ASTM D1218-21

Standard Test Method for Refractive Index and Refractive Dispersion of Hydrocarbon Liquids

SIGNIFICANCE AND USE
5.1 Refractive index and refractive dispersion are fundamental physical properties, which can be used in conjunction with other properties to characterize pure hydrocarbons and their mixtures.
SCOPE
1.1 This test method covers the measurement of refractive index, accurate to four decimal places or better, of transparent and light-colored hydrocarbons in the range of 1.3300 to 1.5000 at temperatures from 20 °C to 30 °C by manual (optical-mechanical) or automatic (digital) procedure.  
1.2 The manual (optical-mechanical) procedure also covers the measurement of refractive dispersion accurate to the fourth decimal place or better.
Note 1: The test method may be suitable for measuring the refractive indices of liquids above 1.5000 and at temperatures both below 20 °C and above 30 °C; however, the test method precision may not apply. Verification of the accuracy of such measurements will depend upon the availability of reliable, certified reference standards that demonstrate the performance of the instrument used under the particular measuring conditions.  
1.3 The test method may not be applicable to liquids that are strongly colored, or that have bubble points so near the test temperature that a reading cannot be obtained before substantial weathering takes place. Liquid color should be limited to No. 4 ASTM Color or lighter, as determined by Test Method D1500.  
1.4 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.  
1.5 WARNING—Mercury has been designated by many regulatory agencies as a hazardous substance that can cause serious medical issues. Mercury, or its vapor, has been demonstrated to be hazardous to health and corrosive to materials. Use Caution when handling mercury and mercury-containing products. See the applicable product Safety Data Sheet (SDS) for additional information. The potential exists that selling mercury or mercury-containing products, or both, is prohibited by local or national law. Users must determine legality of sales in their location.  
1.6  This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.7 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

General Information

• Status: Published
• Publication Date: 31-Oct-2021
• ICS: 75.080 - Petroleum products in general
• Technical Committee: D02 - Petroleum Products, Liquid Fuels, and Lubricants
• Drafting Committee: D02.04.0D - Physical and Chemical Methods

Relations

• Refers: ASTM D6300-24 - Standard Practice for Determination of Precision and Bias Data for Use in Test Methods for Petroleum Products, Liquid Fuels, and Lubricants
• Effective Date: 01-Mar-2024
• Refers: ASTM D1500-24 - Standard Test Method for ASTM Color of Petroleum Products (ASTM Color Scale)
• Effective Date: 01-Mar-2024
• Refers: ASTM D6300-23a - Standard Practice for Determination of Precision and Bias Data for Use in Test Methods for Petroleum Products, Liquid Fuels, and Lubricants
• Effective Date: 01-Dec-2023
• Refers: ASTM D6299-23a - Standard Practice for Applying Statistical Quality Assurance and Control Charting Techniques to Evaluate Analytical Measurement System Performance
• Effective Date: 01-Dec-2023
• Refers: ASTM D6300-19a - Standard Practice for Determination of Precision and Bias Data for Use in Test Methods for Petroleum Products and Lubricants
• Effective Date: 01-Dec-2019
• Refers: ASTM D6299-17b - Standard Practice for Applying Statistical Quality Assurance and Control Charting Techniques to Evaluate Analytical Measurement System Performance
• Effective Date: 15-Dec-2017
• Refers: ASTM D6299-17a - Standard Practice for Applying Statistical Quality Assurance and Control Charting Techniques to Evaluate Analytical Measurement System Performance
• Effective Date: 15-Nov-2017
• Refers: ASTM D6299-17 - Standard Practice for Applying Statistical Quality Assurance and Control Charting Techniques to Evaluate Analytical Measurement System Performance
• Effective Date: 01-Jan-2017
• Refers: ASTM D6300-16 - Standard Practice for Determination of Precision and Bias Data for Use in Test Methods for Petroleum Products and Lubricants
• Effective Date: 01-Apr-2016
• Refers: ASTM D6300-15 - Standard Practice for Determination of Precision and Bias Data for Use in Test Methods for Petroleum Products and Lubricants
• Effective Date: 01-Jun-2015
• Refers: ASTM D6300-14ae1 - Standard Practice for Determination of Precision and Bias Data for Use in Test Methods for Petroleum Products and Lubricants
• Effective Date: 01-Jun-2014
• Refers: ASTM D6300-14a - Standard Practice for Determination of Precision and Bias Data for Use in Test Methods for Petroleum Products and Lubricants
• Effective Date: 01-Jun-2014
• Refers: ASTM D6300-14 - Standard Practice for Determination of Precision and Bias Data for Use in Test Methods for Petroleum Products and Lubricants
• Effective Date: 01-May-2014
• Refers: ASTM D6300-13a - Standard Practice for Determination of Precision and Bias Data for Use in Test Methods for Petroleum Products and Lubricants
• Effective Date: 01-Dec-2013
• Refers: ASTM D6299-13e1 - Standard Practice for Applying Statistical Quality Assurance and Control Charting Techniques to Evaluate Analytical Measurement System Performance
• Effective Date: 01-Oct-2013

