Common Titration Errors and How They Can Affect Results

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higherme
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I did a titration lab to find out the weight percent of CO3^2- in an unknown sample which contains only NaCL and CaCO3^2-.

My results were about 29% for the first titration and 18% for the second titration; I was wondering the types of error that led to this ~10% difference.

Some of the errors I thought of are:
- The buret readings were not very accurate.
- When preparing the 1 L of 0.01M HCl from 6M HCl (dilution) the 6M HCl was not mixed well with water. (used to dissolve the unknown)
- Same for the 1L of 0.01M NaOH from 6M NaOH which was used to titrate the unknown
 

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What volume of titrant did you use? I'll assume roughly 20 mL. You would have to misread the buret by 2 mL for this to be the source of your error.

- When preparing the 1 L of 0.01M HCl from 6M HCl (dilution) the 6M HCl was not mixed well with water. (used to dissolve the unknown)
The poor preparation of the HCl would give you an error of accuracy, not precision. This problem deals with an error of precision.

Why don't you assume that the solutions you used in both replicates was identical. What could have caused you this error under that assumption?
 
"Why don't you assume that the solutions you used in both replicates was identical. What could have caused you this error under that assumption?"

i don't get what is meant here~
 
well the amount of unknown i used for both replicates were pretty much around the same weight... just 0.001g difference... would that matter?

well... the amount of HCl used to dissolve the first sample is 21.31ml and 25.47ml for the second sample... maybe the error is there??

is that kind of right?? :P
 
Describe the solid material you used (NaCl + CaCO3). Was it a fine powder or were there larger grains apparent? Describe your titration endpoint. Were both replicates identical in color. Was the color dark or light? What indicator did you use?
 
higherme said:
"Why don't you assume that the solutions you used in both replicates was identical. What could have caused you this error under that assumption?"

i don't get what is meant here~

I meant that you should assume that the concentration of HCl used in both replicates was identical and that the concentration of NaOH used in both replicates was identical. Insufficient mixing when diluting these reagents is possible but how likely is it? Did you at least try to mix the individual solutions?