ASTM D7237-18

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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: D7237 − 15a D7237 − 18
Standard Test Method for
Free Cyanide and Aquatic Free Cyanide with Flow Injection
Analysis (FIA) Utilizing Gas Diffusion Separation and
Amperometric Detection
This standard is issued under the fixed designation D7237; 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 is used to establish the concentration of free cyanide in an aqueous wastewater, effluent and in-stream free
cyanide concentrations after mixing treated water with receiving water. The test conditions of this test method are used to measure
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free cyanide (HCN and CN ) and cyanide bound in the metal-cyanide complexes that are easily dissociated into free cyanide ions
at the pH of 6. Free cyanide is determined at pH 6 at room temperature. The aquatic free cyanide can be determined by matching
the pH to the water in the receiving environment in the range of pH 6 to 8. The extent of HCN formation is less dependent on
temperature than the pH; however, the temperature can be regulated if deemed necessary for aquatic free cyanide to further
simulate the actual aquatic environment.
1.2 The free cyanide test method is based on the same instrumentation and technology that is described in Test Method D6888,
but employs milder conditions (pH 6–8 buffer versus HCl or H SO in the reagent stream), and does not utilize ligand displacement
2 4
reagents.
1.3 The aquatic free cyanide measured by this procedure should be similar to actual levels of HCN in the original aquatic
environment. This in turn may give a reliable index of toxicity to aquatic organisms.
1.4 This procedure is applicable over a range of approximately 5 to 500 μg/L (parts per billion) free cyanide. Sample dilution
may increase cyanide recoveries depending on the cyanide speciation; therefore, it is not recommended to dilute samples. Higher
concentrations can be analyzed by increasing the range of calibration standards or with a lower injection volume. In accordance
with Guide E1763 and Practice D6512 the lower scope limit was determined to be 9 μg/L for chlorinated gold leaching barren
effluent water and the IQE is 12 μg/L in the gold processing detoxified reverse osmosis permeate waste water sample matrix.
10 %
1.5 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.6 This test method is not recommended for samples that contain reduced sulfur compounds such as sulfides.
1.7 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. Specific hazard statements are given in 8.6 and Section 9.
1.8 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:
D1129 Terminology Relating to Water
D1193 Specification for Reagent Water
D1293 Test Methods for pH of Water
D2036 Test Methods for Cyanides in Water
This test method is under the jurisdiction of ASTM Committee D19 on Water and is the direct responsibility of Subcommittee D19.06 on Methods for Analysis for
Organic Substances in Water.
Current edition approved June 1, 2015Dec. 1, 2018. Published June 2015January 2019. Originally approved in 2006. Last previous edition approved in 2015 as
D7237 – 15.D7237 – 15a. DOI: 10.1520/D7237-15A.10.1520/D7237-18.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’sstandard’s Document Summary page on the ASTM website.
*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
D7237 − 18
D2777 Practice for Determination of Precision and Bias of Applicable Test Methods of Committee D19 on Water
D3856 Guide for Management Systems in Laboratories Engaged in Analysis of Water
D4658 Test Method for Sulfide Ion in Water
D4841 Practice for Estimation of Holding Time for Water Samples Containing Organic and Inorganic Constituents
D5847 Practice for Writing Quality Control Specifications for Standard Test Methods for Water Analysis
D6512 Practice for Interlaboratory Quantitation Estimate
D6696 Guide for Understanding Cyanide Species
D6888 Test Method for Available Cyanides with Ligand Displacement and Flow Injection Analysis (FIA) Utilizing Gas
Diffusion Separation and Amperometric Detection
D7365 Practice for Sampling, Preservation and Mitigating Interferences in Water Samples for Analysis of Cyanide
D7728 Guide for Selection of ASTM Analytical Methods for Implementation of International Cyanide Management Code
Guidance
E691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
E1601 Practice for Conducting an Interlaboratory Study to Evaluate the Performance of an Analytical Method
E1763 Guide for Interpretation and Use of Results from Interlaboratory Testing of Chemical Analysis Methods (Withdrawn
2015)
3. Terminology
3.1 Definitions:
3.1.1 For definitions of terms used in this standard, refer to Terminology D1129 and Guide D6696.
3.2 Definitions of Terms Specific to This Standard:—For definitions of terms used in this test method, refer to Terminology
D1129 and Guide D6696.