Overview

ASTM D1218-21, titled "Standard Test Method for Refractive Index and Refractive Dispersion of Hydrocarbon Liquids," establishes procedures for the precise measurement of the refractive index and refractive dispersion of transparent, light-colored hydrocarbons. Covering a refractive index range of 1.3300 to 1.5000, at temperatures between 20°C and 30°C, this standard offers detailed guidelines for both manual (optical-mechanical) and automatic (digital) refractometric methods. Refractive index and its dispersion are fundamental physical properties that, when coupled with other measurements, serve to characterize pure hydrocarbons and their mixtures for a wide array of applications in the petroleum and chemical industries.

Key Topics

• Measurement Methods:
  • Manual (optical-mechanical, such as Abbe-type refractometers), which also accommodates refractive dispersion measurements.
  • Automatic (digital) refractometers, employing high-accuracy, digital readout and temperature control.
• Precision and Reporting:
  • Results are reported to four decimal places, with documented repeatability and reproducibility guidelines to ensure reliable data.
  • Temperature at which the measurement is performed is included in the report to maintain result accuracy.
• Sample Requirements:
  • Applies to transparent, light-colored hydrocarbon liquids, limited to ASTM Color No. 4 or lighter.
  • Not suitable for strongly colored liquids or samples with high volatility at the test temperature.
• Instrumentation and Calibration:
  • Mandates use of appropriate calibration standards (solid or liquid references), with periodic calibration checks.
  • Provides guidance on temperature control and light source selection (e.g., sodium D-line at 589 nm) for consistent and comparable outcomes.
• Safety Considerations:
  • Advises on the hazards associated with materials like mercury and other reagents, stressing compliance with safety and regulatory requirements.

Applications

ASTM D1218-21 is extensively used in:

• Petroleum Refining and Blending:
  • Quality control of fuels, lubricants, and base stocks through refractive index characterization.
• Chemical Identification:
  • Differentiation and verification of hydrocarbon compounds and detection of impurities in mixtures.
• Product Specification and Certification:
  • Refractive index serves as a specification criterion in supply agreements and regulatory filings.
• Process Control:
  • Inline or laboratory refractometric measurement supports production optimization and consistency.
• Research and Development:
  • Fundamental studies on hydrocarbon properties and mixture behaviors.
• Statistical Quality Assurance:
  • Implementation of control charting and precision practices, as referenced in related ASTM practices.

Related Standards

• ASTM D1193 - Specification for Reagent Water
• ASTM D1500 - Test Method for ASTM Color of Petroleum Products
• ASTM D6299 - Practice for Statistical Quality Assurance in Analytical Measurement
• ASTM D6300 - Practice for Determination of Precision and Bias Data
• ASTM E1 - Specification for ASTM Liquid-in-Glass Thermometers

