Methods for testing lime in the field
Practical Action
‘high-calcium lime’, the chemical specification is:
CaO+MgO
MgO CO2 SO3
>85 %
<6 % <6 % <2.5 %
Note that these values are applicable to all forms of lime. For quicklime these values
correspond to the ‘as delivered’ condition; for all other forms of lime (hydrated lime and
lime putty) the values are based on the water free and bound water free product.
There are two other tests which can be done, but these are rather inaccurate and
subjective, so an evaluation based on these tests alone should not be taken on good
authority. Firstly, both quicklime and hydrated lime are slightly soluble in water, but
calcium carbonate is hardly soluble at all. So washing a small quantity (say 5 to 10
grammes) with excess warm sugar solution (the sugar helps the lime to dissolve quicker)
for a few minutes would allow quick or hydrated lime to dissolve away. Any solid residue,
especially if in the form of a white powder, is likely to be calcium carbonate and a rough
estimate of the proportion could be obtained. Additionally a sample of the quick or
hydrated lime can be treated with phenolthalein indicator. This will turn bright pink if the
sample is substantially quick or hydrated lime, but become weakly pink or not change
colour if the lime has substantially carbonated. Note that these two indicative tests would
work best with high calcium lime. If the lime might be hydraulic, that is contain clay
minerals, the tests could be misleading and indicate that you have a poor lime when in
fact the lime is completely satisfactory.
Annex 1
Determination of available lime by the rapid sugar test (using hydrochloric acid)
The following procedure is that specified by the ASTM. Boynton considered it to be the
simplest, most expeditious method and equal or superior in accuracy to other methods.
Apparatus
300 ml Erlenmeyer flask, i.e. a conical flask having approximate volumes marked at
certain intervals.
100 ml burette, with stand. (A wooden stand can be made locally).
Balance capable of weighing 0.85 g and 0.5 g to an accuracy of 2%, i.e. to 0.01 g.
No.100 mesh sieve. (0.15 mm).
Materials
CO2 free distilled water, if available.
Hydrochloric acid (specific gravity 1.18) 15.7 ml per litre of distilled water.
Methyl orange indicator.
Phenolphthalein indicator.
Sucrose – granulated sugar is satisfactory – 15 g.
Method
Take 0.5 g of 100 mesh lime and brush it into a 300 ml Erlenmeyer flask containing 20
ml of CO2 free distilled water and stopper the flask. Swirl and heat to boiling for 2
minutes. Add 150 ml of water and at least 15 g of sucrose. Stopper the flask, shake at
intervals for 5 minutes and allow to stand for 30 minutes to 1 hour. Add 2 drops
phenolphthalein, wash down stopper and sides of flask with distilled water, then titrate in
the original flask with the standard HCl solution*. Add about 90% of the estimated
amount of acid before shaking the flask and then complete titration, with the final acid
being fed slowly until the pink colour disappears. Note the reading: 1 ml of the acid
solution is equivalent to 1% available lime expressed as CaO.
* Standard HCl solution: 15.7 ml of HCl (specific gravity 1.18) per litre of CO2 -free
distilled water. The solution is standardised against 0.85 g of anhydrous Na2CO3 with
methyl orange as indicator, so that this amount will neutralise exactly 90 ml of standard
HCl solution. In adjusting for this, add more water if it is too strong or more acid if too
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