A New Cation-Exchange Method for 

Accurate Field Speciation of Hexavalent 

Chromium 

Water-Resources Investigations Report 03-4018 

Prepared in cooperation with the 

California Environmental Protection Agency, 

United States Marine Corps MCAGCC Twentynine Palms, and 

Edwards Air Force Base 

U.S. Department of the Interior 

U.S. Geological Survey 
A New Cation-Exchange Method for Accurate Field Speciation of 

Hexavalent Chromium 

By James W. Ball and R. Blaine McCleskey

U.S. Geological Survey 

Water-Resources Investigations Report 03-4018 

Prepared in cooperation with the 

California Environmental Protection Agency, 

United States Marine Corps MCAGCC Twentynine Palms, and 

Edwards Air Force Base 

Front Cover Photography: Sampling a USGS monitoring well at 29 Palms Marine Base, May 2001 

Boulder, Colorado 

2003 
U.S. DEPARTMENT OF THE INTERIOR 

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY 

Charles G. Groat Director 

The use of firm, trade, and brand names in this report is for identification purposes only and does 

not constitute endorsement by the U.S. Geological Survey. 

For additional information write to: Copies of this report can be purchased from: 

Chief, Branch of Regional Research U.S. Geological Survey 

U.S. Geological Survey Branch of Information Services 

Denver Federal Center Denver Federal Center, 

Box 25046, MS-418 Box 25286 

Denver, Colorado 80225 Denver, Colorado 80225-0286 
iii

CONTENTS

Abstract........................................................................................................................................... 1 

Introduction..................................................................................................................................... 1 

Analytical Method .......................................................................................................................... 2 

Apparatus and Supplies .......................................................................................................... 2 

Field Procedure....................................................................................................................... 4 

Method Performance....................................................................................................................... 5 

Comparison of Methods ......................................................................................................... 5

Limitations of the Cation-Exchange Method ......................................................................... 6 

Accuracy of the Cation-Exchange Method: Standards and Spike Recoveries....................... 6 

Comparison of Cation-Exchange Method Results with Results from Alternative 

Methods ...................................................................................................................... 8 

pH Dependence of the Cation-Exchange Separation ........................................................... 12 

Effects of Potential Interfering Species on the Cation-Exchange and USEPA Methods..... 12 

Summary ....................................................................................................................................... 16 

References..................................................................................................................................... 17 

FIGURES

Figure 1. Hexavalent chromium sampling supplies........................................................................ 3 

Figure 2. Hexavalent chromium field separation............................................................................ 4 

Figure 3. Cr(VI) determinations by the modified Hach DPC direct colorimetric method 

and by the cation-exchange method......................................................................................... 9 

Figure 4. Cr(VI) determinations by USEPA method 218.6 and by the cation-exchange 

method.................................................................................................................................... 10 

Figure 5. pH dependence of Cr(VI) separation from synthetic solutions containing 20 

micrograms per liter each of Cr(III) and Cr(VI) .................................................................... 12 

Figure 6. Recovery of 20 micrograms per liter Cr(VI) from solutions containing 0.1 to 10 

milligrams per liter Cr(III). .................................................................................................... 14 

Figure 7. Recovery of 20 micrograms per liter Cr(VI) from solutions containing 20 

micrograms per liter Cr(III) and 10 to 1000 milligrams per liter Na, Ca, or Mg................... 14 

Figure 8. Recovery of 20 micrograms per liter Cr(VI) from solutions containing 20 

micrograms per liter Cr(III) and 1 to 100 milligrams per liter Ba. ........................................ 15 

Figure 9. Recovery of 20 micrograms per liter Cr(VI) from solutions containing 20 

micrograms per liter Cr(III) and 0.1 to 100 milligrams per liter Fe(II) or Fe(III). ................ 15 

TABLES

Table 1. Recovery or retention from cation-exchanged Cr(III) and Cr(VI) standard 

solutions ................................................................................................................................... 7 

Table 2. Results of analyses of four Cr-containing samples, each spiked with three 

concentrations of Cr(III) .......................................................................................................... 8 

Table 3. Comparative Cr(VI) determinations in waters from around the world. ......................... 11 
iv 

 

 Explanation of abbreviations

Cr (chromium) 

DPC (diphenylcarbazide) 

GFAAS (graphite furnace atomic absorption spectrometry) 

IC (ion chromatography) 

ICP-MS (inductively-coupled plasma-mass spectrometry) 

μg/L (micrograms per liter) 