Practical Value

By following ASTM D1218-21, laboratories and process facilities ensure accurate, reproducible measurement of refractive index, supporting critical functions such as quality control, material verification, and regulatory compliance. Consistent use of this standard fosters reliability across inter-laboratory and international petroleum trade, aiding in the transparent exchange of analytical data. Through comprehensive requirements for calibration, sample handling, and safety, the standard helps organizations maintain the integrity of refractive index data, which is vital for hydrocarbon characterization and product quality evaluation.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: D1218 − 21
Standard Test Method for
Refractive Index and Refractive Dispersion of Hydrocarbon
Liquids
This standard is issued under the fixed designation D1218; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. Scope* 1.6 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
1.1 This test method covers the measurement of refractive
responsibility of the user of this standard to establish appro-
index, accurate to four decimal places or better, of transparent
priate safety, health, and environmental practices and deter-
and light-colored hydrocarbons in the range of 1.3300 to
mine the applicability of regulatory limitations prior to use.
1.5000 at temperatures from 20°C to 30°C by manual
1.7 This international standard was developed in accor-
(optical-mechanical) or automatic (digital) procedure.
dance with internationally recognized principles on standard-
1.2 The manual (optical-mechanical) procedure also covers
ization established in the Decision on Principles for the
themeasurementofrefractivedispersionaccuratetothefourth
Development of International Standards, Guides and Recom-
decimal place or better.
mendations issued by the World Trade Organization Technical
NOTE 1—The test method may be suitable for measuring the refractive
Barriers to Trade (TBT) Committee.
indicesofliquidsabove1.5000andattemperaturesbothbelow20°Cand
above 30°C; however, the test method precision may not apply. Verifi-
2. Referenced Documents
cation of the accuracy of such measurements will depend upon the
availability of reliable, certified reference standards that demonstrate the
2.1 ASTM Standards:
performance of the instrument used under the particular measuring
D1193Specification for Reagent Water
conditions.
D1500Test Method forASTM Color of Petroleum Products
1.3 Thetestmethodmaynotbeapplicabletoliquidsthatare
(ASTM Color Scale)
strongly colored, or that have bubble points so near the test
D6299Practice for Applying Statistical Quality Assurance
temperature that a reading cannot be obtained before substan-
and Control Charting Techniques to Evaluate Analytical
tial weathering takes place. Liquid color should be limited to
Measurement System Performance
No. 4 ASTM Color or lighter, as determined by Test Method
D6300Practice for Determination of Precision and Bias
D1500.
Data for Use in Test Methods for Petroleum Products,
1.4 The values stated in SI units are to be regarded as
Liquid Fuels, and Lubricants
standard. No other units of measurement are included in this E1Specification for ASTM Liquid-in-Glass Thermometers
standard.
2.2 ASTM Adjuncts:
Determination of Precision and Bias Data for Use in Test
1.5 WARNING—Mercury has been designated by many
Methods for Petroleum Products and Lubricants, Version
regulatory agencies as a hazardous substance that can cause
4.0.65
serious medical issues. Mercury, or its vapor, has been dem-
onstrated to be hazardous to health and corrosive to materials.
3. Terminology
Use Caution when handling mercury and mercury-containing
products. See the applicable product Safety Data Sheet (SDS)
3.1 Definitions:
for additional information. The potential exists that selling
3.1.1 refractive dispersion, n—the difference between the
mercuryormercury-containingproducts,orboth,isprohibited
refractive indices of a substance for light of two different
bylocalornationallaw.Usersmustdeterminelegalityofsales
wavelengths, both indices being measured at the same tem-
in their location.
perature.
This test method is under the jurisdiction of ASTM Committee D02 on
Petroleum Products, Liquid Fuels, and Lubricants and is the direct responsibility of For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Subcommittee D02.04.0D on Physical and Chemical Methods. contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
