Cement contains oxides of calcium, Aluminium, Magnesium, iron, and small amount of silica Calcium oxide in a prime constituent of cement. Excess lime reduces the strength of cement and presence of lime in amount lesser than needed also reduces the strength of cement and results in quick setting . The general composition or Indian Portland cement is given below.
CaO = 60-66%, SiO2= 17-25 %, Al2O3 = 3-8%, Fe2O3 = 2-6 %
MgO = 0.1 -5.5%, SO3=1-3 %, and Na2O and K2O = 0.5-1.5 %
A known weight of the cement sample is treated with acid and the insoluble residue mainly, containig silica is removed by filtration. Iron and aluminum in the filtrate are precipitated as their hydroxides and filtered. The resulting solution containing calcium magnesium is diluted to 250cm3 and supplied for estimation.
Calcium ions present in the solution is determined by titrating a known volume of the cement solution with EDTA. The solution is treated with diethyl amine to the maintain the pH at 12.5. 4N NaOH is added to precipitate the Mg2+ present in the solution as magnesium hydroxide to that Mg2+ does not interfere in the detection of Ca2+. The solution is then titrated against standard EDTA using Patton and Reeder’s indicator. This indicator can be used in the estimation of calcium in the presence magnesium as the latter is in the form of precipitate. EBT indicator cannot be used at this PH (12.5), since it exists or yellow orange coloured ions at this pH and the colour change from wine – red to yellow orange at the end point in not a striking colour change to detect at the end point. Glycerol is added to get a sharp endpoint. The titration as called rapid EDTA method because, Ca2+ ions in cement solution estimated directly and quickly by using standard EDTA solution without removing the other metal ions
(CaO + SiO2) + 2HCl ? CaCl2 +SiO2 ? + H2O
Cement calcium chloride
Ca2 + Indicator ? Ca- Indicator complex (wine red colour)
Ca – Indicator + EDTA ? Ca – EDTA complex + In (blue colour)
Preparation of standard EDTA salt solution
Solid EDTA given in a weighing bottle is accurately weighed into a 250 ml standard flask. it is dissolved in distilled water, made up to the mark with distilled water and shaken well for uniform concentration. This is taken in a 50ml burette.
25 ml of given cement solution is pipette out into a clean conical flask. 5ml of diethylamine buffer, 5ml of 1:1 glycerol, 10ml of 4N NaOH and 2 drops of Patton and Reeder’s indicator are added. Resulting solution in the conical flask is titrated until wine red colour just changes to clear blue without any reddish tinge. The titration should be conducted slowly near the end point. Titration is repeated until agreeing values are obtained.
Wt. of CaO in the given cement solution = g
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