132 MASS. EXPI]RIMENT STATION BULLETIN 166. 



Gravimetric Process. ^ — After acetylating, a gravimetric process for 

 acetyl number may be conducted in a manner similar to that for the quan- 

 titative determination of insoluble fatty acids, observing all the precau- 

 tions therein noted as to ceresine, washing, drying, weighing, etc. 



This modification is apparently rather more difficult, tedious and sub- 

 ject to error than the saponification or volumetric process. An inaccuracy, 

 due to a deficiency in weight arising from the dehydration of free fatty 

 acids by acetic anhydride during acetylation, is probably unavoidable, 

 although of little consequence where the amount of free acids is relatively 

 small. 



The acetyl number (a) is calculated from the increase in weight (i) by 

 the following formula : — 



^""^016 "'* 1335.39604 i 



In case only one hydroxy compoimd of known molecular weight (m) 

 and number of hydroxyls (d) is present, its amount can be calculated 

 from the increase in weight (i) of the oil, fat or wax on acetylating. The 

 theoretical increase for a hydroxy compound is — 



42.016 d 



The amount (H) of such a compound in an oil, fat or wax is, therefore 



~ 42.016 d °^ 42.016 d 



Molecular Weight of Hijdroxy Compounds. — The molecular weight of 

 the hydroxy compounds can be calculated from the weight (w) of fat 

 taken and the increase (i) on acetylating, provided the number (d) of 

 hydroxyls in the molecule is known: — 



w : w+i :: m : m-|-42.016 d 

 „ 42.016 dw 



The formation of anhydrides during the acetylating process will affect 

 the accuracy of these calculations. 



The computation of the amount of hydroxy compounds by the gravi- 

 metric process is greatly facilitated by use of the following table: — 



' Has. not received sufficient study in this laboratory to warrant positive statements, but is 

 similar to the methods described by Lewkowitsch (loc. cit.) 1, pp. 451-453, 578-580 (1913). 



