Sees aac 
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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 
sae'saath 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 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 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 how 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. 
y ie 
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