ASTM D6501-15

Standard Test Method for Phosphonate in Brines

SIGNIFICANCE AND USE
5.1 This test method is useful for the determination of trace level phosphonate residues in brines. Chemical treatment which contain phosphonates are used as mineral scale and corrosion inhibitors in gas and oil drilling and production operations; and other industrial applications. Often, the decision for treatment is based on the ability to measure low phosphonate concentration and not upon performance criteria. Phosphonate concentrations as low as 0.16 mg/L have been shown effective in carbonate scale treatment. This test method enables the measurement of sub-mg/L phosphonate concentration in brines containing interfering elements.  
5.2 The procedure includes measuring total (see 12.3.8) and free orthophosphate (see 12.4.3) ions and the difference in concentration is the phosphonate concentration. The sample could contain orthophosphate naturally, or from decomposition of the phosphonate during processing or well treatment or from treating compounds containing molecular dehydrated phosphates.
SCOPE
1.1 This test method covers the colorimetric determination of phosphonate (PNA) in brines from gas and oil production operations in the range from 0.1 to 5 mg/L.  
1.2 This phosphonate method is intended for use to analyze low concentration of phosphonate in brine containing interfering elements. This test method is most useful for analyzing phosphonate at 0.1 to 1 mg/L range in brines with interfering elements; however, it requires personnel with good analytical skill.  
1.3 This test method has been used successfully with reagent water and both field and synthetic brine. It is the user's responsibility to ensure the validity of this test method for waters of untested matrices.  
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 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 9.1.3.

General Information

• Status: Published
• Publication Date: 14-Mar-2015
• ICS: 75.020 - Extraction and processing of petroleum and natural gas
• Technical Committee: D19 - Water
• Drafting Committee: D19.05 - Inorganic Constituents in Water

Relations

• Replaces: ASTM D6501-09 - Standard Test Method for Phosphonate in Brines
• Effective Date: 15-Mar-2015
• Refers: ASTM D1129-13(2020)e2 - Standard Terminology Relating to Water
• Effective Date: 01-May-2020
• Refers: ASTM D2777-12 - Standard Practice for Determination of Precision and Bias of Applicable Test Methods of Committee D19 on Water
• Effective Date: 15-Jun-2012
• Refers: ASTM D3856-11 - Standard Guide for Management Systems in Laboratories Engaged in Analysis of Water
• Effective Date: 15-Nov-2011
• Refers: ASTM D4375-96(2011) - Standard Practice for Basic Statistics in Committee D-19 on Water (Withdrawn 2018)
• Effective Date: 01-May-2011
• Refers: ASTM D5810-96(2011) - Standard Guide for Spiking into Aqueous Samples
• Effective Date: 01-May-2011
• Refers: ASTM D3370-10 - Standard Practices for Sampling Water from Closed Conduits
• Effective Date: 01-Dec-2010
• Refers: ASTM D1129-10 - Standard Terminology Relating to Water
• Effective Date: 01-Mar-2010
• Refers: ASTM E275-08 - Describing and Measuring Performance of Ultraviolet and Visible Spectrophotometers
• Effective Date: 15-Oct-2008
• Refers: ASTM D3370-08 - Standard Practices for Sampling Water from Closed Conduits
• Effective Date: 01-Oct-2008
• Refers: ASTM D2777-08 - Standard Practice for Determination of Precision and Bias of Applicable Test Methods of Committee D19 on Water
• Effective Date: 15-Jan-2008
• Refers: ASTM D3370-07 - Standard Practices for Sampling Water from Closed Conduits
• Effective Date: 01-Dec-2007
• Refers: ASTM D1129-06ae1 - Standard Terminology Relating to Water
• Effective Date: 01-Sep-2006
• Refers: ASTM D1129-06a - Standard Terminology Relating to Water
• Effective Date: 01-Sep-2006
• Refers: ASTM D2777-06 - Standard Practice for Determination of Precision and Bias of Applicable Test Methods of Committee D19 on Water
• Effective Date: 15-Aug-2006

Overview

ASTM D6501-15 is a standard test method developed by ASTM International for the colorimetric determination of phosphonate in brines. This analytical procedure is essential for assessing low concentrations of phosphonate-common mineral scale and corrosion inhibitors-used in gas and oil drilling, production operations, and various industrial applications. The method is specifically designed to accurately measure trace levels of phosphonate, even in the presence of interfering elements, within a typical concentration range of 0.1 to 5 mg/L. Ensuring precise monitoring of phosphonate helps optimize chemical treatment strategies, support regulatory compliance, and protect critical equipment from scale and corrosion.

Key Topics

• Phosphonate Compounds: Organic phosphorus derivatives used extensively as scale inhibitors, corrosion inhibitors, dispersants, and chelants in brine treatment.
• Colorimetric Analysis: Utilizes potassium persulfate digestion to convert all phosphonates to orthophosphate, followed by reaction with ammonium molybdate to form a measurable complex.
• Analytical Range: Effective for measuring phosphonate content in brines from 0.1 to 5 mg/L, with optimal precision in the 0.1 to 1 mg/L range.
• Interference Management:
  • Procedure outlined for eliminating sulfide interference.
  • Recommendations for handling high concentrations of copper, silica, and iron.
  • Guidance on acidifying samples to prevent precipitation and ensure accuracy.
• Quality Control: Emphasizes analyst performance checks, calibration verification, and quality control samples to ensure data precision and validity.
• Safety Considerations: Detailed instructions are provided regarding the handling of strong acids and hazardous chemicals commonly used in testing.