μm (micrometer) 

μS/cm (microSiemens per centimeter) 

mg/L (milligrams per liter) 

mL (milliliter) 

mm (millimeter) 

USEPA (U.S. Environmental Protection Agency) 

XANES (x-ray absorption near-edge structure spectrometry) 
1

A New Cation-Exchange Method for Accurate Field Speciation of 

Hexavalent Chromium 

By James W. Ball and R. Blaine McCleskey 

Abstract 

A new cation-exchange method for field speciation of Cr(VI) has been developed to meet 

present stringent regulatory standards and to overcome the limitations of existing methods. The new 

method allows measurement of Cr(VI) concentrations as low as 0.05 micrograms per liter, storage of 

samples for at least several weeks prior to analysis, and use of readily available analytical 

instrumentation. The sensitivity, accuracy, and precision of the determination in waters over the pH 

range of 2 to 11 and Fe concentrations up to 1 milligram per liter are equal to or better than existing 

methods such as USEPA method 218.6. Time stability of preserved samples is a significant 

advantage over the 24-hour time constraint specified for USEPA method 218.6. 

INTRODUCTION 

Chromium (Cr) is usually present in surface and ground water in either the trivalent [Cr(III)] 

or the hexavalent [Cr(VI)] oxidation state. Since Cr(III) is considered an essential nutrient and 

Cr(VI) is considered a toxin and a carcinogen (Daugherty, 1992), it is important to distinguish 

between oxidation states of Cr. Chromium(VI) concentration limits for drinking water now being 

considered by the California Environmental Protection Agency present analysts with the need for 

quantitation limits in the sub-μg/L range. Existing methods for accurately determining Cr(VI) in 

natural waters have one or more of the following weaknesses: (1) poor sensitivity, (2) timeinstability of the Cr redox species, or (3) necessity for expensive instrumentation that is not 

commonly available. 

Concentrations of the redox states of chemical species dissolved in water are inherently 

unstable and subject to change by re-equilibration by reaction with other dissolved components, a 

coexisting solid phase, if present, or the atmosphere. Thus, concentrations of the respective redox 

states must be preserved in their initial state, determined at the time of collection, or separated from 

each other and preserved for later analysis. There are many methods available for determining redox 

species of Cr, including flow injection chemiluminescence (Escobar and others, 1995), x-ray 

absorption near-edge structure spectrometry (XANES; Bajt and others, 1993), ion chromatography 

(IC; Arar and others, 1991; Arar and Pfaff, 1991), separation and preconcentration of the Cr(VI)-

diphenylcarbazide (DPC) complex by ion-exchange (Frenzel, 1998) or by using surfactant-coated 

alumina (Manzoori and others, 1996), and direct measurement of the Cr(VI)-DPC complex (Hach 

Company, 1992). 

One of the most commonly used methods is U.S. Environmental Protection Agency 

(USEPA) method 218.6 (Arar and others, 1991; Arar and Pfaff, 1991). This method consists of 

injecting a sample into the sample loop of an ion chromatograph and using (NH4)2SO4/NH4OH 
2 

eluent solution to pump it through an anion-exchange column where the Cr(VI) is concentrated and 

separated from potential interferents. The Cr(VI) is then eluted from the resin, mixed with DPC 

color reagent, and passed through a flow-through absorbance detector. The absorbance of the 

solution is proportional to its Cr(VI) concentration. This method specifies that samples be preserved 

and adjusted to a pH of 9 to 10 with an (NH4)2SO4/NH4OH buffer solution. More importantly, 

USEPA method 218.6 requires that samples be analyzed within 24 hours of collection to ensure that 

the distribution of dissolved Cr redox species remains unchanged, perhaps as a result of reequilibration of the sample with the gas phase. 

Another popular method is direct colorimetric determination of Cr(VI) using DPC as the 

color reagent (Hach Company, 1992). This method is less sensitive than USEPA method 218.6, 

having a quantitation limit of ≥10 μg/L when used with a battery-powered field spectrophotometer 

and a 2.5-cm measuring cell, but is applicable as a field screening tool for detection of elevated 

Cr(VI) concentrations. In addition, colored solutions caused by the presence of dissolved organic 

compounds in natural waters, for example, constitute a potential source of positive interference. 

Other potential sources of interference include extreme sample pH or buffer capacity and elevated 

concentrations of other transition metals. 

