luminol and bleach experiment

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Luminol and chemiluminescence.

February 6, 2019 English Posts , Light 27,956 Views

Chemiluminescence

Chemiluminescence  is the emission of electromagnetic radiation, particularly in the visible and near infrared, which can accompany a chemical reaction. Considering a reaction between the reagents A and B to give the product P:

A + B → P* → P +  hν

In practice, the reaction leads to the product P in an excited state and the decay to the ground state does not lead to the formation of heat, but of a photon ( hν ). It is therefore necessary that the mechanisms of radiative decay are more efficient than those that are not radiative.

An example of a reaction that leads to chemiluminescence is that of luminol with hydrogen peroxide catalyzed by metal ions.

Luminol  (C 8 H 7 N 3 O 2 ) is a chemical that exhibits chemiluminescence, with a blue glow, when mixed with an appropriate oxidizing agent. Luminol is a white-to-pale-yellow crystalline solid that is soluble in most polar organic solvents, but less soluble in water. Forensic investigators use luminol to detect trace amounts of blood at crime scenes, as it reacts with the iron in hemoglobin. Biologists use it in cellular assays to detect copper, iron, cyanides, as well as specific proteins.

To exhibit its luminescence, the luminol must be activated with an oxidant . Usually, a solution containing  hydrogen peroxide (H 2 O 2 ) and  hydroxide ions in water is the activator. In the presence of a catalyst such as an iron or periodate compound, the hydrogen peroxide decomposes to form oxygen and water :

2 H 2 O 2  → O 2  + 2 H 2 O

Laboratory settings often use  potassium ferricyanide  or potassium periodate for the catalyst. In the forensic detection of blood, the catalyst is the iron present in haemoglobin. Enzymes in a variety of biological systems may also catalyse the decomposition of hydrogen peroxide. Luminol reacts with the hydroxide ion, forming a dianion. The oxygen produced from the hydrogen peroxide then reacts with the luminol dianion. The product of this reaction — an unstable organic peroxide — is made by the loss of a nitrogen molecule, the change of electrons from triplet excited state to ground state, and the emission of energy as a photon. This emission produces the blue glow. The image below shows schematically the reaction that produces the luminescence:

We have prepared two solutions :

• Solution A Mix 5 grams of Sodium Hydroxide in 1000 ml of water. When thoroughly mixed & dissolved, pour some of this solution in a small (50 ml) beaker and add 0.1 grams of Luminol . Luminol is difficult to dissolve so to help, with a glass rod keep smashing the Luminol powder until it all goes into solution. When the Luminol is finally dissolved, pour the contents of the small beaker into the rest of the Sodium Hydroxide solution.
• Solution B Mix 10 ml of 3% Hydrogen Peroxide (regular drug store variety) in 1000 ml of water.

The image below shows the two solutions. The catalyst (Iron, Copper, …) is to be added to the solution B. Mixing the two solutions will produce the light emission from the chemiluminescence of the chemical reaction.

Experimental Setup

For the measurement of luminol chemiluminescence, we used the “dark box” already described in the posts: Photon Counting & Statistics , Glowing in the Dark . The solution “B” with the reaction catalyst is placed inside a glass bottle placed in front of the PMT. The solution with luminol is placed in a syringe outside of the box. After closing the box and starting the acquisition by the PMT, the luminol is introduced into the bottle with the syringe. The image below shows the experimental setup used:

Three different catalysts were used: potassium ferrocyanide (Fe ion), copper sulfate (Cu ion) and bleach (sodium hypochlorite).

Luminol Reaction with Iron Catalyst

The graphs below show the trend of the light emission catalyzed by the iron ion contained in the potassium ferrocyanide. After a first phase in which the emission increases and reaches a maximum, there is a decay with an exponential trend.

Luminol Reaction with Copper Catalyst

The graphs below show the trend of the light emission catalyzed by the copper ion contained in the copper sulphate. The brightness decay follows an exponential trend with two different time constants.

Luminol Reaction with Bleach Catalyst

The graphs below show the trend of light emission catalyzed by sodium hypochlorite. In this case, with respect to iron and copper, the increase in brightness is quite slow and the subsequent decay is exponential with two different time constants.

From the comparison between the three different curves we can say that the first part reflects the kinetics of the chemical reaction between the reactants: the reaction catalyzed by copper is faster than that catalyzed by iron while the reaction with sodium hypochlorite is the slowest one. The subsequent decay of luminescence generally follows an exponential trend (similar to the phenomenon of phosphorescence).

