548 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



10 c.c. of the iron solution are placed in the flask and diluted with 100 

 c.c. of air-free water. 10 c.c. of pure concentrated sulphuric acid are added; 

 when the solution is cold 5 c.c. of 40 per cent, potassium thiocyanate are added 

 as indicator. When the air has been displaced by carbon dioxide, the titanium 

 solution is run in till it is colourless, i.e. in excess. The excess is determined 

 by titrating with the iron solution until a red colour appears. This red colour 

 must be permanent for 5 minutes, e.g. : 



10 c.c. iron solution + 775 c.c. titanium solution + 4-3 c.c. iron solution. 

 7*75 c.c. titanium solution = 143 c.c. iron solution = 14-3 mgm. iron. 

 .'. i c.c. titanium solution = 0-1845 mgm. Fe. 



Owing to the instability of the titanium chloride solution it should be 

 standardised before and after a series of iron estimations. 



(3) Estimation of Iron. 



5 c.c. of concentrated sulphuric acid are added to a measured volume 

 of the solution : when it is cold 5 c.c. of 40 per cent, potassium thiocyanate 

 are added, the air is displaced and excess of titanium solution is run in. The 

 excess is determined by titrating with the iron solution, e.g. 



20 c.c. titanium solution were used and excess titrated with 3-2 c.c. iron solution. 

 20 c.c. titanium solution = 3-690 mgm. Fe - 3'2 mgm. Fe = 3' 370 mgm. Fe. 



Sodium and Potassium. 



20-100 c.c. of urine are oxidised (i) by evaporation and ashing, or (2) with 

 nitric and sulphuric acids. The residue from (i) is dissolved in water or 

 dilute hydrochloric acid and filtered from insoluble salts. The residue from 

 (2) is heated in a platinum basin till nearly all the sulphuric acid has beer* 

 volatilised. It is diluted with water. 



The solution obtained by ejther method is treated with barium chloride as 

 long a as precipitate is formed and baryta water is added till the solution has a 

 distinct alkaline reaction. The precipitate of phosphates and sulphates is fil- 

 tered off and well washed. The filtrate and washings are treated with ammonia 

 and ammonium carbonate to remove barium salts ; these are filtered off and 

 washed. The filtrate and washings are evaporated to dryness. The residue 

 is heated to redness to remove ammonium salts, dissolved and treated again 

 with ammonia and ammonium carbonate and filtered. The filtrate is acidified 

 with hydrochloric acid, evaporated and heated to redness. The weight of this 

 residue is the amount of sodium and potassium chlorides. It is dissolved in 

 water, the potassium is estimated' in it by precipitation with platinum chloride, 

 or better, perchloric acid, and the amount of sodium calculated by difference. 



Potassium. 



As Perchlorate. 



The perchlorate method of estimating potassium has been tested by W. 

 A. Davis x who recommends its use for the analysis of potassium in soil, ash 

 of plants, etc. The advantage of this method is that small quantities of sul- 

 phates, phosphates, barium, magnesium and calcium do not interfere. Large 

 quantities of sulphate, etc., must be removed as above. Iron salts must be 

 absent. 



The extract of soil, the aqueous extract of the ash of plant tissues, or the 

 extract of the ash of animal tissues and urine previously treated as above, 

 is eVdpDrated to dryness in a porcelain basin and heated to dull redness for 

 about 15 minutes. The heating is continued for so long that, on dissolving, 

 a colourless solution free from iron results. 



If sulphates be present in large amount they are removed by adding 5-10 

 c.c. of saturated baryta solution, evaporating and igniting as above. 



a j. Agric. Sci., 1912, 5, 52. 



