1, understand the structure of polarimeter and the measuring principle of optical rotation.
2, master the use of polarimeter and specific optical rotation calculation method.
Preview requirements
Understand the definition; Understand the influencing factors; Understand the meaning of determination; Understand the optical rotation phenomenon of carbohydrates; Think about how to protect the polarimeter in this experiment.
Experimental principle
When a single beam of plane polarized light passes through chiral substances, its vibration direction will change, and the vibration plane of light will rotate at a certain angle, which is called optical rotation. The property that a substance makes the vibration plane of polarized light rotate is called optical rotation, and a substance with optical rotation is called optical rotation or optical rotation. Many natural organic substances are optically active. Because optically active substances can make the vibrating surface of polarized light rotate to the right (clockwise, marked with "+") or to the left (counterclockwise, marked with "-"), optically active substances can be divided into right-handed substances and left-handed substances.
The light emitted by a monochromatic light source (usually a sodium lamp) is converted into plane polarized light (polarized light for short) after passing through a polarizing prism (Nicole prism). When polarized light passes through the optically active substance in the sample tube, the vibration plane rotates at a certain angle. Adjust the calibrated analyzer (also a Nicole prism) to let polarized light pass, and the rotation degree of the analyzer is displayed on the dial, which is the measured optical rotation α of the sample.
Instruments and reagents
Polarimeter, bottle washing, rubber dropper, filter paper;
Distilled water, 5% glucose solution, glucose solution with unknown concentration.
Experimental content
1. Structure of polarimeter
Optical rotation can be measured by polarimeter. There are two kinds of polarimeters: one is an automatic polarimeter that automatically displays the measurement results digitally, and the other is a disc polarimeter that obtains the results through visual calibration.
2. Determination of optical rotation
Preparation of (1) sample solution
Accurately weigh a certain amount of samples and prepare a solution in a 50 ml volumetric flask. Generally, water, ethanol and chloroform can be used as solvents. If a pure liquid sample is used for direct test, it is only necessary to determine its relative density before determination.
Because the glucose solution changes rotation, it is necessary to prepare the glucose solution to be tested 24 hours in advance to eliminate the rotation change phenomenon, otherwise the reading will be unstable during determination.
(2) preheating
Turn on the power switch of the polarimeter, preheat it for 5 ~ 10 minutes, and observe and use it after it completely emits sodium yellow light.
(3) Zero adjustment
Before determining the sample, the zero point of polarimeter must be adjusted with distilled water. After cleaning the sample tube, fill it with distilled water so that the liquid level slightly protrudes from the nozzle. Gently flatten the glass cover along the edge of the nozzle, without bubbles, and tighten the nut (tighten it to be watertight, too tight, the glass slide will easily generate stress, which will cause the brightness change of the visual field and affect the determination accuracy). Dry the sample tube, put it in the polarimeter and cover it. Turn on the sodium light and adjust the dial to about zero, and the field of view will be greater than or less than zero. As shown in a and b in figure 10-3. Rotate the thick and thin handwheels to make the brightness of the three parts in the field of view consistent, that is, the zero field of view, as shown in figure 10-3c. Write down the dial reading, repeat zero adjustment for 4~5 times, and take the average value. If the average value is not zero and there is a deviation value, it should be subtracted or added in the measurement reading.
A. field of vision is greater than or less than zero. B. field of vision C. field of vision is less than or greater than zero.
(4) determination
Sample determination and zero adjustment are the same. Before each measurement, the sample tube must be washed with distilled water 1~2 times, and then wetted with a small amount of liquid to be measured for 2 ~ 3 times to avoid being affected by dirt, and then loaded with samples for measurement. Turn the dial to find a zero-degree field of vision with uniform brightness under dark illumination. If the reading is positive, it is right-handed; The reading is negative, left-handed. The difference between the reading and the zero value is the optical rotation of the sample when the temperature is measured. During the determination, record the temperature of the sample and the length of the sample tube. After the determination, pour out the solution in the sample tube, rinse the sample tube with distilled water, and put it away after drying.
Determine the optical rotation of 5% glucose solution for 4~5 times according to the above method, and fill in the corresponding position in the table below. Then measure the optical rotation of glucose solution with unknown concentration for 4~5 times and fill in the corresponding position in the table below.
(5) Data recording and processing
According to the items designed in the table, the corresponding treatment is carried out, and finally the concentration of glucose solution with unknown concentration is obtained.