Iodide of Potassium, as a Test for Arsenic. 59 



kali, 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, the 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 ^.8 per cent, arsenite ofpotassa. 



Iodine alone is added to a solution > _ _ • r 



. . > 2.8 per cent, arsenite oi potassa. 



containing ) 



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

 tion 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 gJoth 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 repeatedly wash- 

 ed and entirely separated from all excess of the precipitant. 



When thus purified, it possesses the following characteristic pro- 

 perties : — 



1 . Concentrated nitric acid immediately changes the white color 

 to a dark brown, purple, or even black, according to the quantity, 

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

 distinctive of free iodine. 



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

 sarae effects, but at the ordinary temperature, it merely changes the 

 color into a bright yellow. 



3. Strong muriatic acid also immediately imparts a bright yellow 

 color. 



These 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 is the precipitant, and even were such substances originally 



