Test to Check Total Calcium Oxide in Lime

Purpose

The lime should have a purity of at least 70% by weight of CaO when tested as follows:

Procedure

1. Take one gram of powdered lime, accurately weighed, in a platinum crucible. Place it in an oven maintained at 105 + 2⁰C until, on cooling and weighing, constant mass is obtained.
2. Heat the crucible first gently and then ignite in a muffle furnace at 1000⁰ Cool the crucible in a desiccator and weigh to a constant mass.
3. Transfer the ignited lime, to a beaker, add 50 ml of water and mix well. Add 10 to 15 ml of concentrated hydrochloric acid, and boil for 15 min. Filter, wash with hot water, ignite and weight to constant mass.
4. To the filtrate, add concentrated hydrochloric acid, if necessary, in sufficient amount to make the total amount equivalent to 10 to 15 ml of concentrated hydrochloric acid. Add a few drops of nitric acid and boil. Dilute to 200-250 ml. Add slight excess of ammonium hydroxide and keep just below the boiling point until the odour of ammonia is barely perceptible. Filter off the iron and aluminium hydroxide while hot, collecting the filtrate in a 250 ml volumetric flask. Wash with hot water. Ignite the residue in a platinum crucible, blast, cool in a desiccators and weigh as aluminium oxide and ferric oxide. Make up the filtrate to 250 ml.
5. Pipette out 50 ml of the filtrate in a beaker and dilute to 100 ml. Heat to boiling and add slowly about 35 ml of boiling ammonium oxalate solution. Continue boiling for 2 or 3 min and allow the precipitated calcium oxalate to settle for half an hour.
6. Filter the precipitated calcium oxalate through filter paper. Wash thoroughly with small portions of dilute ammonium hydroxide (1 N) and then with hot water until the washing do not decolourise a hot dilute potassium permanganate solution in the presence of dilute sulphuric acid.
7. Puncture the filter paper and transfer the precipitate to the beaker already used for precipitation with a fine jet of hot water. Add about 50 ml of dilute sulphuric acid, heat to 60⁰C and titrate with standard potassium permanganate solution with constant stirring. Towards the end of the titration, introduce the filter paper which was used for filtration into the titration vessel and carry out the titration till the end point is reached.
8. Carry out a blank with all reagents following the procedure as outlined for this method. In the case of magnesium limes, decant through a filter paper, redissolve the calcium oxalate in the beaker and in the filter paper with dilute hydrochloric acid and wash the filter paper four times with hot water and finally with dilute ammonium hydroxide (5 N) in a slight excess and proceed as before. For the second filtration, use the same filter paper as was used before.
9. If it is desired to complete the analysis in as short a time as possible, a portion of 50 ml of the filtrate from the ferric and aluminium oxide determination should be precipitated in the usual way with excess of ammonium oxalate. Boil for about 5 min and let the calcium oxalate settle clear. Decant through a qualitative filter paper and cool the filtrate (with ice water if possible). Add diammonium hydrogen phosphate solution in large excess and 5 to 10 ml of ammonium hydroxide solution. Stir rapidly with rubber ‘policeman’. From the amount of precipitate thus formed, one can judge whether the lime contains sufficient magnesium oxide to require a double precipitate or not. For accurate work, if there is more than a slight amount of magnesium oxide, a double precipitation should be carried out, using a fresh 50 ml aliquot.

Calculation

Where

V₁ = volume, in ml, of standard potassium permanganate solution used for the test;

V₂ = volume, in ml of standard potassium permanganate solution used for the blank;

N = normality of standard potassium permanganate solution; and

M = mass, in g, of the material taken for the test

Reference

IS: 1514 – 1990