A number of inorganic elements are present in plants which also catalyse many enzymatic reactions. The existence of such inorganic elements in plants can be detected by the chemical tests of plant ash.
Requirements for Detection of Ash:
(a) Preparation of Ash Extract:
For detection of various elements in plant ash viz., Ca, Mg, Fe, S, P, CI etc. ash material is dissolved in 50% HC1 (v/v) or 50% HNO3 (v/v) for an hour or more, then filtered through filter paper. Finally selective tests are made for detection of the elements.
(b) Reagents:
1. HCl (50%).
2. HNO3 (50%).
3. Barium chloride (5%).
4. NH4OH solution (dilute).
5. Ammonium oxalate (saturated).
6. Na2HPO4 solution (10%).
7. Potassium ferrocyanide solution (2%).
8. Ammonium thiocyanate solution (5%).
9. Potassium thiocyanate solution (5%).
10. Ammonium molybdate solution (10%).
11. Silver nitrate solution (10%).
12. Disodium hydrogen phosphate solution (10%).
Tests for Detection of Ash:
(a) Tests Using HC1 Extract of Plant Ash:
Experiment:
Test 1:
To about 5-10 ml of filtrate, a few drops of 5% Barium chloride solution is added.
Test 2:
About 20 ml of filtrate is taken in a test tube and made alkaline by addition of a drop of dil. NH4OH. Finally saturated ammonium oxalate solution is added.
Test 3:
After removal of Calcium oxalate ppt. by filtration, the solution is heated to concentrate and then cooled. Finally 10% disodium hydrogen phosphate solution is added to it.
Test 4 :
To about 10 ml of ash extract 2% potassium ferrocyanide solution is added.
Test 5:
To about 10 ml of extract, 5% ammonium thiocyanate or 5% potassium thiocyanate solution is added.
Observations:
A white crystalline precipitate is formed (ppt. of BaSO4).
A white precipitate of Calcium oxalate is formed.
A white precipitate of ammonium magnesium phosphate is formed.
A dark blue coloration (Prussian blue) is formed.
The solution turns blood red.
Inference:
Presence of sulphur in the ash extract.
Presence of Calcium in the ash extract.
Presence of Magnesium in the ash extract.
Presence of Iron in the ash extract.
Presence of Iron in the ash extract.
(b) Tests using HNO3 extract of Plant Ash:
Experiment:
Test 6:
To about 10 ml of extract, a few drops of ammonium molybdate soln. added, then heated for 10 min. and cooled.
Test 7:
To a small quantity of extract a few drops of 10% silver nitrate solution are added.
Observation:
A yellowish crystalline precipitate is formed (ppt. of ammonium phosphomolybdate).
A white precipitate is formed.
Inference:
Presence of Phosphorus in the ash extract.
Presence of Chlorine in the ash extract.