Question

Describe plant ash analysis.

Answer 1

• This method is used to determine the presence of a relative amount of various elements in the plant. 
• In this method when fresh plants are dried at 70 – 80° C in the oven for one or two days so that, all water present in the plant is driven off.
• The remaining part is weighed and the dry weight of the plant is obtained.
• The main constituents of the dry matter are the polysaccharides, lignin, proteins, fats, organic acids and some other elements.
• This dry matter is burnt in a furnace at 600^° C, as a result, all organic compounds are oxidized and are driven off in the form of gases (CO₂ , NH₃ and SO₂).
• The remaining material is called plant ash and contains only mineral elements.
•  The relative amount of various minerals can be determined by analysis of this ash. 
• However, the extent of their utilization and essentiality cannot be determined by this method.

Answer 2

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.