THE PREPARATION OF AMALGAMS. IJ 



PRECAUTIONS USED IN THE PREPARATION OF AMALGAMS. 



Zinc and cadmium amalgams in contact with air or aqueous solutions con- 

 taining air are rapidly oxidized. As early as 1863 Crova complained of the 

 inconstancy of the potential of amalgams " when the proportion of the oxi- 

 dizable metal is extremely small ; " he says that " continual agitation lessens 

 the error but does not nullify it." St. Lindeck, in 1888, mentions the same 

 difficulty. Meyer, in 1891, prepared potassium and sodium amalgams in an 

 indifferent gas, and delivered them through fine jets into a solution free from 

 air. Richards and Lewis weighed and kept their zinc and cadmium amal- 

 gams under solutions of the corresponding sulphate a precaution which 

 we found, from preliminary experiments, to retard oxidation, but not to 

 prevent it altogether. Cady worked with amalgams of calcium and other 

 oxidizable metals under pyridin. In a word, it appeared impossible to 

 determine potentials within one hundred thousandth of a volt unless the 

 exclusion of oxygen gas from every part of the process was complete. As 

 will appear from the description of the experiments, the success of the follow- 

 ing measurements was proportional to the completeness of this exclusion. 

 Finally this cause of error seems to have been wholly overcome. 



Clean, dry amalgams of known concentration were made from the pure 

 materials as follows : 



The best zinc sulphate was dissolved in pure water to form a saturated 

 solution, which was then somewhat diluted. Ammonia was generated in a 

 glass-stoppered distilling flask by heating a strong commercial solution of 

 the gas, and passed into the zinc sulphate solution through a bent delivery 

 tube ; no cork or rubber was used in the apparatus. A heavy precipitate of 

 zincic hydroxide redissolved at length to form a clear solution, but the cur- 

 rent of ammonia was continued a little longer to avoid the formation of 

 zincic hydroxide at the anode during electrolysis. The electrodes were of 

 platinum foil about 1 sq. cm. in area. Platinum wires were welded to 

 them. Before using they were boiled with strong nitric acid and washed 

 with distilled water. The current density was large, but any tendency 

 to form black or spongy deposits was carefully avoided. 40 Since zinc 

 alloys somewhat with a platinum cathode, the layer next the platinum was 

 not disturbed. The outer layer was removed from time to time to a beaker 

 containing dilute distilled ammonia. When a sufficient quantity had been 

 collected it was washed on a bare Gooch crucible with dilute distilled am- 

 monia, distilled water, pure alcohol, and ether. The crystals were then ex- 

 posed over night on a watch glass over strong sulphuric acid in a vacuum 



40 Proc. Am. Acad., 34, 89 (1898). 