To address the need to overcome limitations of existing methods, the U.S. Geological 

Survey, in cooperation with the California Environmental Protection Agency, Department of Toxic 

Substances Control; the United States Marine Corps MCAGCC at Twentynine Palms, CA; and 

Edwards Air Force Base, developed and field tested a new cation-exchange method for fieldspeciation of Cr(VI). 

This report describes the application of existing cation-exchange cartridge technology to the 

implementation of a new field method for collecting and preserving Cr(VI) dissolved in water. The 

method is simple, sensitive, accurate, and precise, the equipment needed is inexpensive and readily 

available, and analysis of samples for the Cr(VI) content can be achieved using commonly available 

instrumentation. The method is compared with the DPC direct colorimetric method and USEPA 

method 218.6. 

The authors are pleased to acknowledge the cooperation and financial support of the 

California Environmental Protection Agency, Division of Toxic Substances Control, the U.S. Marine 

Corps MCAGCC at Twentynine Palms, California, and Edwards Air Force Base, California, and the 

Joshua Basin Water District. The authors also thank John Izbicki and Clinton Church of the USGS 

San Diego District Office for their collaboration and assistance. We also wish to thank John Izbicki 

and Randy Hanson of the USGS for colleague reviews of this manuscript. 

ANALYTICAL METHOD 

Apparatus and Supplies 

Cation-exchange cartridges (IC-H, Alltech), disposable syringes, and suitable containers 

(centrifuge tubes) of size appropriate to process and hold sample volumes consistent with the 

laboratory analytical technique are required. Syringe filters (25-mm, 0.2-μm) are needed for 
3

unfiltered samples. Separated (speciated) samples are preserved with concentrated hydrochloric acid 

(HCl) or nitric acid (HNO3), American Chemical Society Reagent grade, for later measurement in 

the laboratory. The sampling supplies are shown in figure 1. An elemental analysis instrument 

capable of determining total Cr at concentrations in the range sought is needed for the measurement 

step. The new cation-exchange method was developed for use with the Zeeman-corrected graphite 

furnace atomic absorption spectrometry (GFAAS) analytical technique, with a detection limit of 

approximately 0.05 μg/L using end-capped transversely-heated graphite tubes. Consequently, 

sample volumes of 2 mL were collected using a 10-mL disposable syringe and stored in 2-mL 

centrifuge tubes. If a different analytical technique were used, such as inductively coupled plasmamass spectrometry (ICP-MS) with a conventional nebulizer, larger sample volumes would be 

required, necessitating use of larger syringes and storage containers, and possibly larger cationexchange cartridges. 

Figure 1. Hexavalent chromium sampling supplies. 
4 

Field Procedure 

 Water samples collected in the field usually are near redox equilibrium with their immediate 

surroundings. This condition may change significantly after samples are collected. Examples of 

waters for which this may occur are deep ground water, geothermal water issuing from flowing hot 

springs, or interstitial lake bottom water. If the potential exists for re-equilibration of dissolved redox 

species as a result of interaction with post-collection conditions, such as exposure to atmospheric air, 

over a time frame shorter than that required for separating or preserving the Cr species, chemical 

changes in the species of interest should be tested for or prevented using appropriate measures. 

A photograph of the separation procedure is shown in figure 2. Aspirate about 8 mL sample 

into a 10-mL disposable plastic syringe. If the sample has not been filtered, use a syringe filter. 

Invert the assembly and purge of all air. Collect a sample for Cr(total) by filling the green 2-mL 

centrifuge tube. Attach an IC-H cartridge to the syringe/filter assembly. At no more than two drops 

per second, force about 3 mL sample through the assembly to waste. Again at no more than two 

drops per second, force about 2 mL sample through the assembly into the orange centrifuge tube. 

The sample issuing from the cation-exchange cartridge contains only Cr(VI), and because Cr(total) 

is determined in the laboratory measurement step, the oxidation state of Cr is no longer critical. 

Preserve the samples by acidifying to pH less than 2 with about 0.01 mL HCl or HNO3. If necessary, 

the IC-H cartridge can be cleaned for reuse by passing 3 mL 2M HCl followed by 5 mL doubledistilled or deionized water through it. 

Figure 2. Hexavalent chromium field separation. 
5

METHOD PERFORMANCE 

Comparison of Methods 

Methods for determination of Cr require that the steps taken to collect and preserve samples 

are compatible with the analytical technique that will be used in the field or later in the laboratory. 

