Chemical Forums
Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: lablackey on December 01, 2009, 08:50:07 AM
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This seems so simple, but I can't figure out what I'm missing.
I'm doing acetyl substitution analysis on starch. The procedure I'm using was written ages ago by someone who isn't with the company anymore and I see now I should have double checked it earlier, but anyway...
The calculation for DS is given as
Degree of substitution = 162 * % acetyl
4300 – (43 x % acetyl)
But I got some literature the other day and they give the calculation as
162 x %A/[4300-(42x%A)]
Since the molecular weight of an acetyl group (-O-C-CH3) is 43
what's with the 42? ???
Once we've all stopped snickering about the Hitchhiker's reference can someone help me out? Thanks much.
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Presumably if you didn't have an acetyl group on there, you'd just have a hydrogen, and the difference between CH3CO and H is 43 - 1 = 42?
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Yes, I realize that, but since you're calculating the entire acetyl group and that has the whole 3 hydrogens, why drop one for the calculation?
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Okay since I'm still trying to figure this out, I thought I'd add that I found a general calculation for degree of substitution.
DS = 162W/100M - [(M-1)W]
Where W is the weight or weight percent in this case
and M is the molecular weight of the substituent in question
So the 42 makes sense
I guess what I really need is a better understanding of the calculation of degree of substitution. :-\
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162 would appear to refer to the starch monomer here (a hexose, less one molecule of water)