224 THE ANALYSIS OF SOLID MILK PRODUCTS. 



the process for their removal may be omitted, and the titration 

 performed on an aliquot portion of the solution which has been 

 made alkaline, evaporated to dryness, and ignited ; the ash is 

 extracted with hot water, and titrated first with acid till neutral 

 to methyl-orange, and then with alkali in the presence of glycerol, 

 till neutral to phenol-phthalein, the result will be the total boric 

 acid, free and combined. 



It is, of course, obvious that any of the other methods for the 

 estimation of boric acid (pp. 109 to 113) may be used in place of 

 Thompson's method. 



The author and Harrison have devised a rapid method for the 

 estimation of boric acid in butter ; 25 grammes of butter are 

 weighed into a beaker, and just melted in the water-oven, 25 

 c.c. of water are added, and the contents of the beaker mixed 

 well by stirring ; the aqueous portion is allowed to settle ; 

 the contents are again mixed, and allowed to settle. 20 c.c. 

 of the lower layer are withdrawn, and the boric acid estimated 

 therein by the method of Miller and the author. The weight 



of boric acid multiplied by - ^ (W = percentage of water) 



will give the percentage of boric acid ; the factor 5-65 may be 

 used without great error. 



The fat may be filtered, and used for the examination of its 

 composition. 



The boric preservative is usually expressed as boric acid, 



H 3 B0 3 . 



The Preservatives Committee has recommended : 



(D) That the only preservative permitted to be used in butter 

 and margarine be boric acid or mixtures of boric acid and borax, 

 to be used in proportions not exceeding 0'5 per cent, expressed 

 as boric acid. 



A.n estimation of the total sulphurous acid may be made by 

 distilling a portion of the liquid with dilute hydrochloric acid, 

 passing the gas evolved into decinormal iodine solution, and 

 titrating with sodium thiosulphate ; 254 parts of iodine convert 

 64 parts of S0 2 into sulphuric acid. The gas evolved may also 

 be passed into bromine water, and the sulphuric acid formed 

 estimated as barium sulphate, of which 233-5 parts represent 

 64 parts of SO,,. The solution from which the sulphurous acid 

 has been distilled may be advantageously evaporated to dryness 

 after making alkaline and ignited, and the sulphuric acid esti- 

 mated in this ; the sulphuric acid present is probably due to the 

 oxidation of the sulphite. 



Nitrates may be estimated by one of the methods described 

 under " Water Analysis." If much salt be present, the copper- 

 zinc couple method should be employed. 



