Double distilled glycerine

Why is it absolutely necessary to make use of freshly prepared double-distilled glycerine in the cyclization of ort/io-benzoyloxyacetophenone ... 

Chemically pure ylyoerim or double distilled glycerine is produced by redistilling once distilled glycerine. Every care is taken to avoid all fractions which do not withstand the nitrate of silver test. The distillation is very carefully performed under strict supervision. 

The purest is the double distilled glycerine with a percentage of water of only 2-3 per cent. It must be colourless and should not possess any unpleasant smell. Once-distilled glycerine must also answer these requirements, but it generally contains slight quantities of salts. 

Distillation improves the quality of glycerin, but even a double-distilled product often contains color and odor that can be removed only by activated carbon. The ease with which glycerin can be decolorized increases with distillation. A crude glycerin is relatively difficult to decolorize to a water-white product, but the task becomes easier after the glycerin has been distilled and is much easier on a twice-distilled glycerin. A low-soluble ash is required in carbons to be used for pharmaceutical glycerin. 

Double distilled chemically pure glycerine should, according to the indications in the German pharmacopoeia, stand the lollowing tests —... 

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