12 IOWA ACADEMY OF SCIENCES. 



therefore, we are not at present justified in the assumption that elements at 

 the moment of formation or liberation from their compounds possess prop- 

 erties in any way different from those they commonly exhibit. 



SOME PECULIARITIES OF SOLUTIONS OF FERRIC SULFHOCYAN ATE 



BY LAUNCELOT ANDREWS, PH. D. 



The deep blood-red color of solutions of ferric sulphocyauate has fre- 

 quently been taken advantage of for the determination of small quantities 

 of iron in river or spring water, in blood, in alloys, in alumcake, etc. 



The earliest method of this kind, so far as I have been able to ascertain, 

 is due to T. L. Herapath, who proposed to determine minute quantities of 

 iron by the addition of potassium sulphocyanate to the acidified solution con- 

 taining an unknown amount of iron, and also to a standard iron solution of 

 known strength, the latter being then diluted until both showed the same 

 tint. 



Very similar methods have been employed or devised by A. Thomsen 

 {Gh. Soc. Jour., 47, 493), Ad. Joles {Arch. f. Hygiene, XIII, 402 ), L. Lapique 

 {Bull Soc. Chim., 2, 295, and by R. R. Tatlock {Jotir. Soc. Gh. hid. 6, 276). 



Vierordt {Quant., Speklralanalyse), suggested a fundamental modifi- 

 cation of Herapath's colorimetric method, in that he dispensed wholly with 

 a standard comparison solution, substituting for it a direct spectrophoto- 

 metric determination of the amount of light of given wave length, trans- 

 mitted by a layer of the ferric sulphocyanate solution one c. m. thick. In 

 this method the assumption, based upon analogy, is made that the light 

 absorbing power of the solution is directly proportional to the amount of 

 iron contained therein; or in other words, that the negative logarithm of 

 the Jraction of light transmitted is jiroportiotial to the concentration of the 

 solution, and the assumption seems to be confirmed by Vierordt's observa- 

 tions. 



Subsequent investigations by Kriiss and Moraht {Lieb. Ann., 260, 193; 

 Kalorimetrie, u. Spektral analyse, 1891, p. 125), and by Magnaniui {Zeit- 

 p)hys. C/t. 8,1) have shown the assumed proportionality to be non-existent 

 in fact. If aqueous solution of ferric sulphocyanate be diluted its color fades 

 away in a much more rapid ratio than corresponds to the diminishing con- 

 centration of the solution. The depth of color is much enhanced by an 

 excess of either generatrix, i. e., of KSCN or of Fe CI3, and as Magnanini has 

 shown the change follows the laws of mass action qualitatively and quantita- 

 tively. 



Magnanini dismisses the affair at this point ^s a res adjudicata assuming 

 that the sulphocyanate is subject, in its solution in water, to a progressive 

 electrolytic dissociation in the ions, Fe and S C N. In accordance with well 