Applications

ASTM D6501-15 is widely employed in industries where brine systems are integral to operations, including:

• Oil and Gas Production: Monitoring phosphonate as a scale and corrosion inhibitor in produced water and injection brines.
• Industrial Water Treatment: Assessing residual phosphonates in cooling towers, desalination plants, and other closed-loop systems.
• Process Optimization: Supporting formulation decisions for chemical treatment programs by providing accurate data on active inhibitor concentrations.
• Regulatory Compliance: Ensuring water treatment programs meet environmental and operational guidelines regarding chemical usage and discharge.

The ability to detect sub-mg/L levels of phosphonate is crucial when low dosages are sufficient for effective scale inhibition, especially in carbonate-rich systems.

Related Standards

Several ASTM and related standards support and complement D6501-15’s methodologies:

• ASTM D1129: Terminology Relating to Water - provides definitions relevant to water analysis.
• ASTM D1193: Specification for Reagent Water - establishes requirements for water purity in analytical processes.
• ASTM D2777: Determination of Precision and Bias of Committee D19 Test Methods - critical for validating analytical accuracy.
• ASTM D3370: Practices for Sampling Water from Closed Conduits - ensures representative sampling.
• ASTM D3856: Guide for Management Systems in Water Laboratories - promotes quality assurance.
• ASTM D5847: Practice for Writing Quality Control Specifications - sets quality criteria for analytical testing.
• ASTM E275: Performance of Ultraviolet and Visible Spectrophotometers - ensures reliability of instrumentation used in colorimetric assays.

Practical Value

The ASTM D6501-15 standard delivers an industry-approved, quality-controlled approach for detecting trace phosphonate in brine matrices, even with high dissolved solid content and potential interferences. Accurate measurement supports efficient scale and corrosion control, minimizes chemical costs, and extends asset longevity in critical brine-handling operations. Implementing this standard helps laboratories ensure precision, support treatment decision-making, and maintain industry best practices in water analysis and management.

Keywords: ASTM D6501-15, phosphonate analysis, brine, colorimetric method, scale inhibitor, corrosion inhibitor, oil and gas, water treatment, quality control, trace analysis.

