33 



acid vats, distillation being continued until only tar (boiled tar) 

 remains in the retort. 



In both cases the substance produced by treating the acid solution 

 with milk of lime is known as "gray acetate of lime." The liquor 

 containing the acetate, whether brown or gray, is pumped to cop- 

 per evaporating pans (fig. 12, Gj^-GJ, which are usually placed on 

 the brickwork over the retort and the solution evaporated until the 

 acetate begins to crystallize out, when it is transferred to a drying 

 floor and stirred frequently until sufficiently dry. Gray acetate of 

 lime contains from 80 to 85 per cent of actual acetate, the balance 

 being tarry matters, calcium carbonate, and water. The gray acetate 

 is used for the manufacture of 

 acetic acid and other acetates 

 and is largely employed in calico 

 printing. It may be further 

 purified by dissolving in water, 

 filtering through boneblack, and 

 evaporating the solution to 1.16 

 specific gravity, when the ace- 

 tata crystallizes in small odorless 

 needles which constitute the raw 

 material from which acetone is 

 made. 



ACETIC ACID. 



Commercial acetic acid is pro- 

 duced from gray acetate of lime, 

 or from the brown acetate pre- 

 viously heated to about 230 C. 

 to destroy tarry matter, by dis- 

 tilling with concentrated hydro- 

 chloric acid or with sulphuric 

 acid (fig> 16). 



FIG. 15. Liming still. 



The latter is rarely used, as the calcium sulphate 



formed is difficult to remove from the stills and the impurities in the 

 acetate reduce the sulphuric to sulphurous acid, which contam- 

 inates the acetic acid. A single distillation yields a slightly col- 

 ored solution, containing 30 to 50 per cent of acid, which may be 

 further purified by treating with potassium bichromate or perman- 

 ganate and redistilling. The first portion of the distillate is con- 

 taminated with formic, proprionic, and butyric acids and with 

 empyreumatic oils, but the subsequent portions are nearly free from 

 impurities. 



Glacial acetic acid is prepared by heating fused sodium acetate 

 with concentrated sulphuric acid in a porcelain-lined or earthenware 

 still and then distilling, when a nearly anhydrous product is obtained, 



[Cir. 36] 



