# Practical method to measure the rate of luminol reaction

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1. Oct 11, 2017

### ChanYoung Park

I was wondering how I could measure the rate constant k for chemiluminescence reaction of luminol (when oxidised by H2O2. I was told that I can use a lux meter that measures the intensity of light emitted, but I'm not so sure how this can be done since the equation calculating the rate constant uses the concentrations of reactants and products. What will be a practical method to measure the rate of luminol reaction?

2. Oct 11, 2017

### Staff: Mentor

Can you think about any dependence between the light intensity and concentration? Do you know what calibration is?

3. Oct 11, 2017

### ChanYoung Park

I would assume that the light intensity will be greater if the concentrations of reactants being reacted are greater. Calibration is setting the equipments to their standards. Do you mean that I can come up with a relation between the concentrations and the light intensity of the reaction and then find the constant by actually conducting the experiment?

4. Oct 12, 2017

### DrDu

I wonder whether this reaction can be described in terms of a single reaction constant anyhow.

5. Oct 12, 2017

### Staff: Mentor

That for sure.

Beware: "light intensity of the reaction" sound strange. "Reaction" itself doesn't produce light. Reacting molecules do, so the amount of light produced will be proportional to the number of reacting molecules (and in turn will depend on the concentration, or more precisely concentration change).

6. Oct 12, 2017

### ChanYoung Park

I think I phrased it wrong there. What I meant was that I would want to find the rate of reaction.

7. Oct 12, 2017

### ChanYoung Park

So... to rephrase what you said - please check if I'm right on the track - the light intensity of the light emitted at time t will be directly proportional to the number of reactant molecules reacting at time t, and based on this logic, I'll be able to know how the rate of reaction changes as time passes (using light intensity-time graph). Though how can I figure out the exact value of rate of reaction? I'm guessing that I would need to calculate E=hv per exicted mol and divide the energy emitted by the E value to get the mol of reactants. Will this work?

8. Oct 12, 2017

### ChanYoung Park

Also, If I'm measuring the light intensity, which equipment should I use? Photometer? Lux meter? or LDR connected to a multimeter?

9. Oct 13, 2017

### DrDu

I fear it is difficult to obtain absolute concentrations from the light measurements. The point is that it depends strongly on the geometry of your setup (percentage of the light reaching the detector), the sensitivity of the detector, internal absorption of radiation, photon efficiency of the reaction ...
Maybe you could repeat the measurement with different starting concentrations of luminol and then gauge the difference in intensity to the difference in concentration.