Hello to everyone,
I am wondering if someone can help me.
During the photocatalytic degradation of Methylene Blue I observed a blue shift of absorbtion peak in UV-Vis measurments. In order to measure the Concetration of Methylene blue which is the correct method?
According to the literature, this shift is due to some subspecies that appear, such as azure A, Azure B, etc. That is, some other molecules. Which means that the MB molecules are no longer present or their concentration is decreasing and the molecules of each subunit are superior in concentration. Is that right?
Thank you !!
Determine the maximum wavelengths of your dye.
Take different concentration by applying beers law and determine the absorbance vs. concentration at the maximum wavelengths
Draw absorbance (y) vs. concentration (x)
You will need to add a line of best fit to the data points and determine the equation for the line.
The equation should be in y=mx + b form. So if you subtract your y-intercept from the absorbance and divide by the slope, you are finding the concentration of your sample.
Check how the shoulder at 615 (approx) nm does not change during the different concentrations, maybe at the yellow curve you need to calculate the first derivative to detect the shoulder.
Is the shift dependent of concentration? What is the difference between the scans You present? The standard protocol for MB is to use the peak at 664.
Dear Henrik Romar
The concentration is dependent of the shift. This is because the Methylene blue (MB) is photo-degradated by the photocatalyst and the concentration of MB decreased.Also in the same time with the degradation a blue-shift is observed as i explained in my question.
The standard protocol as i know is to use the peak at 664nm only if the beak is this value. In my case this peak is shifted to other smaller values as you see in the diagrams
I hope Ι answered your question.
Michael
Dear Javier Rivera De la Rosa ,
As i understand you suppose to do the calibration curve o Abs(Conc) by taking the peak at 615nm, don't you? I think that your answer has a point, but what happens in the case that the peak at 615nm doesnt exist and i have only one peak as you see in the yellow case at 644nm (approx)?
Michael
Dear Mutasem Z. Bani-Fwaz ,
This is what i was doing to get the concentration and i thought that is was right (by taking the peak of 6 diffenent solutions of MB in different concentrations ->making the Calibration Curve -> linear least-square etc.) . But as you see I have a blue shift during my experiments of Methylene Blue degradation under photocatalysis process. Is it correct, to take the absorbance value of the shifted peak and equate it into the linear least-square ?
Dear Michael Karavasilis , you can use the first derivative in order to evidence the peak that in the normal curve is not possible to see.
Dear Michael Karavasilis ,
In my adsorption experiments I saw exactly what you are observing. and the peak shifted after a long contact time in pH> 11
So I started to change the pH to maintain the peak at 664 in order to function at that long contact time. MB is some sort of indicator so by adding adequate amount of HCl and NaOH I was able to balance the reduction (colorless) and hydrolyzation (purple) of MB which caused the shift in the wavelength.
I am not sure about your experiments but also check the pH of your stock and final solution maybe it helps.
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