Sciencemadness Discussion Board

Discovering what elements are in a homogenous mixture

drakecai - 3-11-2010 at 20:21

How do you discover what elements are in a homogenous mixture

DDTea - 3-11-2010 at 20:34

Mass spectrometry, Atomic Emission Spectroscopy, X-Ray Fluorescence, Electron Microprobe, Instrumental Neutron Activation Analysis (if you happen to have a nuclear reactor at your disposal), wet chemical tests... There are a lot of options. Could you be more specific with your question? What kind of homogeneous mixtures are you interested in? Solids? Liquids? Gasses? Biological samples? Geological samples? Air samples? Are there any particular elements that you are interested in?

[Edited on 11-4-10 by DDTea]

drakecai - 5-11-2010 at 14:49

liquids. to be more exact pen ink from the companies Bic, Papermate, and Pilot. The colors red, blue, and black. I wish to know the common properties all those inks share.

not_important - 5-11-2010 at 15:30

They all contain carbon, hydrogen, oxygen, and mostly likely nitrogen; some also contain sulphur and a few contain chlorine or bromine.

None of which is going to tell you much about inks, as running random organics through such analysis would give similar results. Dyes are less about the particular elements, and more about molecular structure.

You'll find common properties are more likely to be viscosity, surface tension, possibly non-Newtonian flow characteristics, speed of evaporation of the carrier fluid - all physical properties once again much more related to structure than elemental analysis.


Note - copper is fairly common among the blue a green pigments, as the phthalocyanines are strongly coloured and generally quite stable. Iron and manganese are two other likely suspects, from there it branches out into a wide array of inorganic pigments often on/in Al2O3, SiO2, and ZrO2; however these are fairly uncommon.



[Edited on 6-11-2010 by not_important]

drakecai - 6-11-2010 at 19:13

Are there any specific properties that relates to their color? Also can you change their color for say blue to green or blue to red? Is it also possible to rearrange the structure or remove certain elements out of the structure?

Thanks! Just trying to experiment :D

drakecai - 7-11-2010 at 21:28

anyone?

DDTea - 8-11-2010 at 17:24

Quote: Originally posted by drakecai  
Are there any specific properties that relates to their color? Also can you change their color for say blue to green or blue to red? Is it also possible to rearrange the structure or remove certain elements out of the structure?

Thanks! Just trying to experiment :D


At the most fundamental level, color is related to the electronic structure of an atom or molecule. Formally, the best way to begin describing this is through the solution of the energy levels to the particle in a box problem. The take-home message from that, though, is that there are discrete wavelengths that a molecule (or atom) will absorb or emit and a larger "box" size (i.e., a bigger 'field' that electrons can run around) corresponds to absorption/emission of longer wavelength light. This is the principle of quantized energy. So when a molecule is exposed to white light, it absorbs only a small part of the spectrum and reflects or transmits the rest. Therefore, when we see color, we basically see white light minus that part of the spectrum that the molecule has absorbed.

There are two common sources of brightly colored molecules that I can think of: organic molecules with extended pi networks and transition metal complexes. Organic molecules might be the simplest to understand. Basically, the longer a chain of conjugation (i.e., alternating single and double bonds), the bigger the molecule's pi orbital is. As such, the electrons have a bigger field to run around and, as the particle in a box problem says, will absorb longer wavelengths of light. Certain parts of molecules are often called "chromophores" in the context of their behavior upon exposure to light. Examples would be NO<sub>2</sub> groups or benzene rings.

Now if you're asking about how to alter the color absorption properties of molecules, that's easy. Since color is the result of light interacting with different electronic environments, then all you have to do is alter the electronic environment of the molecule! A common example is bleach, which oxidizes several chromophores and breaks up extended pi networks.

As not-important mentioned, transition metal-based compounds also tend to be brightly colored. Different ligands can produce different colors, in that case. Transition metal compounds are nicely described by ligand field theory. However, the basic principle is the same: that energy levels are quantized and only discrete wavelengths of light are absorbed by a particular molecule.

[Edited on 11-9-10 by DDTea]

drakecai - 9-11-2010 at 23:24

are there any types of lasers I can use to change the pi network and the chromophores?

DDTea - 9-11-2010 at 23:53

No, because the the pi network and chromophores are the result of the molecular structure. You would have to alter the structure itself. However, that isn't as complicated as it sounds. For things like proteins or nucleic acids, heating them until they denature can dramatically affect which wavelengths of light they'll absorb. A clever experiment would be to monitor the absorption at a particular wavelength over time to measure the rate of denaturing. This is one good application of UV/Vis spectroscopy, which is based on the absorption of light by chromophores.

Again: what are lasers? A source of coherent light; i.e.: the photons are in-phase and of the same energy. Remember, color is the result of electrons interacting with photons. Formally, this would be explained through quantum electrodynamics. Let me recommend a book to you: Richard Feynman - QED.