Current edition approved Nov. 1, 2021. Published November 2021. Originally Standards volume information, refer to the standard’s Document Summary page on
approved in 1952. Last previous edition approved in 2016 as D1218–12 (2016). the ASTM website.
DOI: 10.1520/D1218-21. Out of print. No longer available from ASTM International Headquarters.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D1218 − 21
3.1.1.1 Discussion—For convenience in calculations, the mercury in glass thermometer, a platinum resistance probe,
value of the difference thus obtained is usually multiplied by thermocouple, or equivalent temperature sensors are accept-
10000. able.
3.1.2 refractive index, n—the ratio of the velocity of light
6.4 Light Sources—The following type of light source can
(of specified wavelength) in air, to its velocity in the substance
beusedinconjunctionwithanoptical-mechanical(Abbetype)
under examination.
refractometer:
3.1.2.1 Discussion—Itmayalsobedefinedasthesineofthe
6.4.1 Sodium Arc Lamp, Na line at 589nm.
D
angle of incidence divided by the sine of the angle of
6.4.2 Mercury Arc Lamp, Hg line at 546nm or Hg line at
c g
refraction, as light passes from air into the substance. This is
436nm.
the relative index of refraction. If absolute refractive index
6.4.3 Cadmium Arc Lamp, Cd line at 644nm.
C’
(that is, referred to vacuum) is desired, this value should be
6.4.4 Mercury-Cadmium Arc Lamp.
multiplied by the factor 1.00027, the absolute refractive index
6.4.5 Helium Discharge Lamp.
ofair.Thenumericalvalueofrefractiveindexofliquidsvaries
inversely with both wavelength and temperature.
NOTE 3—Measurement of refractive dispersion requires more than one
type of light source.
4. Summary of Test Method
6.5 Light Filters—Arc lamps can emit a number of spectral
4.1 Therefractiveindexismeasuredusingahigh-resolution
lines that result in multiple-borderlines observed in the refrac-
refractometer of an optical-mechanical or automatic digital
tometer. Filters can be used to eliminate unwanted lines
type with the prism temperature accurately controlled. The
(borderlines). Depending upon instrument design, the manu-
instrument principle is based on the critical angle concept.
facturer will recommend and supply a suitable filter or adapt a
commercially-available filter (for example, interference filter)
5. Significance and Use
to suit the application.
5.1 Refractive index and refractive dispersion are funda-
mental physical properties, which can be used in conjunction
7. Reagents and Materials
with other properties to characterize pure hydrocarbons and
7.1 n-Pentane, 95 mol % Minimum Purity, for cleaning the
their mixtures.
prism faces. (Warning—Flammable. Harmful if inhaled. Va-
pors may cause flash fire.)
PROCEDURE A—MANUAL (OPTICAL-
NOTE 4—Low boiling hydrocarbon fractions with boiling range 50°C
to 100°C have also been found to be acceptable.
MECHANICAL) PROCEDURE
7.2 Toluene, HPLC Grade, for cleaning the prism faces.
6. Apparatus
(Warning—Flammable. Vapor harmful.)
6.1 Refractometer, high-resolution optical-mechanical re-
7.3 1-Bromonaphthalene, 98 mol % Minimum Purity, con-
fractometer of the “Abbe” type with suitable measuring range
tact liquid when calibrating with solid reference standard.
(1.3300 to 1.5000 or higher) and an accuracy/resolution of
(Warning—Toxic when ingested.)
0.0001 or better refractive index.
7.4 Primary Reference Materials, for calibrating the instru-
NOTE 2—Prior to 2001, Test Method D1218 was based on a Bausch &
ment.
LombRefractometer.However,thisinstrumentisnolongermanufactured.
Currentlyavailablemanualrefractometersareofthe“Abbe”type(critical
7.4.1 Solid Reference Standard, with the value of refractive
anglerefractometers,seeX1.2.3).Thereisavarietyofrefractometersthat
index engraved on its upper face.
have been found to be suitable for this measurement. Some instruments
7.4.2 Distilled or Deionized Water,conformingtoTypeIIor
offer a wider measuring range but it is important to verify the uniformity
III of Specification D1193. At 20°C, n =1.3330; at 25°C,
of accuracy across the entire measuring range and to ensure suitable
D
calibration materials are available for this verification. n =1.3325; and at 30°C, n =1.3319
D D
7.4.3 2,2,4-trimethylpentane, 99 mol % Minimum Purity, at
6.2 Temperature Control Unit, either an external liquid bath
20°C, n =1.3915; at 25°C, n =1.3890.
with both heating and cooling capability and pump for main-
D D
