CHEMICAL TOXICOLOGY 279 



or vegetable origin (ptomaine bases) which when a small 

 quantity of alkaloid is present {e.g.,1 milligramme) may equal 

 or exceed it in weight. Further purification leads to loss 

 of material, which may be so great as to extend even to the 

 vanishing point. 



2. Volumetric methods depend on the performance of a 

 reaction with a solution of a reagent of known strength, the 

 end of the reaction being marked or indicated in various 

 ways. As an illustration, hydrocyanic acid may be deter- 

 mined in the presence of sodium bicarbonate by adding 

 measured quantities of a standard solution of iodine, which 

 may be prepared of such strength that 1 cubic centimetre 

 is equivalent to -^ milligramme (xrVcr of ^ grain). If starch 

 solution is also present the end of the reaction is marked 

 by the production of the blue starch iodine coloration. 

 From the number of cubic centimetres of iodine used the 

 quantity of hydrocyanic acid may at once be calculated. 



Free alkalis such as potash or ammonia, and free acids 

 such as sulphuric, hydrochloric, or nitric, may also be 

 determined by neutralisation with standard acid or alkali, 

 respectively, in the presence of a suitable indicator, such as 

 phenolphthalein, methyl orange, litmus, etc. 



3. Colorimetric methods depend on the production of 

 colour reactions. Having obtained a certain result, the 

 colour may be matched by preparing similar tubes from 

 measured quantities of the pure materials. In illustration : 

 a certain tint of blue may be got by the Prussian blue test 

 from an unknown quantity of a cyanide. This is matched 

 by exactly similarly conducted preparations from known 

 quantities of cyanide. 



4. Allied to colorimetric methods are those dependent 

 on the comparison of the relative intensity or bulk of a 

 turbid precipitate, or a stain, such as an arsenical mirror 

 in Marsh's test. Zinc is precipitated from ammonia 

 solution by sulphuretted hydrogen, and when known 

 amounts of zinc are tested similarly in the same bulk of 

 fluid, it is easy to closely match the unknown by a known 

 quantity. This method has received more study and 



