[lablackey]

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.

[sjb]

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?

[lablackey]

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?

[lablackey]

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.   

[sjb]

162 would appear to refer to the starch monomer here (a hexose, less one molecule of water)