METHOD #: 218.6
Recommended for Approval for NPDES (August, 1991)
TITLE:	Determination Of Dissolved Hexavalent Chromium In Drinking
Water, Groundwater And Industrial Wastewater Effluents By
Ion Chromatography
ANALYTE:	CAS # Chromium 7440-47-3, Cr
INSTRUMENTATION: IC
1.0 Scope and Application
1.1	This method provides procedures for the determinatbn of dissolved hexavalent
chromium in drinking water, groundwater and industrial wastewater effluents.
1.2	The Method Detection Limits (MDL, defined in Sectin 3) for the above matrices
are listed in Table 1. The MDL obtained by an individual laboratory for a
specific matrix may differ from those listed depending on the nature of the
sample and the instrumentation used.
1.3	Samples containing high levels of anionic species such as suHate and chloride
may cause column overload. Samples containing high levels of organics or
sulfides cause rapid reduction of soluble Cr(VI) toCr(III). Samples must be
stored at 4°C and analyzed within twenty-four hours of collection.
1.4	This method should be used by analysts experienced in the use of ion
chromatography and the interpretatbn of ion chromatograms.
2.0 Summary of Method
2.1 An aqueous sample is filtered through a 0.45 um filter and the filtrate is
adjusted to a pH of 9 to 9.5 with a buffer solution. A measured volume of the
sample (50-250 |iL) is introduced into the ion chromatograph. A guard column
removes organics from the sample before the Cr(VI) as Cr042 is separated on
an anion exchange separator column. Post-Column derivatization of the Cr(VI)
with diphenylcarbazide is followed by detection of the colored complex a 530
nm.
3.0 Definitions
3.1	Dissolved-Material that will pass through a 0.45 um membrane filter.
3.2	Method Detection Limit (MDL)- The minimum concentration of an analyte that
can be identified, measured and reported with 99% confidence that the analyte
concentration is greater than zero and determined from analysis of a sample in
a given matrix containing analyte (1).
3.3	Linear Dynamic Range- The concentratbn range over which the analytical
working curve remains linear.
3.4	Laboratory Reagent Blank (LRB)- An aliquot of reagent water that is treated
exactly like a sample including exposure to all glassware, equipment, solvents
and reagents that are used with samples. The LRB is used to determine if the
method analyte is present in the laboratory environment, the reagents or
apparatus.

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3.5	Stock Standard Solution- A concentrated, certified standard solution of the
method analyte. The stock standard solution is used to prepare calibration
standards.
3.6	Calibration Standard (CAL)- A solution prepared from the stock standard and
used to calibrate the instrument response with respect to analyte concentration.
3.7	Laboratory FonHied Blank (LFB)- An aliquot of reagent water to which a
known quantity of method analyte is added in the laboratory. The LFB is
analyzed exactly like a sample, and its purpose is to determine whether the
method is within accepted control limits.
3.8	Laboratory Fortified Sample Matrix (LFM)- An aliquot of an environmental
sample to which a known quantity of method analyte is added in the
laboratory. The LFM is analyzed exactly like a sample, and its purpose is to
determine whether the sample matrix contributes bias to the analytical result.
The background concentration of the analyte in the sample matrix must be
determined in a separate aliquot and the measured value in the LFM corrected
for the concentration found.
3.9	Quality Control Sample (QCS)- A solution containing a known concentration of
analyte prepared by a laboratory other than the laboratory performing the
analysis. The sample is used to check laboratory performance.
3.10	Laboratory Duplicates (LD)- Two albuots of the same sample that ara treated
exactly the same throughout preparative and analytical procedures. Analyses
of laboratory duplicates indicate precision associated with laboratory
procedures.
3.11	Laboratory Performance Check Standards (LPC)- A solution of the analyte
prepared in the laboratory by making appropriate dilutions of the stock
standard in reagent water. The LPC is used to evaluate the performance of the
instrument system within a given calibration Curve.
Interferences
4.1 Interferences which affect the accurate determination of Cr(VI) may come from
several sources.
4.1.1	Contamination- A trace amount of Cr is sometimes found in reagent
grade salts. Since a concentrated buffer solution is used in this method
to adjust the pH of samples, reagent blanks should be analyzed to
assess for potential Cr(VI) contamination. Contamination can also come
from improperly cleaned glassware or contact of caustic or acidic
reagents or samples with stainless steel or pigmented material.
4.1.2	Oxidation of soluble Cr(III) to Cr(VI) can occur in an alkaline medium
in the presence of oxidants such as Fe(III) and oxidized Mn or as a
result of the aeration that occurs in most extraction procedures (2-5).
4.1.3	Reduction of Cr(VI) to Cr(III) can occur in the presence of reducing
species in an acidic medium. At a pH of 6.5 or greater, however,
HCr04 1 is converted to Cr042 which is less reactive than the HCr04_1.
4.1.4	Overloading of the analytical column capacity with high concentrations
of anionic species, especially chloride and sulfate, will cause a loss of
Cr(VI). The column specified in this method can handle samples
containing up to 5% sodium sulfate or 2% sodium chloride (6). Poor
recoveries from fortified samples and tailing peaks are typical
manifestations of column overload.

