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Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: mehc on April 16, 2014, 12:04:32 PM

Title: Analysis of calcium in limestone
Post by: mehc on April 16, 2014, 12:04:32 PM

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

Title: Re: Analysis of calcium in limestone
Post by: Borek on April 16, 2014, 03:43:19 PM

Your calculations look reasonably correct, but there plenty of possible experimental errors.

What indicator have you used?

Title: Re: Analysis of calcium in limestone
Post by: mehc on April 16, 2014, 09:34:53 PM

phenolphthalein. I have considered every possible error but the result is same,

Title: Re: Analysis of calcium in limestone
Post by: Borek on April 17, 2014, 03:25:49 AM

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^-).

Title: Re: Analysis of calcium in limestone
Post by: mehc on April 17, 2014, 04:17:35 AM

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 %.