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: D6501 − 15
Standard Test Method for
Phosphonate in Brines
This standard is issued under the fixed designation D6501; 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.
1. Scope* D4375 Practice for Basic Statistics in Committee D19 on
Water
1.1 This test method covers the colorimetric determination
D5810 Guide for Spiking into Aqueous Samples
of phosphonate (PNA) in brines from gas and oil production
D5847 Practice for Writing Quality Control Specifications
operations in the range from 0.1 to 5 mg/L.
for Standard Test Methods for Water Analysis
1.2 This phosphonate method is intended for use to analyze
E275 PracticeforDescribingandMeasuringPerformanceof
low concentration of phosphonate in brine containing interfer-
Ultraviolet and Visible Spectrophotometers
ing elements. This test method is most useful for analyzing
3. Terminology
phosphonate at 0.1 to 1 mg/L range in brines with interfering
elements; however, it requires personnel with good analytical
3.1 Definitions—For definitions of terms used in this test
skill.
method, refer to Terminology D1129.
1.3 This test method has been used successfully with
3.2 Definitions of Terms Specific to This Standard:
reagent water and both field and synthetic brine. It is the user’s
3.2.1 phosphonate, n—a group of organophosphorus com-
responsibility to ensure the validity of this test method for
pounds typically used for mineral scale and corrosion control,
waters of untested matrices.
as cleaning agents, dispersants, and chelants.
3.2.1.1 Discussion—Typical phosphonate compounds
1.4 The values stated in SI units are to be regarded as
include, but are not limited to, the following phosphonic acid
standard. No other units of measurement are included in this
and their neutralized salts: Aminotri(methylenephosphonic
standard.
acid), 1-hydroxyethylidene-1,1-diphosphonic acid, ethylenedi-
1.5 This standard does not purport to address all of the
aminetetra (methylenephosphonic acid), hexamethylenedi-
safety concerns, if any, associated with its use. It is the
aminetetra (methylenephosphonic acid), and diethylenetri-
responsibility of the user of this standard to establish appro-
aminepenta (methylenephosphonic acid).
priate safety and health practices and determine the applica-
bility of regulatory limitations prior to use. For specific hazard
4. Summary of Test Method
statements, see 9.1.3.
4.1 Phosphonate materials are converted to orthophosphate
by potassium persulfate digestion. The orthophosphate is then
2. Referenced Documents
reacted with ammonium molybdate to form a phosphomolyb-
2.1 ASTM Standards:
date complex.The complex is extracted with a methyl isobutyl
D1129 Terminology Relating to Water
ketone/cyclohexane mixture and measured colorimetrically.
D1193 Specification for Reagent Water
5. Significance and Use
D2777 Practice for Determination of Precision and Bias of
Applicable Test Methods of Committee D19 on Water
5.1 This test method is useful for the determination of trace
D3370 Practices for Sampling Water from Closed Conduits
level phosphonate residues in brines. Chemical treatment
D3856 Guide for Management Systems in Laboratories
which contain phosphonates are used as mineral scale and
Engaged in Analysis of Water
corrosion inhibitors in gas and oil drilling and production
operations; and other industrial applications. Often, the deci-
sion for treatment is based on the ability to measure low
This test method is under the jurisdiction of ASTM Committee D19 on Water
phosphonate concentration and not upon performance criteria.
and is the direct responsibility of Subcommittee D19.05 on Inorganic Constituents
in Water.
Phosphonate concentrations as low as 0.16 mg/L have been
Current edition approved March 15, 2015. Published April 2015. Originally
shown effective in carbonate scale treatment. This test method
approved in 1999. Last previous edition approved in 2009 as D6501 – 09. DOI:
enables the measurement of sub-mg/L phosphonate concentra-
10.1520/D6501-15.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or tion in brines containing interfering elements.
contactASTM Customer Service at [e-mail]. ForAnnual Book ofASTM
5.2 The procedure includes measuring total (see 12.3.8) and
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. free orthophosphate (see 12.4.3) ions and the difference in
*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
D6501 − 15
concentration is the phosphonate concentration. The sample 7.5 Glass Bottles, 60 mL and 240 mL with Teflon-lined
could contain orthophosphate naturally, or from decomposition screw cap closure.
ofthephosphonateduringprocessingorwelltreatmentorfrom
8. Reagents and Materials
treating compounds containing molecular dehydrated phos-
phates.
8.1 Purity of Reagents—Reagent grade chemicals shall be
used in all tests. Unless otherwise indicated, it is intended that
6. Interferences all reagents shall conform to the specifications of the Commit-
teeonAnalyticalReagentsoftheAmericanChemicalSociety.
6.1 Sulfide interferes in this test method, but techniques
Other grades may be used, provided it is first ascertained that
described in the procedure (see 9.1.2) eliminate this interfer-
the reagent is of sufficiently high purity to permit its use
+2
ence. Concentrations less than 1000 mg/L copper (Cu ) and
without lessening the accuracy of the determination.
–2 +4
silica (SiO /SiO /Si ); and less than 200 mg/L of iron
2 3
+2
+3
8.2 Purity of Water—Unless otherwise indicated, reference
(Fe /Fe ) can be tolerated.
towatershallbeunderstoodtomeanreagentwaterconforming
6.2 Produced brines can contain high concentrations of
to Specification D1193, Type I. Other reagent water types may
dissolved solids. Some of these dissolved solids tend to
be used provided it is first ascertained that the water is of
precipitate when produced brines reach new equilibria at
sufficiently high purity to permit its use without adversely
atmospheric temperature and pressure. Phosphonate will co-
affecting the precision and bias of the test method. Type III
precipitate or adsorb onto these newly formed solids and
water was specified at the time of round robin testing of this
become unavailable for analysis. This problem can be mini-
test method.
