Appendix 229 



have been decomposed and the silica volatilised by treatment with 

 hydrofluoric acid is only rarely attempted. The British method, 

 adopted by the Agricultural Education Association, is thus described 

 by Hall: "20 grams of the powdered soil are placed in a flask of Jena 

 glass, covered with about 70 c.c. of strong hydrochloric acid, and 

 boiled for a short time over a naked flame to bring it to constant 

 strength. The acid will now contain about 20-2 per cent, of pure 

 hydrogen chloride. The flask is loosely stoppered, placed on the 

 water-bath, and the contents allowed to digest for about forty-eight 

 hours. The solution is then cooled, diluted and filtered. The washed 

 residue is dried and weighed as the material insoluble in acids. The 

 solution is made up to 250 c.c. and ahquot portions are taken for the 

 various determinations. The analytical operations are carried out in 

 the usual manner, but special care must be taken to free the solution 

 from siUca or organic matter." {The Soil.) As a rule only potash and 

 phosphoric acid are determined, but where other bases are wanted 

 they are estimated in the usual way. 



Potash. 50-100 c.c. of the solution are evaporated to dryness after 

 addition of 0-5 gram of pure CaCOg if the original soil did not effervesce 

 when HCl was added. 



Add 10 c.c. of 5 per cent, baryta solution, evaporate to dryness, 

 ignite and take up with water, add 2-5 c.c. perchloric acid (sp. gr. 

 1-12), concentrate till dense fumes are given off, allow to cool, add 

 20 c.c. of 95 per cent, alcohol and stir. Decant off the clear alcohol, add 

 40 c.c. alcohol containing 0-2 per cent, perchloric acid, transfer to a 

 tared filter paper, wash with 50-100 c.c. of 95 per cent, alcohol till 

 the runnings are no longer acid, dry at 100 C, and weigh as KCIO4 . 



Phosphoric acid. The charred residue from which the potassium 

 chloride has been removed is now extracted with hot dilute H2SO4, 

 the filtrate and washings amounting to about 110 c.c. Add 25 c.c. of 

 ammonium nitrate solution. Warm to 55 C, add 25 c.c. ammonium 

 molybdate solution also at 55, stir, aUow to cool and filter after 

 standing two hours. Decant through a filter: wash by decantation 

 with 2 per cent. XaXOg till the washings are neutral, transfer the 

 precipitate to the beaker used for the precipitation, and add a known 

 volume of standard alkah so that the precipitate completely dissolves. 

 Measure the excess by titration using phenolphthalein as indicator. 

 1 C.C. of X/10 alkali = 00003004 gm. PjOj. 



Ainilable potash avd phosphoric acid. Dyer's directions are as 