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5.0 Safety
5.1 Hexavalent chromium is toxic and a suspected carcinogen and should be
handled with appropriate precautbns (3,4). Extreme care should be exercised
when weighing the salt for preparation of the stock standard. Each laboratory
is responsible for maintaining a current awareness file of OSHA regulations
regarding the safe handling of chemicals specified in this method. A reference
file of material safety data sheets should also be available to all personnel
involved in the chemical analysis (7,8).
6.0 Apparatus and Equipment
6.1	Ion Chromatograph
6.1.1	Instrument equipped with a pump capable of withstanding a minimum
backpressure of 2000 psi and capable of delivering a constant flow in
the range of 1-5 mL/min and containing no metal parts in the sample,
eluent or reagent flow path.
6.1.2	Helium gas supply (high purity, 99.995%).
6.1.3	Pressurized eluent container, plastic, one or two liter size.
6.1.4	Sample loops of various sizes (50-250|iL).
6.1.5	A pressurized reagent delivery module with a mixing tee and beaded
mixing coil.
6.1.6	Guard Column- A column placed before the separator column
containing a sorbent capable of removing strongly absorbing organics
and particles that would otherwise damage the separator column
(Dionex IonPac NG1 or equivalent).
6.1.7	Separator Column- A column packed with a high capacity anion
exchange resin capable of resolving Cr042 from other sample
constituents (Dionex IonPac AS7 or equivalent).
6.1.8	A low-volume flow-through cell visible lamp detector containing no
metal parts in contact with the eluent flow path. Detection wavelength
is at 530 nm.
6.1.9	Recorder, integrator or computer for receiving analog or digital signals
for recording detector response (peak height or area) as a function of
time.
6.2	Labware- All reusable glassware (glass, quartz, polyethylene, Teflon, etc.)
including the sample containers should be soaked overnight in laboratory
grade detergent and water, rinsed with water, and soaked for four hours in a
mixture of dilute nitric and hydrochloric acid (1+2+9) followed by rinsing with
tap water and ASTM Type I water.
NOTE: chromic acid must not be used for the cleaning of glassware.
6.2.1	Glassware- Class A volumetric flasks and a graduated cylinder.
6.2.2	Assorted Class A calibrated pipettes.
6.2.3	10 mL male luer-bck disposable syringes.
6.2.4	0.45 |im syringe filters.
6.2.5	Storage bottle—high density polypropylene, 1 liter capacity.
6.3	Sample Processing Equipment
6.3.1 Liquid sample transport containers- high density polypropylene,
125 mL capacity.