mized by acidifying the brine sample on-site with hydrochloric
8.3 Alcoholic Sulfuric Acid Solution—Cautiously add 20
acid to pH below 2.
mL concentrated H SO (sp. gr. 1.89) to 900 mL methyl
2 4
6.3 Glassware must be cleaned with phosphate free deter-
alcohol (8.7) and dilute to 1 L with methyl alcohol. It is
gent and rinsed with 0.1 N hydrochloric acid to remove all
recommended to dispense the liquid with a bottle top liquid
residual phosphate or phosphonate.
dispenser, which dispenses a 10-mL volume.
8.4 Ammonium Molybdate Solution—Dissolve 39.1 g
6.4 The standard addition method in 12.6 is recommended
(NH ) Mo O ·4H O in 200 mL water. Cautiously add 210
for brine with high matrix interference.
4 6 7 24 2
mLconcentrated HCl (sp. gr. 1.19) to 400 mLwater. Cool, add
molybdate solution, and dilute to 1 L. It is recommended to
7. Apparatus
dispense the liquid with a liquid dispenser, which dispenses a
7.1 Pressure Cooker or Sterilizer (Autoclave).
10-mL volume.
7.2 Spectrophotometer, for measurement above 650 nm
8.5 Glycerol—Reagent grade, 99 % or greater.
with 4-cm light path cells. A longer light path will yield a
8.6 Hydrochloric Acid (6N)—Add 500 mL of concentrated
corresponding higher sensitivity (see 12.5.1). Spectrophotom-
HCl (sp. gr. 1.19) to 500 mL of water.
eter practices prescribed in this test method shall conform to
8.7 Methyl Alcohol—Reagent grade, 99 % or greater.
Practice E275.
8.8 Methyl Isobutyl Ketone/Cyclohexane Solvent—Mix
7.3 Bottle Top Liquid Dispenser, 20-mL capacity, <1 %
equal volumes of methyl isobutyl ketone (MIBK) and cyclo-
accuracy, and <0.1 % precision.
hexane. (Warning—This solvent is highly flammable. It is
7.4 Pipetter, automated, 10-mL capacity with 0.2 to 0.5 %
recommended to dispense the liquid with a bottle top liquid
accuracy.
dispenser, which dispenses a 20-mL volume.)
8.9 Phosphate Solution, standard (1.00 mL= 0.05 mg PO ).
Dissolve 71.6 mg anhydrous KH PO in water and dilute to 1
2 4
Fisher Scientific No. 14-141-S has been satisfactory for this purpose, or
L.
equivalent, should be used. If you are aware of alternative suppliers, please provide
8.10 Phosphonate Solution, (50-mg/L phosphonate)—If the
this information to ASTM International Headquarters. Your comments will receive
careful consideration at a meeting of the responsible technical committee, which
standard addition procedure (see 12.6) is to be used, a stock
you may attend.
solution of 50 mg/L, as phosphonate, should be prepared. To
Varian DMS-100 has been satisfactory for this purpose, or equivalent, should
preparethissolution,aconcentratedsampleofthephosphonate
be used. If you are aware of alternative suppliers, please provide this information to
ASTM International Headquarters. Your comments will receive careful consider-
ation at a meeting of the responsible technical committee, which you may attend.
5 7
Fisher Scientific No. 13-687-21 REPIPET has been satisfactory for this Fisher Scientific No. 03-326-3C and 03-326-3G have been satisfactory for this
purpose, or equivalent, should be used. If you are aware of alternative suppliers, purpose. If you are aware of alternative suppliers, please provide this information to
please provide this information to ASTM International Headquarters. Your com- ASTM International Headquarters. Your comments will receive careful consider-
ments will receive careful consideration at a meeting of the responsible technical ation at a meeting of the responsible technical committee, which you may attend.
1 8
committee, which you may attend. Reagent Chemicals, American Chemical Society Specifications, American
Fisher Scientific No. 21-279-25 Eppendorf Maxipipetter has been satisfactory Chemical Society, Washington, DC. For suggestions on the testing of Reagents not
for this purpose. If you are aware of alternative suppliers, please provide this listed by the American Chemical Society, see Analar Standards for Laboratory
information to ASTM International Headquarters. Your comments will receive Chemicals, BDH Ltd., Poole, Dorset, U.K., and the United States Pharmacopeia
careful consideration at a meeting of the responsible technical committee, which and National Formulary, U.S. Pharmaceutical Convention, Inc. (USPC), Rockville,
you may attend. MD.
D6501 − 15
to be measured along with the wt/wt percent phosphonate showing phosphate ion concentration in mg/L on the X axis
concentration must be obtained from the manufacturer. The with the corresponding absorbance (A) reading of the spectro-
wt/wtpercentphosphonateconcentrationalsocanbecalibrated photometer on the Y axis of linear graph paper.
by this procedure as described in 12.2 and 12.3.
12. Procedure
8.11 Potassium Persulfate, K S O .
2 2 8
12.1 The procedures in 12.2 and 12.3 are applicable to
8.12 Sodium Chloride Solution (1.0 M, Synthetic Brine)—
samples containing 0.5 to 5 mg/L phosphonate. For samples
Dissolve58.44g.NaClin800mLwateranddiluteto1L.This
containing less than 0.5 mg/L phosphonate, a larger sample
solution is used as a synthetic brine.
volume or a different light path cell can be used (see 12.5).
8.13 Sodium Hypochlorite, (5.65–6 %).
12.2 Persulfate Digestion Procedure:
8.14 Stannous Chloride Solution—Mix 0.4 g SnCl ·2H O
2 2
12.2.1 Pipet 20 mL of the following samples (12.2.1.1,
in 100 mL glycerol (8.4). This reagent is stable for at least six
12.2.1.2, 12.2.1.3) into separate 60-mL glass bottles, each
months. The solution is stored in a dropper bottle.
containing 200 mg of potassium persulfate (8.11). Multiple
9. Hazards samples can be digested at the same time.
12.2.1.1 Blank, 1-M sodium chloride (see 8.12).
9.1 Precautions:
12.2.1.2 Phosphate standards (see 11.1).
9.1.1 Most phosphonate inhibitors are strongly adsorbed to
glass or metal; therefore, polyethylene beakers, flasks, pipets, 12.2.1.3 Samples of acidified brine.
etc.,shouldbeusedtocontainandtransferbrinesolutionsfrom
12.2.2 Close the sample bottles loosely with Teflon-lined
the field.
caps.
9.1.2 A glass bottle is recommended for use in the color
12.2.3 Heat the samples for 30 minutes in a pressure cooker
development steps (see 12.2 and 12.3) for better visualization
or sterilizer at 100–120°C (103.4–137.9 kPa (15–20 psig)).
of the reaction. Since the reaction media is acidic, phosphonate
