In this article we will discuss about the tests for detection of organic acids in plants with the help of experiments.
In plants there are a number of organic acids synthesized and stored in the cytoplasm. All organic acids possess at least a carboxyl group (—COOH).
Organic acids are usually formed during carbohydrate metabolism, particularly during Krebs’ cycle reactions or CO2 fixation reaction in the dark phase of photosynthesis. Normally, organic acids are stored as salts/crystals viz. calcium oxalate, tartarate, citrate etc. A number of fungi are also capable of producing organic acids commercially.
Some common plant sources of organic acids are:
Organic acids — Plant sources
1. Oxalic acid — Leaves of Oxalis sp.
2. Tartaric acid — Fruits of tamarind, grape, tomato etc.
3. Citric acid — Fruits of Citrus spp.
4. Malic acid — Fruits of apple, pea, tomato etc.
The organic acids are mostly water-soluble and highly reactive. They perform a number of important functions in the cytoplasm of living system.
Requirements for Detecting Organic Acids in Plants:
(a) Preparation of Sample Solution:
Crystals of organic acids are dissolved in distilled water to prepare the sample solution.
From plant materials, the organic acids are obtained on crushing the material followed by squeezing the extract by a linen cloth.
(b) Reagents:
1. Calcium chloride solution — 5 gm. in 100 ml dist. water.
2. Lead acetate solution — 5 gm. in 100 ml dist. water.
3. KMnO4 solution — 1 gm in 100 ml dist. water.
4. Silver nitrate solution — 5 gm. in 100 ml dist. water.
5. Dilute H2SO4.
6. Denige’s reagent:
(5 gm of yellow mercuric oxide in 100 ml of conc. H2SO4, make it up to 500 ml with water).
7. Glacial acetic acid.
8. Resorcinol solution — 2 gm. in 100 ml dist. water.
9. Saturated FeSO4 solution.
10. Hydrogen peroxide (20 volumes).
11. NaOH solution — 5 gm. in 100 ml dist. water.
12. Ferric chloride solution — 5 gm. in 100 ml dist. water.
13. Ammonium hydroxide (dilute) — 5%
14. Cadmium chloride solution — 5 gm. in 100 ml dist. water.
(c) Neutralization of Sample:
Organic acids can easily be detected by a number of chemical tests after neutralization by ammonium hydroxide solution. Excess alkali can be evaporated by heating. Neutralization point is tested by Litmus test.
Tests for Detection of Organic Acid in Plants:
Following tests are performed with the neutralized sample solution:
I. Tests for Oxalic Acid:
Experiment:
1. Calcium chloride test:
To a small quantity of sample solution a few drops of 5% Calcium chloride solution is added.
2. Lead acetate test:
To a small quantity of sample solution, a few drops of 5% Lead acetate solution is added.
3. Potassium permanganate test:
To a small quantity of sample solution, a few drops of 1% KMnO4 solution is added and then a few drops of dilute H2SO4 is added to the same
4. Denige’s Test:
To a small quantity of sample solution, a few drops of Denige’s reagent is added. The mixture is heated.
Observation:
White precipitation of calcium oxalate is formed immediately.
A white precipitate is formed.
The colour of KMnO4 is discharged immediately.
A stable white precipitate is formed
Inference:
Presence of oxalic acid.
Presence of oxalic acid.
Presence of oxalic acid.
Presence of oxalic acid.
II. Tests for Tartaric Acid:
Experiment:
1. Calcium Chloride Test:
To a small quantity of sample solution, a few drops of 5% calcium chloride solution is added.
2. Ferrous Sulphate Test:
To a small quantity of sample solution a few drops of saturated FeSO4 solution is added, followed by addition of traces of H2O2 and dilute NaOH solution.
3. Silver Mirror Test:
To a small quantity of sample a drop of dilute NH4OH is added. Followed by addition in excess of 5% AgNO3 solution. The solution is then placed in a boiling water bath.
4. Potassium Permanganate Test:
To a small quantity of sample solution 2 drops of 2% resorcinol solution are added and then about 3 ml of conc. H2SO4 is added. The mixture is finally boiled.
5. Denige’s Test:
To a small quantity of sample solution a few drops of Denige’s reagent is added. Then the mixture is heated after addition of a drop of 5% KMnO4 solution.
Observation:
No immediate precipitation, precipitation occurs slowly on shaking the mixture or scratching by a glass rod within the tube containing mixture.
The solution turns violet.
Immediately white precipitate is formed.
Precipitation turns into a shiny mirror on boiling for 15 min. in water bath.
A rosy-red colour is formed which turns violet-red.
Permanganate colour is discharged leaving a clear solution.
Inference:
Presence of Tartaric acid.
Presence of Tartaric acid.
Presence of Tartaric acid.
Presence of Tartaric acid.
Presence of Tartaric acid.
III. Tests for Citric Acid:
Experiment:
1. Calcium Chloride Test:
To a small quantity of sample solution, a few drops of 5% CaCI2 solution is added.
2. Cadmium Chloride Test:
To a small quantity of sample solution, one drop of dilute NH4OH is added, followed by addition in excess of Cadmium chloride solution. The mixture is boiled in a water bath for 15 minutes.
3. Silver Mirror Test:
To a small quantity of sample solution, a drop of dil. NH4OH is added, followed by addition in excess of AgNO3 solution. The mixture is boiled for 15 mins.
4. Denige’s Test:
To a small quantity of sample solution a few drops of Denige’s reagent is added. The solution is then heated after addition of a few drops of dil. KMnO4 solution.
Observation:
No immediate precipitation. No precipitation on shaking or scratching. Precipitation on boiling and followed by cooling only.
A white gelatinous precipitate is formed
A blackish silver mirror formed
Permanganate colour is discharged with the appearance of turbidity.
Inference:
Presence of Citric acid.
Presence of Citric acid.
Presence of Citric acid.
Presence of Citric acid.
IV. Tests for Malic Acid:
Experiment:
1. Calcium Chloride Test:
To a small quantity of sample solution, a few drops of 5% Calcium chloride is added.
2. Ferric chloride test:
A few drops of 5% Ferric chloride solution is added to a small quantity of sample solution.
Observation:
No immediate precipitation. No precipitation on shaking and scratching.
No precipitation on heating. Precipitation on addition of equal volume of absolute alcohol.
Mixture turns yellowish.
Inference:
Presence of Malic acid.
Presence of Malic acid.
