[chadsey] decomposition OF POTASSIUM CHLORATE 17 



effected could be attained in three or four minutes. At the expiration of 

 the time set for heating, the bulb was removed from the furnace, and 

 allowed to cool in a desiccator. By repeatedly exhausting the desiccator 

 the removal of all excess of oxygen from the bulb was ensured so that it 

 could be weighed when filled with air. 



In preliminary experiments the volume of the gas evolved from the 

 chlorate was measured over mercury and its calculated weight compared 

 with the loss of weight of the bulbs. As these corresponded as closely 

 as the readings of the volume allowed, the oxygen set free in subsequent 

 experiments was estimated by the loss of weight, and no attempt was 

 made to measure the gas. 



The solid residue was examined for chloride and undecomposed 

 chlorate, as follows : A solution of the residue in 200^^ was made, and 

 25^^ of this titrated for chloride, with 10" or 20" silver nitrate — using 

 potassium chromate as indicator. From the amount of chloride found, 

 the weight of chlorate decomposed was calculated as well as the total 

 weight of oxygen evolved in this decomposition. 



A second 25'"^ of the 200'^^ solution, was examined for undecomposed 

 chlorate hy the method of N. Blattner and J. Brassner (Zeit. Angorw. 

 Chem., 1900, p. 1185). Sulphur dioxide was passed through this por- 

 tion in order to reduce the chlorate to chloride, thus : 



/iC/Og + 380._ + 3HM — KCl ^ 3H.,S0, 



= KCl + shJo^ 



As the perchlorate is not reduced by sulphur dioxide, this operation may 

 be safely carried out. Excess of sulphur dioxide was immediately re- 

 moved by boiling and the sulphuric acid formed in the solution neutra- 

 lized with C. P. calcium carbonate (Precip). After cooling, the 

 chloride in solution was estimated by titration as before, and the excess 

 over that of the previous estimation used to calculate the amount of un- 

 decomposed chlorate in the residue. 



This method was found to work very well, although some difficulty 

 was at first experienced with silver nitrate titrations when considerable 

 quantities of chlorate had been reduced. The precipitate of silver 

 chloride in these cases formed quickly into curdy masses which entangled 

 the red precipitate of silver chromate, and thus impaired the sharpness 

 of the end point. This was found to be due, not to the excess of calcium 

 carbonate, which exercises no influence upon the reaction, but to the 

 calcium sulphate formed in the neutralization. Even when the precipi- 

 tated sulphate was removed by filtration the chloride was still curdled. 

 Moreover, when a quantity of C. P. calcium sulphate was added to a 

 solution of pure sodium chloride and the excess filtered off, the filtrate 



Sec. III., 1903. 2. 