12.2.4 Make sure the samples are cooled to room tempera-
will not adsorb to the glass surface.
ture before proceeding to color development. The temperature
9.1.3 Personnel performing this test must be familiar with
of solution is critical in procedure 12.2.3. At this point in the
all precautions for handling strong sulfuric acid, hydrochloric
procedure, all of the phosphonate has been oxidized to phos-
acid and sulfide-containing brine. Personnel should consult the
phate.
material safety data sheet for handling strong acids. Protective
12.3 Color development and extraction procedure:
clothing and latex gloves should be worn. The sulfide brine
12.3.1 The timings specified in procedures 12.3.3, 12.3.4,
should be handled in the hood with good ventilation. Sulfide
and 12.3.7 are critical to the test. It is recommended to run
containing brine can be treated with sodium hypochlorite
small numbers of samples at a time in order to manage the
(8.13) prior to analysis to oxidize the hydrogen sulfide.
timing.
10. Sampling
12.3.2 Standard addition method (see 12.6) should be used
10.1 Collect the sample in accordance with Practices
for data quality control.
D3370.
12.3.3 Add 20 mLMIBK/Cyclohexane solvent (8.8) and 10
mLammonium molybdate solution (8.4) to the sample bottles,
10.2 Preserve the samples immediately at the time of
and immediately, vigorously shake each bottle for 15 s.At this
collection by adding 4 mL of 6 N hydrochloric acid 8.6 per
point, the clear and electrically-neutral phosphomolybdate
100-mL brine.
complex has been formed and extracted into the organic
NOTE 1—Alternatively, the pH may be adjusted in the laboratory if the
solvent phase.
sample is returned within 14 days. However, acid must be added at least
12.3.4 Wait exactly five minutes to allow the aqueous and
24hoursbeforeanalysistodissolveanymetalsthatadsorbtothecontainer
walls. This could reduce hazards of working with acids in the field when organic solvent phases to be separated, and withdraw 10.0 mL
appropriate.
of liquid from the organic solvent layer into a clean 60-mL
glass bottle using an automatic pipetter. Care should be taken
11. Calibration and Standardization
not to disturb the solvent/water interface or accidentally
11.1 Prepare standards by adding 2.0, 4.0, 6.0, 8.0, 10.0 mL
withdraw some aqueous solution, since the excess molybde-
each of phosphate standard solution (1.00 mL = 0.05 mg PO )
num in the aqueous phase can also be reduced by stannous
(8.9) to separate 100-mL volumetric flasks. Dilute to 100 mL
chloride to form a deep blue color.
with 1 M sodium chloride solution (8.12). These solutions will
12.3.5 Add 10 mL alcoholic H SO solution (8.3)tothe
2 4
contain 1.0, 2.0, 3.0, 4.0, 5.0 mg/L phosphate as PO.Ifthe
samples, and swirl to mix.
procedure in 12.5 is used for samples with low phosphonate
12.3.6 Add four drops stannous chloride solution (8.14)to
concentrations, then solutions containing 0.2, 0.4, 0.6, 0.8, 1.0
each sample, and mix thoroughly.
mg/L phosphate as PO should be used.
12.3.7 After 10 minutes, but before 20 minutes, pour each
11.2 Followtheprocedurein12.2and12.3todevelopcolor,
sample into a 4-cm cell and read the absorbance against the
and determine the absorbance at 725 nm.
blank at 725 nm.Absorbance readings also can be taken at 650
11.3 Read directly in concentration if this capability is or 700 nm, but with reduced sensitivity. Use the sample blank
provided with the instrument or prepare a calibration curve as reference solution in measuring the sample.
D6501 − 15
TABLE 1
12.3.8 Read the total phosphate concentration (C – )
T PO4
from a calibration curve prepared by analyzing known phos- Molecular
No. of
Common Names Formula Weight
phate standards, as described in Section 11.
P-atoms/mole
(g/mol)
12.4 Procedure for Analyzing Orthophosphate Concentra-
ATMP, Dequest (H O PCH ) N 299 g/mol 3
2 3 2 3
A
tion in the Brine:
DTPMP, Dequest {(H O PCH ) NCH CH } N- 573 g/mol 5
3 2 2 2 2 2
12.4.1 Pipet 20 mL of the acidified brine sample to a
A
2060 CH PO H
2 3 2
separate 60-mL glass bottle.
HEPP, Dequest (H OO P) CCH OH 206 g/mol 2
2 3 2 3
A
12.4.2 Follow the procedure in 12.3.3 – 12.3.7 to develop
A
phosphomolybdate complex and to extract the complex to the Dequest is a registered trade name of the Monsanto Company, St. Louis, MO
63167.
organic liquid phase.
12.4.3 Read the orthophosphate concentration (C – )
F PO4
from a calibration curve prepared in Section 11.
TABLE 2 Composition of Synthetic Brine Samples
12.5 Procedure for brines containing phosphonate concen-
Brine 1, mg/L Brine 2, mg/L Brine 3, mg/L
trations outside the range(s) specified.
NaCl 6.1 31 96.33
12.5.1 The above concentration range is specified for using CaCl 3.9 15 54.47
MgCl 0.064 5 7.71
a 4-cm light path cell. Longer light path cells are suitable for
CaCl ·2H O 5.166 19.87 1696
2 2
analyzing phosphonate at low concentrations (see the follow-
MgCl ·6H O 0.166 10.674 386.3
2 2
ing): Na SO 0.0739 0.037 0.0074
2 4
TDS, mg/L 10 000 51 000 157 000
Approximate Phosphonate Range (mg/L) Light Path (cm)
0.1–2.0 10
12.5.2 Alternatively, the sample size can be adjusted to
analyzebrinescontaininglowphosphonateconcentrationother
where:
than that specified in 12.1.An example of 100-mLsample size
C = Concentration of total phosphate
T–PO
is given below.
(mg/L) read from calibration curve
12.5.2.1 Pipet 100 mL instead of 20 mL into a 240-mL
(see 12.3.8);
bottle. The organic solvent phase in the 240-mL bottle will be
C = Concentration of orthophosphate
F–PO
a thin layer. Care should be taken not to disturb the solvent/
(mg/L) read from calibration curve
water interface or accidentally withdraw aqueous solution
(see 12.4.3);
when removing the phosphomolybdate complex from the
Volume of Standard = Volume (mL) of standard used (see
organic solvent phase.
12.2.1); and,
12.5.2.2 Add 1 g potassium persulfate (8.11) to the sample
Volume of Sample = Volume (mL) of sample used (see
bottle.
12.2.1, 12.5.2.1).
12.5.2.3 Follow 12.2.2 – 12.3.8 to analyze for phosphate
13.1.1 See 10.2 and as follows:
ion.
Field Sample Volume~mL!1Acid Volume~mL!
12.6 Standard Additions Procedure:
Field Dilution 5
S
Field Sample Volume mL
~ !
12.6.1 This procedure is recommended to determine the
(2)
concentration of phosphonate in brine contain
...