3.2.1 aquatic free cyanide, n—free cyanide measured when the buffer or temperature is adjusted to mimic the receiving-water
environment.
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3.2.2 free cyanide, n—sum of the free cyanide (HCN and CN ) and cyanide bound in the metal-cyanide complexes that are
easily dissociated into free cyanide under the test conditions described in this test method at pH 6 and room temperature.
4. Summary of Test Method
4.1 The test is generally performed at room temperature, but temperature of the sample and flow injection reagents can be
regulated to match the aquatic environment if necessary to measure aquatic free cyanide.
4.2 The sample is introduced into a carrier solution of the flow injection analysis (FIA) system via an injection valve and
confluence downstream with a phosphate buffer solution at pH 6 to measure free cyanide or in the range of pH 6 to 8 to measure
aquatic free cyanide. The released hydrogen cyanide (HCN) gas diffuses through a hydrophobic gas diffusion membrane into an
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alkaline acceptor stream where the CN is captured and sent to an amperometric flowcell detector with a silver-working electrode.
In the presence of cyanide, silver in the working electrode is oxidized at the applied potential. The anodic current measured is
proportional to the concentration of cyanide in the standard or sample injected.
4.3 Calibrations and sample data are processed with the instrument’sinstrument’s data acquisition software.
5. Significance and Use
5.1 Cyanide and hydrogen cyanide are highly toxic. Regulations have been established to require the monitoring of cyanide in
industrial and domestic wastes and surface waters.
5.2 It is useful to determine the aquatic free cyanide to establish an index of toxicity when a wastewater is introduced into the
natural environment at a given pH and temperature.
5.3 This test method is applicable for natural water, saline waters, and wastewater effluent.
5.4 Free cyanide measured using this test method is applicable for implementation of the International Cyanide Code Guidance
in accordance with Guide D7728.
6. Interferences
6.1 Sulfide will diffuse through the gas diffusion membrane and can be detected in the amperometric flowcell. Oxidized
- -
products of sulfide can also rapidly convert CN to SCN at a high pH. Refer to 11.3 for sulfide removal.
6.2 Refer to 6.1 of Test Method D6888 and Test Methods D2036 for elimination of cyanide interferences.
The last approved version of this historical standard is referenced on www.astm.org.
40 CFR Part 136.
D7237 − 18
C = carrier (water), R = reagent buffer (variable: pH 6 for free cyanide and pH 6-86–8 for aquatic free cyanide, 0.2 M phosphate buffer), A = acceptor solution (0.1 M
NaOH), S = sample, PPP = peristaltic pump (flow rates in mL/min), I = injection valve (200 μL sample loop), MC = mixing coolcoil (30–60 cm × 0.5 mm i.d.),ID), positioned
in optional constant temperature manifold, D = gas-diffusion cell, FC = amperometric flow cell, PO/DAT = potentiostat/data collection device running data acquisition
software, W = waste flows.
FIG. 1 Example of Flow Injection Manifold for the Determination of Aquatic Free Cyanide
6.1 Residual Sulfide, as H S, will diffuse through the gas diffusion membrane and can be detected in the amperometric flowcell.
Also, residual flotation reagents have been shown to interfere, which is indicated by failure of the amperometric signal to return
to baseline compared to the standards. This effect is attributed to the formation of volatile carbon disulfide. If sulfides are suspected,
confirm by means of Test Method D4658this interference is encountered, verify by comparing with analysis using Test Method.
