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March 30, 2017

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solvent layer: 1
solid precipitate: no

I ran the solvent layer I got in my unknown in the GC and I had sharp peaks in the beginning but at about 4 min on the polar column, I got a broad peak that tailed off considerably.

question is, Is it water?
I'm not sure how water would look if you had it in your sample after placing it in the GC. (Peak can't be the unknown compound because it's not volatile, right?)

I don't know how to make sure that there is no water in solvent layer either. Couldn't there be one layer if you have a polar solvent + water?
I tried to put a drop of the solvent layer on drierite to see if it changed from blue to pink, but it did not.
Does this mean that there is no water? couldn't it mean that there is more organic solvent than water as well?

Second test I did for water was adding solvent layer to water and shaking. It was partially miscible but not completely. (polar solvent?)

Is there any sure method to determine if there is water in solvent layer or not?

Thanks Dr.Bob

  • science(chemistry) -Q for Dr.Bob - ,

    hm...I was just reading that a flame ionization detector does not respond to water?
    now I'm confused.

  • science(chemistry) -Q for Dr.Bob - ,

    I don't know what to tell you. Does more than one sharp peak on the GC mean you had more than one solvent? An alternative might be that the solvent used was not very pure and the GC is picking up the main solvent plus the impurities. Perhaps the broad peak at the end was some of the solute and not water? What do you usually use as a drying agent? Here is a list of some common drying agents but I would try it on a small part of the sample just in case the solvent reacts with the drying agent you choose.
    http://en.wikipedia.org/wiki/Desiccant#Drying_of_solvents

  • science(chemistry) -Q for Dr.Bob - ,

    I did see more than one solvent. That's what I was saying, but the last peak didn't look like a solvent peak. It just looked like a hill compared with the other peaks that were sharp.

    I have never used a drying agent in this particular lab before. I guess Drierite is a desiccant that we have available, and that I mentioned experimenting with above.

  • science(chemistry) -Q for Dr.Bob - ,

    Drierite is CoCl2.
    The problem I have is that I'm not believing what I'm seeing. It did not turn from blue to pink like it would if it had water and the solvent just evaporated quickly after touching the drierite.
    BUT if there was water and there was just more organic solvent in the layer, wouldn't it do the same as the above described situation?

  • science(chemistry) -Q for Dr.Bob - ,

    Drierite is not CoCl2. I believe it is anhydrous CaSO4. INDICATING drierite has CoCl2 added to it; the CoCl2 is the indicator of water. Blue when no water, pink with water. CaSO4 without the CoCl2 would be my first choice as a drying agent if you need one.

  • science(chemistry) -Q for Dr.Bob - ,

    Okay, I'll try that then.

    Thank you,
    Dr.Bob

  • science(chemistry) -Q for Dr.Bob - ,

    Wait, the whole problem here is to determine whether there is or there is not water in the solvent layer.

    Utilizing the CaSO4 to dry the layer would not tell me if there was or was not water.

    basically,
    Is there any way to determine if there is water in solvent layer?

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