LABORATORY WORK 203 



Peroxides. Hydrogen peroxide oxidises lead sulphide to 

 sulphate, which the oxygen of the air does not. But hydrogen 

 peroxide alone does not oxidise lactic acid. The addition of a 

 catalyst, such as iron, results in its oxidation, as shown thus : 



Using the apparatus of Fig. 12 (p. 182), put a dilute solution of 

 lactic acid into A and lime-water into B. The air entering should 

 preferably have passed through caustic soda. No carbon dioxide 

 is formed. Add hydrogen peroxide (the commercial 2O-volume 

 solution will serve). Again suck air through. There is still no 

 formation of carbon dioxide. Add I or 2 c.c. of a dilute solution 

 of ferrous sulphate ; carbon dioxide is produced. 



Peroxidase. Instead of ferrous sulphate as above, take a fresh 

 mixture of lactic acid and hydrogen peroxide and add grated 

 horse-radish root. Carbon dioxide is evolved. 



Guaiacum Reaction. Take an excract of horse-radish in water, 

 add to some of it in a test-tube a drop of freshly made solution of 

 guaiacum resin in alcohol (guaiaconic acid is better). It is precipi- 

 tated by the watery solution, so that any change of colour is 

 difficult to see. Accordingly, add alcohol to dissolve the deposit. 

 It is not blue. In another test-tube, after the addition of guaiacum, 

 add a small quantity of hydrogen peroxide. Alcohol then added 

 will dissolve the blue oxidation product of the guaiacum. 



Solution of guaiacum dropped on the cut surface of a potato is 

 blued at once, so that the peroxide is already there. 



The cut surfaces of potatoes, apples, and other fruits turn brown 

 on exposure to the air. This is because there is a compound in 

 them which turns brown on oxidation. 



The peroxide in these cases is only formed when free oxygen 

 is present. Place a potato with a cut surface uppermost in a wide- 

 mouthed glass bottle, through whose cork three glass tubes pass, one 

 to the bottom of the bottle, another leads from the top to a Bunsen 

 burner, while the end of the third opens just above the potato and 

 is, at its outer end, connected by a rubber tube with a clip on it to 

 a little funnel containing guaiacum solution. Pass coal gas through 

 the bottle and light it at the burner. The peroxide previously 

 present is soon used up. Allow the guaiacum to drop on the 

 potato. No blue colour will be seen. Turn off the gas and empty 

 out the potato into the air. It rapidly turns blue. 



Reduction. Add sufficient solution of methylene blue to quite 

 fresh milk to give a distinctly blue colour. Then a small quantity 

 of formalin. The blue colour disappears more or less rapidly. 

 The reaction does not take place if the milk has been boiled, so 

 that it is due to an enzyme. Milk that has undergone bacterial 

 change reduces methylene blue without the presence of an aldehyde, 

 since some of the bacterial products serve the same purpose. 



