Iodide of Potassium, as a Test for Arsenic. 59 



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kalij but It will not answer for the simple solution of the arsenious 

 acid. When these substances are added to each other, a very char- 

 acteristic white precipitate appears, f^he properties of which will be 

 noticed presently. 



Immediate precipitation takes place when 



Iodide of potassium is added to a C 2.8 per cent, arsenious acid. 



solution containing (1.8 per cent- arsenlle ofpotassa. 



Iodine alone Is added to a solution . „ _ . ^ 



b per cent, arsenite of potassa. 

 conlaunng ^ ^ ^ 



As in all these cases of immediate precipitation, a drop of the solu- 

 {\ox\ upon a plate of glass will furnish enough of the precipitate to 

 judge of its nature, we may consider that the quantity of arsenious 

 acid exhibited does not exceed so^th of a grain ; but much weaker 

 solutions answer the purpose equally well, when the fluid is gradual- 

 ly evaporated. Indeed, they have one important advantage arising 

 from the great tenacity with which the white precipitate, when grad- 

 ually formed, adheres to the glass plate — it may be repeatedlv wash^ 

 cd and entirely separated from all excess of the precipitant. 



When thus purified, it possesses the following characteristic pro-* 

 pertles : — 



!• Concentrated nitric acid Immediately changes tlie white color 

 to a dark brown, purple, or even black, according to the quantit}-, 

 and starch added at the same time, assumes the deep blue tint, so 



r 



distinctive of free iodine. 



2. Strong sulphuric acid, wuth the assistance of heat, produces the 

 same effects, but at the ordinary temperature, it merely changes die 

 color Into a bright yellow. 



3. Strong muriatic acid also immediately imparts a bright yellow 

 color- 



Thesc simple experiments, with ordinary care, are quite sufficient 

 for the purpose of testing, and will enable us to recognize the mineral 

 acid even in very small quantities; other properties will, however, be 

 noticed subsequently in a more general manner. 



It will be seen that the foregoing characteristics are actually those 

 of Iodine and its compounds ] yet this very circumstance is consider^ 

 ed to enhance the value of the test ; since the powder admits, by re^ 

 peated washing, of being completely separated from all the hydrio^ 

 date not chemically combined. Metallic salts are indeed the only 

 compounds which could well occasion confusion, when hydriodate of 

 potassa 15 the precipitant, and even were such substances prigiqdl^ 



