MICRO-NUTRIENT PROBLEMS 47 



sstimation of boron, but if large quantities of other substances 

 present are found to interfere with the determination of boron, 

 :he latter may first be separated by volatilization with methyl 

 ilcohol. 



An electrometric titration method has been specially devised 

 by Wilcox (1940) for the determination of boron in plant 

 material. A quantity of the dried and powdered plant material 

 (say, from 5 to 25 g.) containing not more than 2 mg. of boron 

 is mixed with one-tenth of its weight of calcium oxide and 

 ignited in a furnace at a low red heat. The resulting ash after 

 cooling is moistened with water and taken up in 15-20 ml. of 

 6 N hydrochloric acid and then heated on a steam-bath for 

 30 min. Phosphate is removed from the resulting solution by 

 the addition of N lead nitrate solution to the extent of 1 ml. for 

 each gram of plant material used, followed by sodium bicar- 

 bonate until a precipitate is produced, when the mixture is 

 heated on a steam -bath and more sodium bicarbonate added 

 until the solution is neutral to brom-thymol-blue (about pR 7). 

 The mixture is then made up to 250 ml. and filtered through a 

 dry filter paper. Carbon dioxide is removed by acidifying with 

 6 N hydrochloric acid and heating to boiling, then making 

 alkaline with 0-5 N sodium hydroxide and reacidifying with 2 N 

 hydrochloric acid until 5-10 drops in excess have been added. 

 On making up to 300 ml. the solution is boiled for a few minutes. 

 It is then ready for electrometric titration. For this the quin- 

 hy drone electrode may be used in conjunction with a 0-7A T 

 calomel electrode, these giving a null point at approximately 

 pB. 7. The electrodes having been introduced into the solution, 

 0-5 JV sodium hydroxide is added until the solution is neutral 

 to brom-thymol-blue, when the galvanometer should register 

 approximately zero ; if it does not the null point is obtained by 

 the addition to the solution of either 0-0231 N sodium hydroxide 

 or hydrochloric acid. Five grams of mannitol are then added, 

 and if boric acid is present a galvanometer deflexion results. 

 Standard 0-0231 N sodium hydroxide is then added until the 

 null point is again reached. The volume of sodium hydroxide 

 gives a measure of the amount of boron present, 1 ml. 0-0231 N 

 sodium hydroxide being equivalent to 0-25 mg. boron. This 

 method is claimed by Wilcox to be specially suitable for deter- 



