[kapital]

When we put UV ligh througt some sample, we said that it absobs UV radiation.(for example water solution of phenol). Than we say that phenol absobrs UV raditon.

But even, if we put UV ligt just thugt water, theri is some absorbtion.(or lost ov UV radiation at least) Why is that?(even if water doesnt absob it?)

And why do some thigs and materials dont put UV lig thrugt them at all? For example, if we put UV radiont thrug wall, nothing wont come out, but the wall of course dosent absorb that?

And why can be absorbance of UV light bigger, just because the sample is colored?

Thanx for answers.

[Arkcon]

Um ... yes.  Uh, I mean no, I mean ... what?

Briefly, yes phenol absorbs at a particular wavelength that we can select in a UV spectrophotometer.  But other things, like water, alcohols or solvents such as acetonitrile also absorb, particularly at much shorter wavelengths, and we tend to not make instruments very efficient at those short wavelengths, for that treason.  And yes, a solid opaque object is definitely absorbing all wavelengths, there shouldn't be a surprise in that.

[kapital]

And yes, a solid opaque object is definitely absorbing all wavelengths, there shouldn't be a surprise in that.

Why?(since it doenst have any conjugated bonds or anthing like that)

[Arkcon]

Really?  What are the various opaque objects that surround you made of?  Wood, for example?  Also, no, conjugated double bonds are not the only thing involved in light absorption.

[Jasim]

This is the Beer-Lambert law. Absorbance = (molar absorbity specific to a compound) * (the distance the light travels through the sample) * (the speed of light).

For practical applications we don't care about what absorbs and what doesn't, what we care about is the spectra, or how much a compound absorbs at a specific wavelength of light. Many clear solvents like water and ACN absorb only at lower wavelengths of light, but we don't look at those wavelengths. We look at wavelengths in the range of 210-260nm. Sometimes higher or lower depending on the application, but that is the range. We also look at a SPECIFIC wavelength of light, not a spectrum of wavelengths.


Let's take some compounds that everyone knows absorbs some specific wavelengths of light, those involved in photosynthesis (it was the best spectra image I could find quickly on Google).
You wouldn't normally be looking at wavelengths this high, but for the pedagogic sake lets assume you wanted to See Chlorophyll-a and Chlorophyll-B. Let's also assume in our hypothetical example that you first run at a wavelength (reference the spectra here) around 500 nm. What are you going to see?

Chlorophyll-A does not absorb at that wavelength, at least not very well at all. Carotenoids absorbed well as does Chlorohpyll-B. Lets assume you don't care about carotenoids, but they just happen to be co-eluting (that means coming out on top of) your Chlorophyll-B, so you can't see the Chlorophyll-B that well. You also can't see Chlorophyll-A, so this wavelength doesn't do what you need it to do.

Then you try wavelength around 660nm. Here the carotenoids do not absorb at all, and the Chlorophyll-A and B both absorb, so you can see both of them and don't have the interference from the carotenoids.

Okay back to real life, So water and ACN do not absorb at the typical range of 210-260nm, just like the carotenoids don't absorb at all after about 550 nm. If you try to run an analysis at a lower wavelength almost everything would absorb and you wouldn't be able to distinguish anything, it would all be coming out on top of each other (co-eluting).

I hope this has helped to clarify some points. Let me know if you have more questions on this subject. I actually know a lot about this, it's what I do all day at work.

[kapital]

I do not understand, why can some molecules absorb all UV light, even if they do not have conjugated double bonds or any similar elements.

[Arkcon]

If you must list every opaque substance by functional group, you do realize that the chemical component of wood -- =Lignin , is multiple polymers of aliphatic and aromatic functional groups, so you can see congugated bonds there. There are of course other components in wood. Metals are different, they don't have conjugated double bonds, but are opaque because they, nonetheless, have their electron clouds overlapping.  How exactly various inorganic minerals that make up rocks are all opaque, I don't really know, but should be somewhat analogous to how metals are opaque.  So there you go -- wood, metal, and stone.  All simple examples of opaque materials, with reasonably simple (and I have to simplify that far, because the physical chemistry really get s away from up if you want more) explanations for why they're opaque.