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: D6501 − 09 D6501 − 15
Standard Test Method for
Phosphonate in Brines
This standard is issued under the fixed designation D6501; 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.
1. Scope*
1.1 This test method covers the colorimetric determination of phosphonate (PNA) in brines from gas and oil production
operations in the range from 0.1 to 5 mg/L.
1.2 This phosphonate method is intended for use to analyze low concentration of phosphonate in brine containing interfering
elements. This test method is most useful for analyzing phosphonate at 0.1 to 1 mg/L range in brines with interfering elements;
however, it requires personnel with good analytical skill.
1.3 This test method has been used successfully with reagent water and both field and synthetic brine. It is the user’s
responsibility to ensure the validity of this test method for waters of untested matrices.
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 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 9.1.3.
2. Referenced Documents
2.1 ASTM Standards:
D1129 Terminology Relating to Water
D1193 Specification for Reagent Water
D2777 Practice for Determination of Precision and Bias of Applicable Test Methods of Committee D19 on Water
D3370 Practices for Sampling Water from Closed Conduits
D3856 Guide for Management Systems in Laboratories Engaged in Analysis of Water
D4375 Practice for Basic Statistics in Committee D19 on Water
D5810 Guide for Spiking into Aqueous Samples
D5847 Practice for Writing Quality Control Specifications for Standard Test Methods for Water Analysis
E275 Practice for Describing and Measuring Performance of Ultraviolet and Visible Spectrophotometers
3. Terminology
3.1 Definitions—For definitions of terms used in this test method, refer to Terminology D1129.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 phosphonate, n—a group of organophosphorus compounds typically used for mineral scale and corrosion control, as
cleaning agents, dispersants, and chelants.
This test method is under the jurisdiction of ASTM Committee D19 on Water and is the direct responsibility of Subcommittee D19.05 on Inorganic Constituents in Water.
Current edition approved Oct. 1, 2009March 15, 2015. Published October 2009April 2015. Originally approved in 1999. Last previous edition approved in 20042009 as
D6501–04. DOI: 10.1520/D6501-09. – 09. DOI: 10.1520/D6501-15.
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.
3.2.1.1 Discussion—
Typical phosphonate compounds include, but are not limited to, the following phosphonic acid and their neutralized salts:
Aminotri(methylenephosphonic acid), 1-hydroxyethylidene-1,1-diphosphonic acid, ethylenediaminetetra (methylenephosphonic
acid), hexamethylenediaminetetra (methylenephosphonic acid), and diethylenetriaminepenta (methylenephosphonic acid).
*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
D6501 − 15
4. Summary of Test Method
4.1 Phosphonate materials are converted to orthophosphate by potassium persulfate digestion. The orthophosphate is then
reacted with ammonium molybdate to form a phosphomolybdate complex. The complex is extracted with a methyl isobutyl
ketone/cyclohexane mixture and measured colorimetrically.
5. Significance and Use
5.1 This test method is useful for the determination of trace level phosphonate residues in brines. Chemical treatment which
contain phosphonates are used as mineral scale and corrosion inhibitors in gas and oil drilling and production operations; and other
industrial applications. Often, the decision for treatment is based on the ability to measure low phosphonate concentration and not
upon performance criteria. Phosphonate concentrations as low as 0.16 mg/L have been shown effective in carbonate scale
treatment. This test method enables the measurement of sub-mg/L phosphonate concentration in brines containing interfering
elements.
5.2 The procedure includes measuring total (see 12.3.8) and free orthophosphate (see 12.4.3) ions and the difference in
concentration is the phosphonate concentration. The sample could contain orthophosphate naturally, or from decomposition of the
phosphonate during processing or well treatment or from treating compounds containing molecular dehydrated phosphates.
6. Interferences
6.1 Sulfide interferes in this test method, but techniques described in the procedure (see 9.1.2) eliminate this interference.
+2 –2 +4 +2 +3
Concentrations less than 1000 mg/L copper (Cu ) and silica (SiO /SiO /Si ); and less than 200 mg/L of iron (Fe /Fe ) can
2 3
be tolerated.
6.2 Produced brines can contain high concentrations of dissolved solids. Some of these dissolved solids tend to precipitate when
produced brines reach new equilibria at atmospheric temperature and pressure. Phosphonate will coprecipitate or adsorb onto these
newly formed solids and become unavailable for analysis. This problem can be minimized by acidifying the brine sample on-site
with hydrochloric acid to pH below 2.
6.3 Glassware must be cleaned with phosphate free detergent and rinsed with 0.1 N hydrochloric acid to remove all residual
phosphate or phosphonate.
6.4 The standard addition method in 12.6 is recommended for brine with high matrix interference.
7. Apparatus
7.1 Pressure Cooker or Sterilizer (Autoclave)(Autoclave). .
7.2 Spectrophotometer, ,for for measurement above 650 nm with 4-cm light path cells. A longer light path will yield a
corresponding higher sensitivity (see 12.5.1). Spectrophotometer practices prescribed in this test method shall conform to Practice
E275.
7.3 Bottle Top Liquid Dispenser, ,20-mL 20-mL capacity, <1 % accuracy, and <0.1 % precision.
7.4 Pipetter, automated, , 10-mL capacity with 0.2 to 0.5 % accuracy.
7.5 Glass Bottles, ,60 60 mL and 240 mL with Teflon-lined screw cap closure.
8. Reagents and Materials
8.1 Purity of Reagents—Reagent grade chemicals shall be used in all tests. Unless otherwise indicated, it is intended that all
reagents shall conform to the specifications of the Committee on Analytical Reagents of the American Chemical SocietySociety. .
Other grades may be used, provided it is first ascertained that the reagent is of sufficiently high purity to permit its use without
lessening the accuracy of the determination.
