180 



REPORT — 1900. 



= 1-0 ampere, an E.M.F. of 2-5 to 3-0 volts, and the addition of 8 gv. of 

 ammonium oxalate for 0-1 to 0-3 gr. of iron. From the contradictory 

 nature of these results it became important to ascertain whether the 

 accuracy of our own determinations was really due to small compensating 

 errors. 



To test the complete deposition of the iron in the experiments in 

 Series II. (p. 178) a small quantity of the solution was withdrawn by a 

 capillary tube, and tested with potassium sulphocyanide after acidifying 

 with hydrochloric acid. The reaction is, however, known to be inhibited 

 by the presence of organic acids, such as oxalic, unless a large excess of 

 hydrochloric acid is present to prevent the dissociation of the ferric 

 sulphocyanide ; this addition may so far dilute the solution as to prevent 

 the detection of small quantities of iron. Further, the metal is present 

 as a ferrous salt at the end of the electrolysis, and this fact may also be a 

 cause of any iron present escaping detection. The delicacy of the sulpho- 

 cyanide reaction was, therefore, carefully studied under the conditions of 

 the electrolytic experiments, as well as in presence of ammonium oxalate 

 and of oxalic acid. Our results show that whilst up to 0-4 mgr. of iron 

 can readily escape detection when the test is made by the usual method 

 of withdrawing only a little of the solution, 0-1 mgr. can always be 

 detected with certainty if the whole of the solution, after electrolysis, is 

 tested by acidifying with 75 c.c. of hydrochloric acid (cone), and then 

 adding lO c.c. of a 20 per cent, solution of potassium sulphocyanide. 

 The coloration is quite distinct in presence of ammonium oxalate, oxalic 

 acid, ammonium chloride, or of the salts remaining after the electrolysis 

 of the mixture of these salts as used in the deposition of iron under the 

 conditions of the experiments in Series II. The sequence of the addition 

 of the reagents in no way affects the delicacy of the reaction, nor is the 

 addition of any oxidising agent, such as hydrogen peroxide, necessary to 

 convert the ferrous into ferric iron when solutions containing oxalic acid 

 or its salts, or the products of their electrolysis, are tested ; in their 

 presence a little stirring appears ample to completely oxidise small 

 quantities of iron. As a matter of fact, less than 0-1 mgr. can be 

 detected thus, but this limit is of course sufficient to check the presence 

 of iron left in the solution after electrolysis. With this check on the 

 complete deposition of the metal a series of determinations were made, in 

 which the iron remaining in solution was determined colorimetrically 

 by potassium sulphocyanide after the electrolysis. 



Series IV. 



The above were three consecutive experiments made with a ferric 

 chloride solution prepared for electrolysis as in the previous experiments, 

 .'ind to which 5 gr. of ammonium oxalate were added. Despite the 

 prolonged time of electrolysis a little iron still remained in solution ; other 



