The Colorful World of Reagents

In composing this article, my hopes are that a better understanding of the reagents (such as Marquis, Mandelin, and Mecke) found in testing kits will encourage their use. Too often drugs can be mislabeled or misrepresented, exposing consumers to the possibility of an unwanted experience. Thankfully testing kits are readily available.

Testing kits contain liquid reagents that are an inexpensive and private way to verify the presence or absence of a substance within a pill or powder. For those who are unfamiliar, a testing reagent is a solution that is dripped onto a small amount of the substance in question. A reaction then occurs and the resulting color indicates information about the substance. An understanding of why reagents catalyse a unique drug-dependent color change begins with a brief look the perception of color. 11

As visible light is reflected off of an object, it passes through our cornea, enters the pupil, and is focused by a transparent lens onto the fovea of the retina. Light perception begins at the fovea, which contains a high concentration of photoreceptive cells called rods and cones.

Rods have only a single variety, which is very sensitive to the intensity of light but does not detect information about color. Interestingly, dogs have a much higher concentration of rods instead of cones. As such, they have better night vision and a more keen ability to sense movement.

Our vision is trichromatic. Colors are sensed by cones, of which we have three kinds: red, green, and blue. Together, these three cones can blend to detect all colors of the rainbow. Back to our discussion on dogs, they possess two kinds of cones thus giving them dichromatic vision. The range of visible light for canine and human vision is depicted here:



Our story involves cones. When a color is perceived, cones are stimulated to varying degrees and a signal is sent from the retina through the optic nerve to the visual cortex. What we see as light is a small fraction of the electromagnetic spectrum, known as visible light.22

Visible light can be described as traveling in waves, with low wavelength (high frequency) waves being perceived as a blueish/violet hue and high wavelength (low frequency) waves appearing reddish in color. As visible light interacts with an object, some colors are absorbed and some are reflected. An object appears white when it reflects all wavelengths and black (like our pupil) when it absorbs them all. We perceive the colors that are reflected back to our eyes.

At the molecular level, certain wavelengths of visible light possess the exact amount of energy required to interact with a pigment molecule’s electrons. Organic molecules that act as pigments often contain conjugated electrons, which occur along a series of carbons. Organic pigments absorb and reflect visible light energy of specific wavelengths.

Reagents are advantageous here because they can react with drugs to form conjugated systems of electrons that mimic those found in organic pigments. As such, the products formed when introducing reagents to a substance can then interact with visible light to reflect specific colors. The Marquis reagent is one of the most commonly used and is produced through the addition of 100 mL of concentrated (95–98%) sulfuric acid to 5 mL of (40%) formaldehyde.

Formaldehyde (CH2O) provides a carbon source to link molecules of a drug, forming a conjugated series of electrons. Sulphuric acid acts as a catalyst in this process. Four major events drive the reaction that occurs between Marquis and methamphetamine: protonation, dehydration, oxidation, and ion formation. As the reaction proceeds, the carbon rings of two methamphetamine molecules are bonded together by the carbon in formaldehyde. The carbon that then links these two methamphetamine molecules becomes positively charged, forming an ion that permits conjugated electron movement. The solution then turns orange in color.


Formaldehyde (left) and Methamphetamine (right)


Methamphetamine and Marquis Reagent Reaction Product

Drugs can change color when tested with the Marquis reagent because they react to form a conjugated system of electrons. Some testing solutions, such as the Mandelin reagent, utilize a metal ion when forming a conjugated system. Molecules react in predictable ways and their arrangement of electrons will absorb and reflect very specific wavelengths of light energy.

As helpful as these reagents may be, testing drugs shouldn’t provide a false sense of security. The goal is harm reduction. Simply verifying the presence or absence of a substance does not mean that the drug is safe, as other more deleterious compounds may be present. However, drug testing reagents do represent one of many strategies in the drug user’s safety kit.


Color Changes for Mecke, Marquis, and Mandelin Reagents

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