44 HYDROCYANIC-ACID GAS FUMIGATION IN CALIFORNIA. 



WATER AS A FACTOR IN FUMIGATION. 



There are several reasons why water should always be employed 

 in fumigation. It is very useful in dissolving the potassium cyanid 

 and hastening and completing the chemical reaction with the acid. 

 A piece of cyanid thrown into a mixture of acid and water immediately 

 gives up a portion of its mass in solution. Scarcely has the cyanid 

 dissolved when it is partially converted into gas. The heat liberated 

 during this process assists in forcing the solution of more cyanid 

 which is also partially converted into gas. This continues until 

 the chemicals are exhausted and the reaction ceases. 



Potassium sulphate, a solid, is the by-product resulting from the 

 reaction by which hydrocyanic-acid gas is produced. Water dissolves 

 the potassium sulphate as it forms and prevents it from coating the 

 cyanid not yet in solution. In the presence of an insufficient amount 

 of water, the potassium sulphate is not completely dissolved, but 

 forms a coating on the pieces of cyanid, preventing the sulphuric 

 acid from penetrating to it, and thereby retarding, or even in part 

 preventing, the reaction. In such cases this undissolved potassium 

 sulphate usually solidifies, causing the pots to " freeze." This 

 phenomenon always occurs where the formula is 1-1-1, or where 

 the same amounts of water, acid, and cyanid are used. On agitating 

 the residue by stirring, it is almost always possible to find small 

 pieces of undissolved cyanid enveloped in a coating of the potassium 

 sulphate. Ordinarily, when the residue is stirred the particles of 

 cyanid are removed, to some extent, from this envelope of potassium 

 sulphate, allowing some of the unused acid to reach them, and thus 

 evolving a small amount of gas without the addition of more acid. 

 Under these conditions, however, the reaction is never complete, 

 and it is highly desirable, therefore, to add sufficient water at the 

 beginning to dissolve all the potassium sulphate. 



Recalling the statements made in discussing the amount of sul- 

 phuric acid to use, it is seen that the " congealing" or "freezing" 

 of the residue in the generating jars is due to either or both of two 

 conditions: (1) An insufficient amount of water to completely dis- 

 solve the sulphate of potassium, or (2) a large excess of sulphuric 

 acid, whereb}^" the water is rendered less capable of taking into 

 solution the same amount of sulphate as it otherwise would. 



Another very important function of the water in the reaction is 

 the heat produced by the union of the sulphuric acid and water. 

 Potassium cyanid introduced into this heated mixture gives off 

 hydrocyanic-acid gas much more quickly and thoroughly" than at a 

 lower temperature, and in field work rapid generation of gas is 

 essential. 



The action of pure or highly concentrated sulphuric acid on potas- 

 sium cyanid results in a very different chemical reaction than when 