taining the indicated prism temperature within 0.1°C, or an 7.4.4 Methylcyclohexane, 99 mol % Minimum Purity, at
internal electronic temperature control system (such as Peltier
20°C, n =1.4231; at 25°C, n =1.4206.
D D
system). If an external bath is used, the thermostating liquid
7.4.5 Toluene, 99 mol % Minimum Purity, at 20 °C,
shallpassthetemperaturemeasuringdeviceonleaving,noton
n = 1.4969; at 25 °C, n = 1.4941. (Warning—2,2,4-
D D
entering the prism. The temperature control unit (external or
trimethylpentane,methylcylcohexane,andtolueneareallflam-
internal) shall have the following control specification:
mable. Their vapor can be harmful.)
Stability ±0.1 °C NOTE 5—Other pure materials may be used to calibrate the instrument
Uniformity ±0.1 °C
asprimaryreferencematerial,aslongastheycanbeobtainedin99mol%
Display Resolution 0.1 °C or better
minimum purity and accurate values of their refractive indices at specific
temperatures are available.The precision of the test method (see 15.1 and
6.3 Temperature Measuring Device, for those apparatus
24.1) were obtained using distilled water as the calibrant.
employingmercuryinglassthermometer,ASTMThermometer
17Chavingarangefrom19°Cto27°C,andconformingtothe 7.5 Secondary Reference Materials, for calibrating the
requirements of Specification E1. For apparatus using non- instrument.
D1218 − 21
7.5.1 Mineral Oil Calibration Standards, measured and make adjustment to the instrument following manufacturer’s
certified by suppliers for specific refractive index ranges and instructions so that the observed refractive index corresponds
temperatures. to the listed value.
11.2 For optimum accuracy, use a reference material whose
8. Sampling
refractive index is close to the desired refractive index range
8.1 Asampleofatleast0.5mLisrequired.Thesampleshall
and temperature to calibrate the instrument.
be free of suspended solids, water, or other materials that may
settle onto the prism surface and affect the measured reading. 12. Procedure
Water can be removed from hydrocarbons by treatment with
12.1 Ensure that the prism faces are clean and dry. Check
calcium chloride followed by filtering or centrifuging to
that the prism temperature is within 0.1°C of the desired
remove the desiccant. The possibility of changing the compo-
temperature.
sition of the sample by action of the drying agent, by selective
12.2 Unlock (if necessary) and open the prism assembly.
adsorption on the filter, or by fractional evaporation, shall be
considered. (Warning—Volatile hydrocarbon samples are 12.3 Place one or two drops of the sample on the lower
flammable.) prism face. Close the prism assembly and lock (if necessary).
Turn on the light source. Allow 3min temperature equilibra-
9. Preparation of Apparatus
tion time.
9.1 Therefractometershallbekeptscrupulouslycleanatall
12.4 Look through the eyepiece and observe the field
times. Dust and oil can impair the optical component of the
consisting of a light and dark portion. Follow manufacturer’s
instrument. Thoroughly clean the prism faces with toluene,
instructions to adjust the instrument so that the boundary
followed by n-pentane (see Note 4)(Warning—These mate-
between the light and dark portions of the field is as sharp as
rials are extremely flammable. Harmful if inhaled.Vapors may
possible.
cause flash fire.) using cotton swabs, fresh clean lens tissue, or
12.5 Following manufacturer’s instructions, make any ad-
similar material, in accordance with manufacturer’s instruc-
ditional adjustment until the sharp boundary line intersects the
tions. Do not dry the prism faces by rubbing with dry cotton.
midpoint of the crosshairs superimposed on the field.
9.2 Adjust the thermostat bath/circulator settings or the
12.6 Read the refractive index on the scale. Repeat 12.5 at
electronic temperature control system so that the temperature
least four times, approaching from either side of the sharp
indicatedbytherefractometertemperaturemeasuringdeviceis
boundary line, and average the scale readings.
within0.1°Cofthedesiredvalue.Turnonthelightsourceand
12.7 Record and report the average refractive index value.
allow the refractometer to equilibrate for 30 min.
NOTE 6—The constancy of the prism temperature can be seriously
12.8 If instrument is capable of determining refractive
affected by variations in ambient conditions such as air drafts or changes
dispersion, change the light source to a light source with
inroomtemperature.Reasonableprecautionsshouldbetakentominimize
