PURPOSE:
To expose the student to new equipment and lab procedures. And to determine the specific rotation of a sample using a simple polarimeter.

INTRODUCTION:
The specific rotation of a chemical compound [a] is defined as the observed angle of optical rotation 'a', when plane-polarized light is passed through a sample with a path length of 1 decimeter. The specific rotation of a pure material is an intrinsic property of that material at a given wavelength and temperature. Often the temperature is not specified; in these cases it is assumed to be room temperature. The form of units used for specific rotation vary where deg/dm g/mL is common, but scientific literature many times uses just degrees. A negative value means levorotatory rotation(to the left) and a positive value means dextrorotatory rotation(to the right).

Optical rotation is measured with an instrument called a polarimeter (see below). A polarimeter consists of two polarized glass plates fitted to the ends of a tube. On one end of the instrument is an index wheel attached to one of the polarized plates. This wheel can be turned, and thus the plate, and the amount of rotation measured. The tube is hollowed out in the middle to accommodate the sample cylinder. A light source such as a sodium lamp provides the light that shines through the tube.

If no sample is placed in the polarimeter, the right and left plates line up at the 0 degree mark. This indicates that light passing through the polarimeter has not been changed or rotated. If an optically active solution is placed in the polarimeter, light passing through the sample is rotated. By rotating the index wheel the amount of rotation can be measured. Once the angle of rotation has been observed, the specific rotation can be calculated using the equation:

[a] = a / (L x c)

where [a] is the specific rotation, a = observed angle of rotation (degrees), L = length of the sample (dm) and c = concentration of substance dissolved (g/mL). The sample tubes are normally a uniform dm in length. As mentioned above, specific rotation can be positive or negative based on the direction of rotation.

---*---

The index wheel is composed of a fixed outer wheel graduated in degrees. The center is a polarized glass plate that can be rotated to the left or right. As it is rotated, an index pointer indicates the angle or rotation.

---*---

The substance being measured is housed in a glass tube with a filling spout and clear glass ends. It is a uniform 1.00 dm long on the inside.

---*---

PROCEDURE:

1) You can adjust the background shading by clicking on the "Special" button to the right and selecting "Background". Click on the "Special" button and select "Print Blank Report" to obtain a web page that can be printed and used as a lab report. (the program will not be interrupted.

2) Record the concentrations of samples A and B, and the specific rotation of sample C in the data sheet. Turn the sodium light on by throwing the switch on its side. Specific rotation values are specific to a particular wavelength of light. The bright D line of sodium is most often used.

3) Use the arrows on the index wheel close-up to rotate the polarized glass plate located at the center of the wheel. At the index reading of "0", the back and front planes of polarized glass line up and allow the maximum of light to pass through. Set the angle to "0" with the arrows.

4) Place sample A in the polarimeter. The view through the central polarized glass should have dimmed. This means that the light passing through the sample has been rotated and no longer in line with the plane of the front polarized glass. Use the arrows to obtain the brightest light level passing through the polarized center glass. Record this angle.

5) Return sample A to its rack and place sample B in the polarimeter. Adjust and record its observed angle of rotation. Do the same for sample C.

Note: this is a simple polarimeter and reads rotations up to + - 90 degrees. More sophisticated units can read past this limit and use electronics to read light intensities.

6) Calculate the requested values asked for on the lab sheet and any given by your teacher. For help on these values click on the "Special" button and select "View Data & Hints". Select "File Report" to send a copy to be viewed by your teacher.