Three methods were compared for this study: DPC direct colorimetric method with analysis by 

spectrophotometry, USEPA method 218.6 with separation of the Cr(VI) species by ion 

chromatography and analysis by spectrophotometry, and the new cation-exchange method with 

analysis by Zeeman-corrected GFAAS. They differ in detection limits, potential for interferences, 

availability of analytical equipment, and time-stability of the preserved samples. 

The field DPC direct colorimetric method makes use of a portable battery-powered 

spectrophotometer and a 2.5-cm measuring cell. The cell is filled with sample, and then a packet of 

Hach ChromaVer3 powdered reagent is added. After color development, the absorbance of the 

solution is read and the Cr(VI) concentration is displayed in milligrams per liter. The two most 

important limitations of this method are its higher detection limit compared with other methods and 

the potential for interferences by unknown solution components. 

The most commonly used method is USEPA method 218.6 (Arar and others, 1991). This 

method specifies that samples be preserved in the field and adjusted to a pH of 9 to 10 with an 

(NH4)2SO4/NH4OH buffer solution to ensure that the distribution of the dissolved Cr redox species 

remains unchanged. This method calls for samples to be analyzed in the laboratory by ion 

chromatography with absorbance detection within 24 hours of collection. The ion chromatograph 

equipped with an absorbance detector and a small-volume sample cell is specialized instrumentation 

costing several tens of thousands of dollars. Older ion chromatographs cannot be retrofitted. 

In contrast to the other two methods, the new cation-exchange method is not as timesensitive. Because only Cr(VI) remains in the sample after the cation-exchange separation and 

Cr(total) is determined in the analytical step, samples can be analyzed up to several weeks after 

collection, but care is needed to prevent contamination and ensure that a clean separation is 

achieved. Strong acid cation-exchange resin (AG50W-X8) will retain Cr(III), whereas Cr(VI) will 

pass into the effluent (Johnson, 1990). Sample pH is not critical, but must be between 2 and 11 so 

that all dissolved Cr(VI) is present as either the negatively-charged hydrogen chromate ( − HCrO4(aq) ) 

or chromate ( 2- CrO4(aq) ) ions and all dissolved Cr(III) is present as one or more of the positivelycharged 3+ Cr(aq) , chromic monohydroxide ( 2+ CrOH(aq) ), or chromic dihydroxide ( + Cr(OH)2(aq) ) ions. 

Although the selectivity of the AG50W-X8 resin for 3+ Cr(aq) is very high, the flow rate through the 

cartridge should be no greater than two drops per second so that Cr(III) species will be quantitatively 

retained. 

 The USEPA method 218.6 and the new cation-exchange method also differ in the availability of 

laboratory instrumentation. The ion chromatograph equipped with an absorbance detector and a 

small-volume sample cell is specialized instrumentation costing several tens of thousands of dollars. 
6 

Older ion chromatographs cannot be retrofitted. The graphite furnace atomic absorption 

spectrometer (GFAAS), although similar in cost, is commonly available. Thus, typically no new 

instrument purchase is necessary to implement the analytical step of the new cation-exchange 

method. 

Limitations of the Cation-Exchange Method 

In the field, when processing water samples through the ion-exchange cartridges, the 

capacity of the resin must be considered. Samples of high ionic strength or high Cr(III) 

concentration may displace 3+ Cr(aq) from the cartridge and cause erroneously high results for Cr(VI). 

In such cases larger amounts of cation exchange resin must be used, and the volume of sample 

passed through the cartridge must be minimized. Cartridges are available in several sizes, and may 

be connected in series to achieve increased capacity. Any modifications made to the described 

conditions should be tested and verified before use. 

Waters high in dissolved organic carbon may complex Cr(III) and prevent its retention by the 

cation-exchange resin (Icopini and Long, 2002). If waters to be tested are known or suspected to 

contain dissolved organic carbon at concentrations exceeding 5-10 mg/L this should be tested by 

adding known concentrations of Cr(III) standard to the test samples. Higher than expected apparent 

Cr(VI) concentrations are an indication that Cr(III) is not being retained by the resin. 

