Chemical Forums
Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: mehc on April 16, 2014, 12:04:32 PM
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In the detetmination of calcium carbonate in limestone by classic titration, I got the abnormal value. I can not find what's the mistake. Here is what I did.
I took 0.2 gram sample, added 40 ml of 0.5 M HCl. The unreacted acid was titrated with 0.25 M NaOH. Volume of NaOH used was 49.4 ml.
The moles of acid reacted with Limestone is converted to moles of calcium carbonate, and converted to percent value.
The percent value I get is more than 100. As according to my calculations, there is about 0.3 gram calcium carbonate in 0.2 gram sample which is not possible.
Can anyone kindly indicate where is the mistake.
Thanks
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Your calculations look reasonably correct, but there plenty of possible experimental errors.
What indicator have you used?
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phenolphthalein. I have considered every possible error but the result is same,
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Try to boil the solution after dissolving the carbonate to get rid of dissolved carbon dioxide. Chances are it interferes with the end point detection (you are not detecting detecting end point of HCl titration, but of H2CO3 titration to HCO3-).
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I boil the solution. But if not boiled, the interfering CO2 will lead to lower calcium values.
In this case I am getting values beyond 100 %.