Deck 11: Refractive Index

Question 1

Correct the following refractive indexes to 20˚C.
(a) n²² _D 1.4398
(b) n³⁰ _D 1.4702
(c) n¹⁶ _D 1.3962
(d) n¹⁸ _D 1.4022

Answer 1

(a) n²⁰ _D = 1.4398 + 0.0009 = 1.4407
(b) n²⁰ _D = 1.4702 + 0.0045 = 1.4747
(c) n²⁰ _D = 1.3962 - 0.0018 = 1.3944
(d) n²⁰ _D = 1.4022 - 0.0009 = 1.4013

Question 2

Compound A has a refractive index of 1.4577 at 20˚, while compound B (miscible with com- pound A) has a refractive index of 1.5000 at the same temperature.
(a) How could you experimentally determine if the refractive indexes of mixtures of A and B follow a linear relationship to the mole percent compositions of the mixtures?
(b) If the relationship is linear, what is the composition (in mole percent) of a mixture that has a refractive index of 1.4678 at 20˚C?

Answer 2

(a) Prepare several mixtures of A and B with known compositions. Determine their refractive indexes. Construct a graph of mol% versus nD and see if the points fall on the line joining 1.4577 (100%A) and 1.5000 (100%B).
(b) 76% A-24% B (from the graph not shown here)

Question 3

When measuring a refractive index, what should you do if
(a) no dividing line between light and dark areas can be observed through the eyepiece?
(b) the dividing line can be seen, but it is fringed with color?

Answer 3

(a) Either (1) use the fine or the coarse adjustment control knob on the refractometer, the knobs that control the refractive index setting of the instrument, or, if (1) does not work, (2) place more compound on the prism.
(b) Adjust the dispersion correction knob.