Accuracy of the Cation-Exchange Method: Standards and Spike Recoveries 

Demonstrating accuracy of the cation-exchange separation is challenging because no 

reference standards exist for redox species, and low concentrations of prepared standards may be 

difficult to maintain in their initial redox state. A series of standard solutions of single redox state 

was prepared in the laboratory, and Cr redox species were separated using the new method. The 

results, presented in table 1, demonstrate that recoveries appear to have considerable variability for 

these solutions, and that the average recovery over the concentration range tested was about 91 

percent. The recoveries were divided by the ratio of Cr found to Cr added for solution 1, which was 

not cation-exchanged, to obtain the recoveries listed in column 5. Retention of Cr(III) was excellent, 

with an average Cr(III) retention of about 99.8 percent.
7

Table 1. Recovery or retention from cation-exchanged Cr(III) and Cr(VI) standard solutions 

[Solution ID, solution identification; μg/L, micrograms per liter] 

 Concentration (μg/L) 

Recovery1or 

Retention2

No. Solution ID Cr(VI) Added Cr Found Percent 

1 Cr(VI)-10.0 μg/L3

 10.0 9.2 100.0 

2 Cr(VI)-0.1 μg/L 0.10 0.12 126 

3 Cr(VI)-0.2 μg/L 0.20 0.15 80.9 

4 Cr(VI)-0.5 μg/L 0.50 0.34 74.0 

5 Cr(VI)-1.0 μg/L 1.0 0.85 92.5 

6 Cr(VI)-2.5 μg/L 2.5 2.1 89.1 

7 Cr(VI)-5.0 μg/L 5.0 3.9 85.5 

8 Cr(VI)-10.0 μg/L 10.0 8.4 90.9 

9 Cr(III)- 2.5 μg/L 0.000 <0.05 100.0 

10 Cr(III)- 5.0 μg/L 0.000 <0.05 100.0 

11 Cr(III)- 10.0 μg/L 0.000 0.07 99.3 

1

Recoveries, calculated for solutions 1-8, are adjusted for measured concentration of Cr standard 

(solution 1) by dividing recovery by the ratio of Cr found to Cr added. 

2

Retentions are calculated for solutions 9-11. 

3

Solution 1 was not cation-exchanged. 

 Four Cr-containing samples from the United Arab Emirates, North Carolina, Arizona, and 

the state of Washington were prepared for analysis by spiking them in the laboratory with 10, 20, or 

30 μg/L Cr(III). Each solution was then divided into two subsamples. One set of subsamples was 

separated by the cation-exchange method and analyzed by GFAAS. The second set of subsamples 

was preserved, and then analyzed by ion chromatography according to USEPA method 218.6. 

Results, shown in table 2 and figure 4, demonstrate that even for this diverse set of samples the two 

preservation and analysis methods have virtually equivalent capabilities to distinguish between 

Cr(VI) and Cr(III). The average recovery of Cr(VI) by USEPA method 218.6 was 98.3 percent, 

whereas the average recovery by the cation-exchange method analyzed by GFAAS was 98.0 percent. 

The anomalous result for sample UAE-30 from the cation-exchange method was not included in the 

recovery calculations. This sample is saline water with a specific conductance of more than 55,000 

μS/cm for which procedural modifications may be needed to achieve better cation-exchange 

separation of the Cr redox species. 
8 

Table 2. Results of analyses of four Cr-containing samples, each spiked with three concentrations of 

Cr(III) 

[μg/L, micrograms per liter; USEPA 218.6, U.S. Environmental Protection Agency method 218.6; 

CE/GFAAS, cation-exchange method analyzed by graphite furnace atomic absorption spectrometry] 

 Cr(III) Cr(III)+ - - - - - - - - - - - - - - - Cr(VI), μg/L - - - - - - - - - - - - - - -

 Added Cr(VI) USEPA 218.6 CE/GFAAS 

Sample Location (μg/L) (μg/L) Expected Found Recovery 

(Percent) Found Recovery (Percent) 

United Arab 

Emirates 10 19.2 9.2 10.1 110 9.5 104 

United Arab 

Emirates 20 29.2 9.2 9.9 107 9.9 108 

United Arab 

Emirates 30 39.2 9.2 9.4 102 1

3.7 40.4 

North Carolina 10 14.8 4.8 4.6 96.3 4.9 101 

North Carolina 20 24.8 4.8 4.3 90.1 4.7 97.8 

North Carolina 30 34.8 4.8 4.4 92.7 4.8 99.0 

Arizona 10 18.1 8.1 8.3 103 7.8 96.6 

Arizona 20 28.1 8.1 7.7 94.8 8.1 99.8 

Arizona 30 38.1 8.1 7.7 95.2 7.7 94.9 

Washington 10 19.7 9.7 9.3 96.1 8.6 89.0 

Washington 20 29.7 9.7 9.3 96.5 9.1 93.9 

Washington 30 39.7 9.7 9.3 96.0 9.1 94.4 

1

This result was not included in average recovery calculations. 

