Chapter 3.3. Measurement of refractive index¹

The optical properties of crystals are complex, and it is planned to include a full account in Volume D . What follows is restricted to a brief description of immersion media for use in the measurement of indices of refraction. WARNING. Many of the media, particularly those of high refractive index, are poisonous, or corrosive, or both.

The immersion media listed in Table 3.3.2.1 are easily prepared, stable, and generally satisfactory. They were selected because they require only a small number of liquids for their preparation. In general, each liquid is miscible in the liquids with higher and lower indices of refraction so that any intermediate mixture can be easily prepared.

Table 3.3.2.1| top | pdf | Immersion media for general use in the measurement of index of refraction Note: Further lists are given by Hartshorne & Stuart (1960).   Temperature coefficient (dn/dT) Dispersion Water 1.333 1 × 10−4 Slight Glycerol 1.473 2.2 × 10−4 Slight n-Octane 1.400 4.8 × 10−4 – n-Hexadecane 1.434 3.8 × 10−4 Slight Kerosene (Paraffin) 1.448 3.5 × 10−4 Slight Petroleum oil (Nujol) 1.477 4 × 10−4 Slight α-Chloronaphthalene 1.626 4 × 10−4 Moderate Methylene iodide 1.740 6.4 × 10−4 Rather strong Methylene iodide saturated with sulfur 1.778 6 × 10−4 Rather strong AsBr3 plus 10% sulfur (mix with methylene iodide or α-bromonaphthalene for lower n) 1.814 (25° C) 7 × 10−4 Rather strong 2S, 2As2S2, 6AsBr3 (mix with 10% sulfur in AsBr3 for lower n) 2.003 (25° C) 6 × 10−4 Rather strong 2Se, 2As2S2, 6AsBr3 (mix with 10% sulfur in AsBr3 for lower n) 2.11 (25° C) 6 × 10−4 Rather strong

The liquids up to n = 1.770 are measured on an Abbe refractometer; those with higher values for n are measured in a hollow glass prism prepared from selected object glasses and mounted on a goniometer or on a spectrometer (Butler, 1933; Larsen & Berman, 1934, pp. 18–20).

A set of immersion liquids with indices of refraction differing by one unit in the second decimal place (1.510, 1.520, 1.530, ) is used for routine work. The liquids are best kept in 15 ml dropping bottles with plastic caps and glass dropping rods. These bottles are more satisfactory than the more expensive dropping bottles with solid glass stoppers and ground-glass caps because there is less trouble with the stopper cementing to the bottle. The bottle should be about half full.

Some crystals dissolve rapidly in the liquids tested. A measurement can usually be made by performing the reading rapidly. If the crystal and the liquid have nearly the same indices of refraction, the index of the liquid is not much changed by the solution of the crystal.

Refractive indices greater than 2.1 present special difficulties. Merwin & Larsen (1912) used melts of sulfur and selenium, satisfactory up to index 2.72. Mixtures of selenium and As₂Se₃ can be used up to index 3.17 (Larsen & Berman, 1934). Above about 2.2, the index must be determined at a wavelength for which selenium is transparent; the red line of lithium is convenient.

Organic substances are usually soluble in organic liquids, but not usually in aqueous solutions. The saturated solutions listed in Table 3.3.4.1 (Jelley, 1934, p. 245) are generally satisfactory. For substances of low solubility, the organic liquids listed in Table 3.3.4.2 may be useful; the refractive index of diluted aqueous solutions changes with time because of evaporation.

Table 3.3.4.1| top | pdf | Aqueous solutions for use as immersion media for organic crystals Salt of saturated solution Lithium iodide 1.490 Sodium iodide 1.496 Potassium iodide 1.456 Barium iodide 1.528 Tetrasodium dioxypentathiostannate 1.615

Table 3.3.4.2| top | pdf | Organic immersion media for use with organic crystals of low solubility Compound Diethyl oxalate 1.41 Di-n-butyl carbonate 1.41 Triethyl citrate 1.44 Tri-n-butyl citrate 1.44 n-Butyl phthalate 1.49 α-Bromonaphthalene 1.66 α-Iodonaphthalene 1.70 Methylene iodide 1.74

If the index of refraction is greater than about 1.7, the media at the end of Table 3.3.2.1 have to be used. These usually dissolve organic crystals, so that the immersion liquid becomes saturated with the compound. The refractive index of the saturated medium is then measured with a microrefractometer (Jelley, 1934, pp. 236–240).