262 SMITH'S INTERMEDIATE CHEMISTRY 



The efficiency of the contact agent depends on the amount of 

 surface it presents to the gases. The action may be illustrated by 

 dipping asbestos in a solution of chloroplatinic acid and then 

 heating the mineral in the Bunsen flame : 



H 2 PtCl 6 -> Pt + 2HC1 1 + 2C1 2 T 



The platinum is thus spread in a fine grey powder on the fibers 

 of the asbestos. The latter is placed in a tube (Fig. 48, p. 140), 

 where a mixture of oxygen (or air) and sulphur dioxide may be 

 passed over the heated material. The sulphur trioxide issues as 

 vapor at the other end of the tube, where its presence is recognized 

 by the dense fumes (droplets of sulphuric acid), produced when 

 it meets the moisture in the air. The vapor can be condensed 

 to liquid form in a cooled flask. 



In practice the contact agent employed is platinum, dispersed 

 in a very finely-divided condition throughout a suitable carrying 

 material, or base. The Grillo process uses as a base magnesium 

 sulphate. This gives a catalytic mass just as active as platinized 

 asbestos, and requires only one-hundreth part the amount of 

 platinum. With silica gel (p. 360) as a base, the platinum con- 

 tent of the contact mass can be still further reduced. This is a 

 very important point in the economics of the process. 



It is absolutely necessary, in employing the contact process, 

 to remove from the sulphur dioxide all traces of substances such 

 as arsenious oxide and similar impurities derived from the cal- 

 cining of pyrite or some other mineral sulphide. The most 

 minute quantities of these substances act as poisons on the cataly- 

 tic agent, and soon render it quite inoperative. The sulphur 

 dioxide is therefore very carefully purified before reaching the 

 contact chamber. Excess oxygen is used in the reacting mixture, 

 in order to drive the reaction more readily towards complete 

 formation of S0 3 (compare p. 234). The temperature in the 

 contact chamber is kept between 380 and 450. The system has 

 a tendency to get hotter during the reaction, owing to the heat 



