Chemical Forums
Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: xanthocyanopia on March 13, 2006, 10:19:53 PM
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In an instrumental analysis lab, we performed standard addition, however my results did not come out so I had to use someone else's data, and they did not do it correctly.
Instead of adding a minimal amount of a solution to the unknown to "spike" the sample, the amount added sometimes would comprise ~30% of the total solution (by volume). I wanted to know whether or not this was mathematically correctable.
For example, 25 mL of Tonic Water was used in each case, and a different amount of 0.1 M NaCl was added. This solution was then fed into the atomic absorption and the absorbance was read. The data set goes as follows
Amount NaCl Added Absorbance
0 024
2 584
4 656
6 920
Now I know if done correctly I could just plot Absorbance vs. Amount Added to find the concentration of unknown, however (especially in the latter cases, some which I excluded) the unknown became diluted by the salt solution. What would I plot though if I wanted to correct for this dilution?
I was messing around with the A=ebc (e for molar absorptivity) and using c = [25x + 0.1(mL NaCl used)]/(25+mL NaCl used),
but I don't know how I would plot this. I don't even know if it is mathematicaly possible. Any help would be appreciated.
Thanks,
Ray
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Generally speaking, when you plot data, in this kind of problem, you should use the quadratic mean.
However, according to my experience, it's almost impossible to draw a line that fits ALL the points.
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Instead of adding a minimal amount of a solution to the unknown to "spike" the sample, the amount added sometimes would comprise ~30% of the total solution (by volume). I wanted to know whether or not this was mathematically correctable.
Correcting for dilution is an easy task. Amount of the substance doesn't change, only volume changes. C1V1 = C2V2. That's all.