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6.3.2	Supply of dry ice or refrigerant packing and styrofoam shipment
boxes.
6.3.3	pH meter- to read pH range 0-14 with accuracy ± 0.03 pH.
6.3.4	0.45 um filter discs, 7.3 cm diameter (Gelman Aero 50A, Mfr.
No.4262 or equivalent)
6.3.5	Plastic syringe filtration unit (Baxter Scientific, Cat. No. 1240 IN
or equivalent).
Reagents and Consumable Materials
7.1	Reagents- All chemicals are ACS grade unless otherwise indicated.
7.1.1	Ammonium hydroxide, NH4OH, (sp.gr. 0.902), (CAS RN 1336-21-6).
7.1.2	Ammonium sulfate, NH2S04, (CAS RN 7783-20-2).
7.1.3	1,5 Diphenylcarbazide, (CAS RN 1 40-22-7).
7.1.4	Methanol, HPLC grade.
7.1.5	Sulfuric acid, concentrated (sp.gr. 1.84).
7.2	Water- For all sample preparations and dilutions, ASTM Type I water (ASTM
D1193) is required. Suitable water may be obtained by passing distilled water
through a mixed bed of anion and cation exchange resins.
7.3	Cr(VI) Stock Solution- Dissolve 4,501 g of Na2Cr04*4H20 in ASTM Type I
water and dilute to one liter. Transfer to a polypropylene storage container.
7.4	Laboratory Reagent Blank (LRB)- Aqueous LRBs can be prepared by adjusting
the pH of ASTM Type I water to 9-9.5 with the same volume of buffer as was
used for the samples.
7.5	Laboratory Fortified Blank (LFB)- To an aliquot of reagent blank add an
aliquot of stock standard to produce a final concentration of 100 Hg/L of
Cr(VI). The LFB must be carried through the entire sample preparation and
analysis scheme.
7.6	Quality Control Sample (QCS)- A quality control sample must be obtained
from an outside laboratory. Dilute an aliquot according to instructions and
analyze with samples.
7.7	Eluent- Dissolve 33 g of ammonium sulfate in 500 mL of ASTM Type I water
and add 6.5 mL of ammonium hydroxide. Dilute to one liter with ASTM Type
I water.
7.8	Post-column Reagent- Dissolve 0.5 g of 1,5 diphenylcarbazide in 100 mL of
HPLC grade methanol. Add to about 500 mL of ASTM Type I water
containing 28 mL of 98% sulfuric acid while stirring. Dilute with ASTM Type I
water to one liter in a volumetric flask. Reagent is stable for four or five days
but should only be prepared in one liter quantities as needed.
7.9	Buffer Solution- Dissolve 33 g of ammonium sulfate in 75 mL of ASTM Type I
water and add 6.5 mL of ammonium hydroxide. Dilute to 100 mL with ASTM
Type I water.
Sample Collection, Preservation and Storage
8.1 Prior to the collection of the sample, consideration should be given to the type
of data required so that appropriate preservation and pretreatment steps can
be taken. Filtration and pH adjustment should be performed at the time of
sample collection or as soon thereafter as practically possible.

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8.2	For the determination of dissolved Cr(VI), the sample should be filtered
through a 0.45 um filter. Use a portion of the sample to rinse the syringe
filtration unit and filter and then collect the required volume of filtrate. Adjust
the pH of the sample to 9-9.5 by adding dropwise a solution of the buffer,
periodically checking the pH with the pH meter. Approximately 10 mLs of
sample are sufficient for three IC analyses.
8.3	Ship and store the samples at 4°C. Bring to ambient temperature prior to
analysis. Samples should be analyzed within twenty-four hours of collection.
Calibration
9.1	Calibration- At the time samples are analyzed a calibration should be
performed using a minimum of three calibration solutbns that bracket the
anticipated concentratbn range of the samples. Calibration standards should be
prepared from the stock standard (Sectbn 7.3) by appropriate dilution with
ASTM Type I water (Sectbn 7.2) in volumetric flasks. The solution should be
adjusted to a pH of 9-9.5 with the buffer solution (Section 7.9) prior to final
dilution.
9.1.1 Establish ion chromatographic operating conditions as indicated in
Table 2. The flow rate of the eluent pump is set at 1.5 mL/min and the
pressure of the reagent delivery module adjusted so that the final flow
rate from the detector is 2.0 mL/min. This requires manual adjustment
and measurement of the final flow using a graduated cylinder and a
stop watch. A warm up period of approximately 30 minutes after the
flow rate has been adjusted is recommended and the flow rate should
be checked prior to calibration and sample analysis.
9.1.2	Injection loop size is chosen based on standard and sample
concentrations and the selected attenuator setting. A 250 uL loop was
used to establish the method detection limits in Table 1. A 50 uL loop is
normally sufficient for higher concentrations. The sample volume used
to had the injection loop should be at least 10 times the loop size so
that all tubing in contact with sample is thoroughly flushed with new
sample to prevent cross contamination.
9.1.3	A calibration curve of analyte response (peak height or area) versus
analyte concentration should be constructed. The coefficient of
correlation for the curve should be 0.999 or greater.
9.2	Instrument Performance- Check the performance of the instrument and verify
the calibration using data gathered trom analyses of laboratory blanks,
calibration standards and the quality control sample.
9.2.1	After the calibration has been established, it should be verified by
analyzing the QCS (Section 7.6). If the measured value of the QCS
exceeds ± 10% of the established value, a second analysis should be
performed. If the value still exceed the established value, the analysis
should be terminated until the source of the problem is identified and
corrected.
9.2.2	To verify that the instrument is properly calibrated on a continuing
basis, run an LRB and an LPC after every ten analyses. The results of
the analyses of the standards will indicate whether the calibration
remains valid. If the measured concentration of the analyte deviates
from the true concentration by more than ± 5%, the instrument must be

