554 Intelligence and Miscellaneous Articles. 



mercial carbonate contains only about 50 per cent. Now in the 

 several alkalimetrical processes employed in the arts, the 50 per 

 cent, of alkali is estimated, and not the 8*5 per cent, of impurities. 

 Constituting, as these impurities invariably do, by far the smaller 

 proportion of the commercial alkalies or alkaline carbonates, it will 

 in practice, I believe, be found advantageous to employ a method 

 which, by indicating the amount of impurities present, will give the 

 available proportion of alkali. 



As 1 have previously remarked, the primary difficulty to be over- 

 come is the entire expulsion of carbonic acid, the presence of but 

 a small quantity of which entirely prevents accurate determination, 

 not only owing to the change of tint produced by its presence on a 

 solution of litmus, but owing to the diminished sensibility of litmus 

 thus tinted. 



In 4he hope of avoiding those sources of error to which I have 

 alluded, and of facilitating the determination of the true per-centage 

 of alkali, I adopted a method of alkalimetry which in substance is 

 the following : — To the alkali under examination is added a known 

 excess of a normal solution of oxalic acid ; and after the expulsion 

 of carbonic acid from the solution by boiling, the excess of oxalic 

 acid remaining is determined by means of a standard solution of 

 ammonia. 



Some objection might be made to the employment of a solution 

 of ammonia ; but it will be found that a dilute solution of ammonia, 

 if kept in properly constructed apparatus, will remain more constant 

 than might be expected. The standard solutions of ammonia and 

 of oxalic acid are most easily prepared by means of a standard solu- 

 tion of sulphuric acid, care being taken that perfectly pure acid be 

 employed, and that the amount of real acid be carefully determined. 



Having prepared the standard solutions of a desired strength, the 

 determination of an alkali or of an alkaline carbonate may be thus 

 effected: — 10 grs. of an alkaline carbonate, carbonate of soda for 

 example, after having been placed in a flask, a solution of oxalic 

 acid corresponding to 10 grs. of pure carbonate of soda is added ; 

 the solution is then boiled until the expulsion of carbonic acid be 

 effected, when the solution is diluted with distilled water ; and after 

 the addition of a few drops of a solution of litmus, the excess of 

 oxalic acid is determined by a standard solution of ammonia. The 

 excess of oxalic acid remaining will of course indicate the impurities 

 present, or the absence of alkali, which, by deduction from the 

 quantity originally taken, will give the amount of available alkali. 



Care must be taken that the solution be only tinted with litmus, 

 and not too deeply coloured, as the more feeble the coloration within 

 certain limits, the more easily detected is the change of tint produced 

 by an excess of alkali or of acid. 



I have found it necessary to employ distilled water for diluting 

 the solutions, failing, as I have done, to obtain accurate results with 

 other water, owing to the presence of carbonic acid. The prepara- 

 tion of standard solutions is so well understood, that it is unneces- 

 sary to enter into further details. 



