PROCEDURE A - MANUAL (OPTICAL-MECHANICAL) PROCEDURE
6. Apparatus
6.1 Refractometer, high-resolution optical-mechanical refractometer of the "Abbe" type with suitable measuring range (1.3300 to 1.5000 or higher) and an accuracy/resolution of 0.0001 or better refractive index.
NOTE 2 - Prior to 2001, Test Method D1218 was based on a Bausch & Lomb Refractometer. However, this instrument is no longer manufactured. Currently available manual refractometers are of the "Abbe" type (critical angle refractometers, see X1.2.3). There is a variety of refractometers that have been found to be suitable for this measurement. Some instruments offer a wider measuring range but it is important to verify the uniformity of accuracy across the entire measuring range and to ensure suitable calibration materials are available for this verification.
6.2 Temperature Control Unit, either an external liquid bath with both heating and cooling capability and pump for maintaining the indicated prism temperature within 0.1°C, or an internal electronic temperature control system (such as Peltier system). If an external bath is used, the thermostating liquid shall pass the temperature measuring device on leaving, not on entering the prism. The temperature control unit (external or internal) shall have the following control specification:
Stability: +/-0.1°C
Uniformity: +/-0.1°C
Display Resolution: 0.1°C or better
6.3 Temperature Measuring Device, for those apparatus employing mercury in glass thermometer, ASTM Thermometer 17C having a range from 19 to 27°C, and conforming to the requirements of Specification E 1. For apparatus using non-mercury in glass thermometer, a platinum resistance probe, thermocouple, or equivalent temperature sensors are acceptable.
6.4 Light Sources - The following type of light source can be used in conjunction with an optical-mechanical (Abbe type) refractometer:
6.4.1 Sodium Arc Lamp, NaD line at 589 nm.
6.4.2 Mercury Arc Lamp, Hgc line at 546 nm or Hgg line at 436 nm.
6.4.3 Cadmium Arc Lamp, CdC’ line at 644 nm.
6.4.4 Mercury-Cadmium Arc Lamp.
6.4.5 Helium Discharge Lamp.
NOTE 3 - Measurement of refractive dispersion requires more than one type of light source.
6.5 Light Filters - Arc lamps can emit a number of spectral lines that result in multiple-borderlines observed in the refractometer. Filters can be used to eliminate unwanted lines (borderlines). Depending upon instrument design, the manufacturer will recommend and supply a suitable filter or adapt a commercially-available filter (for example, interference filter) to suit the application.
7. Reagents and Materials
7.1 n-Pentane, 95 mol % Minimum Purity, for cleaning the prism faces. (Warning - Flammable. Harmful if inhaled. Vapors may cause flash fire.)
NOTE 4 - Low boiling hydrocarbon fractions with boiling range 50 to 100°C have also been found to be acceptable.
7.2 Toluene, HPLC Grade, for cleaning the prism faces. (Warning - Flammable. Vapor harmful.)
7.3 1-Bromonaphthalene, 98 mol % Minimum Purity, contact liquid when calibrating with solid reference standard. (Warning - Toxic when ingested.)
7.4 Primary Reference Materials, for calibrating the instrument.
7.4.1 Solid Reference Standard, with the value of refractive index engraved on its upper face.
7.4.2 Distilled or Deionized Water, conforming to Type II or III of Specification D 1193. At 20°C, nD = 1.3330; at 25°C, nD = 1.3325; and at 30°C, nD = 1.3319
7.4.3 2,2,4-trimethylpentane, 99 mol % Minimum Purity, at 20°C, nD = 1.3915; at 25°C, nD = 1.3890.
7.4.4 Methylcyclohexane, 99 mol % Minimum Purity, at 20°C, nD = 1.4231; at 25°C, nD = 1.4206.
7.4.5 Toluene, 99 mol % Minimum Purity, at 20°C, nD = 1.4969; at 25°C, n D = 1.4941. (Warning - 2,2,4-trimethylpentane, methylcylcohexane, and toluene are all flammable. Their vapor can be harmful.)
NOTE 5 - Other pure materials may be used to calibrate the instrument as primary reference material, as long as they can be obtained in 99 mol % minimum purity and accurate values of their refractive indices at specific temperatures are available. The precision of the test method (see 15.1 and 24.1) were obtained using distilled water as the calibrant.
7.5 Secondary Reference Materials, for calibrating the instrument.
7.5.1 Mineral Oil Calibration Standards, measured and certified by suppliers for specific refractive index ranges and temperatures.
8. Sampling
8.1 A sample of at least 0.5 mL is required. The sample shall be free of suspended solids, water, or other materials that may settle onto the prism surface and affect the measured reading. Water can be removed from hydrocarbons by treatment with calcium chloride followed by filtering or centrifuging to remove the desiccant. The possibility of changing the composition of the sample by action of the drying agent, by selective adsorption on the filter, or by fractional evaporation, shall be considered. (Warning - Volatile hydrocarbon samples are flammable.)