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Tags Luminol

Detection of beta and alfa radiation with KC761B

Abstract: in this article, we continue the presentation of the new KC761B device. In previous posts, we described the device in general terms and its functionality as a gamma spectrometer. In this post, we describe its use as a beta and alpha radiation detector. To detect beta and alpha particles, the device uses a PIN-type semiconductor sensor positioned on the back of the device.

Experimental Techniques

If your school has palm-top computers with scientific interfaces then this will make your job a lot easier. For my project I was able to create a basic light detector by using an LDR (light dependant resistor) connected through a support system to a palm-top which could be adjusted to take and automatically log measurements of light intensity over a set period and with set intervals. After calibrating the LDR with a light source of known intensity, I was able to set-up the equipment, press “go” and after thirty seconds I had more than 250 sets of data in a spreadsheet.

It should be noted that often, an increase in rate will compromise total light yielded (light sticks can glow for weeks in a freezer).

Without this equipment a more traditional (but less accurate) approach can be adopted by laying out a long thin piece of transparent tubing around a clamp stand with a funnel at the top through which to pour your reactants. An estimation of relative rate can be made by noting the time or point on the tube at which luminescence stops.

Tip : Always try to secure your glassware and spend a bit of extra time putting items necessary for the reaction in easily accessible places because you are likely to spend a lot of time fumbling around in the dark.

Variables to Explore

Concentration We’ll start with the obvious ones, this is GCSE stuff. The more concentrated your solution is, the more light it will produce (unless of course there is a significant amount of colour from your reactants – in this case there will be an awkward relationship as concentration increases because your solution will begin to absorb its own light). This is obviously due to the fact that there are more molecules reacting, producing more light. One thing you might want to explore is the Beer-Lambert Law. This should be in textbooks but will allow you to make a quantified analysis of increasing light production.

Temperature Temperature is another obvious one. Higher temperature means a higher rate. For “mickey mouse” points you could show that the reaction is less bright in an ice bath than in a warm water bath. Note that not all chemiluminescent reactions get faster with increasing temperature (do chemistry at university to find out about this one).

The rate of the reaction of luminol is a little more complicated than you will have come across in school but I will attempt to explain it using some interesting concepts. Talking about the Arrhenius rate law will no-doubt get you points so do this and you should be able to get a good mark. For an excellent mark, read on.

The Steady-State Approximation The luminol reaction could be seen as being made up of two steps; the attack by base and then the subsequent oxidation (the last step is so fast that it will have no effect on the overall rate). To explain the diagram, the first step is in equilibrium so will have a forward and reverse rate constant, k 1 and k -1 . The second step has rate constant, k 2 .

The overall rate law is going to be a combination of both these steps but because there is a lot more water around in an aqueous solution than there is oxygen, k -1 will be much larger than k 2 . (A) is going to be an awkward thing to measure, it will proceed to products very quickly and therefore be at low concentrations and very short lived.

The steady-state approximation allows us to deal with this problem by assuming that the concentration of (A) will be low and steady (or in other words d[(A)]/dt will be approximately zero).

Writing the rate law in terms of (A) (d[(A)]/dt) allows us to remove it by saying that this is zero.

The rate law is d[(A)]/dt = k 1 [luminol][NaOH] 2 – k -1 [(A)][H 2 O] 2 = 0

Which becomes (remembering that water has unit activity (ideal concentration)),

k 1 [luminol][NaOH] 2 = k -1 [(A)]

(White et al have shown that the reaction is strictly pseudo first order (reaction behaves as first order although may not necessarily be first order, this might occur when one rate constant is much smaller than another and becomes negligible in comparison) where k' = 2.5 x 10 -2 sec -1 )

I found chemiluminescence to only appear at pHs higher than 8-9 and found luminol to only dissolve easily at approximately 11.

Dyniewski et al have shown that three separate pKa values exist at pH = 1.5, 8, and 11.8 corresponding to each of the three hydrogen atoms able to dissociate.

Due to this, complete dissociation wil occur at around pH 11.8 making the luminol fully ionised and the most soluble in aqueous solvent. Below pH 8, the dissociation necessary for the reaction to proceed cannot occur and so there is no luminescence.

The pH chemistry of luminol chemiluminescence is very complicated and I failed to get any kind of conclusive data from my own experiments.

Addition / Concentration of Catalyst

Other Ideas

For a further investigation into luminol, try investigating electrochemiluminescence.