International
Tables for
Crystallography
Volume C
Mathematical, physical and chemical tables
Edited by E. Prince

International Tables for Crystallography (2006). Vol. C, ch. 3.3, pp. 160-161
https://doi.org/10.1107/97809553602060000587

## Chapter 3.3. Measurement of refractive index1

E. S. Larsen Jr,a R. Meyrowitzb and A. J. C. Wilsonc

aUS Geological Survey, Washington 25, DC, and Harvard University, Cambridge, MA, USA,bUS Geological Survey, Washington 25, DC, USA, and cSt John's College, Cambridge, CB2 1TP, England

This chapter gives a brief description of immersion media for use in the measurement of indices of refraction. Details of immersion media for general use and of aqueous and organic immersion media for use with organic crystals are tabulated.

### 3.3.1. Introduction

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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.

### 3.3.2. Media for general use

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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.

### 3.3.3. High-index media

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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.

### 3.3.4. Media for organic substances

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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).

### References

Butler, R. D. (1933). Immersion liquids of intermediate refraction. Am. Mineral. 18, 386–401.
Hartshorne, N. H. & Stuart, A. (1960). Crystals and the polarising microscope, 3rd ed. London: Arnold.
International Tables for X-ray Crystallography (1962). Vol. III. Birmingham: Kynoch Press.
Jelley, E. E. (1934). A microfractometer and its use in chemical microscopy. J. R. Microsc. Soc. 54, 234–245.
Larsen, E. S. Jr & Berman, H. (1934). The microscopic determination of nonopaque minerals, 2nd ed. US Geol. Survey Bull. No. 848.
Merwin, H. E. & Larsen, E. S. Jr (1912). Mixtures of amorphous sulphur and selenium as immersion media for the determination of high refractive indices with the microscope. Am. J. Sci. 34, 42–47.