Comparison of Cation-Exchange Method Results with Results from Alternative Methods 

The Hach Company (1992) DPC direct colorimetric method was modified by using a diodearray spectrophotometer and a 5-cm measuring cell to determine Cr(VI) in a mobile laboratory truck 

at a Mojave Desert field site in Califormia. This resulted in improvement of the detection limit from 

about 10 μg/L to about 0.3 μg/L. In June 2002, more than 100 ground water samples from the 

Mojave Desert field site were analyzed for Cr(VI) using the modified Hach Company DPC direct 

colorimetric method and the new cation-exchange method with analysis by GFAAS. The results, 

shown in figure 3, demonstrate excellent comparability between the two methods indicating that, to 

the extent the Hach DPC method is accurate, the new cation-exchange method is at least as accurate 

and, owing to its 10-fold lower detection limit, is more sensitive. 
9

Figure 3. Cr(VI) determinations by the modified Hach DPC direct colorimetric method and by the 

cation-exchange method.

Many samples from diverse sources were analyzed using the cation-exchange method and 

USEPA method 218.6 (Arar and others, 1991). The results (fig. 4, table 3) illustrate that, for these 

samples and over the Cr(VI) range covered, the new cation-exchange method produces Cr(VI) 

concentrations essentially identical to USEPA method 218.6. The large percentage differences in 

table 3 are for samples having low Cr concentrations; thus, actual differences in the Cr(VI) 

concentrations are small. 
10 

Figure 4. Cr(VI) determinations by USEPA method 218.6 and by the cation-exchange method. 
11

Table 3. Comparative Cr(VI) determinations in waters from around the world. 

[μg/L, micrograms per liter; Cation-exchange, cation-exchange method with analysis by graphite furnace 

atomic absorption spectrometry; USEPA 218.6, U.S. Environmental Protection Agency method 218.6; 

---, not calculated] 

Sample Cr(VI) (μg/L) Difference

Description Cation-exchange USEPA 218.6 (percent) 

United Arab Emirates well LWS-1 100 110 -9.5 

United Arab Emirates well LWS-15 170 180 -5.7 

United Arab Emirates well LWS-18 140 140 0.0 

United Arab Emirates well QWS-37 230 160 36 

United Arab Emirates well LWS-2 130 120 8.0 

United Arab Emirates well LWS-16 140 140 0.0 

Maqarin, Jordan ground water 6600 6800 -3.0 

Elizabeth City, North Carolina ground water 2700 2400 11.8 

Ranegras, Arizona well RAN-10 69 69 0.0 

Hughes Missile Systems, Arizona well E-12 39 42 -7.4 

Hanford, Washington well 100-K 93 100 -7.3 

Pantex, Texas well DM-20 1400 1400 0.0 

El Mirage, California well 25M2 53 51 4.1 

Hinkley, California well MW15 1120 1110 1.4 

Hinkley, California well PMW6 1470 1520 -3.5 

29 Palms MCAGCC, California well 32D4 10 9.4 8.2 

29 Palms MCAGCC, California well 32D6 11 10 8.2 

29 Palms MCAGCC, California well CW2-1 0.5 0.1 117 

29 Palms MCAGCC, California well CW2-2 10 9.0 10.2 

29 Palms MCAGCC, California well CW2-3 14 13 7.8 

29 Palms MCAGCC, California well CW3-1 8.4 7.6 10.2 

29 Palms MCAGCC, California well CW3-2 13 13 -1.8 

29 Palms MCAGCC, California well CW3-3 14 14 0.0 

29 Palms MCAGCC, California well SSR-1 25 22 11.4 

29 Palms MCAGCC, California well SSR-3 18 16 13.3 

29 Palms MCAGCC, California well SSR-4 27 26 4.3 

29 Palms MCAGCC, California well SW-5 20 19 8.4 

29 Palms MCAGCC, California well SW-9A 17 15 15.5 

29 Palms MCAGCC, California WW POND 2.8 0.5 142 

Joshua Basin Water District well 16-360' 1.9 0.4 135 

Joshua Basin Water District well 16-410' 2.0 <0.1 --- 

Joshua Basin Water District well 16-590' 1.7 <0.1 --- 

Joshua Basin Water District well 16-615' 36 34 6.0 

Joshua Basin Water District well 16-680' 37 36 3.4 

Joshua Basin Water District well 16-750' 37 35 6.3 

Joshua Basin Water District well 16-820' 35 33 8.0 

Joshua Basin Water District well 16-bulk sample 33 33 -0.2 

Joshua Basin Water District blank 0.3 <0.1 --- 
12 

pH Dependence of the Cation-Exchange Separation 

 A set of laboratory standards was prepared containing 20 μg/L each of Cr(III) and Cr(VI). 

