H. C. Bradley — Color Reaction for Copper. 327 



That is, while under the most favorable conditions ferrocy- 

 anide of copper is formed visibly in solutions of one part cop- 

 per in 100,000 parts of water, blue starch iodide in solutions 

 of one part copper in 1,000,000 parts of water, the copper- 

 haematoxylin compound is distinctly recognizable in solutions 

 of one part of copper in 1,000,000,000 parts of water. This is, 

 we believe, one of the most delicate reactions known, chemical 

 or physical, and is comparable with the physiological effects 

 of copper salts on certain algae, with the catalytic effect of 

 copper in certain oxidations, and with the reactions for detect- 

 ing radio-active bodies of extreme dilution. It is a thousand 

 times more delicate than the ferrocyanide test for copper. 



The possibilities for the use of such a reaction as a qualita- 

 tive test for copper in drinking water from reservoirs treated 

 with copper sulphate to destroy algae, is at once apparent. 

 Whether the reaction can be applied directly to the proximate 

 analyses of drinking waters, what the conditions for optimum 

 results are, and what the intensely blue copper compound is, 

 are problems still to be worked out. 



Zinc. — In carrying out some investigations on the normal 

 presence and distribution of zinc in certain marine gastropods,* 

 the difficulty of recognizing definitely small amounts of that 

 metal in tissue ash containing relatively large amounts of cop- 

 per, iron, calcium, and phosphoric acid, was found to be very 

 great. Zinc forms practically no colored compounds by which 

 it may be identified in such a mixture, and the ordinary pro- 

 cesses of separation are tedious and unsatisfactory. The desir- 

 ability of finding some rapid and reliable test for zinc led to a 

 thorough canvassing of the less common laboratory reagents 

 for precipitating that metal, and the finding of a microchemi- 

 cal test which proved to be adequate in every way. The 

 reaction is by no means a new one, but its possibilities as a 

 reliable test for zinc seem to have been overlooked. 



A moderately concentrated solution of a zinc salt when 

 treated with sodium nitroprusside throws down a salmon-pink 

 precipitate of zinc nitroprusside, fairly insoluble in cold water, 

 much less so in hot. The characteristic feature of this pre- 

 cipitate is its definite and readily identified crystal form. All 

 the other insoluble nitroprussides of the heavy metals are 

 amorphous, slimy precipitates resembling the ferrocyanides in 

 general physical properties. Thus even in a mixture of several 

 metallic salts', such as copper, silver, cobalt, zinc, etc., the zinc 

 nitroprusside can be recognized under the microscope by the 

 presence of its characteristic crystals in the amorphous mass of 

 the other nitroprussides. In performing the test, it is desir- 

 able to have the solution of the salts fairly concentrated — about 

 * Bradley : Science, 1903, xix, p. 196. 



