66 DAIRY ANALYSIS 



If found, the estimation is carried out by weighing 

 25 grammes of butter, adding 25 c.c. of water, melting 

 the butter at a low temperature, and stirring well ; the 

 aqueous portion is allowed to settle, and 20 c.c. are 

 withdrawn, and placed in a small beaker and boiled ; 

 at least 10 c.c. of phenolphthalein solution (see Appendix) 

 is added, and the solution titrated while still boiling with 



N 



- caustic soda solution (see Appendix) till a faint pink 



colour occurs ; the reading of the burette is noted, 12 c.c. 

 glycerine are added, and the solution titrated again 

 (boiling is not necessary) till a pink colour appears ; 

 the difference between the reading of the burette and 

 the first reading, corrected for the acidity of the 

 glycerine, multiplied by 0.031, will give the weight 

 of boric acid (as H 3 B0 3 ), and this multiplied by 

 5 + 0.05 W (W = percentage of water) will give the 

 percentage of boric acid. Where the percentage of 

 water is not estimated the figure 5.65 may be used 

 for 5 + 0.05 W, and as an approximate figure the 



N 

 number of cubic centimetres of soda solution used 



2 



between the first and second titrations may be multiplied 

 by 0.17. 



Sulphites are detected by the smell of sulphurous 

 acid given off on acidifying the aqueous portion ; formal- 

 dehyde will give its characteristic reaction (p. 54) if a little 

 of the aqueous portion is added to milk ; fluorides are 

 detected by adding a solution of calcium chloride to the 

 aqueous portion, filtering, igniting the precipitate, taking 

 up with dilute acetic acid, and treating the insoluble 

 portion with strong sulphuric acid in a platinum dish 

 over which is inverted a glass plate coated with bees- 

 wax through which one or two lines are scratched ; on 

 warming the sulphuric acid, hydrofluoric acid is given 

 off in the presence of fluorides, and this etches the 

 portions of the glass exposed by scratching through the 

 beeswax, and the marks are visible on melting and 

 wiping off the beeswax. 



