chemical reaction is known as a “limiting reagent” while the reactant that is partially consumed in a chemical reaction is called as “excess reagent”. Chemical transformations or chemical reactions simply imply breaking of old bonds among participating atoms of the reactants and formation of new bonds that result in formation of new chemical substances. The following chemical equation typifies the scheme for any chemical reaction:

Regents can be ‘inorganic compounds’ like hydrochloric acid (HCl), sulphuric acid (H2SO4), sodium hydroxide (NaOH), Bayer’s reagent (KMnO4), calcium chloride (CaCl2), Tollen’s reagent (Ag (NH3)2OH), hydrogen sulphide (H2S), Fenton’s reagent (mixture of FeSO4& H2O2), or ‘organic compounds’ like formaldehyde (HCHO), ethanol (C2H5OH), acetic acid (CH3COOH), citric acid (C6H8O7) ammonium oxalate (C2H8N2O4), Collins reagent (C10H10CrN2O3) and Grignard reagents (R-MgX).

Attacking reagent – “Attacking reagent” generally induceschemical transformation under suitable reacting conditions. Attacking reagents are classified on the basis of the type of chemical reaction they induce such as ‘oxidizing or reducing agents’ induce redox reactions, ‘homolytic or heterolytic agents’ induce fission reactions, ‘substituting, displacement or eliminating agents’ induce acid-base reactions, ‘nucleophilic or electrophilic reagents’ induce organic reactions and ‘catalytic agents’ induce catalytic transformations.The attacking reagent approaches the reactive center of the primary reactant to cause chemical transformation such as oxidizing agent oxidizes the reactant molecule by combining with oxygen and reducing agents reduces the reactant molecule by removing oxygen or combining with hydrogen. Nucleophiles (Nu-) are electron-rich species characterized by organic functional group molecules such as hydroxides (OH-), cyanides (CN-), aldehydes (CHO-), amines (NH2-) or carbanions (R3C-); and, electrophiles (E+) are electron-deficientspecies characterized by organic functional group molecules such as nitro group (NO2+), carbonyl group (CO2+), proton (H+) or carbcations (H3C+).

Catalyst – A “catalyst” is a type of a chemical reagent that does not directly participate in the chemical reaction as a reactant rather enhances its reacting capability. The reactant that combines with the catalyst is called a ‘substrate’ molecule. A catalyst only increases or decreases the speed of chemical reaction and can be recovered unaltered after completion of the chemical reaction. Catalysts that participate in chemical reactions inside living organisms are known as “biological catalysts or enzymes”.The following chemical equation typifies the scheme for a catalytic reaction:

Some typical chemical species that act as catalysts are alumina (Al2O3), manganese oxide (MnO2), titanium dioxide (TiO2), ferric chloride (FeCl3), aluminum tri chloride (AlCl3), calcium carbonate (CaCO3), aluminosilicate (mixture of Al & SiO2), zeolites and so on. Enzymes act on substrate biomolecules like carbohydrates, proteins, fats and hormones. Some typical enzymes present inside living organisms are amylase, protease, pepsin, trypsin, lipase, urease, carboxylase, pyridoxal phosphates and so on.

Indicator – Indicators are “analytical reagents”that are widely used in chemistry for identification and quantification of chemical reactions. Analytical reagents are characterized on the basis of a qualitative property such as color change, appearance of turbidity, opaqueness, suspension or precipitation indicating the presence of test substance (substrate)
during the chemical reaction.

For example, Iodine solution is used to test the presence of starch, Bayer’s reagent is used to test the presence of unsaturated organic compounds (C=C, C=C) and Tollen’s reagent is used to determine the presence of aldehyde among organic compounds. Ionic salts and organic acids that cause precipitation to occur in chemical reactions are used to determine the presence of specific elements (ions). Some typical pH indicators used in acid-base titration reactions are phenolphthalein, phenol red, methyl orange, methyl violet, bromophenol blue, thymol, alizarin yellow, and indigo caramine. Some typical staining dyes specific for nucleic acids and proteins in microbiology practice are acridine&rodhamine.

Solvent – “Solvent” is a pure compound that is capable of dissolving another substance called ‘solute’ resulting in a homogenous mixture called ‘solution’. Depending upon the bonding chemistry of chemical compounds, solvent may be a ‘polar compound’such aswater (H2O), ammonia (NH3), ethyl alcohol (C2H5OH), acetone (CH3COCH3), chloroform (CHCl3), formaldehyde (HCHO) or ‘non-polarcompounds’such ascarbon di sulphide (CS2), acetylene (C2H2), carbon tetra chloride (CCl4), dimethylether (CH3OCH3) and turpentine oil. Solvent chemistry is another important branch of chemistry that explores bonding relationship among various compounds and identifies techniques for extraction of a pure substance from homogenous mixture of substances.

Chemical reagents are analytical or reactive precursors, which are an extremely important constituent of the scientific experiment. Hence, they find diverse applications in the field of chemistry, biology, pharmacology, life sciences, forensic and medical sciences as diagnostic or productive agents.Although reagents are extensively studied to provide rare insights into experimental procedures of chemistry and biology but their preparation and use is an equally critical component in investigation science.

The correct composition, mixing, dilution and concentration of reagents are important for the success of an experiment as well as obtaining correct observations orqualitative results of the experiment.Chemical purity is the measure of the amount of impurity present in the sample. Typical specifications that areused as a measure of purity of the reagent are– scientific grade, pharmaceutical grade, and industrial grade.

Scientific or laboratory grade is used for educational purposes because these are less pure and useful for experimental investigations rather technical or industrial work. Technical grade of reagent indicates its purity to be used as a formulating agent in food, beverage or drug compound. This is usually the official description of chemical reagent used for technical or commercial economic activity. The purity gradeis set by the United States Pharmacopeia (USP)specifiesreagent to be used in drug formulation for pharmaceutical or therapeutic use. The purity standards of chemical reagents set by American Chemical Society (ACS) is the highest level that meets industrial or technical specifications for manufacturing purposes.

Many accidents in the chemistry laboratory also take place while preparation of reagents. Most lab reagents are highly classified under one of the following hazard categories as – irritant, toxic, corrosive, inflammable, exposable, mutagenic or carcinogenic agents. Adequate knowledge and care can prevent accidents in the lab. Therefore, these must be used with adequate precautionary measures while handling or preparation such as use of personal safety gears, safe cabinets, proper sampling and equipment. General precautions recommended while working with chemical reagents are:  

  • Never touch, taste, or smell any chemical, whatever it may be
  • Use personal protective gears like lab-coat, safety gloves, goggles and face shield
  • Maintaining safe distance while using the chemicals for preparation
  • Hold containers away from the body when transferring a chemical or solution from one container to another
  • Use of clean, dry glass apparatus while preparing the reagents
  • Follow the specified procedure and amounts/volumes of chemical or solvents to be used for reagent preparation
  • Add concentrated acid to water slowly. Never add water to a concentrated acid.
  • Proper labeling instructions on apparatus like warning sign, name and date of preparation
  • Proper storageof reagents after preparation
  • Proper disposal ofany waste paper or any kind of solid waste
  • Make sure no flammable solvents are in the surrounding area when lighting a matchstick or a burner
(The author is M.D.of VMG Biotech Consultants, New Delhi)

 

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