Analysis of Sea Water. 13 



delicacy of electrolytic tests of iodine, a current of electricity 

 produced by voltaic induction was passed through a suitable 

 glass tube, filled with 300 grains of distilled water containing 

 so o,'oo 0^^^ part of its weight of iodide of potassium and a small 

 quantity of starch, but no action was observed until a few drops 

 of nitric acid were added, which assisting the electric current, 

 developed, after a few brisk revolutions of the coils of the mag- 

 net, the blue color of the iodide of starch. Even a current of 

 electricity from a single constant galvanic battery passed through 

 the same glass tube, in which the proportion of iodide of potas- 

 sium was only one millionth part of the w^eight of the Avaier, 

 indicated the presence of iodine by a pure blue speck of iodide 

 of starch at the anode or negative extremity of the electric cir- 

 cuit. When iodide of potassium diluted in the same manner 

 was properly treated with starch, sulphuric acid, and chlorine, 

 the blue iodide of starch likewise became visible, but the small- 

 est additional proportion of chlorine occasioned a pinkish sedi- 

 ment. The presence of chlorides and bromides, however, does 

 not interfere with the action of the electric current upon traces 

 of iodine ; for a solution of salts containing, in 500 grains of 

 water, 100 grains of chloride of sodium, 10 grains of bromide 

 of sodium, and the five hundred thousandth part of iodide of 

 potassium, gave a deposit of iodide of starch of a dark pinkish 

 color. A concentrated solution of bromide of sodium, contain- 

 ing the millionth part of iodide of potassium, also gave by the 

 action of the electric current a slightly pinkish deposit. 



It is always necessary, when we wish to detect by means of 

 chlorine minute quantities of an iodide, to employ the chlorine in 

 a very diluted state, as when in excess it forms a soluble chloride 

 of iodine which will not act on starch. 



The sulphates and chlorides present in salt waters do not in- 

 terfere with the delicacy of the starch test ; on the contrary a 

 concentrated solution of the chlorides will show the presence of 

 one millionth part of iodide of potassium more distinctly than an 

 equal volume of distilled water. This appears to arise from the 

 iodide being a little soluble in pure water. I thought at first that 

 a trace of an iodide might be contained in the common chloride 

 of sodium, and thus cause a deeper tinge of blue color ; but by 

 employing a chloride of sodium prepared from pure hydrochloric 

 acid and pure soda, I found the same degree of increased reaction. 



