466 R. N. Maxson — Small Amounts of Gold. 



Art. XLIV. — The Limit of Error in the Volumetric 

 Determination of Small Amounts of Gold ; by Ralph N. 



Maxson. 



[Contributions from the Kent Chemical Laboratory of Yale University — CXXVIL] 



In a former paper from this laboratory, Gooch and Morley* 

 established the accuracy of a method for the determination of 

 small amounts of gold, based upon the reaction 



AuCl a + 3KI = 3KC1 + Aul + I 2 . 



Recently the attempt has been made by Ruppf to show that this 

 method is inaccurate, and to bring forward a process based upon 

 the reduction of gold by standard arsenious acid, and titration of 

 the residual arsenious acid by standard iodine. In a study of 

 these two methods,:); I have verified the results obtained by 

 Gooch and Morley, but have been unable to determine similar 

 .amounts of gold with even approximate exactness by the method 

 of Rupp. 



More recently, § Rupp has disclaimed accuracy for his method 

 when applied to amounts less than a few milligrams, and pro- 

 ceeds to discuss the absurdity of attempting to determine volu- 

 metrically tenths and hundredths of a milligram of material, 

 however accurate the method may be — and this in the face of 

 his statement, in his own previous paper, of volumetric results 

 carried to thousandths of a milligram. 



While quite in agreement with Rupp so far as to admit the 

 absurdity of attempting to determine amounts of material 

 measured in tenths and hundredths of a milligram by means 

 of N/2 arsenious acid, as did Rupp in three out of the six 

 determinations upon which he rested his process, I take the 

 view, that with properly made standard solutions of suitable 

 dilution, it is quite possible for a skillful analyst to determine 

 tenths and hundredths of a milligram of material. 



By the use of approximately N/100 iodine and thiosulphate, 

 Gooch and Morley got results with a mean error of 5/100 

 milligram between extremes of -f- 3/100 mg. and —10/100 mg. 

 in twelve determinations, a single small divided drop of the 

 iodine solution and of the thiosulphate solution being sufficient 

 to produce an immediate reaction with starch, and a bleaching 

 of the starch color, respectively. 



When using N/1000 solutions of iodine it was necessary to add 

 to the volume used 0*l cm3 of iodine, in order that the first small 

 drop of gold should bring out the starch reaction. This was a per- 

 fectly definite correction and was equivalent to 1/100 mg. of gold. 



*This Journal (4), viii, 261, 1899. 

 f Ber. Dtsch. Chem. Ges., xxxv, 2011. 

 i This Journal (4), xvi, 155, 1903. 

 §Ber. Dtsch. Chem. Ges., xxxvi, 3961. 