Aside from this method, ion chromatography may be used as a reliable means for D6888 including bismuth nitratedetermining
sulfide interference. See Appendix X1in the acidification reagent on a solution without sodium hydroxide preservation, which
should provide confirmation due to lower results. For National Pollutant Discharge Elimination System reporting, however, the
user is advised to analyze the sample using Test Method D6888 for samples containing reduced sulfur compounds.
6.2 Oxidants can continue to react with free cyanide, if present, lowering the concentration, if not immediately treated with a
reducing reagent during sampling.
7. Apparatus
7.1 The instrument must be equipped with a precise sample introduction system, a gas diffusion manifold with hydrophobic
membrane, and an amperometric detection system to include a silver working electrode, a Ag/AgCl reference electrode, and a Pt
or stainless steel counter electrode. An example of the apparatus schematic is shown in Fig. 1. Example instrument settings are
shown in Table 1.
NOTE 1—The instrument and settings in Fig. 1 and Table 1 are shown as examples. The analyst may modify these settings as long as performance of
the method has not been degraded. Contact the instrument manufacturer for recommended instrument parameters.
7.2 An autosampler is recommended but not required to automate sample injections and increase throughput. Autosamplers are
usually available as an option from the instrument’sinstrument’s manufacturer. If the sample is to be analyzed at a constant
temperature other than the temperature of the room, manual injections may be required unless the autosampler is equipped to
maintain constant temperature.
7.3 If aquatic free cyanide at a temperature other than room temperature is required, a constant temperature bath capable of
maintaining the temperature of the aquatic environment within 60.5°C should be used to regulate the temperature of the flow
injection reagents and samples.
7.4 Data Acquisition System—Use the computer hardware and software recommended by the instrument manufacturer to
control the apparatus and to collect data from the detector.
7.5 Pump Tubing—Use tubing recommended by instrument manufacturer. Replace pump tubing when worn, or when precision
is no longer acceptable.
7.6 Gas Diffusion Membranes—A hydrophobic membrane which allows gaseous hydrogen cyanide to diffuse from the donor
to the acceptor stream at a sufficient rate to allow detection. The gas diffusion membrane should be replaced when the baseline
becomes noisy, or every 1 to 2 weeks.
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TABLE 1 Flow Injection Analysis Parameters
FIA Instrument Recommended
Parameter Method Setting
FIA Instrument Recommended
Parameter Method Setting
Pump Flow Rates 0.5 to 2.0 mL/min
Pump flow rates 0.5 to 2.0 mL/min
Cycle period (total) Approximately 120 seconds
Sample load period At least enough time to completely fill the
sample loop prior to injection
Injection valve rinse time At least enough time to rinse the
between samples sample loop
Peak Evaluation Peak height or area
Peak evaluation Peak height or area
Working Potential 0.0 V vs. Ag/AgCl
Working potential 0.0 V versus Ag/AgCl
7.7 Use parts and accessories as directed by instrument manufacturer.
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 American Chemical Society, where such specifications are available. 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.
8.2 Purity of Water—Unless otherwise indicated, references to water shall be understood to mean reagent water that meets the
purity specifications of Type I or Type II water, presented in Specification D1193.
8.3 Sodium Hydroxide Solution (1.00M (1.00 M NaOH)—Dissolve 40 g NaOH in laboratory water and dilute to 1 L.
8.4 Sodium Hydroxide and Acceptor Solution (0.10 M NaOH)—Dissolve 4.0 g NaOH in laboratory water and dilute to 1 L.
NOTE 2—Acceptor solution concentration of 0.025 M NaOH has also been found to be acceptable.
8.5 Carrier—Water, as described in 8.2.
NOTE 3—Carrier solution containing 0.025 M NaOH has also been found to be acceptable.
