. 40 MILK ANALYSIS 



contain more than 3 per cent, of formaldehyde, are mixed with 

 25 c.c. ~ iodin solution and sufficient strong sodium hydroxid 

 solution added to make the liquid bright yellow. After standing 

 10 minutes, hydrochloric acid is slowly added until a marked 

 brown liquid is produced. The iodin is then titrated with thio- 

 sulfate in the usual way. The amount of iodin that has been 

 taken up, multiplied by 0.118, will give the amount of formalde- 

 hyde. A blank experiment should be made and any necessary 

 correction applied. 



For dilute solutions, the potassium cyanid method is best. 



30 c.c. of— silver nitrate solution are acidulated with 15 drops 

 of nitric acid. 10 c.c. of this solution are mixed with 10 c.c. of 

 normal potassium cyanid solution (6.5 grams in 1000 c.c), then 

 water to make 50 c.c, the liquid shaken, filtered through a dry 

 filter and 25 c.c. set apart for titration as below (Volhard's 

 method). 



Another 10 c.c. of cyanid solution are mixed with a measured 

 amount of the formaldehyde solution (which must not contain 

 more than 0.03 gram of formaldehyde), the mixture added to 

 another 10 c.c. of the acid silver nitrate solution, shaken, made 

 up to 50 c.c, filtered and 25 c.c. of the filtrate taken as before. 

 The two solutions contain excess of silver, but the second con- 

 tains more, because the formaldehyde converts the cyanid into 

 a compound that does not precipitate silver. 



Standard thiocyanate solution is prepared by dissolving 10 

 grams of potassium thiocyanate (or 8 grams of ammonium 

 thiocyanate) in water to make 1000 c.c. The solution is approx- 

 imately — . Its value in silver must be determined thus : 

 -' 10 



<,o c.c. of -^ silver nitrate are mixed with i c.c. of nitric acid 

 ^ 10 



and I c.c. of saturated solution of ammonium ferric sulfate, and 

 thiocyanate solution added until a faint permanent brown is 

 produced. 



The titration of the acid filtrates is conducted in the same 