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recalibrated and the previous ten samples re-analyzed. The instrument
response from the calibration check may be used for recalibration
purposes.
10.0 Quality Control
10.1	Each laboratory using this method is required to operate a formal Quality
Control (QC) program. The minimum requirements of this program consist of
an initial demonstration of laboratory capability, and the analysis of laboratory
reagent blanks, fortified blanks and samples as a continuing check on
performance. The laboratory is required to maintain performance records that
define the quality of the data thus generated.
10.2	Initial Demonstration of Performance
10.2.1	The initial demonstration of performance is used to characterize
instrument performance (method detection limits and linear calibration
ranges) for analyses conducted by this method.
10.2.2	Method Detection Limit (MDL)- Method Detection Limit should be
established using reagent water (blank) fortified at a concentration of
two to five times the estimated detection limit. To determine the MDL
value, take seven replicate aliquots of the fortified reagent water and
process through the entire analytical method. Perform all calculations
defined in the method and report the concentratbn values in the
appropriate units. Calculate the MDL as follows:
MDL = (t) x (s)
where:
t = students' t value for a 99% confidence level and a standard
deviation estimate with n-1 degrees of freedom [t = 3.143 for seven
replicates].
s = standard deviation of the replicate analyses.
10.2.3	Linear dynamic range- linear dynamic ranges are governed by Beer's
Law. A set of at least five standards covering the estimated linear range
should be prepared fresh from the stock solution and one analysis of
each performed. A log vs. log plot of peak height vs. analyte
concentration having a slope between 0.98 and 1.02 will indicate
linearity (7). The linear dynamic range for this method covered four
orders of magnitude (1 Hg/L to 10,000 Hg/L) when peak height was
used.
10.3	Assessing Laboratory Performance Reagent and Fortified Blanks
10.3.1 Laboratory Reagent Blank (LRB)- the laboratory must analyze at least
one reagent blank (Section 7.4) with each set of samples. Reagent blank
data are used to assess contamination from a laboratory environment. If
the Cr(VI) value in the reagent blank exceeds the determined MDL,
then laboratory or reagent contaminatbn should be suspected. Any
determined source of contaminatbn should be corrected and the
samples re-analyzed.

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10.3.2	Laboratory Fortified Biank (LFB)- the laboratory must analyze at least
one fortified blank (Section 7.5) with each set of samples. Calculate
accuracy as percent recovery (see 10.4.2). If the recovery of Cr(VI) falls
outside the control limits (seelO.3.3), then the procedure is judged out
of control, and the source of the problem should be identified and
resolved before continuing the analysis.
10.3.3	Until sufficient data become available (usually a minimum of twenty to
thirty analyses), the laboratory should assess laboratory performance
against recovery limits of 90-110%. When sufficient internal
performance data becomes available, develop control limits from the
percent mean recovery (x) and the standard deviation (s) of the mean
recovery. These data are used to establish upper and lower control
limits as follows:
Upper Control Limit = x + 3s
Lower Control Limit = x - 3s
10.4 Assessing Analyte Recovery- Laboratory Fortified Sample Matrix
10.4.1	The laboratory must add a known amount of Cr(VI) to a minimum of
10% of the routine samples. The concentration level can be the same as
that of the laboratory fortified blank (Section 7.5) for liquid samples.
10.4.2	Calculate the percent recovery for Cr(VI) corrected for background
concentration measured in the unfortified sample, and compare this
value to the control limits established in SectionlO.3.3 for the analysis of
LFBs. Fortified recovery calculations are not required if the fortified
concentration is less than 10% of the sample background concentration.
Percent recovery may be calculated in units appropriate to the matrix,
using the following equation:
(CF - C) x 100
R. - 	
F
Where:
R = percent recovery.
CF = fortified sample concentration.
C = sample baekground concentration.
F = concentration equivalent of Cr(VI) added to sample.
10.4.3 If the reeovery of Cr(VI) falls outside control limits, while the reeovery
obtained for the LFB was shown to be in control (Section 10.3), the
recovery problem encountered with the fortified sample is judged to be
matrix related, not system related. The result for Cr(VI) in the
unfortified sample must be labelled "suspect matrix".
10.5 Quality Control Sample (QCS)- Each quarter, the laboratory should analyze
one or more QCS (if available). If criteria provided with the QCS are not met,
corrective action should be taken and documented.

