Impure acetic acid, or acetic anhydride?

[spectralmagic]

I got it into my head that I wanted some glacial acetic acid, and went about the route of converting vinegar + baking soda to sodium acetate (wasn't the tri-hydrate, mp was above 120C), adding that to sulfuric acid, and distilling acetic acid from that.  I used a large excess of sulfuric acid.  The liquid I distilled, whatever it is, smells strongly of acetic acid, but won't freeze, even in the freezer.  I'm thinking, maybe water got into it somehow, if perhaps that would lower the melting point below that of either water or acetic acid...  or maybe some other contaminant?  What could lower the mp below -15C?  Oh, the temp at which it distilled was 115-125C (the range surprised me somewhat).

Could I, by some miracle, have created some acetic anhydride in this process?  It has a mp of -73C...  I thought it required oleum to do that, sulfuric acid wasn't strong enough...  any tests I could perform to determine concentration and/or presence?

[Mountain_Girl]

From Ullmann's Encyclopedia of Industrial Chemistry:
*
Glacial acetic acid is very hydroscopic. The presence of 0.1 wt % water lowers the freezing point significantly. Measuring the freezing point is a convenient way to evaluate acetic acid purity.
Freezing points for various acetic acid – water mixtures:
% m/m HoAc  °C
100           16.75
99.6          15.84
99.2          15.12
98.8          14.49
98.4          13.86
98.0          13.25
97.6          12.66
97.2          12.09
96.8          11.48 
96.4          10.83
96.0          10.17
93.5           7.1
80.6         – 7.4
50.6         –19.8
18.1         – 6.3
*

Maybe you can cross check with density:
*
% m/m HoAc    Density g/ml @ 15°C
  1                  1.007
  5                  1.0067
 10                 1.0142
 15                 1.0214
 20                 1.0284
 30                 1.0412
 40                 1.0523
 50                 1.0615
 60                 1.0685
 70                 1.0733
 80                 1.0748
 90                 1.0713
 95                 1.0660
 97                 1.0625
 99                 1.0580
100                1.0550
*

[spectralmagic]

Thanks, that explains it perfectly.

[Chromic]

I've done this before, search for my nick. The acetic acid that I produced did freeze in the fridge... personally, I prefer to buy glacial acetic acid, but I wanted to prove it's very possible to make your own.