One of the most convenient methods of assessing a liquid's purity is to measure its refractive index.  The method can also be used to easily analyze binary liquid mixtures.
    Refactive index is defined as the ratio of the speed of light in a vacuum to the speed of light in the medium in question:
  c_light / v_medium =  n (the refractive index)

Since the  velocities cancel, and since nothing travels faster than the speed of light in a vacuum, n is a unitless number that is greater than one.  For example, the refractive index of water is 1.333 [CRC - 62nd Ed.]

OPERATION:

You will use a refractometer to measure refractive indices of liquids.  Their operation is simple.  Flip the stage cover (prisms) open to the left (the photo shows it open).  Place a few drops of the liquid to be analyzed on the stage of the refractometer.  Close the cover.  Don't worry if a little liquid squirts out the side; just wipe it up with a Kimwipe.
    Turn the three-position toggle switch on the left to its middle postion.  Look through the eyepiece and adjust the wheel on the right until you see a "half-moon" image (half light - half dark).  Line up the light/dark border with the intersection of the crosshairs in the scope.  Then press the toggle switch down to view the refractive index scale.  From the scale you can read  the refractive index to the nearest +/^_0.0001.

MEASUREMENTS:

You will use the refractometer to judge the purity of a compound.  Compare the experimental refractive index to the literature value.  Literature values may vary from source to source - the index varies with temperature and wavelength, so try to look up values close to room temperature, and obtained with white light if possible, not with the yellow "sodium D line".

Simple Disillation Results:  Measure the refractive indices of your distilled water, our lab deionized water, and our tap water.  Compare the results.

Fractional Distillation Results:  For the fractional distillation project, you will use refractometry to analyze mixtures.  You will not use the calibration curve method outlined in the CHM 245/246 lab manual.
Rather, you will use the equation

n_mixture  =  X_a n_a +  X_b n_b

where X_a and X_b are the mole fractions of two components in a mixture, and n_mixture is the measured refractive index of the mixture.  At first this looks like one equation with two unknowns (the two mole fractions), but keep in mind that for a two component mixture,  X_a +  X_b = 1.00.

Measure the refractive index of each of your fractions (I, II, III) and samples of pure ethyl acetate and pure butyl acetate.  Use these values to calculate the mole fraction of each ester in each of your fractions.  Compare these results to your gas chromatographic results.