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11.0 Procedure
11.1	Sample Preparation Filtered, pH adjusted samples at 4°C should be brought to
ambient temperature prior to analysis.
11.2	Initiate instrument operating configuration and calibrate (Section 9).
11.3	Draw into a new, unused syringe (6.2.3) approximately 3 mL of sample and
attach a syringe filter to the syringe. Discard 0.5 mL through the filter and load
10X the sample loop volume. Samples having concentrations higher than the
established linear dynamic range should be diluted into the calibration range
and re-analyzed.
12.0 Calculations
12.1	From the calibration curve the concentration of the sample can be determined.
For the above procedure, if there is no dilution, the concentration of the
sample should be reported as Hg/L. Data should be corrected for any Cr(VI)
contamination found in reagent blanks.
12.2	The QC data obtained during the analyses provide an indication of the quality
of the sample data and should be provided with the sample results.
13.0 Precision and Accuracy
13.1	Instrument operating conditions used for single laboratory testing of the
method are summarized in Table 2. Dissolved Cr(VI) method detection limits
determined using the procedure in 10.2.2 are listed in Table 1.
13.2	Data obtained from single laboratory testing of the method are summarized in
Table 3 for five water samples representing drinking water, deionized water,
groundwater, treated municipal sewage wastewater and treated electroplating
wastewater. Samples were fortified with 100 and 1000 Hg/L of Cr(VI) and
recoveries determine (Section 10.4.2).
14.0 References
1.	Glaser, J.A., Foerst, D.L., McKee, G.D., Quave, S.A. and Budde, W.L., "Trace
Analyses for Wastewaters," Environmental Science and Technology, Vol. 15,
No. 12,1981, PP. 14261435.
2.	Bartlett, R. and James, B., "Behavior of Chromium in Soils: ill. Oxidation," J.
Environ. Qual., Vol. 8, No. 1,1979, pp. 31-35.
3.	Zatka, V.J., "Speciation of Hexavalent Chromium in Welding Fumes
Interference by Air Oxidation of Chromium," Am. Ind. Hyg. Assoc.J., Vol. 46,
No. 7,1985, pp. 327-331.
4.	Pedersen, B., Thomsen, E. and Stern, R.M., "Some Problems in Sampling,
Analysis and Evaluation of Welding Fumes Containing Cr(VI)", Ann. Occup.
HYg., Vol. 31, No. 3,1987, pp.325-338.
5.	Messman, J.D., Churchwell, M.E., et. al. Determination of Stable Valence States
of Chromium in Aqueous and Solid Waste Matrices - Experimental Verification
of Chemical Behavior. EPA/600/S4-86/039, U.S. Environmental Protection
Agency, Cincinnati, Ohio, 1987, 112 pp.
6.	Dionex Technical Note No. 26, May 1990.

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7.	"Proposed OSHA Safety and Health Standards, Laboratories," Occupational
Safety and Health Administratbn, Federal Register, July 24, 1986.
8.	"OSHA Safety and Health Standards, General Industry," (29 CFR 1910),
Occupational Safety and Health Administration, OSHA 2206, revised January
1976.
9.	Johnson, D.C., Anal. Chim. Acta, Vol. 204, No. 1 1988.
Table 1. Method Detection Limit for Cr(VI)
Cone, used to compute MDL Method Detection Limit (a)
Matrix Type	(pg/L)	(pg/L)
Reagent Water	1	0.4
Drinking Water	2	0.3
Ground Water	2	0.3
Primary Sewage Wastewater	2	0.3
ElectroplatingWastewater	2	0.8
(a) MDL concentrations are computed for final analysis solution (Section 11.2).
Table 2. Ion Chromatographic Conditions
Columns:	Guard Column - Dionex IonPac NG1
Separator Column - Dionex IonPac AS7
Eluent:
250 mM (NH4)2S04
100 mM NH4OH
Flow rate - 1.5 mL/min
Post-Column
Reagent:
Detector:
2 mM Diphenylcarbohydrazide
10% v/v CH3OH
1 N H2S04
Flow rate - 0.5 mL/min
Visible 530 nm
Retention Time: 3.8 min.

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Table 3. Single- Laboratory Precision and Accuracy
Percent Mean
Sample Type	Cr(VI) ^ Recovery RPD ^
(l-ig/L)
Reagent Water
100
100
0.8

1000
100
0.0
Drinking Water
100
105
6.7

1000
98
1.5
Ground Water
100
98
0.0

1000
96
0.8
Primary Sewage Wastewater
100
100
0.7

1000
104
2.7
Electroplating Wastewater
100
99
0.4

1000
101
0.4
(a)	Sample fortified at this concentration level.
(b)	RPD - relative percent difference between duplicates.

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