9. Preparation of Apparatus
9.1 The refractometer shall be kept scrupulously clean at all times. Dust and oil can impair the optical component of the instrument. Thoroughly clean the prism faces with toluene, followed by n-pentane (see Note 4) (Warning - These materials are extremely flammable. Harmful if inhaled. Vapors may cause flash fire.) using cotton swabs, fresh clean lens tissue, or similar material, in accordance with manufacturer's instructions. Do not dry the prism faces by rubbing with dry cotton.
9.2 Adjust the thermostat bath/circulator settings or the electronic temperature control system so that the temperature indicated by the refractometer temperature measuring device is within 0.1°C of the desired value. Turn on the light source and allow the refractometer to equilibrate for 30 min.
NOTE 6 - The constancy of the prism temperature can be seriously affected by variations in ambient conditions such as air drafts or changes in room temperature. Reasonable precautions should be taken to minimize these factors.
10. Calibration of Refractometer Using Solid Reference Standard
10.1 Thoroughly clean the prism faces and surfaces of the solid reference standard (see 7.4.1). Open the prism assembly. Apply a drop of 1-bromonaphthalene contact liquid, about 1.5 mm in diameter, to the center of the polished surface of the solid reference material. Press the reference standard against the surface of the prism face with the polished end towards the light source.
10.2 Follow the manufacturer's instructions on how to calibrate the instrument using the solid reference standard.
10.3 If the observed refractive index differs from the value engraved on the solid reference standard by more than 0.0001, adjust the refractometer's scale reading to match the certified value, following manufacturer's instructions.
11. Calibration of the Refractometer Using Liquid Reference Standards
11.1 Using the procedure described in Section 12, determine the refractive index of any of the Reference Materials specified in 7.4 or 7.5 for a specific test temperature. If the observed refractive index for the chosen reference material at a specified test temperature differs by more than 0.0001 of the listed value, make adjustment to the instrument following manufacturer's instructions so that the observed refractive index corresponds to the listed value.
11.2 For optimum accuracy, use a reference material whose refractive index is close to the desired refractive index range and temperature to calibrate the instrument.
12. Procedure
12.1 Ensure that the prism faces are clean and dry. Check that the prism temperature is within 0.1°C of the desired temperature.
12.2 Unlock (if necessary) and open the prism assembly.
12.3 Place one or two drops of the sample on the lower prism face. Close the prism assembly and lock (if necessary). Turn on the light source. Allow 3 min temperature equilibration time.
12.4 Look through the eyepiece and observe the field consisting of a light and dark portion. Follow manufacturer's instructions to adjust the instrument so that the boundary between the light and dark portions of the field is as sharp as possible.
12.5 Following manufacturer's instructions, make any additional adjustment until the sharp boundary line intersects the midpoint of the crosshairs superimposed on the field.
12.6 Read the refractive index on the scale. Repeat 12.5 at least four times, approaching from either side of the sharp boundary line, and average the scale readings.
12.7 Record and report the average refractive index value.
12.8 If instrument is capable of determining refractive dispersion, change the light source to a light source with different wavelength. Determine the refractive index at the secondary wavelength following 12.4-12.7.
NOTE 7 - When determining refractive dispersion, it is expected that the instrument would have been calibrated at both wavelengths used.
13. Quality Control
13.1 Confirm the performance of the test procedure by analyzing a quality control (QC) sample, which is stable and representative of the sample of interest.
13.1.1 When quality control/quality assurance (QC/QA) protocols are established in the testing facility, these may be used to confirm the reliability of the test result.
13.1.2 When there are no QC/QA protocols established in the testing facility, Appendix X4 can be used to provide guidelines in performing this function.
14. Calculation and Report
14.1 Report the average refractive index to four decimal places and the test temperature at which the test was made, for example:
where D or the 589 signifies that the sodium D spectral line was used.
14.2 If a refractive dispersion measurement was made, calculate the absolute value of the difference between the refractive indices at the two wavelengths and multiply the difference by 10 000. Report the calculated value and the test temperature, for example:
where g and D signifies that the mercury g and sodium D spectral lines were used in the measurement.
15. Precision and Bias
15.1 Precision - Refractive Index:
15.1.1 The precision for the manual (optical-mechanical) refractive index procedure, as determined by the statistical examination of interlaboratory test results conducted in 1996 on eight different samples from ten laboratories, is as follows:
15.1.2 Repeatability - The difference between successive test results, obtained by the same operator with the same apparatus, under constant operating conditions, and on identical test material, would, in the long run, in the normal and correct operation of the test method, exceed the following values only in one case in twenty (95 % confidence level):
15.1.3 Reproducibility - The difference between two, single and independent results obtained by different operators working in different laboratories on identical test material would, in the long run, in the normal and correct operation of the test method, exceed the following values only in one case in twenty (95 % confidence level):
15.1.4 The precision of this test method for automatic and manual refractive index was obtained in accordance with Practice D6300 using the D02 Precision Program (D2PP) software.
15.2 Precision - Refractive Dispersion:
15.2.1 Repeatability = 0.00012
15.2.2 Reproducibility = 0.00012
15.2.3 The precision for refractive dispersion was not obtained in accordance with Practice D6300, and was not part of the 1996 interlaboratory study.