The following points highlight the four common organic acids identified in plant tissues. The organic acids are: (1) Oxalic Acid (2) Tartaric Acid (3) Malic Acid and (4) Citric Acid.
Organic Acid # 1. Oxalic Acid:
Occurs most commonly as calcium oxalate crystals, e.g., raphides, sphaeraphides, etc.
Material:
Oxalis leaves.
Make an extract by rubbing the leaves in a mortar with sand and dilute HC1. Use litmus paper. Neutralise the filtrate with NH4OH. Boil off excess NH3.
(a) Treat with 5% CaCl2 soln. Observe that the ppt. of calcium oxalate is insoluble in acetic acid even on heating but soluble in dilute mineral acids.
(b) To 5 ml of the neutral extract, add a few drops of 5% lead acetate soln. A white ppt. of lead oxalate is formed. Add an equal quantity of strong acetic acid and warm; the ppt. is insoluble.
(c) Add KMnO4 to the neutral extract and then dil. H2SO4. The colour of KMnO4 is discharged showing reduction of KMnO4 and the oxidation of oxalic acid.
Organic Acid # 2. Tartaric Acid:
Material:
Fruits of tamarind, Ananas, grapes, tomatoes, etc. Make aqueous extracts. Filter and neutralise the filtrate with NH4OH and boil off the excess ammonia.
(a) Add 5% CaCl2 soln. and observe the characteristic rhombic crystals (prisms) with octahedral faces or as needles of calcium tartarate. Observe that the crystals are soluble in both HC1 and acetic acid.
(b) Add ammoniacal AgNO3 in a part of the extract in test tube and warm gently in a water bath. A silver mirror is formed showing the reducing properties of tartaric acid.
(c) Heat a soln. of tartarate. Notice the odour of burnt sugar.
(d) Make a strong aqueous extract of grapes. Acidify with glacial acid and add a little potassium acetate. A white ppt. of potassium hydrogen tartarate is formed.
(e) Add extract to a soln. of Na2GO3. Observe the liberation of CO2 by effervescence.
(f) To 2-3 ml of neutral grape extract add a few drops of FeSO4. Place the tube in a beaker of cold water and add a few drops of H2O2 followed by an excess of NaOH soln. A deep violet or blue colour is obtained.
(g) To a neutral extract of grapes add 2 drops of a 2% soln. of resorcinol and 3 ml strong H2SO4. Heat gently. A rosy colour is formed which soon turns to violet-red.
Organic Acid # 3. Malic Acid:
Material:
Fruits of tomato, apples, pears, succulents of the family Crassulaceae, etc. Make aqueous extracts; filter, and neutralise the filtrate with NH4OH. Boil off the excess ammonia.
(a) Add a few drops of 5% CaCl2 soln. Boil and cool. No ppt. Add an equal volume of 95% alcohol. A white ppt. of calcium malate is seen.
(b) Add a few drops of 5% lead acetate soln. A white ppt. of lead malate, soluble in warm acetic acid (difference from oxalic acid) is obtained.
(c) To a neutral soln. add dil. FeCl3 soln. Yellow colour is observed.
Organic Acid # 4. Citric Acid:
Material:
Fruits of the genus, Citrus, e.g., orange, lime, lemon, etc., also fruits of tomato. Unripe lemon has about 6% citric acid.
Make an extract of lemon in water. Filter and test the filtrate with litmus paper.
(a) In a neutral or slightly alkaline soln. of the extract add 5% of CaCl2 soln. No ppt. Heat to boiling and observe now the thick ppt. of calcium citrate which is soluble in acetic acid.
(b) To the aqueous extract add ammoniacal AgNO2. A silver mirror is formed very slowly.
(c) Add 5% lead acetate soln. The white ppt. of lead citrate is soluble when an equal volume of acetic acid is added and warmed.
If an unknown solution is given, an idea about the identity of the organic acid in it could be found by simply treating the solution with CaCl2. The soln. must be made neutral (add dilute NH4OH and test with litmus paper; if ammonia is added in excess, drive off the excess by boiling) before adding CaCl2 in the cold. If there is a white crystalline ppt. with CaCl2 in the cold, the solution must be either oxalic or tartaric acid. If the ppt. is soluble in acetic acid, it is probably tartaric acid; if not, oxalic.
If there is no ppt. with CaCI2 in the cold, the neutral solution is boiled. If now there is a thick ppt., the solution contains citric acid. If there is no ppt. even on heating, the solution is thoroughly cooled and an equal volume of 95% alcohol added. A thick white ppt. shows the presence of malic acid. Further confirmatory tests indicated before should be done to identify the individual acid completely.
Sometimes Denige’s reagent (dissolve slowly about 5 g. of yellow mercuric oxide in 100 ml of conc. H2SO4; make it up to 500 ml with water) has been successfully used in the identification of three of these acids—oxalic, tartaric and citric. In all the cases given below, only neutral solutions, as previously, are to be used-
Oxalic acid treated with Denige’s reagent gives a stable, white ppt., which remains insoluble even on heating.
When tartaric acid is treated in a test tube with a few drops of the reagent and a drop or two of 5% KMnO4 solution is added, the colour of the permanganate is completely discharged and the solution remains clear; if citric acid is similarly treated, even to boiling, and the permanganate is added while the solution is still hot, the colour of the permanganate disappears no doubt, but is followed immediately by a heavy white ppt.