ANALYSIS OF CRUDE RUBBER 87 



The determinations of the ash, resin, protein, and 

 caoutchouc are usually done on the dry washed rubber. 



(1) Loss on Washing.— ^The " loss on washing " includes 

 {(t) tlie moisture present in the crude rubber ; (6) the solid 

 impiu'ities ; and (c) any substances soluble in water. 



A weighed quantity of the rubber is treated in a 

 rubber- washing machine until any impurities present 

 are removed and it is converted into thin crepe. The 

 crepe rubber is cb'ied, at first in the air and finally in a 

 vacuous desiccator. The difference between the final 

 weight of the dry rubber and the original weight is the 

 " loss on washing." 



The amount of moisture present in the original sample 

 may be determined if desired (1) by heating a weighed 

 quantity of the rubber in vacuo until it ceases to 

 lose weight ; or (2) by heating the rubber in a current 

 of hydrogen or carbon dioxide and weighing the water 

 given off. 



(2) Ash. — A weighed quantity of the washed rubber is 

 incinerated and the residue is heated in a muffle furnace 

 until all the carbon disappears. 



(3) Resin. — -A weighed quantity of the washed crepe 

 rubber, cut into fine shreds, is thoroughly extracted with 

 acetone in a Soxhlet apparatus. The acetone is then 

 distilled off from a weighed flask, and the flask with the 

 residue is dried in a vacuous desiccator until of constant 

 weight. The portion soluble in acetone is returned as 

 resm. 



Glacial acetic acid is sometimes used as a solvent for 

 the resin in place of acetone. 



(4) Protein. — The amount of nitrogen is determined 

 by Kjeldahl's method, and the result multiplied by 6*25 

 to represent protein. This assumes that all the nitrogenous 

 substances present in rubber are protein, which, however, 

 is improbable. 



(5) Caoutchouc. — The percentage of caoutchouc in the 

 chy washed rubber is usually obtained by difference — 

 viz. 100 minus the sum of the percentages of ash, resin, 

 and protein. 



The determination of the caoutchouc by difference is 

 not satisfactory, but at present there is no method of 

 direct estimation which can be used successfully. Pro- 

 posals have been made to determine the caoutchouc by 



