156 Composition of Waters of Land-Drainage and of Rain. 



Avhlch is an Indiarubber joint with a brass spring as in Mohr's 

 burettes. 



To ascertain by means of these solutions the quantity of iodine 

 which has been liberated by the action of the nitric acid, the 

 contents of the small flask are washed out carefully (and by the 

 help of a little iodide of potassium to assist in the solution of 

 the iodine) into a larger flask, the quantity of liquid being made 

 up with distilled water to about 5000 or 6000 grs, A measured 

 quantity of the sulphurous acid solution is now added by means 

 of a pipette ; if sufficient it entirely destroys the colour of the 

 liquid. A few drops of solution of starch are now introduced, 

 and the standard iodine liquid is added drop by drop, until the 

 blue colour of the iodide of starch becomes permanent. A 

 simple calculation founded upon the known relation of the two 

 liquids, as before explained, gives the quantity of nitric acid in 

 the pint of water operated upon in the analysis. If iodine has 

 been liberated during the process, the sulphurous acid will re- 

 quire the addition of a smaller quantity for its destruction. 

 Supposing the standard to be 30 measures of iodine liquid, and 

 that in an experiment only 20 are required — then as each measure 

 indicates '00356, the ten measures not required will indicate 

 • 0356 of nitric acid present in the water examined. 



Iodide of potassium was originally employed in this process 

 instead of iodide of silver. To this substance, however, there 

 were found to exist two objections : the first of these was, that if 

 the waters contained sulphates, which would be the case in nine 

 out of ten, a separation of iodine occurred even in the entire 

 absence of nitric acid. It is well known that the re-action upon 

 which Bunsen's process is founded, is reversed when the solutions 

 are strong ; that is to say, iodide of potassium or hydriodic acid and 

 sulphuric acid, unless very dilute, mutually decompose one another 

 with formation of sulphurous acid and liberation of iodine. 



In the trials which were made with iodide of potassium, the 

 hydrochloric acid employed must have liberated sulphuric acid, 

 which was then acted upon by the hydriodic acid formed at the 

 same time.* 



This objection to the use of iodide of potassium was indeed 

 successfully removed by the employment of caustic baryta in the 

 place of lime in the boiling dow;i of the water under examination, 

 as in this way sulphuric acid was removed from the liquid ; but 

 another difficulty still remained. 



We found that a mixture of hydrochloric acid and iodide of 



* This is an additional illustration of the law of Berthollet of the distribution 

 of acids and bases, as it is clearly seen that hydrochloric acid can decompose a 

 sulphate. There is no doubt tliat a very delicate process for sulphuric acid and 

 sulphates might be founded on this circumstance. 



