298 VETEEINAEY TOXICOLOGY 



made for this, and a quantitative determination of alkali is 

 imperative in order to establish the existence of an excess. 

 The alkalis caustic soda and potash are soluble in alcohol, 

 and may be thus separated and distinguished from the 

 carbonates. A further discrimination is afforded by their 

 different behaviour on 'neutralisation by acids with par- 

 ticular indicators. When phenolphthalein is used as the 

 indicator, the neutral point reached on adding acid gives 

 the whole of the alkali. Carbonates, on the other hand, 

 show neutrality at the half point, and when a second in- 

 dicator (methyl orange) is added, neutralisation may be 

 pursued to completion. 



As regards acids, it must be remembered that the diges- 

 tive stomach is normally acid through the presence of 

 free hydrochloric acid, and it must also be remembered 

 that organic acids {e.g., lactic and butyric) may be present. 

 In order to distinguish mineral (strong) acids from organic 

 (weak) acids, several methods may be adopted : 



(a) A dilute solution of methyl violet is made by adding 

 water to a few drops of the 1 per cent, alcoholic solution, 

 until a clear violet colour is given. When a solution of 

 mineral acid, such as nitric, hydrochloric, or sulphuric, is 

 added to this, the colour passes through blue and green to 

 yellow. Amongst organic acids, oxalic gives a blue, but 

 not yellow, colour. 



(6) Neutral ferric acetate does not give a red colour with 

 potassium or ammonium sulphocyanide, and a dilute solu- 

 tion scarcely shows the faint red colour of the acetate. 

 When a mineral acid is added, the non-ionised ferric 

 acetate yields the ionised ferric salt of the mineral acid, 

 which gives the blood -red coloration with the sulphD- 

 cyanide. 



(c) On similar grounds neutral ferric acetate, starch 

 solution, and potassium iodide show no blue coloration. 

 But the addition of mineral acid produces the ferric ion, 

 which liberates iodine from the potassium iodide, and thus 

 produces the blue coloration with the starch. 



