46 METHODS OF INVESTIGATING 



So far it has not been found possible to estimate boron 



polarographically . 



A colorimetric method for the determination of small quantities 

 of boron which has been applied to the estimation of boron in 

 plant material depends on the colour change of quinalizarin 

 effected by boric acid (see e.g. Smith, 1935). To 1 ml. of solution 

 containing from 1 to 40/xg. of boric acid 9 ml. of concentrated 

 sulphuric acid are added followed by 0-5 ml.. of a 0-01 per cent 

 solution of quinalizarin in 93 per cent sulphuric acid. A colour 

 change from reddish violet to blue results, the complete pro- 

 cess taking about 5 min. Nitrate, dichromate and fluoride 

 must not be present, but the common metals do not interfere 

 with the reaction. 



While this method appears to be quite satisfactory for the 

 estimation of boron in plant material, the necessity of using 

 concentrated sulphuric acid is something of a drawback. Another 

 colorimetric method for the determination of small quantities 

 of boron, which does not involve the use of this reagent, has 

 more recently been described by Naftel (1939). This method 

 depends on the colour produced when a solution of boric acid 

 is treated with oxalic acid and either curcumin or an extract of 

 turmeric and the mixture evaporated to dryness. According to 

 the procedure recommended by Naftel, the soil or plant-ash 

 extract containing from 0-5 to 8/xg. of boron is first rendered 

 alkaline by the addition of 5 ml. or more of 0-1 N calcium 

 hydroxide and then evaporated to dryness on a water-bath. 

 After cooling there are added to the residue 1 ml. of a freshly 

 prepared solution of oxalic and hydrochloric acids (made by 

 adding 80 ml. of a saturated solution of oxalic acid to 20 ml. of 

 concentrated hydrochloric acid) and 2 ml. of a 0-1 per cent 

 solution of curcumin or a 1 per cent freshly prepared extract of 

 turmeric in 95 per cent ethyl alcohol. The mixture is evaporated 

 to dryness on a water-bath at 55° C, heated for a further 

 30 min. at this temperature and then cooled, extracted with 

 95 per cent ethyl alcohol and the colour of the clear solution 

 obtained after filtering or centrifuging compared with that of 

 standard solutions prepared in the same manner. Quantities of 

 boron down to 0-5ju.g. can be determined by this method. In 

 soils other elements present do not appear to interfere with the 



