SANGER-RIEGEL. — PYROSULPH. CHLORIDE-CHLORSULPH. ACID. 717 



hwer temperature, the chlorsulphonic acid was more quickly hydrated 

 and therefore not evident in the product of the subsequent distillation. 



On this point depends much of the success of the Prandtl and 

 Borinski method, as our study of it shows. If ice be added to a cooled 

 mixture of the two bodies, the chlorsulphonic acid will be decomposed 

 and a part of the pyrosulphuryl chloride. The latter must not be 

 separated at once and distilled, but the mixture must stand for some 

 time. The sulphuric acid does not always form the upper layer, since 

 the specific gravities of the two are so nearly the same. Addition of 

 concentrated sulphuric acid or a little water will bring the pyrosul- 

 phuryl chloride to the bottom. After separation, the addition of 

 phosphorus pentoxide to the moist pyrosulphuryl chloride is not op- 

 tional, as Prandtl and Borinski contend, but necessary, if the minimum 

 amount of chlorsulphonic acid is to be formed on distillation. With 

 proper precautions, the method can be made to give a fairly pure prod- 

 uct, though the entire absence of chlorsulphonic acid is not assured. 

 The 3^ield is, however, not over 70 per cent, as Prandtl and Borinski 

 state, and may be less. 



In this connection, the behavior of pyrosulphuryl chloride toward 

 concentrated sulphuric acid is of interest. There is no immediate 

 action, but a partial hydration takes place on standing. The two im- 

 miscible substances were sealed in a tube and allowed to stand in the 

 cold for a month. The product was homogeneous. On opening the 

 tube there was no excess of pressure ; on mixing a portion of the con- 

 tents with water, a violent reaction took place and hydrochloric acid 

 was given off. This indicates the formation of chlorsulphonic acid. 

 Since we have shown that chlorsulphonic acid is partially dehydrated 

 by sulphur trioxide (p. 673), we may express both results again by 

 writing the reaction (6) in this place also : 



S2O5CI2 + H2SO4 1:; 2SO3HCI -f SOs, 

 or S2O5CI2 + 2H2SO4 U 2SO3HCI -f H2S2O7. 



This condition of equilibrium explains why, in the preparation of 

 chlorsulphonic acid by the Williamson method, the formation of pyro- 

 sulphuryl chloride is impossible, since the large amount of sulphuric 

 acid would cause the reaction to run to the right. 



^&^ 



Summary. 



The work of previous investigators on the action of sulphur trioxide 

 upon certain chlorides is reviewed as briefly as possible and the results 

 summarized in tabular form (Table I). 



