Refractive Index
wavelength of light. Because the velocity of light in air is always greater than that through a liquid, the refractive index is a number greater than 1; for example, hexane n20D 1.3751. The superscript 20 indicates that the measurement was made at 20 oC, and the subscript D refers to the yellow D-line of a sodium vapor lamp, light with a wavelength of 589 nm.
The measurement is made with a refractometer using a few drops of liquid. Compensation is made within the instrument for the fact that white light and not sodium vapor light is used, but a temperature correction must be applied to the observed reading by adding 0.00045 for each degree above 20 oC. n20D = ntD + 0.00045(t – 20oC)
The refractive index can be determined to 1 part in 10,000, but because the value is quite sensitive to impurities, there is not always very good agreement with the literature with regard to the last figure. To master the technique of using the refractometer, measure the refractive indices of several known, pure liquids before measuring an unknown.
For more information on the Abbe-3L refractometer (the instrument we have in the lab) and some video clips demonstrating its us, please go to http://web.uccs.edu/bgaddis/chem337/expts/nD/nD.htm
Procedure
Two or three drops of the sample are placed on the open prism using a polyethylene pipette (to avoid scratching the prism face). The prism is closed, and the light is turned on and positioned for maximum brightness as seen through the eyepiece. If the refractometer is set to a nearly correct value, then a partially gray image will be seen. Turn the knob so that the line separating the dark and light areas is at the crosshairs. Sometimes the line separating the dark and light areas is fuzzy and colored. Turn the chromatic adjustment until the demarcation line is sharp and colorless. Then read the refractive index by pressing the button down to light up the scale in the field of vision. Read the temperature on the thermometer attached to
The measurement is made with a refractometer using a few drops of liquid. Compensation is made within the instrument for the fact that white light and not sodium vapor light is used, but a temperature correction must be applied to the observed reading by adding 0.00045 for each degree above 20 oC. n20D = ntD + 0.00045(t – 20oC)
The refractive index can be determined to 1 part in 10,000, but because the value is quite sensitive to impurities, there is not always very good agreement with the literature with regard to the last figure. To master the technique of using the refractometer, measure the refractive indices of several known, pure liquids before measuring an unknown.
For more information on the Abbe-3L refractometer (the instrument we have in the lab) and some video clips demonstrating its us, please go to http://web.uccs.edu/bgaddis/chem337/expts/nD/nD.htm
Procedure
Two or three drops of the sample are placed on the open prism using a polyethylene pipette (to avoid scratching the prism face). The prism is closed, and the light is turned on and positioned for maximum brightness as seen through the eyepiece. If the refractometer is set to a nearly correct value, then a partially gray image will be seen. Turn the knob so that the line separating the dark and light areas is at the crosshairs. Sometimes the line separating the dark and light areas is fuzzy and colored. Turn the chromatic adjustment until the demarcation line is sharp and colorless. Then read the refractive index by pressing the button down to light up the scale in the field of vision. Read the temperature on the thermometer attached to