ASTM D1747 Standard Test Method for Refractive Index of Viscous Materials
10. Preparation of Apparatus
10.1 The refractometer shall be kept scrupulously clean at all times. Dust and oil, if allowed to accumulate on any part of the instrument, will find its way into the moving parts, causing wear and eventual misalignment. If permitted to collect on the prism, dust will dull the polish, resulting in hazy lines.
10.2 Thoroughly clean the prism faces with fresh clean lens tissue or surgical grade absorbent cotton saturated with a suitable solvent. Pass the swab very lightly over the surface until it shows no tendency to streak. Repeat the procedure with a fresh swab and solvent until both the glass and adjacent polished metal surfaces are clean. Do not dry the prism faces by rubbing with dry cotton.
10.3 Adjust the thermostat so that the temperature as indicated by the thermocouple inserted between the prism faces and wet with oil is within 0.2°C of the desired test temperature. This temperature is to be held constant to within 0.02°C during the test. Observe and record the thermometer reading corresponding to the test temperature. Turn on the sodium arc lamp and allow it to warm up 30 min.
11. Standardization with Reference Liquids
11.1 Introduce a sample of the API Standard trans-decahydronaphthalene to the prism which is adjusted to the chosen test temperature of 80 or 100°C, turn the telescope adjustment screw until a refractive index scale reading corresponding to the certified refractive index for trans-decahydronaphthalene is observed, and adjust the instrument according to the instructions given by the manufacturer until the sharp boundary between the light and dark portions of the field passes through the intersection of the cross hairs of the telescope.
11.2 Check the accuracy of this setting by loading a fresh sample of trans-decahydronaphthalene and measure its refractive index at the test temperature following the procedure described in Section 12. If the value for the refractive index differs from the certified value by 0.0001 or more units, then repeat the procedure given in 11.1 until a satisfactory check is obtained.
11.3 Measure the refractive index of API Standard n-hexadecane and 1-methylnaphthalene at the test temperature following the procedure described in Section 12.
11.4 Construct a calibration curve for use at the chosen test temperature. Plot the difference between the observed refractive index for n-hexadecane and its certified value along the ordinate against the refractive index level along the abscissa. Also plot the difference between the observed and certified refractive indices for 1-methylnaphthalene in the same manner. Draw a straight line from the point representing the deviation found for n-hexadecane to zero at the certified refractive index of trans-decahydronaphthalene. Likewise, draw a straight line from this same zero point to the deviation found for 1-methylnaphthalene.
11.5 If it is desired to measure the refractive index of samples at a temperature other than 80 or 100°C, obtain calibration data by repeating 11.1-11.4 at this desired temperature. Determine the refractive indices for the API Standard compounds, n-hexadecane, trans-decahydronaphthalene, and 1-methylnaphthalene at the desired temperature by plotting the certified refractive indices at 20, 25, 30, 80, 100°C against temperature and drawing a smooth curve between the points.
11.6 Precautions - In using pure liquids for calibration or checking of calibration of an Abbé-type refractometer, the following precautions should be observed:
11.6.1 Before inserting the hydrocarbon calibrating liquids, the prisms should be flushed with solvents and cleaned as described in 8.2. It is advisable to preheat the solvent before use to minimize thermal shock to the prism. This should be followed by several such flushings with the test liquid and wiping with lens paper. After such cleaning a reading with the test liquid should be taken as described in Section 11. This should be followed by another flushing with the test liquid before taking another reading of the test liquid in the prescribed manner. The prisms cannot be considered free from contaminating substances until two such determinations on the test liquid agree within the limits given in 11.6.2.
11.6.2 In setting the edge of the field on the cross hairs, readings should be taken in pairs, approaching the alidade setting from one direction only as recommended by the manufacturer. Several such sets will probably be necessary before satisfactory agreement is obtained. Satisfactory agreement is 0.00005 to 0.0001.
11.6.3 For results of highest accuracy, the calibration with hydrocarbons of known properties should be made immediately before the determination on the sample.
11.6.4 Fluctuations in ambient temperatures should be minimized as much as possible during the test.
