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. When 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 sodium 
carbonate or sodium chlorid, or a mixture of both, aluminum forms, 
aluminum 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 may 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 2 C0 3 +2H 2 — H 2 0+C0 2 +2NaOH. 
Aluminum then displaces hydrogen from a boiling solution of the 
alkali. 
(2) Al+3NaOH -* N"a 8 A10 8 +3H. 
The atomic hydrogen supplied by this reaction reduces the silver 
sulphid. 
(3) Ag 2 S+2H -> H 2 S+2Ag. 
When an excess of hydrogen ions is continually being formed, the 
sulphid ions are gradually removed to form molecules of H 2 S. In 
this way the equilibrium between Ag 2 S (undissociated) and its ions 
is disturbed, and accordingly more Ag 2 S 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. 
