a. N. Maxson — lodometric Determination of Gold. 157 



ground that \X\q percentage errors are found to be large. It is 

 to be noted, however, that large percentage errors are inevita- 

 ble, in the application of volumetric methods to the determina- 

 tion of total weights measured in hundredths of a milligram, or 

 tenths of a milligram, or even in a few milligrams, inasmuch as 

 the reasonable errors to be expected in the measuring of solu- 

 tions strong enough to be visibly reactive are of themselves 

 large when reckoned as percentages of small absolute amounts. 

 These errors, small as they are, Rupp attributes to the second- 

 ary reactions of potassium iodide on aurous iodide, in which 

 gold is reduced and iodine set free. I have been unable, how- 

 ever, to find evidence of such action under the conditions 

 of experimentation during periods much longer than those 

 required for the completion of the analytical process. Thus 

 aurous iodide, obtained by treating a solution of auric chloride 

 containing 0'0125 grm. of gold, with potassium iodide accord- 

 ing to the directions of Gooch and Morley, adding starch and 

 bleaching the starch with sodium thiosulphate, gave no color of 

 starch blue after the interval of an hour. A second similar 

 experiment gave the same result. Inasmuch as an interval of 

 ten minutes is enough for the complete manipulation of a 

 single determination, it is plain that the stability of the aurous 

 iodide does not figure in the accuracy of the determination of 

 the small amounts of gold for which the process w^as designed. 

 As a substitute for the method described by Gooch and 

 Morley and sustained by the fifty-five analytical determinations 

 mentioned, Rupp announces a process for the estimation of 

 gold depending upon the precipitation of that element from 

 auric chloride by means of standard arsenious acid in excess, 

 according to the equation 



3 As A + 4 AUCI3 + 6H,0 = 3As,0, -1- 12HC1 -}- 4Au, 



and the titration of the excess of arsenious acid by iodine. 

 To show the reliability of the process Rupp cites six determi- 

 nations, in one of which the titrations were made in two por- 

 tions. Four of these six deal with 61*2™^" of gold each, while 

 in two the gold amounted to '6'V^^. These figureSj which cover 

 the entire range of the amounts used in Rupp's analytical 

 determinations, do not represent the order of magnitude or range 

 of the amounts of gold handled by Gooch and Morley, which 

 varied from S'T""^ to 0-052'^^. Only two determinations upon 

 amounts comparable with those used by Gooch and Morley 

 were made, and in only these two were suitably dilute solu- 

 tions of iodine (— — ) and of arsenious acid ( tt^tt ) employed. 



The gold solution employed in all the determinations was of 

 such strength that a single error of O'Ol^'''^ in reading would 