Lasers can be a good tool for the experimenter because of this, but you really have to know how to use them. Another very cool experiment is to analyze lasing media in UV-Vis and Fluorescence spectrometers. This might help you to understand how Ligand Field Theory works. If you aren't familiar with this equation, this is what links energy to the wavelength of light:

E = (hc)/λ

where E = energy of a photon, h = Planck's constant, c = speed of light in a vacuum (constant), and λ = the wavelength of the photon. With this in mind, if you know the wavelength that a molecule absorbs or emits light, you can learn about the energy separation between its ground state and excited state.

I like you. This is the sort of stuff I find exciting :)

drakecai - 11-11-2010 at 00:39

hahaha thanks. i'll be sure to order that book on Amazon. Is there any way I can figure out which bleach to use perhaps to change perhaps black to blue? etc.

DDTea - 11-11-2010 at 01:12

Aha! Now I see what you are trying to do, and that's a more complicated problem. The short answer: there's no easy way to do it :P

drakecai - 12-11-2010 at 14:29

hahaha what about the long answer? :D

DDTea - 13-11-2010 at 03:20

Well, I could only begin to touch the long answer. First of all, you have to define what the chemical source of the black color is. Again, color is the result of the molecular and electronic structure of molecules. Black is the total absorption of white light such that none is reflected back. In reality though, true black is rarely found.

In any case, what you want to do is tweak the absorption properties of a molecule from absorbing most of the visible spectrum to absorbing only the redder (longer wavelength) parts. That isn't easy to do and the route that you'd take to accomplish that would depend upon the specific chemistry of the compound responsible for the black color.

In the simple case of say, 1,3-butadiene, we know which wavelengths it absorbs (it's in the ultraviolet region). By expanding the pi network by somehow lengthening the carbon chain while keeping the conjugation, e.g., 1,3,5-hexatriene, we can lengthen the "box" that the electrons are free to roam around. From the solution to the particle in a box problem, this would decrease the energy separation between the HOMO and LUMO of the molecule, thus it would absorb lower energy wavelengths. That would mean that it would delete "redder" parts of the visible spectrum when it is reflected back to your eyes, which would make it appear more blue.

This just touches on the theory behind color. This is saying nothing of the actual chemical steps you'd need to do to change a compound of one color to a compound of another color. That, as I mentioned earlier, depends upon the specific chemistry of the compound. Nevertheless, it is an interesting field because such techniques are useful in, for example, wet chemical tests or colorimetric determination. You may want to read up on the chemistry of indicators.

Truthfully, I don't know as much about this subject as I wish I did! I hope I can give you enough information to at least spark some interest in this field :)

drakecai - 13-11-2010 at 17:26

mhmm, is there any difference between changing black to blue and just deleting the black color completely?

DDTea - 13-11-2010 at 22:57

Let me clarify a few more things to give you some better perspective:
-Truly white light is noncoherent light comprising all wavelengths of light from radio waves all the way up to gamma rays. The light from the sun that actually reaches our eyes has many of the more energetic frequencies attenuated (i.e.: diminished) by Earth's atmosphere.

-When white light hits a molecule, some of the wavelengths are absorbed by the molecule. Which specific frequencies are absorbed depends upon the electronic energy levels of the molecule because electronic transitions are quantized. To solve for the specific energy levels, you would have to solve the Schrodinger equation for the system or use other nefariuos mathematical techniques. For this explanation, a qualitative description will work. Those wavelengths of light that are absorbed by the molecule are "deleted" from the spectrum of white light that is incident on the molecule. So when the incident light beam is reflected back to our eyes, we see the spectrum minus the wavelength that was absorbed due to the specific, quantized, electronic transitions of the molecule.

-When something appears black, and I mean truly black, to our eyes, it is because it is reflecting *nothing* back to our eyes--i.e., it has done something evil with the incident light. So black is not a color, per se, in the same way white light is not truly colored. White light is *all* wavelengths of light, while black is the absence of all wavelengths.

What I'm trying to say is that to simply ask if you can change "black" to "blue" is a question that overlooks all the details I'm talking about. You aren't simply changing the color, you are changing the entire molecule--and thus its electronic energy levels, which in turn corresponds to which wavelengths of light it is absorbing and not reflecting back to your eyes.

Your original question was concerning pen inks though, right? Are you familiar with chromatography? Black pen ink has many different color-causing molecules in it! A cool experiment is to take a strip of printer paper, scribble a few lines of pen ink onto it, and put it about 1/8" into a cup of nail polish remover. Watch how a whole rainbow of colors separates. This is a very crude chromatography experiment. What it shows, though, is how many components are involved in making that black color.

drakecai - 14-11-2010 at 01:20

mhmm my old chem teacher used to give us as a lab. I guess changing the entire molecule is a bit out there.