Fisher Scientific No. 14-141-S has been satisfactory for this purpose, or equivalent, should be used. If you are aware of alternative suppliers, please provide this
information to ASTM International Headquarters. Your comments will receive careful consideration at a meeting of the responsible technical committee, which you may
attend.
Varian DMS-100 has been satisfactory for this purpose, or equivalent, should be used. If you are aware of alternative suppliers, please provide this information to ASTM
International Headquarters. Your comments will receive careful consideration at a meeting of the responsible technical committee, which you may attend.
Fisher Scientific No. 13-687-21 REPIPET has been satisfactory for this purpose, or equivalent, should be used. If you are aware of alternative suppliers, please provide
this information to ASTM International Headquarters. Your comments will receive careful consideration at a meeting of the responsible technical committee, which you may
attend.
Fisher Scientific No. 21-279-25 Eppendorf Maxipipetter has been satisfactory for this purpose. If you are aware of alternative suppliers, please provide this information
to ASTM International Headquarters. Your comments will receive careful consideration at a meeting of the responsible technical committee, which you may attend.
Fisher Scientific No. 03-326-3C and 03-326-3G have been satisfactory for this purpose. If you are aware of alternative suppliers, please provide this information to ASTM
International Headquarters. Your comments will receive careful consideration at a meeting of the responsible technical committee, which you may attend.
Reagent Chemicals, American Chemical Society Specifications , American Chemical Society, Washington, DC. For suggestions on the testing of Reagents not listed by
the American Chemical Society, see Analar Standards for Laboratory Chemicals, BDH Ltd., Poole, Dorset, U.K., and the United States Pharmacopeia and National
Formulary, U.S. Pharmaceutical Convention, Inc. (USPC), Rockville, MD.
D6501 − 15
8.2 Purity of Water—Unless otherwise indicated, reference to water shall be understood to mean reagent water conforming to
Specification D1193, Type I. Other reagent water types may be used provided it is first ascertained that the water is of sufficiently
high purity to permit its use without adversely affecting the precision and bias of the test method. Type III water was specified at
the time of round robin testing of this test method.
8.3 Alcoholic Sulfuric Acid Solution—Cautiously add 20 mL concentrated H SO (sp. gr. 1.89) to 900 mL methyl alcohol (8.7)
2 4
and dilute to 1 L with methyl alcohol. It is recommended to dispense the liquid with a bottle top liquid dispenser, which dispenses
a 10-mL volume.
8.4 Ammonium Molybdate Solution—Dissolve 39.1 g (NH ) Mo O · 4H O in 200 mL water. Cautiously add 210 mL
4 6 7 24 2
concentrated HCl (sp. gr. 1.19) to 400 mL water. Cool, add molybdate solution, and dilute to 1 L. It is recommended to dispense
the liquid with a liquid dispenser, which dispenses a 10-mL volume.
8.5 Glycerol—Reagent grade, 99 % or greater.
8.6 Hydrochloric Acid (6N)—Add 500 mL of concentrated HCl (sp. gr. 1.19) to 500 mL of water.
8.7 Methyl Alcohol—Reagent grade, 99 % or greater.
8.8 Methyl Isobutyl Ketone/Cyclohexane Solvent—Mix equal volumes of methyl isobutyl ketone (MIBK) and cyclohexane.
Warning:(Warning—This solvent is highly flammable. It is recommended to dispense the liquid with a bottle top liquid dispenser,
which dispenses a 20-mL volume. This solvent is highly flammable. It is recommended to dispense the liquid with a bottle top
liquid dispenser, which dispenses a 20-mL volume.)
8.9 Phosphate Solution, standard (1.00 mL = 0.05 mg PO ). Dissolve 71.6 mg anhydrous KH PO in water and dilute to 1 L.
4 2 4
8.10 Phosphonate Solution, (50-mg/L phosphonate)—If the standard addition procedure (see 12.6) is to be used, a stock solution
of 50 mg/L, as phosphonate, should be prepared. To prepare this solution, a concentrated sample of the phosphonate to be measured
along with the wt/wt percent phosphonate concentration must be obtained from the manufacturer. The wt/wt percent phosphonate
concentration also can be calibrated by this procedure as described in 12.2 and 12.3.
8.11 Potassium Persulfate, K S O .
2 2 8
8.12 Sodium Chloride Solution (1.0 M, Synthetic Brine)—Dissolve 58.44 g. NaCl in 800 mL water and dilute to 1 L. This
solution is used as a synthetic brine.
8.13 Sodium Hypochlorite, (5.65–6 %).
8.14 Stannous Chloride Solution —Solution—Mix 0.4 g SnCl · 2H O in 100 mL glycerol (8.4). This reagent is stable for at least
2 2
six months. The solution is stored in a dropper bottle.
9. Hazards
9.1 Precautions:
9.1.1 Most phosphonate inhibitors are strongly adsorbed to glass or metal; therefore, polyethylene beakers, flasks, pipets, etc.,
should be used to contain and transfer brine solutions from the field.
9.1.2 A glass bottle is recommended for use in the color development steps (see 12.2 and 12.3) for better visualization of the
reaction. Since the reaction media is acidic, phosphonate will not adsorb to the glass surface.
9.1.3 Personnel performing this test must be familiar with all precautions for handling strong sulfuric acid, hydrochloric acid
and sulfide-containing brine. Personnel should consult the material safety data sheet for handling strong acids. Protective clothing
and latex gloves should be worn. The sulfide brine should be handled in the hood with good ventilation. Sulfide containing brine
can be treated with sodium hypochlorite (8.13) prior to analysis to oxidize the hydrogen sulfide.
10. Sampling
10.1 Collect the sample in accordance with Practices D3370.
10.2 Preserve the samples immediately at the time of collection by adding 4 mL of 6 N hydrochloric acid 8.6 per 100-mL brine.
NOTE 1—Alternatively, the pH may be adjusted in the laboratory if the sample is returned within 14 days. However, acid must be added at least 24
hours before analysis to dissolve any metals that adsorb to the container walls. This could reduce hazards of working with acids in the field when
appropriate.
11. Calibration and Standardization
