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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: Zainb on July 06, 2011, 09:05:57 AM

Title: concentration for optical rotation
Post by: Zainb on July 06, 2011, 09:05:57 AM

any units used for optical rotation publications,, density units or molarity units ?
for example
 +27° (c 0.08??, CHCl3); for c here any units used usually?

Thanks

Title: Re: concentration for optical rotation
Post by: Honclbrif on July 06, 2011, 09:39:45 AM

http://en.wikipedia.org/wiki/Specific_rotation (http://en.wikipedia.org/wiki/Specific_rotation)

Title: Re: concentration for optical rotation
Post by: Zainb on July 06, 2011, 11:19:21 AM

Thanks Honclbrif,

I have seen it before.
In this article the unit which used, was density units g/mL.

Title: Re: concentration for optical rotation
Post by: fledarmus on July 06, 2011, 04:01:02 PM

Quote from: Zainb on July 06, 2011, 11:19:21 AM

Thanks Honclbrif,

I have seen it before.
In this article the unit which used, was density units g/mL.

Read down a touch further - only for pure liquids do you use the density. Everything else uses the concentration, which oddly enough also has the units g/mL, but it is grams of solute per milliliter of solution.

Title: Re: concentration for optical rotation
Post by: Dan on July 06, 2011, 07:07:22 PM

c usually has units g/100 mL unless otherwise stated (not g/mL). This is also stated in the wikipedia article.

Quote from: Wikipedia

The rotation is reported using degrees, and no units of concentration are given (it is assumed to be g/100mL).

Title: Re: concentration for optical rotation
Post by: Zainb on July 07, 2011, 05:19:37 AM

Thanks Honclbrif and Dan...

I try to confirm the optical rotation for some compounds but I use small scale like 5 mg in 0.6 ml of solvent with optical rotation tube length 0.05 dm..because I worked on small scale synthesis.

if I use g/100ml ,,, I hope it will be ok  ???.

Title: Re: concentration for optical rotation
Post by: Honclbrif on July 07, 2011, 09:01:13 AM

"5 mg in 0.6 ml of solvent"

If you had 100 mL of that solution, how many grams would be present?

Title: Re: concentration for optical rotation
Post by: Zainb on July 07, 2011, 09:56:22 AM

I worked on small scale,,, so I want to change units only.



http://www-jmg.ch.cam.ac.uk/tools/magnus/optRot.html

Title: Re: concentration for optical rotation
Post by: Dan on July 07, 2011, 02:35:26 PM

You shouldn't change the units. Specific rotation in the organic chemistry literature is reported invariably with units:

10^-1 degrees m² g^-1

and is calculated from the equation:

[a] = 100a/(c x l)

where a is the rotation in degrees measured on the polarimeter; c is concentration in g/100 mL; l is path length in dm.

The units are archaic, very odd, and rarely explicitly mentioned in the literature. I will now never forget them since I was asked to derive them in my PhD defence. That was fun.

Zainb, there is no reason to change your units, why on Earth do you want to?

Title: Re: concentration for optical rotation
Post by: Zainb on July 08, 2011, 05:26:40 AM

Thanks Dan  :-*,,,

I used the optical rotation calculator which available from this site,,,

http://www-jmg.ch.cam.ac.uk/tools/magnus/optRot.html

it give you the same result which can obtained by the equation

[a] = a/(c x l)

my question was on concentration between bracts:

 (c ...??, CHCl₃); so i will use g/100 mL for it.

Thanks again Dan

Title: Re: concentration for optical rotation
Post by: Zainb on July 13, 2011, 05:48:49 AM

I like to add the units are as of mass concentration

g/100mL (=g/dL)

so will be  (g/dL)

where dL is deciliter.