ELEMENTAEY EXPERIMENTAL PHYSIOLOGY 3 



element, to the copper or negative element; outside the cell the 

 current passes from the copper binding-screw, the positive pole or 

 anode of the battery, to the zinc binding-screw, the negative pole or 

 kathode. 



If plates of copper and zinc were simply immersed in 10 per cent, 

 sulphuric acid, the chemical action set up would soon cause the copper 

 plate to be covered with bubbles of hydrogen gas. This would cause a 

 resistance to the flow of current inside the cell, and further, hydrogen 

 being electro-positive to zinc, a polarisation current in the opposite 

 direction to the original battery current would be set up in the cell 

 and rapidly reduce its E.M.F. Daniell, by placing the copper plate in a 

 solution of copper sulphate, which the hydrogen splits up, prevented 

 polarisation from taking place within the battery. 1 Therefore as long 

 as there is free sulphuric acid present and the copper sulphate is 

 saturated, the current produced by the cell remains constant. Pro- 

 vided too that the porous pot, which is to prevent the deposition 

 of copper on the zinc rod, remains permeable to the H ions. 



The zinc rod has to be amalgamated because commercial zinc con- 

 tains iron and other metallic impurities ; these in the presence of the 

 sulphuric acid would, with the zinc, constitute a number of minute 

 batteries. By covering the impurities with zinc amalgam their dis- 

 turbing action is removed, and as the zinc is dissolved away, the 

 mercury combines with fresh zinc so that the electromotive properties 

 of the zinc rod remain constant. 



1 A more accurate description of the chemistry of a Daniell cell is as follows : 

 The cell consists of two metals, zinc and copper, dipping into an electrolyte 

 containing various ions in solution ; these are H, S0 4 , OH, Cu and S0 4 , of which 

 Cu and H, being positive ions, will work their way towards the negative element, 

 the copper plate and the OH and SO 4 being negative ions towards the zinc. 

 When in use chemical changes take place around both metallic plates. The zinc 

 is attacked by the S0 4 ions discharging, forming ZnS0 4 , and energy is liberated, 

 which is conducted across the electrolyte by the ions in solution. Around the 

 copper plate the copper sulphate is being split up into S0 4 and Cu ions, in which 

 process energy is stored up. But the energy liberated at the zinc plate is greater 

 than that stored in the neighbourhood of the copper plate, therefore the cell, 

 when working, is always liberating a balance of energy which appears as an 

 electric current. The S0 4 ions, constantly being liberated in the copper sulphate 

 solution and charged with electricity, migrate through the porous pot towards 

 the zinc, discharge forming ZnS0 4 and a liberation of energy as explained. 

 Towards the copper plate both H and Cu ions charged with electricity are 

 constantly streaming. That it is the Cu ions and not the H ions which discharge 

 and become precipitated on the plate depends simply upon the fact that it 

 requires a less energy and a lower E.M.F. to separate Cu than H ions. Therefore 

 as long as there are sufficient Cu ions present to conduct the current, Cu ions and 

 not H ions will discharge and be precipitated on the copper plate. 



