Calibration graphs

• We often want to analyze a solution for the concentration of one component
  • today we want to analyze alcohol-water mixtures
  • the samples are produced by distillation
  • We start with a sample that's about 10-20% alcohol
  • we expect to initially collect solutions fairly rich in alcohol
  • after the first few samples are distilled, what remains is mainly water
  • the remaining samples will contain very little water

• What we need is some process that responds to solution concentration
  • On Aug. 30 we looked at light emitted by a flame
    • a solution with Na, K or Li was introduced
    • the stronger the solution, the brighter the emitted light
  • We also looked at fluorescence
    • again, more concentrated solutions will emit more light
  • We also looked at the absorption of light
    • again, the amount of light absorbed increases with concentration
    • (the % transmission decreases with concentration)

• We will use a similar method today
  • In this case we will use the refractive index of the solution
    • Refractive Index is the ability of glass or liquid to bend light
    • We choose this method because it is very fast and easy to measure RI
      • Assuming, of course, that you own a refractometer

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• One difficulty is that we don't know an exact relationship between the reading and the concentration
  • Of course, we had exactly the same problem analyzing for K, Na and Li
  • The flame intensity is higher as concentration rises
  • But we need a way to translate the intensity into concentration

• We solve this problem by creating a calibration curve
  • we carefully prepare a set of solutions
    • these have accurately known concentrations
  • we measure these solution with the instrument
  • we then plot instrument reading vs. concentration
    • reading on the Y axis, conc. on the X-axis
  • data should follow a simple curve
  • draw the best curve corresponding to the data

• Now a solution of unknown concentration
  • measure it with the same instrument
    • keeping all the other conditions the same
  • fit this reading to the graph
    • draw a horizontal line at the reading
    • where it intersects the curve, draw a vertical line
    • read where this line intersects the X-axis
  • that's the concentration of this solution

Refractive Index Curve

Isopropanol / Water Mixtures

• Isopropanol and Isopropyl Alcohol are the same species

• Here's the graph we got for a set of alcohol water solutions
  • we chose to use isopropyl alcohol for this study
  • the graph is not a simple straight line
  • but the data follows a very obvious smooth curve
    • Example-- if the refractive index of a sample is 1.3642
      • the graph allows us to read off 38.3% alcohol
• There is a problem near the upper right of the graph
  • refractive index changes only slightly with % alcohol from about 85% to 100%
    • in fact, it is a maximum around 95_% alcohol.
  • If we have a solution with a refractive index of 1.3751
    • it could be either 85% or 98% from the graph
• We will simply label solutions with RI >1.3750 as % alcohol > 80 %

• Solutions from 0-80% alcohol will be easy to evaluate

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  Would you like directions on using the refractometer?

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