What Is A Reagent?

11th September 2020


If you’ve ever used a pregnancy test, a blood glucose test kit, or a coronavirus rapid test kit at home, then you’ve already used reagents. Technically, the reagent that you use in home test kits are your own body fluids. Usually, this is in the form of either urine or blood, since these are needed in order for test kits to work. 

However, a reagent is more broadly defined as any substance or compound that has been added to a system to trigger a chemical reaction. In this article, we’ll discuss the various aspects and applications of reagents. 

Lab assisstant using reagents in chemical analysis test tubes

A reagent is used to trigger a chemical reaction and is often used in chemical analysis

What Is A Reagent In Chemistry?

In terms of chemical classifications, a reagent is any organic or inorganic substance that can be added to  a mixture to trigger a chain of chemical reactions. It’s also used to test for the presence of other substances in a solution. This makes certain types of reagents very useful as testing tools in experiments.

The term ‘reagent’ is usually used interchangeably with the term ‘reactant’, but the two have different and precise definitions: a reactant is consumed during a chemical reaction, while a reagent is not. 

For example, solvents are used to dissolve chemicals, but they usually don’t participate in chemical reactions. Similarly, in biochemical reactions, especially those that involve catalytic enzymes, the reactants are called substrates, while the reagents are called catalysts.

What Are Some Examples Of Reagents?

When used in organic chemistry, the term reagent means a type of chemical ingredient that is added to an organic mixture or solution in order to transform it into another type of substance. A reagent can be a compound or a mixture of organic or inorganic substances. Some common examples of reagents used in organic chemistry are:

  • Collins reagent: This reagent is a metal-pyridine complex – specifically, the complex of chromium(VI) oxide with pyridine in dichloromethane. It’s solid, red in colour, and has a primary use of converting alcohols to aldehydes and ketones. It’s very useful in the oxidation of compounds that are sensitive to acids. 
  • Fenton’s reagent: This is a solution of hydrogen peroxide mixed with ferrous iron, like iron (II) sulphate. It’s a catalyst that’s used mainly to oxidise contaminants in waste waters. For example, it can neutralise and destroy toxic organic compounds like trichloroethylene and tetrachloroethylene.
  • Grignard reagents: This group of reagents has a generic formula of R-Mg-X, where the X represents a halogen and the R represents an organic group of molecules. Typically, the R is an alkyl or aryl. The most common example of a reagent in this group is methylmagnesium chloride. Grignard reagents are commonly used to create new carbon bonds.

In terms of analytical chemistry, reagents are used to determine the presence or absence of another type of substance. Change in colour is the usual indicator. The concentration can be estimated by the degree of colour change. Some examples of these reagents are the following:

  • Fehling’s reagent: This is a reagent that’s useful in differentiating between two molecular functional groups: carbohydrates and ketones. It’s used as a test for reducing sugars and non-reducing sugars. As an analytical reagent, it’s used as a supplementary reagent to the Tollen’s reagent test.
  • Tollen’s reagent: This is a mixture of three compounds dissolved in a solution, namely silver nitrate, ammonia, and a small amount of sodium hydroxide. The alkaline compound is important to maintain the necessary pH of the reagent. It’s useful in identifying molecular functional groups.
  • Millon’s reagent: A few drops of this reagent can detect the presence of soluble proteins in a test solution. When this happens, the reagent turns reddish brown, or precipitates are formed, which indicate the presence of tyrosine residue.
Reagent kit with blood samples on slides

A reagent is an ingreditent that is added to a solution in order to transform it into something else. Thought it’s often a chemical, a reagent can also be something like blood or urine

What Is A Reagent Kit?

Reagent kits are testing kits that can be used in the field or in the lab to detect certain substances. They’re commonly used to test for the use of illicit drugs. Reagent test kits are typically simple to use, and usually involve colour changes when specific substances have been detected. Some testing kits are a little more complicated, and need to be processed in the laboratory using thin chromatography techniques.

Some examples of commonly used reagent kits are:

  • Mandelin reagent kits: These are used to detect alkaloids and other compounds. The reagent turns dark green if amphetamine is present in a test solution, and a deep orange when cocaine is present.
  • Marquis reagent kits: These are very useful in simple spot tests, where the reagent can detect a wide range of illicit drugs. For instance, it turns olive black in a test solution containing LSD. It’s also good in detecting methamphetamine, which makes it turn orange or brown.
  • Mecke reagent kits: These are also used in simple spot tests. When mixed with a solution containing the psychedelic drug, like MDA, the reagent turns very dark blue. In the presence of heroin, it turns a deep bluish green.
  • Froehde reagent kits: These are good at detecting some opioids and alkaloids, such as 2-CB, codeine, and ecstasy. In the presence of these drugs, the reagent turns yellow, dark green, red brown, and black with greenish brown, respectively.

What Is A Limiting Reagent In Chemistry?

As the name suggests, a limiting reagent is a reagent that limits or stops a chemical reaction when it’s completely consumed. This means that the chemical reaction cannot go any further because no more substance is available for the reaction to continue. Therefore, a limiting reagent determines when the reaction stops.

You can actually calculate the amount of reactants needed to complete a chemical reaction. If one of the reactants or reagents is exhausted, the reaction will stop at a certain point, leaving behind excess reactants. The stoichiometry of a reaction, which is simply the proportions of the reactants as indicated in the balanced equation, can be established with precision. You can calculate this by determining the respective mole proportions of the reactants. You can also measure the weight of the final products to determine the limiting reagent.

Novel coronavirus 2019 nCoV RT-PCR diagnostics kit

The novel coronavirus RT-qPCR test kit contains reagents

How Are Reagents Being Used In Coronavirus Testing?

Two main methods are used for coronavirus testing. The first method is rapid antibody testing, which detects whether antibodies for the virus are present in the body. Using a patient’s blood sample as the reagent, it’s a form of serology test. It may only take several minutes to determine the results, and it’s good for testing past infections.

The second method is known as the ‘real time reverse transcription polymerase chain reaction’ (RT-PCR). This determines the presence of genetic materials in the test sample that are specific to the virus. Both of these methods need reagents, which are in the form of enzymes.

Serology test reagents bind with certain types of antibodies like IgG, IgM, and IgA. When one or more of these antibodies are present, lines will show on the test window like a pregnancy test. The RT-PCR test, on the other hand, requires genetic materials to be amplified in order for them to be detected. The reagents used here are enzymes and chromatography techniques.


All content published on the ReAgent.co.uk blog is for information only. The blog, its authors, and affiliates cannot be held responsible for any accident, injury or damage caused in part or directly from using the information provided. Additionally, we do not recommend using any chemical without reading the Material Safety Data Sheet (MSDS), which can be obtained from the manufacturer. You should also follow any safety advice and precautions listed on the product label. If you have health and safety related questions, visit HSE.gov.uk.

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