118 



ELECTRO-PLATING. 



47. 



any free cyanide of potassium, we should not obtain a deposit 

 beyond a momentary blush ; as the silver plate or electrode would 

 get an instantaneous coating of cyanide of silver, and this not being 

 dissolved the current would stop. The quantity of free cyanide of 

 potassium required in the solution varies according to the amount 

 of silver that is present, and the rapidity of the deposition. If 

 there be too little of it, the deposit will go on slowly ; if there be 

 too much, the silver plate will be dissolved in greater proportion 

 than the quantity deposited, and the solution will consequently 

 get stronger. The proportion we have found best is about half by 

 weight of free cyanide of potassium to the quantity of silver in 

 solution ; thus, if the solution contains two ounces of silver to the 

 gallon, it should have one ounce of free cyanide of potassium per 

 gallon. This is known by taking some nitrate of 

 silver, dissolving it in distilled water, and placing 

 it in a common alkalimeter, graduated into 100 

 parts, fig. 47. The proportion of the nitrate of 

 silver in the solution is, that every two graduations 

 of the solution should contain 1 grain. A given 

 quantity of the plating solution is now taken 

 say 1 ounce by measure, and the test solution of 

 nitrate of silver is added to it by degrees, as long 

 as the precipitate formed is redissolved : when this 

 ceases, the number of graduations is then noted, 

 and the following equation gives the quantity of 

 free cyanide. Every 175 nitrate of silver is equal 

 to 130 cyanide of potassium in solution. Suppose 

 20 graduations were taken, equal to 10 grains 

 nitrate of silver, then 175 : 130 : ; 10 : 7'4 grains 

 free cyanide of potassium. This, multiplied by 

 160, the number of fluid ounces per gallon, will 

 make about 2^ ounces. We have taken 2 gradua- 

 ^ tions to one grain of nitrate of silver, that the 



solution may be considerably dilute and less liable 

 to error. The following table is calculated at a half grain nitrate of 

 silver to the graduation, and will be a guide to the student or work- 

 man : the quantity of solution tested is one ounce by measure. 



Number of 

 graduations used. 



Free cyanide per gallon. 



oz. dwt. gr. 

 . 2 13 

 .053 



7 16 



2 



3 



4 10 6 



5 12 19 



6 15 9 



