ASTM D4928-00
(Test Method)Standard Test Methods for Water in Crude Oils by Coulometric Karl Fischer Titration
Standard Test Methods for Water in Crude Oils by Coulometric Karl Fischer Titration
SCOPE
1.1 This test method covers the determination of water in the range from 0.02 to 5 mass or volume % in crude oils. Mercaptan (RSH) and sulfide (S- or H2S) as sulfur are known to interfere with this test method, but at levels of less than 500 ug/g (ppm), the interference from these compounds is insignificant (see Section 5).
1.2 This test method can be used to determine water in the 0.005 to 0.02 mass % range, but the effects of the mercaptan and sulfide interference at these levels has not been determined.
1.3 This test method is intended for use with standard commercially available coulometric Karl Fischer reagent.
1.4 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 and health practices and determine the applicability of regulatory limitations prior to use. For specific hazard statements, see Section 7.
General Information
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Standards Content (Sample)
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Contact ASTM International (www.astm.org) for the latest information.
Designation: D 4928 – 00 An American National Standard
Designation: MPMS Chapter 10.9
Designation: 386/99
Standard Test Methods for
Water in Crude Oils by Coulometric Karl Fischer Titration
This standard is issued under the fixed designation D 4928; 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 (e) indicates an editorial change since the last revision or reapproval.
This test method has been approved by the sponsoring committees and accepted by the cooperating societies in accordance with
established procedure.
This standard has been approved for use by agencies of the Department of Defense.
1. Scope D 5854 Practice for Mixing and Handling Liquid Samples
of Petroleum and Petroleum Products
1.1 This test method covers the determination of water in
E 203 Test Method for Water Using Karl Fischer Titration
the range from 0.02 to 5 mass or volume % in crude oils.
−
Mercaptan (RSH) and sulfide (S or H S) as sulfur are known
3. Summary of Test Method
to interfere with this test method, but at levels of less than 500
3.1 After homogenizing the crude oil with a mixer, an
μg/g (ppm), the interference from these compounds is insig-
aliquot is injected into the titration vessel of a Karl Fischer
nificant (see Section 5).
apparatus in which iodine for the Karl Fischer reaction is
1.2 This test method can be used to determine water in the
generated coulometrically at the anode. When all the water has
0.005 to 0.02 mass % range, but the effects of the mercaptan
been titrated, excess iodine is detected by an electrometric
and sulfide interference at these levels has not been deter-
end-point detector and the titration is terminated. Based on the
mined.
stoichiometry of the reaction, one mole of iodine reacts with
1.3 This test method is intended for use with standard
one mole of water, thus the quantity of water is proportional to
commercially available coulometric Karl Fischer reagent.
the total integrated current according to Faraday’s Law.
1.4 This standard does not purport to address all of the
3.2 The precision of this test method is critically dependent
safety concerns, if any, associated with its use. It is the
on the effectiveness of the homogenization step. The efficiency
responsibility of the user of this standard to establish appro-
of the mixer used to achieve a homogeneous sample is
priate safety and health practices and determine the applica-
determined by the procedure given in Practice D 5854.
bility of regulatory limitations prior to use. For specific hazard
3.3 Two procedures are provided for the determination of
statements, see Section 7.
water in crude oils. In one procedure, a weighed aliquot of
2. Referenced Documents sample is injected into the titration vessel and the mass % of
water is determined. The other procedure provides for the
2.1 ASTM Standards:
direct determination of the volume % of water in the crude oil
D 1193 Specification for Reagent Water
by measuring the volume of crude oil injected into the titration
D 4057 Practice for Manual Sampling of Petroleum and
vessel.
Petroleum Products
D 4177 Practice for Automatic Sampling of Petroleum and
4. Significance and Use
Petroleum Products
4.1 A knowledge of the water content of crude oil is
important in the refining, purchase, sale, or transfer of crude
1 oils.
This test method is under the jurisdiction of ASTM Committee D02 on
Petroleum Products and Lubricants and API Committee on Petroleum Measurement
5. Interferences
and is the direct responsibility of Subcommittee D02.02 on Static Petroleum
Measurement.
5.1 A number of substances and classes of compounds
Current edition approved Sept. 10, 2000. Published October 2000. Originally
published as D 4928 – 89. Last previous edition D 4928 – 96.
2 4
Annual Book of ASTM Standards, Vol 11.01. Annual Book of ASTM Standards, Vol 05.03.