Each solution was subsampled using the new cation-exchange method and preserved for analysis by 

GFAAS, then preserved and analyzed by USEPA method 218.6. The results, shown in figure 5, 

demonstrate that, for these simple standard solutions, the cation-exchange separation is effective 

over the range pH 2 to pH 11. 

Figure 5. pH dependence of Cr(VI) separation from synthetic solutions containing 20 micrograms 

per liter each of Cr(III) and Cr(VI) 

Effects of Potential Interfering Species on the Cation-Exchange and USEPA Methods 

 The effects of large concentrations of Cr(III), three major cations (Na, Ca, and Mg), one alkaline earth 

cation with a relatively high selectivity (Ba), Fe(II), and Fe(III) were tested by spiking solutions containing 20 

μg/L each of Cr(III) and Cr(VI). The results, shown in figures 6-9, illustrate that (1) Cr(III) concentrations at 

least 50 times that of the Cr(VI) concentration can be separated from simple solutions containing Cr(III) and 
13

Cr(VI) (fig. 6); (2) Na, Ca, and Mg at concentrations as large as 1000 mg/L and Ba at concentrations as large 

as 100 mg/L have a negligible effect on the separation of Cr(III) and Cr(VI) (figs. 7 and 8); and (3) 

concentrations of Fe(II) or Fe(III) greater than 50 times those of Cr(VI) dramatically affect the recovery of 

Cr(VI) (fig. 9). 

 The Cr(III) stock solution used was not specified as containing only Cr(III). This solution contained 

approximately 0.38 μg/L Cr(VI) per 1 mg/L Cr(III), or about 0.038 percent Cr(VI). The results, corrected for 

the Cr(VI) content of the stock solution and shown in fig. 6, indicate potential for Cr(III) to pass through the 

cation-exchange cartridge when the Cr(III) / Cr(VI) concentration ratio is 500. While the source of this 

problem is not known, it is most likely a function of liquid flow within the cartridges, whereby there is 

insufficient contact of the solution with the cation-exchange resin. The maximum concentration of Cr(III) 

passing through the cartridge is about 8 μg/L in the presence of 10,000 μg/L Cr(III). This constitutes at worst 

better than 99.9 percent retention of Cr(III), and is consistent with the results of the experiment described 

earlier in this report (table 1) using Cr(III) concentrations three orders of magnitude smaller. For solutions 

with Cr(VI) concentrations less than 10 μg/L and Cr(III) / Cr(VI) concentration ratios greater than 500, the 

error could approach 100 percent. If a solution having these Cr(III) and Cr(VI) concentrations is identified, 

multiple replicates should be separated using larger cation-exchange cartridges. 

 In the case of Na, Ca, Mg (fig. 7), and Ba (fig. 8), little effect on recoveries is seen using either the new 

cation-exchange method or USEPA method 218.6. These results demonstrate that the new method and 

USEPA method 218.6 are equivalent in their ability to accurately determine Cr(VI) in the presence of large 

excesses of these major cations. 

 In the case of Fe(II), USEPA method 218.6 appears to provide accurate recoveries at an excess of at 

least 500-fold [10 mg/L Fe(II) in solution with 20 μg/L each of Cr(III) and Cr(VI)], whereas the new cationexchange method yields less than 50 percent recovery at these concentrations. The Fe(II) most likely reduced 

Cr(VI) to Cr(III) before the cation-exchange separation could be completed. USEPA method 218.6 does not 

produce accurate recoveries at a 5-fold excess concentration of Fe(III) whereas the new cation-exchange 

method produces accurate recoveries at up to a 500-fold Fe(III) concentration excess. Because of the necessity 

in USEPA method 218.6 to adjust the sample pH to values above 9, Fe(III) is most likely hydrolyzing and 

coprecipitating with or adsorbing Cr. At 100 mg/L in the case of USEPA method 218.6, both Fe(II) and 

Fe(III) hydrolyze during pH adjustment, removing virtually all Cr from the solutions. The new cationexchange method also performs less effectively at Fe(II) and Fe(III) concentrations of 100 mg/L.
14 

Figure 6. Recovery of 20 micrograms per liter Cr(VI) from solutions containing 0.1 to 10 milligrams 

per liter Cr(III). 

