412 PRACTICAL PHYSIOLOGY 



The sulphates of copper and potassium accelerate the process very 

 greatly, the former by acting as a carrier of oxygen, the latter by 

 raising the boiling point of the mixture. 



Distillation, (2nd stage). When the acid mixture has cooled, add 

 100 c.c. of distilled water (free from ammonia if obtainable), and mix. 

 If the incineration was performed in a small flask, it will be necessary 

 to transfer its contents to a large Jena flask for the distillation, 1 and 

 rinse with portions of about 100 c.c. water, until the distilling flask is 

 nearly half full. If the mixture is already in the distilling flask the 

 requisite amount of water is added. Add a little powdered pumice or 

 mica to prevent bumping. Then, holding the flask in a slanting 

 position, pour strong caustic soda (40 to 50 per cent.) solution, enough 

 to render the liquid alkaline, down the neck and wall of the flask so as 

 to form a bottom layer of alkali. By this means any ammonia set free 

 by the alkali will be caught by the overlying acid mixture. The 

 amount of alkali required to render the mixture alkaline must be 

 previously determined ; 40 c.c. is usually an excess. 



Attach the flask to the distilling apparatus (see Fig. 239). Place 50 



c.c. yrr sulphuric acid, about an equal volume of distilled water and a 



few drops of methyl orange solution, or other suitable indicator, in 

 the receiving flask. Adjust the delivery tube of the apparatus so that 

 it just dips below the surface of the liquid in the receiver. Mix the 

 contents of the distilling flask. (If enough alkali has been added a 

 blue colour will develop owing to the liberation of cuprous hydroxide.) 

 Light the burner, and continue the distillation for at least 15 minutes, 

 taking care to prevent the sucking back of the contents of the receiver 

 into the distilling flask. 2 



Lower the receiver so that the delivery tube is above the liquid, and 

 continue the distillation for a few minutes so as to wash out the inside 

 of the tube. Finally wash the outside of the tube with a jet of water 

 from the wash bottle, so as to remove adhering acid, and remove the 

 receiver for the titration. Should the contents of the receiver become 

 neutralised during the distillation, as shown by the indicator changing 



7? 



colour, a further measured quantity of ^ acid must be added. 3 



1 750-1000 c.c. capacity. 



2 A vertical condenser, replacing the descending portion of the delivery tube, 

 reduces the risk of sucking back and prevents the heating of the liquid in 

 the receiver, and is, therefore, desirable in accurate work. 



3 It is often necessary to continue the distillation for considerably more than 

 15 minutes. When it is doubtful whether all of the ammonia has distilled over 

 in this time, distillation should be continued into a second flask containing a 

 few c.c. decinormal acid and distilled water. 