[kapital]

OK. But what if we have a solution of beta-carotene at wave-lengt that it doesnt absorbs, but if we add a powder of beta carotene in cuvette, it will definitly be absorbtion(probably 100%)?

[fledarmus]

Absorbing and transmitting aren't the only things that a substance can do to electromagnetic radiation. Two other options are reflecting and refracting. White opaque materials are reflecting essentially all of the light that comes to it. Black opaque materials absorb all of the light that comes to it. "Transparent" materials will transmit most of the light that comes to it - there is always some scattering. Translucent materials will absorb part of the light, transmit part of the light and reflect part of the light - if the material is thick enough, none of the light will make it through.

Only a pure vacuum will perfectly transmit all of the light that is directed through it - all other materials will scatter it to some extent, either by reflection, refraction, or absorbance. Even a lake of pure water, if it is deep enough, will be dark at the bottom.

[kapital]

Why does a powder(of beta-caroten for example) absors all radiation(or reflects it), but solution of beta-carotene does not?

[Arkcon]

Why does a powder(of beta-caroten for example) absors all radiation(or reflects it), but solution of beta-carotene does not?

Specifically, how do you mean this?  If I have a pile of beta-carotene, it looks to my eye as yellowish orange, because it reflects those associated wavelengths, and absorbs some others.  My eyes are not calibrated to nm scales, so I do not report any such observation, just that it looks yellow orange.  A solution of beta carotene looks to me to have similar color, and transmits -- allows the spectrophotometer to "shine through" a certain wavelength range.  Is that the question?  How a solution -- by definition a dispersed mixture, transmits the wavelengths that are reflected in bulk?  Because you can do that with anything, even a solid, if you slice it thinly enough.

[kapital]

What I woud like to know is what changes with molecules, when they are put in solution? When we put solid object(power, pil,..) in cuvette and put uv ligt thrut it it will absorb some and it will probably also absorb and reflect some. But in solution, just it just absorb.

[Arkcon]

What I woud like to know is what changes with molecules, when they are put in solution? When we put solid object(power, pil,..) in cuvette and put uv ligt thrut it it will absorb some and it will probably also absorb and reflect some. But in solution, just it just absorb.

And it will transmit some wavelengths, hold the cuvette up to your eye and see -- suspiciously, the same wavelengths are transmitted in solution that are reflected in bulk.  Look at a green plant leaf, see the wavelengths reflected, then, hold it up to the light, and the the wavelengths transmitted ... suspiciously, the same.

[kapital]

What does a leaf have with my question? You can not prepare solution from leaf.
When we put powder of some substance in solution and some of its optical properties change and I do not know why. (but yes if we put red powder in solution it will still be red)

[Enthalpy]

I do not understand, why can some molecules absorb all UV light, even if they do not have conjugated double bonds or any similar elements.

Light absorption needs charge carriers with some mobility, so they interact with light: free electrons, electrons whose orbitals can deform, ions or polarized molecules with some mobility like rotation or vibration... There are many ways!

Now, if you want to produce a dye, it must absorb some portion of visible light. Though, electrons (the most efficient absorbers) tend to be on rather stiff orbitals because these are small. The solution is to produce molecules with huge orbitals, which are easier to deform by the incoming light, so the resonant frequency (frequency band, more often) is lower and falls within the visible spectrum.

The most common method to obtain huge orbitals is to have electrons delocalized over many atoms, typically by conjugated ethylenic bonds. Phenyl groups at the ends also contribute to orbital "elasticity" by sharing an electron lack or excess among many atoms.

Though, these tricks for visible light aren't needed for UV when light frequency naturally matches the transitions between small orbitals.

More: for light absorption, the main molecule isn't necessarily the absorber. The slightest amount of impurity or dopant already colours a crystal; for instance quartz takes other names depending on tiny quantities of other components that change its colour.

Interesting for lasers for instance. Lasing needs a population inversion, meaning that >50% of lasing molecules are excited: this would destroy the lasing medium if pure. But in a YAG, the transparent garnet matrix holds the lasing ions, which are the active medium despite being in dopant quantity.