400 Browning— Influence of Acids 



sufficient excess, but rather renders the precipitate more crys- 

 talline, and therefore more easily and quickly filtered, suggested 

 a similar series of experiments having- as their object an inves- 

 tigation of the influence of free nitric acid on the same pre- 

 cipitation. Certain qualitative preliminary experiments showed 

 a similar effect to that brought about by hydrochloric acid as 

 regards the crystalline form and rapid settling of the precipi- 

 tate. It therefore remained to determine whether the nitric 

 acid present had any solvent effect upon the precipitate. 

 Accordingly a standard solution of barium nitrate was pre- 

 pared, free from alkali, by precipitating a soluble barium salt 

 with ammonium carbonate, washing thoroughly with hot water 

 and dissolving in nitric acid, care being taken to avoid an 

 excess of the acid, and making up to measure. Definite por- 

 tions of this solution were drawn from a burette into counter- 

 poised beakers, and weighed as a check on the burette reading. 

 Several precipitations of the barium in the form of sulphate 

 were made, both in the presence of free hydrochloric acid and 

 in neutral solution, and the mean of closely agreeing deter- 

 minations was taken as the value of the standard solution. 

 The mode of procedure in the testing was simple and may be 

 outlined as follows: — Into a counterpoised beaker a definite 

 amount of the standardized solution of barium nitrate was 

 drawn and the weight taken as before described, the amount 

 of nitric acid to be used was then added, and the whole volume 

 brought up to 90 cm 3 by the addition of water. This acid 

 solution was then brought to the boiling point and 10 cm 3 of 

 the dilute sulphuric acid added, and the whole was allowed to 

 stand for the length of time shown by the table before filter- 

 ing on asbestos, igniting and weighing. . It will be seen that 

 the total volume of liquid taken in each determination was 

 uniformly 100 cm 3 , the percentage of acid by volume being 

 thus easily regulated. In no case did the amount of barium 

 salt present exceed 04 gm. considered as the sulphate, and con- 

 sequently the uniform amount of 10 cm 3 of dilute (1:4 by 

 volume) sulphuric acid employed was always enough to assure 

 the excess which Fresenins* has shown to be necessary in the 

 precipitation of barium as the sulphate in the presence of hy- 

 drochloric acid. By reference to Series I it is evident that in 

 the presence of five per cent of nitric acid very little solvent 

 action is shown, and it appears also that the sulphate may be 

 safely filtered after an hour's time. In the presence of ten to 

 fifteen per cent of the acid the solvent effect is very small 

 when the solution is allowed to stand six hours or more. With 

 twenty to twenty-five per cent of acid present we find the 

 solubility to be slightly increased, but even then the average 

 loss is less than 0*001 grm. 



* Zeitschr. f. anal. Chem., xxx, 455. 



