Eerie Blue Light of Luminol

The Eerie Blue Light of Luminol

Crime scene investigators use Luminol. It is used to detect blood on a surface at crime scenes even if it has been cleaned up or removed. When it detects blood it glows blue chemiluminescence. Chemiluminescence is the emission of light with limited emission of heat (luminescence), as the result of a chemical reaction. (Wikipedia, chemiluminescence, 4 March 2009). The crime scene investigators make up a solution of containing luminol and then they spray it on the area they think has blood. The iron in the blood catalyzes the chemical reaction and makes the blue light. The blue light glows for only about thirty seconds. To see the blue glow one would need to be in a dark room. Biologists to detect transition metals and cyanides also use Luminol. A transition metal is any of the metallic elements within Groups 3 to 12 in the Periodic Table that have an incomplete inner electron shell and that serve as transitional links between the most and the least electropositive in a series of elements. They are characterized by multiple valences, colored compounds, and the ability to form stable complex ions (, transition element.) Some examples of transition metals are titanium, copper, silver, etc. The transition metals, iron in particular, are important because they have multiple valence states, which allow them to regularly give up or receive electrons. An electron is a subatomic particle that carries a negative electric charge. (Wikipedia, electron, January 29, 2010)

Some factors that affect the rate of a chemical reaction are temperature, solvent, medium, surface area, catalysts and concentration. Usually, an increase in temperature is accompanied by an increase in the reaction rate. Temperature is a measure of the kinetic energy of a system, so higher temperature implies higher average kinetic energy of molecules and more collisions per unit time. A general rule of thumb for most (not all) chemical reactions is that the rate at which the reaction proceeds will approximately double for each 10°C increase in temperature. Once the temperature reaches a certain point, some of the chemical species may be altered (e.g., denaturing of proteins) and the chemical reaction will slow or stop. (Helmenstine, Ph d)

The first step of the reaction is the luminol and hydroxide producing a dianion. This dianion exists in equilibrium in two states. The side reaction H2O2 and the transition metal produce water and oxygen. This oxygen then reacts with the dianion to produce 1, 2-aminopothalic acid in the exited state. The final step of the reaction is when the aminopothalic acid relaxes and releases photons (Science Buddies, the eerie blue light of luminol.) The luminol solution glows blue when it is mixed with a transition metal such as copper, etc. In order to glow luminol has to be activated with an oxidant. This solution has an oxidant that is usually hydrogen peroxide and a hydroxide salt in water. When a catalyst is present, the hydrogen peroxide is converted into oxygen and water. Hydrogen peroxide is a very pale blue liquid, slightly more viscous than water, which appears colorless in dilute solution. It is a weak acid, has strong oxidizing properties, and is a powerful bleaching agent. It is used as a disinfectant, antiseptic, oxidizer, and in rocket fuel oxidizer as a propellant. (Wikipedia, hydrogen peroxide, 18 December 2009) Then the oxygen that is made from the hydrogen peroxide reacts with the luminol dianion. An anion is an ion with more electrons than protons, giving it a net negative charge (since electrons are negatively charged and protons are positively charged) (Wikipedia, Ion, 13 January 2010) and a dianion is an atom with two negative charges. The product is very unstable and quickly decomposes with the loss of nitrogen to make 5 - aminophthalic acid with electrons in the exited state. So when the exited state relates to the ground state, the leftover energy liberated as a photon, and it looks like blue light. a photon is an elementary particle, the quantum of the electromagnetic field and the basic "unit" of light and all other forms of electromagnetic radiation. (Wikipedia, Photon, 15 January 2010)

The amount of catalyst that is needed for the reaction to occur is a very small amount compared to the amount of luminol making it possible for the detection of even trace amounts of blood. Catalysis is the process in which the rate of a chemical reaction is either increased or decreased by means of a chemical substance known as a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. The catalyst may participate in multiple chemical transformations. Catalysts that speed the reaction are called positive catalysts. Catalysts that slow down the reaction are called negative catalysts or inhibitors. Substances that increase the activity of catalysts are called promoters and substances that deactivate catalysts are called catalytic poisons (Wikipedia, Catalysis, 22 January 2010).

The ground state is the state of a physical system having the lowest possible potential energy. For example, an electron in the lowest energy orbital in a hydrogen atom is in a ground state. The ground state of a physical system tends to be stable unless energy is applied to it from the outside; states that are not the ground state have a tendency to revert to the ground state, giving off energy in the process. (The Free Dictionary, ground state)

Increasing the concentration of the reactants will increase the frequency of collisions between the two reactants. You also need to discuss kinetic theory in an experiment where you vary the concentration. Although you keep the temperature constant, kinetic theory is relevant. This is because the molecules in the reaction mixture have a range of energy levels. When collisions occur, they do not always result in a reaction. If the two colliding molecules have sufficient energy they will react. The kinetic theory is the theory that gases are made up of a large number of small particles (atoms or molecules), all of which are in constant, random motion. The rapidly moving particles constantly collide with each other and with the walls of the container.

If reaction is between a substance in solution and a solid, you just vary the concentration of the solution. The experiment is straightforward. If the reaction is between two solutions, you have a slight problem. Do you vary the concentration of one of the reactants or vary the concentration of both? You might find that the rate of reaction is limited by the concentration of the weaker solution, and increasing the concentration of the other makes no difference. What you need to do is fix the concentration of one of the reactants to excess. Now you can increase the concentration of the other solution to produce an increase in the rate of the reaction. (purchon, chemistry, rates)

The rate of a chemical reaction depends on the medium in which the reaction occurs. It may make a difference whether a medium is aqueous or organic; polar or nonpolar; or liquid, solid, or gaseous. (Helmenstine, Ph. D.) Catalysts affect them a lot, because they speed up chemical reactions. Only a small amount of the catalyst is needed to produce a dramatic change in the rate of the chemical reaction. This is because the catalyst allows the reaction to proceed by a different pathway where the heat of activation is smaller. (Brian Viers)

The things that we know is that even the smallest amount of a transition metal will activate the luminol. And changing the amount of iron will change the rate of the reaction. The luminol hydroxide to the dianion state is in equilibrium to each other and will probably not affect the overall rate of the reaction. The factor that affects the rate of a chemical reaction the most is temperature. When the temperature of a substance is decreased, the rate of a chemical reaction will also decrease, because the higher the temperature will cause the particles to move faster and to increase the number and strength of the collisions. So, a lower temperature will slow down at which the particles move and lower the number and strength of the collisions.


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