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8.6 Stock Cyanide Solution (1000 μg/mL CN )—Dissolve 2.51 g of KCN and 2.0 g of NaOH in 1 L of water. Standardize with
silver nitrate solution as described in Test Methods D2036, 16.2. Store the solution under refrigeration and check concentration
approximately every 6 months and correct if necessary. (Warning—Because KCN is highly toxic, avoid contact or inhalation.)
Reagent Chemicals, American Chemical Society Specifications, Am.American Chemical Soc.,Society, Washington, DC. For suggestions on the testing of reagents not
listed by the American chemicalChemical Society, see Analar Standards for Laboratory Chemicals, BDH Ltd., Poole, Dorset, U.K., and the United States
Pharmacopeia.Pharmacopeia and National Formulary, U.S. Pharmacopeial Convention, Inc. (USPC), Rockville, MD.
Commercial Solutions of Stock Cyanide may be substituted.
D7237 − 18
8.7 Intermediate Cyanide Standards:
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8.7.1 Intermediate Standard 1 (100 μ g/mL CN )—Pipette 10.0 mL of stock cyanide solution (see 8.6) into a 100 mL volumetric
flask containing 1 mL of 1.0 M NaOH (see 8.3). Dilute to volume with laboratory water. Store under refrigeration. The standard
should be stable for at least 2 weeks.
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8.7.2 Intermediate Cyanide Solution 2 (10 μg/mL CN )—Pipette 10.0 mL of Intermediate Cyanide Solution 1 (see 8.7.1) into
a 100 mL volumetric flask containing 1.0 mL of 1.00 M NaOH (see 8.3). Dilute to volume with laboratory water. The standard
should be stable for at least 2 weeks.
8.8 Working Cyanide Calibration Standards—Prepare fresh daily as described in 8.8.1 and 8.8.2 ranging in concentration from
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2 to 500 μg/L CN .
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8.8.1 Calibration Standards (20, 50, 100, 200, and 500 μg/L CN )—Pipette 20, 50, 100, 200, and 500 μL of Intermediate
Standard 1 (see 8.7.1) into separate 100 mL volumetric flasks containing 1.0 mL of 0.10 M NaOH (see 8.4). Dilute to volume with
laboratory water.
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8.8.2 Calibration Standards (2, 5, and 10 μg/L CN )—Pipette 20, 50, and 100 μL of Intermediate Cyanide Solution 2 (see 8.7.2)
into separate 100 mL volumetric flasks containing 1.0 mL of 0.10 M NaOH (see 8.4). Dilute to volume with laboratory water.
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8.9 Cyanide Electrode Stabilization Solution (Approximately 5 ppm as CN )—Pipette 500 μL of Stock Cyanide (see 8.6) into
a 100 mL volumetric flask containing 1.0 mL of 0.10M M NaOH (see 8.4). Dilute to volume with laboratory water. The solution
should be stored under refrigeration.
8.10 Acetate Buffer—Dissolve 410 g of sodium acetate trihydrate (NaC H O ·3H O) in 500 mL of laboratory water. Add glacial
2 3 2 2
acetic acid (approximately 500 mL) to yield a pH of 4.5.
8.11 Buffer Solution A, 2M Sodium phosphate monobasic solution—Weigh 276 g sodium phosphate monobasic monohydrate
(NaH PO ·H O) in a 1 L volumetric flask. Dissolve and dilute to volume with water.
2 4 2
8.12 Buffer Solution B, 2 M Sodium phosphate dibasic solution—Weigh 284 g sodium phosphate dibasic, anhydrous (Na HPO )
2 4
in a 1-L volumetric flask. Dissolve and dilute to volume with water. If necessary, warm to approximately 40°C on a hot plate and
stir to completely dissolve the sodium phosphate dibasic into the water. Allow the solution to cool prior to use.
8.12.1 Alternatively, prepare a 1 M solution by dissolving 142 g sodium phosphate dibasic, anhydrous in 1 L.
8.13 1 M Phosphate Buffer pH 7.0 Stock Solution—Add 97.5 mL Buffer Solution A and 152.5 mL Buffer Solution B to a 500-mL
volumetric flask. Dilute to volume with water.
