In the field of forensics, luminol is often used to detect traces of blood in crime scenes, as the reaction between luminol and hemoglobin in blood results in a visible glowing effect, making the blood easier to spot. This reaction occurs because hemoglobin is an oxidant, a chemical with the ability to accept electrons from luminol, resulting in a phenomenon called chemiluminescence. Chemiluminescence is light emission as a product of a chemical reaction, and can be physically measured in lux, a metric of illuminance or light intensity. In this lab we tested the reaction between a luminol solution and two different oxidants, one acid and one base, to determine when the strongest phenomenon of luminescence occurs, using light intensity as a dependent variable. Oxidants vary in chemical characteristics, and so there are many potential factors that dictate a reaction’s light emission and the phenomenon of chemiluminescence and so this lab attempts to hone in on some of the characteristics that affect a reaction's ability to emit light. Essentially, we tried to determine what factors produce the greatest chemiluminescence. Obviously, there are factors that aren’t quantifiable or identifiable by only using one acid and one base, but using one acid and once base can still give us some conclusions about what oxidants work best.

Chemiluminescence occurs when two reactants produce an intermediate product that exhibits an excited state (a state as a result of an electron in a state of higher energy), which then, in an attempt to reach its ground state, releases some of it’s extra energy in the form of light photons, essentially a phenomenon of glowing in the dark.

In order to determine the effect of the two different oxidants on luminol, we had to first create a stock luminol solution to use for all of the reaction trials. The luminol stock solution was created by dissolving 4.0 grams of sodium carbonate, 0.2 grams of luminol, 24.0 grams of sodium bicarbonate, 0.5 grams of ammonium carbonate monohydrate, and 0.4 grams of copper (II) sulfate pentahydrate to a final volume of one litre. This is the necessary solution to see a chemiluminescent reaction in a reaction with an oxidant. However, in the reaction, Luminol is the only thing reacting with the oxidant.

For our acidic oxidant, we used hydrogen peroxide. The reaction between luminol and hydrogen peroxide looks as so:

Here, hydrogen peroxide accepts luminol’s nitrogen’s electrons, ultimately allowing for the formation of an intermediate with an excited state. This excited molecule eventually reverts to its ground state as an aminophthalate, omitting energy in the form of light waves at 425 nm.

For our basic oxidant, we used sodium hypochlorite (NaClO), otherwise commonly known as chlorine bleach. Chlorine bleaches are oxidizing agents; when chlorine reacts with water, it produces hydrochloric acid and atomic oxygen. The oxygen reacts easily to remove electrons from a nearby susceptible molecule, oxidizing it, essentially the same way that hydrogen peroxide removes electrons from luminol in Figure 1.

When observing both of these reactions, we learnt a few things. When hydrogen peroxide reacts with luminol, the reaction and chemiluminescent effect is immediate, whereas when sodium hypochlorite reacts with luminol, the chemiluminescent occurs slowly and in spikes. This data will be explored more in depth in the data section of our DYO. However, we did learn a few things from this lab. For one, we learned about the oxidizing nature of both acids and bases. Like hydrogen peroxide, most acids are able to accept electrons instantaneously. This is shown in the instant chemiluminescent effect shown in the reaction between luminol and hydrogen peroxide. The slow and fluctuating occurrence of chemiluminescence in the reaction between our base, sodium hypochlorite, gives us a perception of how weak bases oxidize: slowly. Obviously it is unfair to generalize the abilities of all acids and bases, but it would be decrediting this lab to say that we have not gotten insight on how different types of oxidants affect reaction rates. Who would have thought that in a lab where we attempted to explore what oxidant would produce the most chemiluminescent would have taught us about what oxidants work best and most efficiently.

Sources:

• Moore, Justin Shorb, Xavier Prat-Resina, Tim Wendorff, E. V., John W., & Hahn, A. (2020, August 17). A Greener Bleach. Retrieved April 26, 2021, from https://chem.libretexts.org/@go/page/50875

• “What Is Chemiluminescence?” Science in School, www.scienceinschool.org/2011/issue19/chemiluminescence#:~:text=Chemiluminescence%20is%20the%20production%20of,see%20Figure%201%2C%20below).

• Boudreaux, Kevin A. Demonstrations - Luminol, www.angelo.edu/faculty/kboudrea/demos/luminol/luminol.htm.

• Shakhashiri, Bassam Z. Chemical Demonstrations. University of Wisconsin Press, 1983.