Figure 7. Recovery of 20 micrograms per liter Cr(VI) from solutions containing 20 micrograms per 

liter Cr(III) and 10 to 1000 milligrams per liter Na, Ca, or Mg. 
15

Figure 8. Recovery of 20 micrograms per liter Cr(VI) from solutions containing 20 micrograms per 

liter Cr(III) and 1 to 100 milligrams per liter Ba. 

Figure 9. Recovery of 20 micrograms per liter Cr(VI) from solutions containing 20 micrograms per 

liter Cr(III) and 0.1 to 100 milligrams per liter Fe(II) or Fe(III). 
16 

SUMMARY 

 A new cation-exchange method for field speciation of Cr(VI) for later determination of its 

concentration has been developed. The method is simple, rapid, portable, and accurate, and makes 

use of readily available, inexpensive materials. Determination of Cr(VI) concentrations can be done 

later in the laboratory using any elemental analysis instrument that is sufficiently sensitive to 

measure the Cr(VI) concentrations of interest. Chromium(VI) can be separated from Cr(III) at 

Cr(VI)/Cr(III) concentration ratios as low as 0.001 and between pH 2 and pH 11. The new method, 

using analysis by graphite furnace atomic absorption spectrometry, has demonstrated excellent 

comparability with two commonly used methods, the Hach Company direct colorimetric method and 

USEPA method 218.6 using the ion chromatographic/colorimetric analytical technique. The new 

method is superior to the Hach direct colorimetric method owing to its relative simplicity and 

sensitivity, and to USEPA method 218.6 because significantly increased sample holding times after 

collection are allowed. The determination is superior to USEPA method 218.6 in the presence of 

Fe(II) concentrations up to 1 mg/L and Fe(III) concentrations up to 10 mg/L. 
17

REFERENCES 

Arar, E.J., Long, S.E., and Pfaff, J.D., 1991, Determination of dissolved hexavalent chromium in 

drinking water, groundwater and industrial wastewater effluents by ion chromatography: 

U.S. Environmental Protection Agency, Method 218.6, revision 3.0. 

Arar, E.J., and Pfaff, J.D. 1991, Determination of dissolved hexavalent chromium in industrial 

wastewater effluents by ion chromatography and post-column derivatization with 

diphenylcarbazide: Journal of Chromatography, v. 546, p. 335-340. 

Bajt, S., Clark, S.B., Sutton, S.R., Rivers, M.L., and Smith, J.V., 1993, Synchrotron x-ray 

microprobe determination of chromate content using x-ray absorption near-edge structure: 

Analytical Chemistry, v. 65, p. 1800-1804. 

Daugherty, M. L., 1992, Toxicity Summary for Chromium: Oak Ridge, Oak Ridge National 

Laboratory Chemical Hazard Evaluation and Communication Group, no. 26. 

Escobar, R., Lin, Q., Guiraum, A., and de la Rosa, F.F., 1995, Determination of trivalent and 

hexavalent chromium in waste water by flow injection chemiluminescence analysis: 

International Journal of Environmental Analytical Chemistry, v. 61, p. 169-175. 

Frenzel, W., 1998, Highly sensitive semi-quantitative field test for the determination of chromium 

(VI) in aqueous samples: Fresenius Journal of Analytical Chemistry, v. 361, p. 774-779. 

Hach Company, 1992, Chromium, hexavalent, for water and wastewater, Method 8023, In DR/2000 

Spectrophotometer Procedures Manual, 7th Ed.: Loveland, CO, Hach Co., p. 113-117. 

Icopini, G.A., and Long, D.T., 2002, Speciation of aqueous chromium by use of solid phase 

extractions in the field: Environmental Science and Technology, v. 36, p. 2994-2999. 

Johnson, C.A., 1990, Rapid ion-exchange technique for the separation and preconcentration of 

chromium(VI) and chromium(III) in fresh waters: Analytica Chimica Acta, v. 238, p. 273-

278. 

Manzoori, J.L., Sorouraddin, M.H., and Shemiran, F., 1996, Preconcentration and 

spectrophotometric determination of chromium (VI) and total chromium in drinking water 

by the sorption of chromium diphenylcarbazide with surfactant coated alumina: Analytical 

Letters, v. 29, p. 2007-2014.