126 MODERN METHODS OF TESTING MILK 



will all be precipitated in the form of the white silver 

 chloride before the brick-red silver chromate forms 

 permanently. 



How are the foregoing facts applied in finding the 

 percentage of salt in a solution? For simplicity, we 

 will use the following illustration experiments: Dis- 

 solve 29.06 grams of pure silver nitrate in pure water 

 and then add enough more water to make just 1000 

 cc. of solution. (Standard silver nitrate solutions are 

 best obtained ready-made from a reliable chemical-sup- 

 ply house.) One cubic centimeter of this solution is 

 just equal to o.oi gram of salt. Next dissolve 2 grams 

 of salt in pure water to make 100 cc. of salt solution. 

 In a cup or beaker measure 10 cc. of this salt solution 

 and add 2 or 3 drops of a solution of potassium chro- 

 mate (made by dissolving as much potassium chro- 

 mate as possible in 100 cc. of cold water) ; then add 

 slowly from a burette some of the silver nitrate solu- 

 tion and stir or shake the mixture in the cup. On the 

 first addition of silver nitrate, the mixture becomes 

 turbid from the formation of silver chloride and on 

 further addition sooner or later there will appear a 

 deep brick-red or blood-red coloration due to the for- 

 mation of silver chromate. So long as any chloride is 

 left in solution in the mixture, the red silver chromate 

 will disappear on agitation. When the red color begins 

 to become prominent, then run in only o.i cc. of silver 

 nitrate at a time, agitating vigorously after each addi- 

 tion and continue until a faint reddish tinge colors the 

 whole mixture and does not disappear on agitation. 

 The color change can best be seen on a white back- 

 ground as in a white cup or a glass beaker set on white 



