62 Iodide of Potassium^ as a Test for Arsenic. 



converting some of it into arseniale, a change which is indicated hy 

 an odor of garlic and the dark color of the salt. 



The most uniform results were obtained by adding to a cold but 

 concentrated solution of the arsenious acid alone^ as much iodide as 

 was necessary to effect complete precipitation. For tliis purpose, 

 arsenious acid was boiled with water until the solution upon cooling, 

 w^as found, by experiment, to contain 2.85 per cent. 100 grammes 

 of tliis solution were decomposed by the cautious addition of iodide 

 of potassium, well dried at about 600^ Fahr. When 4.13 grammes 

 of this substance had been added, precipitation entirely ceased, and 

 the powder, upon being well dried, was found to weigh 7. grammes, 

 making a difference of 2 centigrammes more than the sum of the bor 

 dies mixed- Alcohol, marking 34,5 on Cartier's scale, was added in 

 order to remove the excess of iodide, by which operation the powder 

 was reduced to 4.5 grammes. Of this amount, it is known, from the 

 fa*st operation, that there must be 2.85, owing to the presence of ar- 

 Benious acid, "Hence the composition is, 



Arsenious acid, 2.85, > ( 63.3 



T j-i r * 1 rr f or per cent. < _^ ^ 



Iodide oi potassium, 1.65, > * C *3o.7 



The accuracy of this synthetical process was next tested analytic- 

 ally, by determining the amount of the alkali, as follows : — 1 gramme 

 of the powder, resulting from the last experiment, was exposed to a 

 heat regulated by a mercurial thermometer. It withstood a temper- 

 ature of 500^ without suffering any further change than a loss of ^ per 

 cent, in weight. By elevating the temperature, arsenic was driven 

 off with free iodine, both maicing a loss of 31 per cent. To the re- 

 mainder sulphuric acid was added, which changed the color to deep 

 brown, and liberated a great deal of iodine and iodide of arsenic. 

 When all excess of sulphuric acid was removed by a red heat, the 

 sulphate was decomposed by nitrate of baryta, and the amount of 

 potassa determined from the insoluble sulphate, after repeated wash- 

 ing in dilute muriatic acid and exposure to a red heat. The sulphate 

 of baryta weighed 0.267 millegramraes, equivalent to 34.7 per cent. 

 iodide of potassium or to 36.6 per cent, of the hydrlodate of polassn, 

 supposing that this salt or the elements of water are present. 



Notwithstanding the novelty of such a compound, in which it i? iin- 

 }X)ssible to tell whether the white arsenic acts the part of an acid or 

 of a base, (although it is present nearly to the extent of five atoms,) 

 and where, moreover, we do not perceive even any analogy to the 

 composition of a double salt, It appears obvious, by tlie following ad- 



