Refractive Index - Traceability with Reference Liquids

Up to now, refractive indices are traced with precision glass prisms the customer has to supply to PTB. This procedure is difficult and expensive. Alternatively, PTB can now offer certified reference liquids for this purpose filled in special phials for one-time use. These reference liquids have special properties: they are sufficiently stable with respect to their chemical composition, non-hazardous in terms of occupational medicine and easy to apply. Furthermore, they cover a range of refractive indices required in practical applications and enabled the data to be circulated with a sufficiently small uncertainty.

The following hydrocarbons are liquid at ambient temperature and have been found to be well suited for use as reference liquids: n-heptane (n_D = 1,388), isooctane (n_D = 1,392), methylcyclohexane (n_D = 1,423), cyclohexane (n_D = 1,427), tetrachlorethylene (n_D = 1,508). n_D denotes the refractive index at spectral line D of the mercury-vapour lamp. At PTB, the refractive indices are calibrated for six wavelengths in the visible light range between 404,7 nm and 643,8 nm, with an uncertainty of 10^Ð5. Using these reference liquids measurements of refractive indices with a refractometer can be traced back over almost the entire spectral range of visible light. Dispersion correction of the refractive index is not necessary as long as the wavelength applied in the measurement coincides with the wavelength declared on the calibration certificate to within ± 0,1 nm. Such small uncertainties require a temperature stability during calibration within ± 0,01 K around a well-defined value.