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What if the concentration of an unknown is higher than the range of concentration used when you were doing an Absorption v. Concentration graph? How can you figure out the concentration of an unknown solution, basically, if it falls above the range? I have literally no idea how to go about doing it.

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  • $\begingroup$ Fit a line to the standard measurements and extrapolate. There's no guarantee, of course, that the absorbance vs concentration response is linear beyond your standards. $\endgroup$ – Michael DM Dryden Nov 10 '14 at 5:07
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    $\begingroup$ ... or dilute the sample and remeasure the spectrum. $\endgroup$ – ron Nov 10 '14 at 14:37
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This can be an issue in many calibration experiments, and falls into the broader and extremely important field of "design of experiments" within the field of chemometrics. Clearly, your range of calibration data is not quite sufficient for your application, but you have several options available to you, each with pros and cons, some of which have been mentioned in the comments

  1. Go back to the calibration curve, and take some standard measurements at higher/lower concentrations than you did. Pros: this will be the most accurate method, and will help you to avoid this problem with future measurements. Cons: takes time and requires more measurements.

  2. As commented, extrapolate from your model. Pros: extremely quick. Cons: likely to be inaccurate unless you are 100% sure Beer's Law holds for your sample at all concentrations. There is also no way of estimating the errors.

  3. As commented, dilute your sample with a known quantity of solvent and remeasure. Then multiply your answer by the dilution factor. Pros: Fairly accurate, takes less time than performing more measurements for your model. Cons: Depending upon your lab skills and precision of your equipment, errors could start to propagate and become larger.

Good luck!

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