3 5
Annual Book of ASTM Standards, Vol 05.02. Annual Book of ASTM Standards, Vol 15.05.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
NOTICE: This standard has either been superseded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
D 4928
associated with condensation or oxidation-reduction reactions where such specifications are available. Other grades may be
interfere in the determination of water by Karl Fischer. In crude used, provided it is first ascertained that the reagent is of
oils, the most common interferences are mercaptans and sufficiently high purity to permit its use without lessening the
sulfides (not total sulfur). At levels of less than 500 μg/g (ppm) accuracy of the determination.
(as sulfur), the interference from these compounds is insignifi- 7.2 Purity of Water—Unless otherwise indicated, references
cant. Most crude oils, including crude oils classified as “sour to water shall be understood to mean reagent water as defined
crude”, have mercaptan and sulfide levels of less than 500 μg/g by Type IV of Specification D 1193.
(ppm) as sulfur. For more information on substances that 7.3 Xylene, reagent grade. Less than 0.05 % water.
interfere in the determination of water by Karl Fischer titration (Warning—Flammable. Vapor harmful.)
method (see Test Method E 203). 7.4 Karl Fischer Reagent—Standard commercially avail-
able reagents for coulometric Karl Fischer titrations.
5.2 The significance of the mercaptan and sulfide interfer-
7.4.1 Anode Solution, shall be 6 parts of commercial Karl
ence on the Karl Fischer titration for water levels in the 0.005
Fischer anode solution with 4 parts of reagent grade xylene.
to 0.02 mass % range has not been determined experimentally.
Fresh Karl Fischer anode solution shall be used. Anode
At these low water levels, however, the interference may be
solution shall not be used past its expiration date. Anode
significant for mercaptan and sulfide levels of less than 500
solution should be replaced after 7 days in the titration vessel.
μg/g (ppm) (as sulfur).
(Warning—Flammable, toxic by inhalation and if swallowed,
avoid contact with skin.)
6. Apparatus
NOTE 1—Other proportions of anode solution and xylene can be used
6.1 Karl Fischer Apparatus, using electrometric end-point.
and should be determined for a particular reagent and apparatus. The
Presently there are available on the market a number of
precision and bias were established using the designated anode solution
commercial coulometric Karl Fischer titration assemblies. and xylene.
Instructions for operation of these devices are provided by the
7.4.2 Cathode Solution, use standard commercially avail-
manufacturer and not described herein.
able Karl Fischer cathode solution. Cathode solution shall not
6.2 Mixer, to homogenize the crude sample. be used after the expiration date and should be replaced after 7
days in the titration vessel. (Warning—Flammable, can be
6.2.1 Non-Aerating, High-Speed, Shear Mixer—The mixer
fatal if inhaled, swallowed, or absorbed through skin. Possible
shall be capable of meeting the homogenization efficiency test
cancer hazard.)
described in Practice D 5854. The sample size is limited to that
suggested by the manufacturer for the size of the mixing probe.
8. Sampling and Test Specimens
6.2.2 Circulating sample mixers, such as those used with
8.1 Sampling is defined as all the steps required to obtain an
automatic crude oil sampling receivers, are acceptable provid-
aliquot representative of the contents of any pipe, tank, or other
ing they comply with the principles of Practice D 5854.
system and to place the sample into a container for analysis by
6.3 Syringes:
a laboratory or test facility. The laboratory sample container
and sample volume shall be of sufficient dimensions and
6.3.1 Samples are most easily added to the titration vessel
volume to allow mixing as described in 8.4.
by means of accurate glass syringes with LUER fittings and
8.2 Laboratory Sample—The sample of crude oil presented
hypodermic needles of suitable length. The bores of the
to the laboratory or test facility for analysis by this test method.
needles used should be kept as small as possible but large
Only representative samples obtained as specified in Practice
enough to avoid problems arising from back pressure and
D 4057 and Practice D 4177 shall be used to obtain the
blocking while sampling. Suggested syringe sizes are as
laboratory sample.
follows:
NOTE 2—Examples of laboratory samples include sample bottles from
6.3.1.1 Syringe, 10 μL with a needle long enough to dip
manual sampling, receptacles from automatic crude oil samplers, and
below the surface of the anode solution in the cell when
storage containers holding a crude oil from a previous analysis.
inserted through the inlet port septum. This syringe is used in
8.3 Test Specimen—The sample aliquot obtained from the
the calibration step (Section 10). It should be of suitable
laboratory sample for analysis by this test method. Once
graduations for readings to the nearest 0.1 μL or better.
drawn, the entire portion of the test specimen will be used in
6.3.1.2 Syringes, 250 μL, 500 μL, and 1000 μL (1 mL), for
the analysis. Mix the laboratory sample properly as described
crude oil samples. For the volumetric determination procedure,
in 8.4 before drawing the test specimen.
the syringes should be accurate to 5 μL, 10 μL, and 20 μL (0.02
8.4 Mix the laboratory sample of crude oil immediately
mL), respectively.
