In chemistry, reagents are compounds, substances, or mixtures used to examine chemical reactions. Commonly, a reagent is incorporated into a test to either produce a chemical reaction, or to check if a reaction would occur to begin with. Read on to discover more about how chemical reagents work, and what role they’re playing in coronavirus test kits.

What Is A Reagent?

A reagent is an organic or inorganic substance that triggers chemical reactions. It can also be used to determine whether certain chemicals are present in a solution. Although you might not be familiar with reagents in everyday life, you’ve probably encountered them without even realising it. Reagents are commonly used in a variety of testing kits. This includes things like pregnancy tests, COVID-19 tests, blood glucose tests, and water quality test kits.

For instance, in pregnancy test kits, antibodies are used as reagents to detect the presence of human chorionic gonadotropin (hCG) – a hormone released by the placenta during pregnancy. The hCG reacts with the antibody-dye conjugate, causing the dye molecules on the test strip to change colour.

Reagents are also used in industrial settings both for analytical and manufacturing purposes. For example, they’re often used to test the quality of products based on certain standards before they’re distributed. The type of reagent involved varies depending on the specific substances and parameters being tested.

Reagents Vs Reactants Vs Solvents And Catalysts

While reagents are used to cause, if not test for a chemical reaction, reactants are substances or compounds that are specifically affected by the chemical reaction. During a chemical reaction, reactants undergo change and form a set of unique products. 

An example of this is the double replacement reaction that occurs when hydrochloric acid is mixed with sodium hydroxide. These are reactants because they’re consumed and altered by the reaction, forming sodium chloride and water:

HCl + NaOH ? NaCl + H2O

Solvents, on the other hand, are distinct from reactants because they’re used to dilute a solution, or dissolve soluble substances, not to cause a chemical reaction that alters compounds and forms new products. 

Catalysts are also different from reactants because they’re not actively affected or consumed during a reaction. Instead, catalysts are used to speed up chemical reactions, like in the decomposition of hydrogen peroxide. If left to occur naturally, hydrogen peroxide decomposes very slowly, which is why some products containing this chemical have a shelf-life of around six months. But, with the help of the following catalysts, the rate of reaction can be increased to a matter of seconds: 

As a side note, both solvents and catalysts may act as reagents, but not as reactants, since the former are not consumed during a chemical reaction.

Researcher is dropping the reagent into test tube for reaction testing in chemical laboratory.
Reagents are added to a solution in order to test for, or cause a reaction

What Is A Tollens’ Reagent Test?

Named after its German chemist discoverer, Bernhard Tollens, the Tollens’ reagent test is used to identify aldehydes and their functional groups, as well as to distinguish between these and ketones

Central to this test is the Tollens’ reagent, which is composed of ammonia and silver nitrate, as well as sodium hydroxide, in order to stabilise the pH level of the reagent solution. A positive result using this test is determined when a precipitated ‘silver mirror effect’ is produced on the surface of the reaction vessel, demonstrating that the reagent has been utilised. Meanwhile, the presence of ketones would yield a negative result, except in the case of alpha-hydroxy ketones.

The short shelf life of the Tollens reagent means that a fresh laboratory preparation must be made prior to its immediate usage. There are three main applications that this reagent is used in:

  • Quantitative organic analysis, where the Tollens reagent is used to differentiate between aldehydes and ketones
  • Anatomic pathology, where it’s used to detect chromaffins, melanin, argentaffin, and lipofuscin in histology (i.e. the study of tissue)
  • Material science, where it’s used to apply a high-quality silver mirror on glass surfaces of commercial products, including scientific apparatus

What Is A Benedict’s Reagent Test?

American chemist Stanley Rossiter Benedict is the namesake behind Benedict’s reagent test. This involves a complex chemical reagent composed of copper(II) sulfate pentahydrate, sodium citrate, and sodium carbonate. 

The test is often used to detect reducing substances, like sugars, by checking for a colour transition from blue to red-brown, with a precipitate:

  • To test for the presence of monosaccharides or reducing disaccharides in certain ingredients, a sample is first dissolved in water
  • A certain amount of Benedict’s reagent is then added to the solution, and the resulting colour is what will determine the results

Like the Tollens’ reagent test, the Benedict reagent test can detect the presence or absence of aldehydes and alpha-hydroxy ketones, as well as hemiacetals (alcohols with carbonyl groups). It can also be used to identify high amounts of glucose present in urine, which could be indicative of something like diabetes mellitus. However, this test isn’t the main method used for glucose analysis, since its high sensitivity can lead to false positives.

Science experiment diagram show Benedict's test for sugar determination with result analysis.

About Iodine Reagent Tests

Yellow or brown iodine solutions are commonly used to detect starch. If starch is present, iodine turns blue or black in colour, and this method can be used to distinguish glucose and carbohydrates from starch. Unlike Benedict’s reagent, iodine reagents react strongly with starches, making them more suitable for this application. 

Which Reagent Is Used To Test For Starch?

Starch has neither odour nor taste. It’s a white substance in its pure form, and can be found in plant tissues, although it’s commonly obtained from cereals and potatoes. Starch is also called amylum, which is a polymeric carbohydrate composed of various units of glucose. It’s technically a polysaccharide, or a complex sugar used as food or energy storage by plants. 

Iodine solution is used as a reagent to test for starch because its sensitivity means that it can differentiate starch from glucose and other carbohydrates. If you want to see this reagent in action, put some iodine on a potato and watch as it turns black! 

Which Reagent Is Used To Test For Alkenes?

Alkenes are double-bonded hydrocarbons used for alcohols, plastics, detergents, and fuels. Bromine water is the reagent of choice when it comes to testing for alkenes

When dissolved in reagent-grade water, bromine water (Br2) is an orange solution that becomes colourless when shaken with an alkene. Due to the double-bonded nature of alkenes, adding bromine water creates ‘dibromo’ products. For instance, when ethene and bromine combine, they form dibromoethane:

C2H4 + Br2 → C2H4Br2

Non-dibromo products can also be produced with this reagent. For example, if distilled vapour or steam is incorporated with an alkene, an alcohol is produced – for instance, when ethene and water form ethanol

C2H4 + H2O → C2H5OH

Bromine water can turn alkenes to alkanes as well: if you combine ethene with the dihydrogen from bromine water, ethane will be produced:

C2H4 + H2 → C2H6

Biology chemistry show experiment of iodine test for detect starch (or carbohydrate) in sample such as food, milk, cosmetic or other

Are Reagents Used In Coronavirus Test Kits?

Since mass testing has proven necessary to help flatten the curve of COVID-19 cases, laboratory tests are required to yield accurate results from screened samples. In order to ensure this, pre-approved reagent grade substances, compounds, enzymes, and other primers must be used for the specific analytical application of test samples. Doing this will ensure a verifiable set of data. 

Reagents used in coronavirus test kits are well-studied for safe and effective applications in biochemical tests, specifically towards an accurate detection of certain virus strains via RT-PCR-based screening. In rapid antibody tests, the reagent used is actually the patient’s blood, which is used to detect COVID-19 antibodies.

Above anything else, chemical reagents are indispensable requirements of coronavirus tests. Without them, we would just be left with inaccurate, epidemiological guesses.


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