Fundamentals of Soil Science (1+1)

Exercise 11: Determination of Calcium and magnesium

Exercise: Determination of calcium
Material required
Equipment and glassware

• Beaker
• Filter pater (Whatman No. 40)

Reagents

• Ammonium chloride
• Ammonium hydroxide
• Ammonium oxalate
• Methyl red indicator
• HCl
• Sulfuric acid
• Potassium per manganate

Procedure

• Concentrate the combined filterate and washing to about 50 ml.Add about 5g of ammonium chloride. Make it slightly alkaline with NH₄OH (1+1).
• While still hot , add saturard solution of ammonium oxlate as long as any precipitate is formed and then add excess of it to convert the calcium ions in to calcium oxlate.
• Add two drops of methyl red indicator and then add either HCl (0.1N) or NH₄OH (1+4) till the colours turns light pink. Heat to boiling and allow to stand for 3 hours or longer.
• Decant the clear solution through filter paper Whatman No. 40. Wash the precipitate on the filter paper with hot water.
• Dissolve any precipitate remaining on the filter paper washing with HCl(1+10) in to the original beaker in which it was precipitated. Wash the filter paper with hot water 5-6 times to ensure the complete transfer of the precipitate to the original beaker.
• Reprecipitate by adding ammonium hydroxide and ammonium oxlate solution.
• Allow to stand as before and filter through the same filter paper.Wash it free from chlorides with hot water.
• Reserve the filterate and washings for the estimation of Mg.
• The precipitate of calcium oxlate may be dissolved in dilute sulphuric acid and titrated with N/20 KMnO₄ solution.

1ml N/20 KMnO₄ = 0.001 g Ca

Determination of Magnesium
Material required
Equipment and glassware

• Beaker (500ml)
• Filter paper (Whatman No. 40)
• Gooch crucible

Reagent

• Citric acid
• Ammonia solution
• Absolute alcohol
• Ammonium phosphate
• Ammonium nitrate

Procedure

• To the combined Mg filterate and washing from Ca determination, add 2 ml of citric acid, 100ml of ammonia and 50ml of alcohol and then add with constant stirring 25 ml of 10% ammonium phosphate solution.Let it stand overnight.
• Filter the solution in to original beaker, make the volume to 100-150 ml.To this solution in to the original beaker, add 10-15 ml of ammonia and 2 drops of ammonium phosphate solution.
• Stir vigorously and alow to stand for 3 hours or longer, filter through a gooch crucible and wash with ammonia(1+9).
• Moisten filter paper with saturated ammonical solution of ammonum nitrate. Ignite the precipitate and weigh as Mg₂P₂O₇.
• Calculate the percentage of Mg as:

Weight of Mg = Weight of Mg₂P₂O₇ x 0.2162