11.1 Prepare standards by adding 2.0, 4.0, 6.0, 8.0, 10.0 mL each of phosphate standard solution (1.00 mL = 0.05 mg PO ) (8.9)
to separate 100-mL volumetric flasks. Dilute to 100 mL with 1 M sodium chloride solution (8.12). These solutions will contain
1.0, 2.0, 3.0, 4.0, 5.0 mg/L phosphate as PO . If the procedure in 12.5 is used for samples with low phosphonate concentrations,
then solutions containing 0.2, 0.4, 0.6, 0.8, 1.0 mg/L phosphate as PO should be used.
11.2 Follow the procedure in 12.2 and 12.3 to develop color, and determine the absorbance at 725 nm.
D6501 − 15
11.3 Prepare Read directly in concentration if this capability is provided with the instrument or prepare a calibration curve
showing phosphate ion concentration in mg/L on the X axis with the corresponding absorbance (A) reading of the
spectrophotometer on the Y axis of linear graph paper.
12. Procedure
12.1 The procedures in 12.2 and 12.3 are applicable to samples containing 0.5 to 5 mg/L phosphonate. For samples containing
less than 0.5 mg/L phosphonate, a larger sample volume or a different light path cell can be used (see 12.5).
12.2 Persulfate Digestion Procedure:
12.2.1 Pipet 20 mL of the following samples (12.2.1.1, 12.2.1.2, 12.2.1.3) into separate 60-mL glass bottles, each containing
200 mg of potassium persulfate (8.11). Multiple samples can be digested at the same time.
12.2.1.1 Blank, 1-M sodium chloride (see 8.12).
12.2.1.2 Phosphate standards (see 11.1).
12.2.1.3 Samples of acidified brine.
12.2.2 Close the sample bottles loosely with Teflon-lined caps.
12.2.3 Heat the samples for 30 minutes in a pressure cooker or sterilizer at 100–120°C (103.4–137.9 kPa (15–20 psig).psig)).
12.2.4 Make sure the samples are cooled to room temperature before proceeding to color development. The temperature of
solution is critical in procedure 12.2.3. At this point in the procedure, all of the phosphonate has been oxidized to phosphate.
12.3 Color development and extraction procedure:
12.3.1 The timings specified in procedures 12.3.3, 12.3.4, and 12.3.7 are critical to the test. It is recommended to run small
numbers of samples at a time in order to manage the timing.
12.3.2 Standard addition method (see 12.6) should be used for data quality control.
12.3.3 Add 20 mL MIBK/Cyclohexane solvent (8.8) and 10 mL ammonium molybdate solution (8.4) to the sample bottles, and
immediately, vigorously shake each bottle for 15 s. At this point, the clear and electrically-neutral phosphomolybdate complex has
been formed and extracted into the organic solvent phase.
12.3.4 Wait exactly five minutes to allow the aqueous and organic solvent phases to be separated, and withdraw 10.0 mL of
liquid from the organic solvent layer into a clean 60-mL glass bottle using an automatic pipetter. Care should be taken not to disturb
the solvent/water interface or accidentally withdraw some aqueous solution, since the excess molybdenum in the aqueous phase
can also be reduced by stannous chloride to form a deep blue color.
12.3.5 Add 10 mL alcoholic H SO solution (8.3) to the samples, and swirl to mix.
2 4
12.3.6 Add four drops stannous chloride solution (8.14) to each sample, and mix thoroughly.
12.3.7 After 10 minutes, but before 20 minutes, pour each sample into a 4-cm cell and read the absorbance against the blank
at 725 nm. Absorbance readings also can be taken at 650 or 700 nm, but with reduced sensitivity. Use the sample blank as reference
solution in measuring the sample.
12.3.8 Read the total phosphate concentration (C – ) from a calibration curve prepared by analyzing known phosphate
T PO4
standards, as described in Section 11.
12.4 Procedure for Analyzing Orthophosphate Concentration in the Brine:
12.4.1 Pipet 20 mL of the acidified brine sample to a separate 60-mL glass bottle.
12.4.2 Follow the procedure in 12.3.3 – 12.3.7 to develop phosphomolybdate complex and to extract the complex to the organic
liquid phase.
12.4.3 Read the orthophosphate concentration (C – ) from a calibration curve prepared in Section 11.
F PO4
12.5 Procedure for brines containing phosphonate concentrations outside the range(s) specified.
12.5.1 The above concentration range is specified for using a 4-cm light path cell. Longer light path cells are suitable for
analyzing phosphonate at low concentrations (see the following):
Approximate Phosphonate Range (mg/L) Light Path (cm)
0.1–2.0 10
12.5.2 Alternatively, the sample size can be adjusted to analyze brines containing low phosphonate concentration other than that
specified in 12.1. An example of 100-mL sample size is given below.
12.5.2.1 Pipet 100 mL instead of 20 mL into a 240-mL bottle. The organic solvent phase in the 240-mL bottle will be a thin
layer. Care should be taken not to disturb the solvent/water interface or accidentally withdraw aqueous solution when removing
the phosphomolybdate complex from the organic solvent phase.
12.5.2.2 Add 1 g potassium persulfate (8.11) to the sample bottle.
12.5.2.3 Follow 12.2.2 – 12.3.8 to analyze for phosphate ion.
12.6 Standard Additions Procedure:
12.6.1 This procedure is recommended to determine the concentration of phosphonate in brine containing interfering
components.
12.6.2 Prepare a blank and three samples, as in 12.2.1. Add 100 μL of 50 mg/L phosphonate standard solution (8.10) to one of
the sample bottles. Add 200 μL of 50 mg/L phosphonate standard solution (8.10) to a second sample bottle.
D6501 − 15
12.6.3 Complete the procedures in 12.2.2 – 12.3.7 to digest phosphonate and to analyze for phosphate ion concentration.
12.6.4 Plot the absorbance versus concentration of added phosphonate. Draw a straight line through these three data points.
Extend this line to intersect the X axis at a negative value of phosphonate concentration. The absolute value of this intersection
is the concentration of phosphonate in the sample of interest.
13. Calculation
13.1 Calculate the phosphonate concentration in the sample (as mg/L PO ) as follows:
Volume of Standard
mg/L PO 5 C 2 2 C 2 Field Dilution (1)
@~ ! ~ !#F G ~ !
4 T PO F PO
4 4
Volume of Sample
where:
C = Concentration of total phosphate (mg/L) read from calibration curve (see 12.3.8);
T–PO
C = Concentration of orthophosphate (mg/L) read from calibration curve (see 12.4.3);
F–PO
Volume of Standard = Volume (mL) of stand
...