[heisenbergy]

Hey guys! I hope someone can explain this to me, since its really boggling my mind for quite a while now, and I just can't seem to find the answer on the internet.

We had this little experiment on Medical Biochemistry where we have we would measure the Absorbance of Methylene Blue using a spectrophotometer (the methylene blue was diluted with water, by the way.) The Absorbance was measured in 4 different wave lengths; 440 nm, 520 nm, 580 nm, and 660 nm.

At 440 nm the Absorbance was 0.01. Absorbance was increasing as the wavelength use gets longer and it was 0.155 at 580 nm. I expected it to go higher as we try longer wavelengths, but when we tried 660 nm, the Absorbance went back to 0.01. We repeated the process a few more times but the result was constant.

So my question is, what in the world is going on? It is a chemical with a blue color so it, at least for me, makes sense that it doesn't absorb that much short wavelength but absorbs long wavelengths. So why does it suddenly stops absorbing wavelengths at around 660 nm? If it doesn't absorb light at that wavelength then shouldn't it exhibit a combination of bluish and reddish color?

[Arkcon]

It a little hard to pin down what your question is, although I do get the gist, I think you're building the wrong conclusions for your observations. Lets start with a spectrum.  You can see it here, arbitrary absolution units, across a range of wavelengths.  We have a rainbow under the curve, so you can see what we'd call the colors being absorbed.  This spectrum is pretty typical for molecules that absorb visible light.  Maybe you can ask your question again?  telling us exactly where you don't expect an absorbance and why?

[heisenbergy]

Actually I realized my mistake and source of confusion after reading your reply and checking the methylene blue absorption spectrum graph that you included.

You're right that my conclusions were wrong, and it was the main source of my confusion. I should have checked the Methylene Blue absorption spectrum graph or atleast graphed the results of our experiment. By doing so, I should have noticed the downward curvature towards the shorter wavelengths and the abrupt rise and fall on the longer wavelength end. The range of wave length not absorbed at the short wavelength end of the visible spectrum is much bigger than the range of wave length not absorbed at the long wavelength end. Now it all makes sense! Ha!

Thank you for the reply!

[Babcock_Hall]

The spectrum that is shown appears to have a λ_max near 660 nm.  What am I missing?