Naked Science Forum
Non Life Sciences => Chemistry => Topic started by: bones89 on 24/02/2009 21:29:51
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I have an assignment and have been given a scenario and a calibratiojn curve for a set of results. it is askling me to account for its shape. how do i do this? then it ses 'using the results from the calibration curve evaluate the values of the output of the colorimeter (another list of results i have been given)
HELP PLEASE!!
Thank You
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What is the shape like? Like A, B or C? Or something else?
[diagram=435_0]
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Calibration curves are usually straight lines, I believe. Or nice smooth curves.
For UV Spectroscopy, we use the Beer-Lambert Law. I suspect it would be very close for colorimetry.
Beer-Lambert Law:
A=εcl
A: Absorbance
ε: Constant
c: Concentration
l: Path Length of Quartz Cell, which is Constant
You can determine ε via the slope of the curve since the term εl is constant and you obviously know what the Path Length is.
Using this, you can find the concentration of solution based on its absorbance.
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Oh! I have no idea why I thought titration curves there when I was drawing [:I][:I]
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"Chemistry Board... Curves... Account for Shape... Ah, Titration!"
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Eh?
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I suspect that was how you arrived at the calibration curves being titration curves.
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Ahhhhh..... yes. Indeed [:)]
Spot on.
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Hi Bones,
We are not going to be able to help you without more information.
What shape is the calibration curve?
A copy of the data would do- we can stuff it into a spreadsheet and see what it looks like..
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Does Calibration Curve from Wikipedia (http://en.wikipedia.org/wiki/Calibration_curve) help you at all?
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1) I presume you should have done this in class / or you're expected to look it up yourself.
2) You need to plot the calibration graph. I'm not going to do your homework for you, but just from the numbers it is clearly not a straight line through the origin. From what you've been taught in class, you are evidently expected to be able to figure out why it is the shape it is. I suspect it may have something to do with absorbtion being a logarithmic/exponential function... but don't take my word for it.
3) "Using the calibration curve" means plotting the points given, drawing a smooth curve through the points, then following the absorbance values given to you on the y-axis of the graph to see what x-value (concentration) they correspond to.
4) Think about the sort of lab practicals you do. Is the sort of variation in numbers fairly normal-looking? So what causes the variation? It looks to me like they're fairly consistent - so they're not playing trick-questions with you.
5) Is asking you whether something is "significant". Assuming this is GCSE-level, this is a simple "does it look like there's an obvious problem with some of the sugar or is it all a bit marginal and not clear-cut"? Is there a clear pattern in the data? You should be comparing the reference/background reading from the "lab standard sucrose" with the readings from your samples. You got to use some judgement, but a good starting point is "is the scatter (variation) within each set of data much less than the difference between the sets of data (ie reference vs sugar1 vs sugar2). If so, your results are probably significant.
5b) the most likely source of the contamination? I don't know, but I'd expect that they've taught you or given you some clues. Sugar is a plant product, so maybe it's nitrate-based fertilisers (they must have taught you about fertilizers and the nitrogen cycle and all that) - but I don't know whether the purification processes would remove that, and I'd have thought that if it were a likely problem then the sugar refiners would test/control for it anyway. Maybe its a cheap import or counterfeit goods from China??? Apparently (according to Google) sodium nitrite can be used as a preservative (more commonly in meat products, and not encouraged). Maybe its terrorist/sabotage!!! (I'm being a bit facetious here - which probably won't get you many marks. Again your class should have given you some ideas)