l^»19] AGRICULTURAL CHEMISTRY — AGROTECHNY. 711 



Electrometric titrations, with special reference to the determination of 

 ferrous and ferric iron, J. C Hostetteb and H. S. Roberts (Jour. Anier, Chem. 

 Soc, 41 {1919), No. 9, pp. 1S37-1357, flgs. 15).— An electrometric titration method 

 with standard dichromate is described, whicli is said to permit the determina- 

 tion of a few tenths of a milligram of ferrous iron in the presence of any 

 quantity (up to 100 gm.) of ferric iron and which can be applied to other 

 oxidation and reduction reactions. The various factors affecting the determina- 

 tion have been studied in detail, and the technique has been so developed that 

 as sharp end points can be obtained with N/100 or even with N/2000 dichromate 

 solutions as with the more concentrated solutions usually employed. 



A new feature of the apparatus employed consists of a titrating head used to 

 cover the flask which contains the solution to be titrated, in order to control the 

 atmosphere above the solution and to support the calomel half-cell and the 

 platinum electrode. Readings are made with the slide wire potentiometer of 

 Roberts, previously noted (E. S. R., 41, p. 503). 



Some of the advantages of the electrometric method for oxidizing and reduc- 

 ing reactions are summarized as follows: 



" This method permits the tise of potassium dichromate with its numerous 

 advantages. The reduction of the solution with electrometric control eliminates 

 the removal of excess reducing agent, which must be done with the usual 

 methods of reduction. Conditions, such as acidity, need not be controlled 

 except within very wide limits, and hydrochloric, sulphuric, or hydrofluoric 

 acid, or mixtures of thesti, may all be used. . . , 



"The sensitivity and accuracy of the method makes possible (a) the deter- 

 mination of a few tenths of a milligram of tin, chromium, ferrous or ferric 

 iron, and probably many other elements, in the presence of large quantities of 

 some other element, and (b) the determination of blanks involved in some 

 of the ordinary determinations by reducing or oxidizing agents. 



" The time within which a determination can be carried out is gi'eatly short- 

 ened. The content of ferrous and ferric iron in a silicate, for instance, can be 

 determined in from 15 to 30 minutes. The precision attainable is comparable 

 to the best of the ordinary volumetric determinations." 



The hydrochloric acid color method for determining' iron, J. C. Hostetter 

 (Jour. Amer. Chcm. Soc, 41 (1919), No. 10, pp. 1531-15^3, figs. 4).— This paper 

 gives the results of experiments undertaken to establish conditions for the 

 development of a method for the determination of iron by means of the color 

 developed in hydrochloric acid solution, and certain applications of the method 

 as finally developed. 



The temperature coefficient for the color varied from 2 to 3 per cent per 

 degree, depending on the concentration of iron and probably also on the acidity. 

 The color developed by a given amount of iron varied with the acid concentra- 

 tion, reaching a maximum intensity at from 2G to 28 per cent of hydrochloric 

 acid. The xise of constant-l)oiling acid is recommended as being the most satis- 

 factory concentration. Sulphates were found to cause bleaching, and chlorids, 

 particularly calcium chlorid, intensification of color. 



A few results are quoted which show good agreement between the hydro- 

 chloric color method and the electrometric method of Hostetter and Roberts 

 noted above. 



The g'enesis of petroleum as revealed by its nitrogen constituents, C. F. 

 Mabery (Jour. Amer. Chem. Soc, 4I (1919). No. 10, pp. 1690-1697) .—A special 

 volumetric method of analysis for the determination of minute amounts of 

 nitrogen in oils is described, which is essentially a combination of the Dumas 

 procedure for nitrogen and the oxygen combustion procedure for carbon. A 



