52 Dr. G. Schweitzer's Analysis qfSea-*mater as it exists 



alkali, and in a solution also containing bromides and chlo- 

 rides. 



From experiment I have ascertained that a minute quan- 

 tity of iodine in distilled water, equal to no more than 

 1,500,000th part of the whole, will be distinctly indicated 

 when mixed with starch, dilute sulphuric acid, and chlorine. 



For the production of such delicate reaction, I add to every 

 500 grains of fluid one drop of diluted sulphuric acid, a small 

 quantity of paste of potato starch, and two drops of a weak 

 solution of chlorine, consisting of one part of a saturated 

 solution diluted with 20 to 25 times its volume of distilled 

 water. The solution gives no indication of the presence of 

 iodine in the fluid until a sufficient time has been allowed for 

 the separation of the starch, when a decided pink hue will be 

 visible on the surface of the precipitate if iodine be present. 

 It has been supposed that the substitution of pink for blue in 

 the ioditle of starch produced arises from the presence of 

 bromine; but this I have ascertained is not correct, as it de- 

 pends entirely on the minute quantity of the precipitate acted 

 upon by free chlorine or bromine. The following experiment 

 will prove this fact. In order to ascertain the delicacy of 

 electrolytic tests of iodine, a current of electricity produced 

 b}' voltaic induction was passed through a suitable glass tube, 

 filled with 300 grains of distilled water containing joq.Voo^^ 

 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, de- 

 veloped, after a kwf brisk revolutions of the coils of the mag- 

 net, the blue colour 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 potassium was only one millionth part of the weight of the 

 water, indicated the presence of iodine by a pure blue speck of 

 iodide of starch at the anode or negative extremity of the elec- 

 tric circuit. 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 smallest additional proportion of chlorine occasioned a 

 pinkish sediment. The presence of chlorides and bromides, 

 however, do not interfere with the action of the electric cur- 

 rent upon traces of iodine ; for a solution of salts containing, in 

 500 grains of water, 100 grs. of chloride of sodium, 10 grs. 

 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 colour. A concentrated solution of bromide of 

 sodium, containing the millionth part of iodide of potassium, 