8.13.1 Alternatively, substitute 305 mL of 1 M sodium phosphate dibasic for the 152.5 mL of Buffer Solution B.
8.14 0.2 M Phosphate Buffer pH 7.0—In a 1 L volumetric flask, add 200 mL 1 M Phosphate Buffer Solution pH 7.0 and dilute
to volume with water. The pH should be pH 7.0 6 0.1. Verify the pH as described in Test Methods D1293 (Test Method A) and
adjust if necessary with dilute sodium hydroxide or sulfuric acid. This buffer solution is to be used in the FIA system when aquatic
free cyanide is to be determined at pH 7.0.
8.15 1 M Phosphate Buffer pH 6.0 Stock Solution—Add 219.25 mL Buffer Solution A and 30.75 mL of Buffer Solution B to
a 500 mL volumetric flask. Dilute to volume with water.
8.15.1 Alternatively, substitute 61.5 mL of 1 M sodium phosphate dibasic for the 30.75 mL of Buffer Solution B.
8.16 0.2 M Phosphate Buffer pH 6.0—In a 1-L volumetric flask, add 200 mL 1 M Phosphate Buffer Solution pH 6.0 and dilute
to volume with water. The pH should be pH 6.0 6 0.1. Verify the pH as described in Test Methods D1293 (Test Method A) and
adjust if necessary with dilute sodium hydroxide or sulfuric acid. This buffer solution is to be used in the FIA system when free
cyanide or aquatic free cyanide is to be determined at pH 6.0 or if the pH of the aquatic environment has not been specified.
8.17 1 M Phosphate Buffer pH 8.0 Stock Solution—Add 10.0 mL Buffer Solution A and 240 mL Buffer Solution B to a 500-mL
volumetric flask. Dilute to volume with water.
8.18 0.2 M Phosphate Buffer pH 8.0—In a 1-L volumetric flask, add 200 mL 1 M Phosphate Buffer Solution pH 8.0 and dilute
to volume with water. The pH should be pH=8.0 pH = 8.0 6 0.1. Verify the pH as described in Test Methods D1293 (Test Method
A) and adjust if necessary with dilute sodium hydroxide or sulfuric acid. This buffer solution is to be used in the FIA system when
aquatic free cyanide is to be determined at pH 8.0.
8.19 Ag/AgCl Reference Electrode Filling Solution—Fill the reference electrode as recommended by the instrument
manufacturer.
9. Hazards
9.1 Warning—Because of the toxicity of cyanide, great care must be exercised in its handling. Acidification of cyanide
solutions produces toxic hydrocyanic acid (HCN). All manipulations must be done in the hood so that any HCN gas that might
escape is safely vented.
9.2 Warning—Many of the reagents used in these test methods are highly toxic. These reagents and their solutions must be
disposed of properly.
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9.3 All reagents and standards should be prepared in volumes consistent with laboratory use to minimize the generation of
waste.
10. Sample and Sample Preservation
10.1 Collect the sample in accordance with latest version of Practice D7365. This practice is applicable for the collection and
preservation of water samples for the analysis of cyanide. Responsibilities of field sampling personnel and the laboratory are
indicated. Usually 100 mL sample volume is sufficient. Samples must be collected and stored in dark (amber or low actinic)
containers to minimize reactions of ultra violet light.
10.2 The sample must be stabilized at time of collection with the addition of sodium hydroxide. Add 1 mL of 0.1 M NaOH to
100 mL of the sample or until the sample is pH 11.
10.3 See Section 11 if oxidizing agents or sulfide are suspected to be present in the sample.
10.4 Samples must be stored in dark bottles that minimize exposure to ultraviolet radiation and refrigerated.
NOTE 4—Practice D7365 recommends refrigeration by storing the sample between its freezing point and 6°C.