(within 15 min) before drawing the test specimen to ensure
7. Reagents and Materials
Reagent Chemicals, American Chemical Society Specifications, American
7.1 Purity of Reagents—Reagent grade chemicals shall be
Chemical Society, Washington, DC. For suggestions on the testing of reagents not
used in all tests. Unless otherwise indicated, it is intended that
listed by the American Chemical Society, see Analar Standards for Laboratory
all reagents shall conform to the specifications of the Commit-
Chemicals, BDH Ltd., Poole, Dorset, U.K., and the United States Pharmacopeia
and National Formulary, U.S. Pharmacopeial Convention, Inc. (USPC), Rockville,
tee on Analytical Reagents of the American Chemical Society,
MD.
NOTICE: This standard has either been superseded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
D 4928
complete homogeneity. Mix the sample at room temperature water titrated is a direct function of the coulombs of electricity
(15 to 25°C) or less in the laboratory sample container and consumed. However, reagent performance deteriorates with
record the temperature of the sample in degrees Celsius use and shall be regularly monitored by accurately injecting 10
immediately before mixing. The type of mixer depends on the μL of pure water. Suggested intervals are initially with fresh
quantity of crude oil in the laboratory sample container. Before reagent and then after every ten determinations (see Section
any unknown mixer is used, the specifications for the homog- 11.1.3). If the result is outside 10 000 6 200 μg, replace both
enization test, Practice D 5854, shall be met. Reevaluate the the anode and cathode solutions.
mixer for any changes in the type of crude, volume of crude in
11. Procedure
the container, the shape of the container, or the mixing
11.1 Mass Determination of Sample Size:
conditions (such as mixing speed and time of mixing).
11.1.1 Add fresh solvents to the anode and cathode com-
8.5 For small laboratory sample containers and volumes, 50
partments of the titration vessel and bring the solvent to
to 500 mL, a non-aerating, high-speed, shear mixer is required.
end-point conditions as described in Section 9.
Use the mixing time, mixing speed, and height of the mixer
11.1.2 Add an aliquot of the crude oil test specimen to the
probe above the bottom of the container found to be satisfac-
titration vessel immediately after the mixing step described in
tory in Practice D 5854. For larger containers and volumes,
8.4 using the following method.
appropriate mixing conditions shall be defined by following a
11.1.2.1 Starting with a clean, dry syringe of suitable
set of procedures similar to those outlined in Practice D 5854
capacity (see Table 1 and Note 4), withdraw at least three
and Practice D 4177 but modified for application to the larger
portions of the sample and discard to waste. Immediately
containers and volumes. Clean and dry the mixer between
withdraw a further portion of sample, clean the needle with a
samples.
paper tissue, and weigh the syringe and contents to the nearest
8.6 Record the temperature of the sample in degrees Celsius
0.1 mg. Insert the needle through the inlet port septum, start the
immediately after homogenization. The rise in temperature
titration and with the tip of the needle just below the liquid
between this reading and the initial reading before mixing (8.4)
surface, inject the sample. Withdraw the syringe and reweigh
is not to exceed 10°C, otherwise loss of water can occur or the
the syringe to the nearest 0.1 mg. After the end-point is
emulsion can become unstable.
reached, record the titrated water from the digital readout on
8.7 Select the test specimen size as indicated in Table 1
the instrument.
based on the expected water content.
NOTE 4—If the concentration of water in the sample is completely
9. Preparation of Apparatus
unknown, it is advisable to start with a small trial portion of sample to
9.1 Follow the manufacturer’s directions for preparation
avoid excessive titration time and depletion of the reagents. Further
and operation of the titration apparatus.
adjustment of the aliquot size can then be made as necessary.
9.2 Seal all joints and connections to the vessel to prevent
11.1.2.2 When the background current or titration rate
atmospheric moisture from entering the apparatus.
returns to a stable reading at the end of the titration as
9.3 Add to the anode (outer) compartment the mixture of
discussed in 9.5, additional samples can be added in accor-
xylene and Karl Fischer anode solutions which has been found
dance with 11.1.2.1.
suitable for the particular reagent and apparatus being used.
11.1.3 Replace the solutions when one of the following
Add the solutions to the level recommended by the manufac-
occurs and then repeat the preparation of the apparatus as in
turer.
Section 9.
9.4 Add to the cathode (inner) compartment the Karl Fis-
11.1.3.1 Persistently high and unstable background current.
cher cathode solution to a level 2 to 3 mm below the level of
11.1.3.2 Phase separation in the anode compartment or
the solu
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