different wavelength. Determine the refractive index at the
these factors.
secondary wavelength following 12.4 – 12.7.
10. Calibration of Refractometer Using Solid Reference
NOTE7—Whendeterminingrefractivedispersion,itisexpectedthatthe
instrument would have been calibrated at both wavelengths used.
Standard
10.1 Thoroughly clean the prism faces and surfaces of the
13. Quality Control
solid reference standard (see 7.4.1). Open the prism assembly.
13.1 Confirm the performance of the test procedure by
Apply a drop of 1-bromonaphthalene contact liquid, about
analyzing a quality control (QC) sample, which is stable and
1.5mmindiameter,tothecenterofthepolishedsurfaceofthe
representative of the sample of interest.
solid reference material. Press the reference standard against
13.1.1 When quality control/quality assurance (QC/QA)
thesurfaceoftheprismfacewiththepolishedendtowardsthe
protocols are established in the testing facility, these may be
light source.
used to confirm the reliability of the test result.
10.2 Follow the manufacturer’s instructions on how to
13.1.2 When there are no QC/QA protocols established in
calibrate the instrument using the solid reference standard.
the testing facility, Appendix X4 can be used to provide
guidelines in performing this function.
10.3 If the observed refractive index differs from the value
engraved on the solid reference standard by more than 0.0001,
14. Calculation and Report
adjust the refractometer’s scale reading to match the certified
14.1 Report the average refractive index to four decimal
value, following manufacturer’s instructions.
places and the test temperature at which the test was made, for
11. Calibration of the Refractometer Using Liquid
example:
Reference Standards
n 5 x.xxxxat20 °C or n 5 x.xxxxat20 °C (1)
D 589
11.1 UsingtheproceduredescribedinSection12,determine
where D or the 589 signifies that the sodium D spectral line
therefractiveindexofanyoftheReferenceMaterialsspecified
was used.
in 7.4 or 7.5 for a specific test temperature. If the observed
refractiveindexforthechosenreferencematerialataspecified 14.2 If a refractive dispersion measurement was made,
testtemperaturediffersbymorethan0.0001ofthelistedvalue, calculate the absolute value of the difference between the
D1218 − 21
refractive indices at the two wavelengths and multiply the 17. Reagents and Materials
difference by 10000. Report the calculated value and the test
17.1 See Section 7.
temperature, for example:
4 18. Sampling
n 2 n 310 at t 520 °C (2)
~ !
? g D ?
18.1 See Section 8.
where g and D signifies that th
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: D1218 − 12 (Reapproved 2016) D1218 − 21
Standard Test Method for
Refractive Index and Refractive Dispersion of Hydrocarbon
Liquids
This standard is issued under the fixed designation D1218; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. Scope Scope*
1.1 This test method covers the measurement of refractive index, accurate to four decimal places or better, of transparent and
light-colored hydrocarbons in the range of 1.3300 to 1.5000 at temperatures from 20 °C to 30 °C by manual (optical-mechanical)
or automatic (digital) procedure.
1.2 The manual (optical-mechanical) procedure also covers the measurement of refractive dispersion accurate to the fourth
decimal place or better.
NOTE 1—The test method may be suitable for measuring the refractive indices of liquids above 1.5000 and at temperatures both below 20 °C and above
30 °C; however, the test method precision may not apply. Verification of the accuracy of such measurements will depend upon the availability of reliable,
certified reference standards that demonstrate the performance of the instrument used under the particular measuring conditions.
1.3 The test method may not be applicable to liquids that are strongly colored, or that have bubble points so near the test
temperature that a reading cannot be obtained before substantial weathering takes place. Liquid color should be limited to No. 4
ASTM Color or lighter, as determined by Test Method D1500.
1.4 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.5 WARNING—Mercury has been designated by many regulatory agencies as a hazardous materialsubstance that can cause
central nervous system, kidney and liver damage. serious medical issues. Mercury, or its vapor, may has been demonstrated to be
hazardous to health and corrosive to materials. Caution should be taken Use Caution when handling mercury and mercury
