88 THE VEGETABLE ACIDS [ch. vi 



C. Heat gently a few crystals of citric acid for some time with an equal weight 

 of resorcinol and a few drops of concentrated sulphuric acid. Add excess of alkali ; 

 the solution shows a fine blue fluorescence due to the presence of a product, resocyan. 



Expt. 89. Preparation of citric acid from lemons. Squeeze the juice from three 

 lemons and filter through muslin. Measure the volume of the juice, and add strong 

 caustic soda solution, carefully, until the reaction is slightly alkaline. Filter and for 

 every 10 c.c. of juice, add 5 c.c. of a 10% solution of calcium chloride. No precipitate 

 is formed. Now heat to boiling and a copious precipitate of calcium citrate is formed. 

 Filter off, while hot, on a filter-pump, wash with a little boiling water, drain well and 

 dry in the air. Weigh and add the requisite amount of sulphuric acid (1 gm. of 

 citrate=15 c.c. of normal sulphuric acid). Allow the mixture to stand for a short 

 time, filter and concentrate the filtrate in a glass dish on a water bath. Crystals of 

 citric acid separate out on concentrating considerably. (If, for any reason, insufficient 

 sulphuric acid has been added, some calcium citrate may separate out first on con- 

 centrating. If so, add a few drops of sulphuric acid, filter and continue to concen- 

 trate.) Drain off the citric acid on a filter-pump, dissolve in water and make the 

 tests in Expt. 88. 



The acids of the ethylene series have not as yet been very widely 

 detected. 



Fumaric acid, COOH • CH = CH * COOH, occurs in the Fumariaceae 

 (Fumaria, Corydalis) and Papaveraceae (Glaucium). 



Aconitic acid, COOH • CH^ * C • COOH • CH • COOH, is found in the 

 Monkshood (Aconitum) and other genera of the Ranimculaceae. 



The best known acid of the acetylene series is sorbic acid, found in 

 berries of the Mountain Ash {Pyrus Aucuparia). 



REFERENCES 



1. Bobbin, L. On the Presence of Formic Acid in the Stinging Hairs of the 

 Nettle. Proc. Roy. Soc, Edinburgh, 1920, Vol. 39, pp. 137-142. 



2. Ordonneau, Ch. De I'acidit^ des raisins verts et de la preparation de 

 I'acide malique. Bull, de la soc. ckim., 1891, Vol. 6, pp. 261-264. 



3. Staebelin, M. Die RoUe der Oxalsaure in der Pflanze. Enzymatischer Abbau 

 des Oxalations. Biochem. Zeitschr., 1919, Vol. 96, pp. 1-49. 



