SANGER-RIEGEL. — PYROSULPH. CHLORIDE-CHLORSULPH. ACID. 693 



sure of 19 mm., it boils at 56-57° without decomposition. It crystal- 

 lizes easily at low temperature and melts at —37.5° to —37°. The specific 

 gravity at 20°, referred to water at 4°, is 1.837 ; at 0° (water at 4°), 

 1.872. There is no appreciable dissociation at the boiling point of ani- 

 line. It is not colored by the addition of finely divided tellurium or 

 selenium. 



For the properties of mixtures of pyrosulphuryl chloride and chlor- 

 sulphonic acid, see page 712 and Table III (page 713). 



The Preparation of Chlorsulphonic Acid. 



In carrying out the Williamson reaction 



H2SO4 + SO3 + HCl = H2SO4 + SOsHCl, 



the oleum should contain as much sulphur trioxide as possible. It is 

 placed in a roomy distilling flask, closed by a three-holed rubber stopper 

 and mounted on a tripod. Through one hole of the stopper passes a right- 

 angle tube slightly constricted at its lower end, extending to the bottom 

 of the flask. Through the second hole is attached a distilling tube in 

 one piece, surrounded by a condenser jacket, and the third hole carries 

 a thermometer. 



The hydrochloric acid gas is conveniently generated by slowly adding 

 crude, concentrated sulphuric acid from a dropping funnel to crude con- 

 centrated hydrochloric acid contained in a generating flask. The gas is 

 carefully dried by passing at least two Emmerling towers containing 

 glass beads saturated with concentrated sulphuric acid, and a (J-tube 

 with phosphorus pentoxide. The gas is passed through the oleum 

 slowly, at room temperature, until there is no further evidence of ab- 

 sorption, the flask and condenser being inclined so that any condensed 

 vapor may run back. At the end, the contents of the flask are distilled 

 in a current of dry hydrochloric acid gas, the portions distilling below 

 145° and above 160° being rejected. A rectification of the middle 

 portion in hydrochloric acid results in a product of sufficient purity for 

 all purposes for which the substance may ordinarily be required, but an 

 absolutely pure product is obtained only by crystallization at low tem- 

 perature, as will be shown in the Experimental Part. 



The Properties of Chlorsulphonic Acid. 



Chlorsulphonic acid is a colorless liquid which fumes very strongly in 

 moist air. If added to water, it reacts with explosive violence, and is 

 immediately decomposed into hydrochloric and sulphuric acids. Chlo- 

 rine and sulphur dioxide are also formed in small amount. It cannot 



