ASTM D1747 Standard Test Method for Refractive Index of Viscous Materials
12. Procedure
12.1 Thoroughly clean the prism faces as described in 10.2. Adjust the thermostat so that the temperature indicated by the thermocouple placed between the faces of the closed prism (loaded with oil) is within 0.2°C of the desired value. The thermocouple is used for establishing the correct temperature level and may be removed during measurements of refractive index. The observed reading of the thermometer at this temperature must be held constant to 0.02°C in the measurements to follow.
12.2 Close the prism box and let it stand for 3 to 5 min to ensure temperature equilibrium between the prisms and the circulating bath liquid. Melt samples which are normally solid in a small container and charge as a liquid to the prism. Charge the sample from a small pipet or medicine dropper through the refractometer opening or onto the prisms open just enough to admit the sample. About 0.2 to 0.5 mL of the sample should be allowed to flush through before completely closing the prisms. Samples of low volatility or high viscosity may be placed directly onto the prism surface by means of a stirring rod, being careful not to touch the prism surface with the rod. If not enough sample has been used to fill the space between the prisms completely or evaporation causes the field division in the telescope to become uneven, clean the prisms thoroughly before employing a new portion of the sample.
12.3 Set the light source to pass rays into the illuminating prism. Move the telescope by means of the adjustment screw until the field division is visible in the telescope.
12.4 Adjust the angle of incidence until the field consists of a light and a dark portion separated by a sharp boundary which may be brought into focus by sliding the telescope eyepiece. In the case of colloidal or suspended matter in the sample, the boundary line may appear diffused or hazy, even though the telescope is perfectly focused. Filtration of the sample will sometimes correct this condition. Water in the sample may also give this hazy effect.
12.5 Turn the adjustment screw so that the field division passes through the intersection of the crossed lines in the telescope. With the exception of extremely volatile samples, an interval of 2 to 3 min should be allowed before taking readings to permit the sample to come to the same temperature as the refractometer. In setting the edge of the field on the cross hairs, at least two determinations should be obtained which agree within 0.00005 to 0.0001 for precision work. The final reading is best obtained by approaching this setting successively from both light and dark sides of the field. Read the scale through the microscope provided. Focus by sliding the eyepiece in or out. Measure and record the refractive index using at least two separate loadings of sample.
NOTE 9 - For more reliable results with a sample which shows a diffused or hazy boundary line even after filtration, there are two possible remedies: (1) back window reflection if the refractometer is so equipped, or (2) dilution with a suitable solvent.
(1) Back window reflection also permits measurements of samples too dark for normal operation. The sample is charged in the normal manner. Remove the cover plate on the back of the prism housing and position the light source so that it shines directly into the opening. Bring the boundary line into view. The fields will be reversed and considerably reduced in contrast. Read the refractive index as described above. Care should be exercised during the measurement since the sharp boundary line is faint and difficult to distinguish.
(2) The dilution procedure is generally unsatisfactory and should only be used if it is impossible to obtain consistent readings in any other way. The sample is diluted with an equal volume of high boiling solvent whose density and refractive index are known and an appropriate correction applied to the results. Highly aromatic or olefinic samples in the gas oil range can be measured by this procedure. The possibility of volume change on mixing should be considered. A check on the density of the mixture will show if this occurs.
