4 BULLETIN 449, U. S. DEPARTMENT OF AGRICULTURE. 



liminary experiments were of value in supplying information as to 

 the general nature of the factors involved in the electrolytic method. 



PRINCIPLE OF THE ELECTROLYTIC METHOD. 



Success in using the electrolytic silver cleaners depends upon 

 bringing the tarnished silver into actual contact with a more active 

 metal when both are immersed in a solution of some suitable elec- 

 trolyte. Wlien so immersed aluminum and zinc are electrolytically 

 more active than silver, or, chemically speaking, they are said to be 

 electropositive referred to silver. In the presence of either sodimn 

 carbonate or sodium chlorid, or a mixture of both, aluminum forms 

 {fluminum ions in the solution and itself becomes negatively charged. 

 The silver accordingly becomes positively charged as the current 

 flows from the aluminum to the silver through the solution. In 

 other words, such an arrangement of metals in an electrolyte ma}^ be 

 considered to be an electrolytic cell. 



Since silver sulphid is slightly soluble, a small number of silver 

 and sulphid ions are always present in the solution, and the silver 

 ions will give up their positive charges of electricity and plate out on 

 the silver or negative pole as silver atoms. Any agency making the 

 silver sulphid more soluble will increase the number of silver and 

 sulphid ions and, provided the silver ions are plated out as rapidly 

 as they are formed, this will increase the rate of the reaction. More- 

 over, in accordance with the law of mass action, the greater the 

 number of aluminum ions formed in the solution, the greater will 

 be the tendency for silver sulphid to be reduced to metallic silver. 

 The conditions are apparently most favorable to the completion of 

 the reaction when a dilute solution of sodium carbonate is used as 

 the electrolyte. The hydrolysis of this salt furnishes a fairly strong 

 alkaline solution. 



(1) Na.COg-f 2H2O -» H30+C0,-f 2NaOH. 

 Aluminum then displaces hydrogen from a boiling solution of the 



alkali. 



(2) Al-fSNaOH-^NagAlOs+SH. 



The atomic hydrogen supplied by this reaction reduces the silver 

 sulphid. 



(3) Ag,S^2H-.H,S+2Ag. 



'\\Tien an excess of hj^drogen ions is continually being formed, the 

 sulphid ions are gradually removed to form molecules of H^S. In 

 this way the equilibrium between AggS (undissociated) and its ions 

 is disturbed, and accordingly more AgoS dissolves. The reaction 

 finally is completed and, since the excess of aluminum ions plates 

 out the silver on the silver pole, practically no silver is lost. 



