IMIDES OF BENZENEDISULPHONIC ACIDS. o 



be reprecipitated by an acid. AVlien the crystals of the oxime 

 are heated iu a capiUary tube they begin to darken at 180°, 

 become almost black at 210°, and decompose suddenly at 216° 

 with evolution of a gas containing the oxides of nitrogen, and 

 leaving a carbonaceous viscous residue of a strongly acid reaction. 

 This temperature of decomposition is so sharply defined that it 

 can be employed for the identification of the substance. The 

 decomposition temperature is, however, very markedly lowered by 

 the presence of even a small quantity of impurities. 



The methods described above are not well suited for the 

 preparation of the oxime on a large scale, as the yield of pure 

 m-benzenedisulphinic acid is too small. The following procedure 

 is found to give better results. 



20 grammes of crystallised m-benzenedisulphochloride in 

 powder are added in three or four portions to a flask containing 

 32 grammes of zinc dust, 20 c.c. of alcohol and 30 c.c. of water. 

 Slight warming causes the reaction to begin. The addition of 

 alcohol prevents the floating of the sulphochloride on the surface, 

 this precaution being advantageous to the progress of the réaction. 

 When the reaction is once started the temperature rises con- 

 siderably, and care must be taken to maintain it at 50°-60°, 

 the flask being shaken constantly during the process and any 

 sulphochloride adhering to the neck of the flask being, from time 

 to time, washed down into the flask with a fine jet of water. 

 When all the sulphochloride has been added and a sensible 

 heat evolution is no longer observed, the flask is w^armed on a 

 water-bath to complete the reaction and to boil off" the greater 

 part of the alcohol. While the flask is still kept warm on the 

 water-bath a solution of sodium carbonate is added little by little 

 until its coutentb acquire a faint alkaline reaction, so as to 



