This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol1", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
Morphiae Acetas. U. S. - To prepare this salt, the U S. Pharmacopoeia directs morphia, deprived of narcotina by means of ether, to be mixed with water, and acetic acid to be gradually dropped in till the morphia is dissolved. The solution is then evaporated, by means of a water-bath, to the consistence of syrup, dried by a gentle heat, and rubbed into powder. As thus obtained, it is amorphous, slightly coloured, and in general not wholly soluble in water. This is owing to the escape, during the drying, of a small portion of acetic acid, which leaves a corresponding portion of the morphia unsaturated, and of course, insoluble. All that is necessary to effect a perfect solution is to add a little distilled vinegar or diluted acetic acid. This salt is soluble in alcohol. It is known to be an acetate by giving forth an acetous odour on the addition of sulphuric acid. It may be given in pill or solution, and in the same dose as the sulphate. It is sometimes preferred for endermic and hypodermic application, under the impression that it is less irritant, and more readily absorbed.