CH. XV] ACIDS. 297 



the methods on mixtures of known composition before 

 attempting to identify particular acids in actual extracts. 



To determine the ' acidity ' of an extract. 



Any exactly standardised caustic alkali may be used 



N 

 with phenolphthalein for indicator. — baryta is generally 



convenient and sometimes gives a sharper reaction than 

 soda or potash. The results may be calculated in grs. BaO 

 neutralised, or as oxalic or acetic acids. 



If an extract should be alkaline and it is desired to 

 determine the alkalinity, it is better not to titrate directly 

 with the standard acid, unless methyl orange is used as 

 indicator, because the alkalinity will be partly at least due 

 to alkaline carbonates. 



The alkalinity is best determined by adding a known 



volume of standard acid (excess), warming for some time 



N 

 to expel CO., and then finding the excess of acid by — 



baryta and phenolphthalein in cold solution. 



The results may be calculated in grs. acid neutralised. 



Inorganic salts. 



Consult : — 



Fresenius. Quantitative Analysis. 6th English edition. 

 Special part, pp. 678—690. 



Fresenius. Qualitative Analysis. 10th English edition. 

 Ash of plants. 



Read : — 



Palladin. Ber. d. d. hot. Oes. ix. (18.91). 



Lawes and Gilbert. /. C. S. Trans, xlv. (1874). 



