Water Contents and Minerals in Plants: 4 Experiments

The following points highlight the four experiments on the water contents and minerals in plants. The experiments are: (1) Determination of Dry Weight, Water Content and Ash Content of Plant Tissues (2) Demonstration of the Presence of Volatile Elements in Plant Tissues (3) Demonstration of the Presence of Nitrates in Plant Tissues and (4) Demonstration of the Elements Present in Plant Ash—Qualitative Ash Analysis.

Experiment # 1. Determination of Dry Weight, Water Content and Ash Content of Plant Tissues:

(a) Weigh out carefully 5-10 g of different plant materials (root, stem, leaf, flowers, fruits, etc.; also succulents, aquatics, etc.). Particular care should be taken with root samples to ensure complete removal of sand and soil particles adhering to the roots (should be done in running water and the adhering water removed by carefully pressing with blotting paper).

Put the samples in an oven at about 90°C. overnight. Cool in a desiccator and weigh again. Calculate the percentage of dry weights and water contents as % of fresh weights and water contents also as % of dry weights.

(b) Remove accurately weighed dried samples of tissues carefully into crucibles and ignite over a Bunsen flame or an electric muffle furnace at low red-heat (300-400°C.) for about 3-4 hours until the samples are completely free of black or grey colour (all combustible materials are removed).

The soft, grey-white powdery stuff now contains only oxides of elements (formed during incineration) present in the plant tissues. When the incineration is complete remove the samples from the furnace, cool in a desiccator and weigh again (repeated weighing is necessary to obtain correct reading of as near ±0.1 mg). Express the amounts of ash present in different tissues as percentages of both fresh and dry weights.

Experiment # 2. Demonstration of the Presence of Volatile Elements in Plant Tissues:

(a) Place one or two crushed dry seeds (e.g., wheat, oat, pea, barley, rice, etc.) in a test-tube and heat carefully over a Bunsen flame. Observe the moisture which condenses on the sides of the tube.

(b) Heat a few crushed seeds of wheat or pea mixed with soda-lime. The evolution of NH₃, tested by smell and also by its action on litmus paper, proves the presence of nitrogen which escapes as ammonia.

(c) In a hard glass or metal test tube, held horizontally by a clamp, put some wheat or pea seeds and the open end of the tube is fitted with a cork, carrying a delivery tube which can be conveniently dipped in different jars, containing different reagents.

The horizontal tube is strongly heated for some time and the delivery tube is put in turn into jars containing:

(i) A little anhydrous, whitish CuSO₄; turns blue (formation of CuSO₄, 5H₂O) showing escape of water.

(ii) Filter paper strips soaked in 5% cobalt chloride solution and dried. The treated dry, blue paper turns pink, showing again escape of water from burnt plant samples.

(iii) A solution of 10% lead acetate; black ppt. of PbS indicates the presence of sulphur (as H₂S).

(iv) Nessler’s reagent; turns brown proving evolution of NHs (presence of nitrogen).

(v) baryta water (or lime water); turns milky-white showing evolution of CO₂.

Experiment # 3. Demonstration of the Presence of Nitrates in Plant Tissues:

Nitrates give a deep blue colour with diphenylamine sulphate (0.5% in conc. H₂SO₄) solution. The coloured compound is soluble in water and thus fades on standing. Place carefully made thickish sections of leguminous plants (roots, stems, leaves, etc.), on a dry slide in a few drops of diphenylamine reagent.

Observe quickly under microscope and look for the blue-coloured localised areas, especially abundant in or around the veins.

Experiment # 4. Demonstration of the Elements Present in Plant Ash—Qualitative Ash Analysis:

To demonstrate the presence of various elements in plant ash, perform the following qualitative tests on ash sample (e.g., ash obtained preferably from mature tobacco leaves):

A. Dissolve about 0.5-1 g. of ash in 10 ml of warm 20% (by volume) HCl (conc. HCl is approximately 37%; one part conc. HCl and four parts distilled water, i.e., 20 ml of conc. HCl in 100 ml of water). Make up to 100 ml or stronger with water.

Filter and perform the following tests with the filtrate:

Sulphur:

Add a few drops of 5% BaCl₂ solution to a 10-ml portion of the filtrate.

A white very fine crystalline ppt. of BaSO₄ proves the presence of sulphur:

Calcium:

Take a second portion of 20 ml of the filtrate and make it slightly alkaline with dilute NH₄OH (equal portions of conc. NH₄OH and water). Filter and add a few drops of saturated ammonium oxalate solution. A white ppt. of calcium oxa­late proves the presence of calcium. Save the solution.

Magnesium:

Add an excess of ammonium oxalate to precipitate all calcium of the calcium-test solution. Filter and evaporate the filtrate to a volume of about 5 ml; this is important as conc. extracts give good results. Add about 1 ml of saturated disodium hydrogen phosphate.

Cool and allow to stand. Crystals of ammonium- magnesium-phosphate—(NH₄)₃Mg.PO₄ prove the presence of magnesium. Precipi­tation may be hastened by rubbing the inside of the test-tube with a glass rod.

Iron:

(a) Add a few drops of 2% solution of potassium ferrocyanide to about 10 ml aliquot of ash solution. A dark-blue colour—prussian blue—due to ferrocyanide being converted into ferricyanide proves the presence of iron.

(b) A few drops of 10% solution of potassium or ammonium thiocyanate (sulphocyanide) are added to about 10 ml of ash solution. A blood-red colour­ation due to the formation of iron thiocyanate—Fe(CNS)₆—proves the presence of iron.

Sodium:

Test a loopful of ash solution, concentrated, if necessary, in a clear flame on sodium-free wire. A yellow-colour flame shows the presence of sodium. B. Dissolve a second sample of ash in 10% (by volume) of HNO₃ and make it up to 100 ml (stronger solutions may be prepared, if desired). Filter.

Chlorine:

To a small portion of the filtrate, add a few drops of AgNO₃ solution. Observe heavy, white precipitate of silver chloride, indicating the presence of chlorine.

Phosphorus:

To another 10 ml aliquot of the filtrate in a test-tube, add ammonium molybdate solution. Heat on a steam-bath for few minutes and cool. A profuse yellow crystalline ppt. of ammonium-phospho-molybdate—(NH₄)₃ PO₄ (MoO₃),₂— proves the presence of phosphates.

Sodium:

To another portion of the filtrate, add uranyl acetate and zinc acetate in acetic acid. Pale yellow ppt. confirms the presence of sodium.