10.5 Synthetic samples have been shown to be stable for at least 14 days and up to 30 days, but in actual samples the cyanide
concentrations may decrease significantly prior to this holding time if there are undetectable traces of chlorine, reduced sulfur
species, Caro’s acid, reduced sulfur, or hydrogen peroxide present. Analyze Treat, filter and measure the sample as soon as
possible to avoid degradation. Holding times can be estimated in accordance with Practice D4841.
11. Elimination of Interferences
11.1 Sulfides—Practice Test for sulfides by placing a drop of sample on lead acetate paper previously moistened with acetate
buffer solution (see 8.10). If the paper turns black, sulfide is present. An odor of “rotten eggs” also indicates the presence of sulfide.
Lead acetate test strips may not be sensitive enough to detect sulfide concentrations below approximately 50 mg/L. If the presence
of sulfide below concentrations of 50 mg/L is suspected, confirm using Test Method D7365D4658 specifies mitigation of
interference procedures. Mitigate using Test Method D6888 for testing water samples for cyanide.to measure available weak and
dissociable cyanide without sulfide interference. Although also effective in overcoming sulfide interference, ion chromatography
is not an approved test procedure for determining free and aquatic free cyanide in accordance with 40 CFR 136.3.
11.2 Oxidizing Agent—Test for the presence of oxidizing agents. Add a drop of the sample to acidified KI starch test paper
(acidify KI starch paper with acetate buffer, see 8.10) as soon as the sample is collected; a blue color indicates the need for
treatment. If oxidizing agents are present, add 0.1 g/L sodium arsenite to the sample and mix until a drop of sample on the test
strip shows no blue color, to avoid degradation of cyanide.
11.3 Sulfide—Test for sulfide by placing a drop of sample on lead acetate paper previously moistened with acetate buffer
solution (see 8.10). If the paper turns black, sulfide is present. Add lead acetate, or if the sulfide concentration is too high, add
powdered lead carbonate to avoid significantly reducing the pH. Repeat this test until a drop of treated sample no longer darkens
the acidified lead acetate test paper. The supernatant containing cyanide must be filtered immediately to avoid the rapid loss of
cyanide due to the formation of thiocyanate.
NOTE 5—Lead acetate test strips may not be sensitive enough to detect sulfide concentrations below approximately 50 mg/L; therefore, treatment may
be performed on samples where sulfide is suspected. Interference can be confirmed by analyzing the sample with and without treatment. If the measured
cyanide in the untreated sample is significantly higher, sulfide is likely present and treatment described in 11.3 should be performed to remove sulfide.
12. Calibration and Standardization
12.1 Turn on the power to the apparatus and the autosampler (if equipped). Start the data acquisition system.
12.2 Clamp the pump tube platens in place and start pumping reagents in the flow injection system. Allow the system to warm
up at least 15 min or until a stable baseline is achieved. Take care not to over-tighten the pump tube platens as this greatly reduces
the lifetime of the tubing.
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12.3 If recommended by the instrument manufacturer, aspirate the Cyanide Stabilization Solution (5 ppm CN ) from 8.9. After
at least 30 s, inject the stabilization solution into the apparatus and record the amperometric response (pA value) after the cycle
period has completed. Repeat this procedure until the peak responses are less than 2 % RSD. This process will ensure that the
electrode system has stabilized.
12.4 After the electrode system has stabilized, aspirate the highest working standard (see 8.8) into the flow injection apparatus.
Follow the instrument manufacturer’smanufacturer’s instructions to store the retention time window for cyanide using the data
acquisition software.
Reduced sulfur samples will cause high results due to direct interference, or low results through the formation of SCN.
D7237 − 18
12.5 Select the buffer to be used for instrumental analysis of the sample, which is pH 6 for free cyanide or the closest pH to
that of the receiving water for the sample for aquatic free cyanide.
12.6 Inject each working standard and a reagent blank into the apparatus and
...