ORGANIC COMPOUNDS OF PLANTS 2Oy 



some plants, it is present in sufficient amounts to be 

 poisonous. Oxalic acid can be produced, in the labo- 

 ratory, by the action of nitric acid upon sugar and other 

 carbohydrates. 



278. Nitric Acid is present in lemons and many small 

 fruits. It is found in small amounts in peas, beans, 

 vetches, and lupines, and with malic acid in cherries, 

 strawberries and currants. 



Experiment 55. Citric acid from lemons. Measure out with 

 the pipette 10 cc. of the prepared lemon juice solution. Dilute 

 with about 25 cc. distilled water, and add 5 to 7 drops phenolphthal- 

 ein indicator. Then add standard KOH from the burette until a 

 faint pink tinge remains permanent. One cc. KOH will neutralize 

 0.004 gram citric acid. How much citric acid is present ? The 

 lemon juice solution is prepared by diluting the clear lemon juice 

 with ten times its bulk of distilled water and filtering. 



Questions (i) Why was KOH used in this experiment? (2) 

 What chemical change took place when KOH was added to the 

 diluted lemon juice? (3) What change in color was observed? 

 (4) What was the final product formed in this experiment? (5) 

 How does this experiment compare in principle with Experiment 10? 



279. Tannic Acid. In many seeds and leaves of 

 plants, bitter astringent compounds, as tannin and tannic 

 acid, are found. They may lessen the value of a food 

 because they retard the natural process of digestion. 

 Tannic acid is not present in food plants in any appre- 

 ciable amounts. Commercially, the tannins are valuable 

 for the tanning of leather and for other purposes. Tannin- 

 yielding plants are occasionally grown as marketable 

 crops. 



280. Function and Food Value of the Organic Acids. 



