An Investigation of a Case of gradual Chemical Change. 397 



The measurement of these intervals of time (whether constant 

 or increasing) during which the same amount of chemical decom- 

 position takes place, can be effected by taking advantage of a com- 

 paratively instantaneous change which may be made to go on in the 

 same liquid, and one which is very familiar. 



When iodine is produced in a liquid by the action of hydrogen 

 dioxide, or some other oxidising agent, on hydrogen iodide, the action 

 is a gradual one, but the introduction of a drop of a concentrated 

 solution of sodium thiosulphate at once converts the iodine into 

 sodium iodide, and every molecule of iodine produced in the liquid 

 after the introduction of the drop will be instantly thus converted 

 until the thiosulphate present is exhausted. If a small quantity of 

 starch is present in the solution, the moment at which the last trace 

 of thiosulphate disappears will be signalised by the appearance of a 

 blue colour in the liquid, the effect of the free iodine upon the starch. 

 Thus, then, by the introduction of constant measured quantities of 

 sodium thiosulphate, the rate of progress of the action between 

 potassium or hydrogen iodide and some oxidising substance may be 

 readily measured. This is the principle and method of division into 

 intervals in Messrs. Harcourt and Esson's classical research, and we 

 have adopted it for the investigation of a similar case of chemical 

 change. 



The reaction chosen for investigation in the present case was one 

 which liberated iodine indirectly. When solutions of potassium 

 chlorate and hydrogen chloride are mixed together, the mixture soon 

 acquires a chlorous smell, and at once liberates iodine from potassium 

 iodide, and as time goes on continues to liberate more. The exact 

 nature of the primary reaction, producing the oxidising agent, has 

 not been ascertained, nor whether the product is chlorine, some oxide 

 of chlorine, or a mixture of both. Various reactions are possible. 

 The one fact which is certain seems to be that in presence of an 

 iodide each molecule of chlorate salt is reduced to the corresponding 

 chloride entirely and without intermediate stages, and the equivalent 

 in iodine of all three atoms of oxygen set free. If it were the case 

 that the decomposition of the chlorate molecule took place in stages 

 there would be observed a considerable variation in the intervals 

 depending on the amount of intermediate products present, which 

 was not the case. 



A mixture of hydrogen chlorate and hydrogen chloride, both 

 dilute, reacts exactly in the same way as the mixture above, slowly 

 producing oxidising material which liberates iodine from potassium 

 iodide. It is probably a reaction common to most soluble chlorates. 

 Part of the investigation has been concerned with such mixtures of 

 the two acids without any metallic salt. These have advantages, as 

 the reaction is not complicated by the presence of such salts. Bat 



2 D 2 