containing mercury-containing products. See the applicable product Material Safety Data Sheet (MSDS) for details and EPA’s
website—http://www.epa.gov/mercury/faq.htm—for additional information. Users should be aware (SDS) for additional informa-
tion. The potential exists that selling mercury and/or mercury containing products into your state or country may be prohibited by
law.or mercury-containing products, or both, is prohibited by local or national law. Users must determine legality of sales in their
location.
1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.
This test method is under the jurisdiction of ASTM Committee D02 on Petroleum Products, Liquid Fuels, and Lubricants and is the direct responsibility of Subcommittee
D02.04.0D on Physical and Chemical Methods.
Current edition approved Oct. 1, 2016Nov. 1, 2021. Published November 2016November 2021. Originally approved in 1952. Last previous edition approved in 20122016
as D1218 – 12.D1218 – 12 (2016). DOI: 10.1520/D1218-12R16.10.1520/D1218-21.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D1218 − 21
1.7 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
D1193 Specification for Reagent Water
D1500 Test Method for ASTM Color of Petroleum Products (ASTM Color Scale)
D6299 Practice for Applying Statistical Quality Assurance and Control Charting Techniques to Evaluate Analytical Measure-
ment System Performance
D6300 Practice for Determination of Precision and Bias Data for Use in Test Methods for Petroleum Products, Liquid Fuels, and
Lubricants
E1 Specification for ASTM Liquid-in-Glass Thermometers
2.2 ASTM Adjuncts:
Determination of Precision and Bias Data for Use in Test Methods for Petroleum Products and Lubricants, Version 4.0.65
3. Terminology
3.1 Definitions:
3.1.1 refractive dispersion, n—the difference between the refractive indices of a substance for light of two different wavelengths,
both indices being measured at the same temperature.
3.1.1.1 Discussion—
For convenience in calculations, the value of the difference thus obtained is usually multiplied by 10 000.
3.1.2 refractive index, n—the ratio of the velocity of light (of specified wavelength) in air, to its velocity in the substance under
examination.
3.1.2.1 Discussion—
It may also be defined as the sine of the angle of incidence divided by the sine of the angle of refraction, as light passes from air
into the substance. This is the relative index of refraction. If absolute refractive index (that is, referred to vacuum) is desired, this
value should be multiplied by the factor 1.00027, the absolute refractive index of air. The numerical value of refractive index of
liquids varies inversely with both wavelength and temperature.
4. Summary of Test Method
4.1 The refractive index is measured using a high-resolution refractometer of an optical-mechanical or automatic digital type with
the prism temperature accurately controlled. The instrument principle is based on the critical angle concept.
5. Significance and Use
5.1 Refractive index and refractive dispersion are fundamental physical properties, which can be used in conjunction with other
properties to characterize pure hydrocarbons and their mixtures.
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
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Out of print. No longer available from ASTM International Headquarters.
D1218 − 21
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 °C to 27 °C, and conforming to the requirements of Specification E1. 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, Na line at 589 nm.
D
6.4.2 Mercury Arc Lamp, Hg line at 546 nm or Hg line at 436 nm.
c g
6.4.3 Cadmium Arc Lamp, Cd line at 644 nm.
C’
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 °C 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 D1193. At 20 °C, n = 1.3330; at 25 °C,
D
n = 1.3325; and at 30 °C, n = 1.3319
D D
7.4.3 2,2,4-trimethylpentane, 99 mol % Minimum Purity, at 20 °C, n = 1.3915; at 25 °C, n = 1.3890.
D D
D1218 − 21
7.4.4 Methylcyclohexane, 99 mol % Minimum Purity, at 20 °C, n = 1.4231; at 25 °C, n = 1.4206.
D D
7.4.5 Toluene, 99 mol % Minimum Purity, at 20 °C, n = 1.4969; at 25 °C, n = 1.4941. (Warning—2,2,4-trimethylpentane,
D D
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.
D1218 − 21
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:
n 5 x.xxxx at 20 °C or n 5 x.xxxx at 20 °C (1)
D 589
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:
~n 2 n ! 310 at t 5 20 °C (2)
? g D ?
where g and D sign
...