Polarimetry

At the other end of the tube is placed a second nicol

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Unformatted text preview: icol prism at the end of an enclosed tube. At the other end of the tube is placed a second Nicol prism mounted on a dial calibrated from 0° to 360° and attached to an eyepiece. Within the tube is placed a sample cell consisting of a glass tube of known length sealed at both ends with caps containing a glass lens. Figure 1: Figure 2: Schematic of Polarimeter Light Source Nicol Prism Plane of Polarized Light Observed Rotation α Calibrated Nicol Prism Sample Tube New Plane of Polarized Light To measure the rotation of a pure compound, first turn on the sodium lamp and allow it to warm up for about 10 minutes. Fill the sample tube with the solvent you will use to dissolve the sample and insert it in the sample compartment. Make sure no bubbles are in the cell or at least are out of the light path. Many cells have enlargements at one end to accommodate a small bubble resulting from filling the cell. The solvent should be optically inactive, i.e., the solvent should not rotate the plane of polarized light. Typical solvents are methanol, ethanol, dichloromethane, and water. View the sample through the eyepiece and turn the dial until you see the brightest light passing through the sample. This should be at 0°, but may be slightly off if the polarimeter is out of calibration. (If the reading is not 0°, note the reading and use it as a zero po...
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