U.S. Department of the Interior USGS Fact Sheet FS-006-03

U.S. Geological Survey August 2003

New Cation-Exchange Method 

for Field Speciation of Hexavalent Chromium 

INTRODUCTION

Chromium (Cr) is usually present in surface and ground water in either 

the +3 or the +6 oxidation state. Because Cr(III) is considered an essential 

nutrient and Cr(VI) is considered a toxin and a carcinogen, it is important 

to distinguish between oxidation states of Cr. Hexavalent chromium 

[Cr(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-parts-per-billion (ppb or 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, (3) necessity for expensive instrumentation 

that is not commonly available. A new cation-exchange method for field 

speciation of Cr(VI) has been developed to meet the stringent regulatory 

standards and to improve upon the weaknesses of existing methods. The 

capability of the new technique compares favorably with existing methods 

such as the diphenylcarbazide direct colorimetric and EPA 218.6 ionchromatographic/colorimetric methods.

Figure 1. Hexavalent chromium field speciation 

supplies. 

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 analytical 

technique are needed. Syringe 

filters [25-millimeter (mm), 

0.2-micrometer (μm)] are 

needed for 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, and a photograph of 

the separation procedure is shown 

in figure 2. An elemental analysis 

instrument capable of determining 

total Cr at concentrations in 

the range sought is needed for 

the measurement step. The cationexchange method was developed 

for analysis using graphite furnace 

atomic absorption spectrometry 

(GFAAS), with a detection limit of 

approximately 0.05 μg/L using endcapped 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 instrument were to be 

used, such as inductively coupled 

plasma-mass spectrometry (ICP-MS) 

with a conventional nebulizer, larger 

sample volumes would be required, 

necessitating use of larger syringes 

and storage containers, and possibly 

larger cation-exchange cartridges.

FIELD SPECIATION 

PROCEDURE

Aspirate about 8 mL sample into a 

10-mL disposable plastic syringe. 

If the sample has not been filtered, 

attach a syringe filter. Invert the 

assembly and purge of all air. Collect 

a sample for Cr(total) by filling one 

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 

second 2-mL centrifuge tube. The 

sample issuing from the cationexchange 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<2 with about 10 

μL HCl or HNO3. 
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Figure 3. Recovery or retention from cation-exchanged Cr(III) and Cr(IV) standard solutions.

Figure 2. Hexavalent chromium field 

separation.

COMPARISON OF METHODS

The diphenylcarbazide direct 

colorimetric method (Hach 

Company, 1992) uses a portable 

spectrophotometer and a 1-inch 

measuring cell. The cell is filled 

with sample, and then a packet of 

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 (mg/L). 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 steps taken to collect and preserve 

samples must be compatible with the 

intended analytical technique that 

will be used later in the laboratory. 

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 ammonium sulfate/ammonium 

hydroxide buffer solution to ensure 

that the distribution of the dissolved 

Cr redox species remains unchanged. 

Samples are analyzed in the 

laboratory by ion chromatography 

with post-column derivatization with 

diphenylcarbazide and absorbance 

detection within 24 hours of 

collection. 

In contrast to the other two methods, 

the new cation-exchange method is not 

as time-sensitive. Because only Cr(VI) 

remains in the sample after the cationexchange separation and Cr(total) 

is determined in the analytical 

step, samples can be processed up 

to several weeks after collection. 

The determination starts with field 

sampling using the cation-exchange 

system. Strong acid cation-exchange 

resin (AG50W-X8) will retain Cr(III), 

whereas Cr(VI) will pass into the 

effluent (Johnson, 1990). Care is 

needed to prevent contamination 

and to ensure a clean separation. 

Sample pH is not critical, but must 

be between 2 and 11, so that all 

Cr(VI) is present as either hydrogen 

chromate or chromate 

and all Cr(III) is present 

as , chromic monohydroxide 

, or chromic dihydroxide 

. Although the 

selectivity of the AG50W-X8 resin 

for 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 smallvolume sample cell is specialized 

instrumentation that is not 

commonly available, and 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 
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Figure 4. Results of analyses of four CR-containing samples spiked with varying 

concentrations of Cr(III).

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 cation-exchange 

cartridges, the capacity of the resin 

must be considered. Samples of 

high ionic strength or high Cr(III) 

concentration may displace Cr(III) 

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) (Icopini 

and Long, 2002) and prevent its 

retention by the cation-exchange resin. 

If waters to be tested are known or 

suspected to contain dissolved organic 

carbon at concentrations exceeding 

several mg/L, this should be tested 

by adding known amounts of Cr(III) 

standard to the sample. Higher than 

expected Cr(VI) concentrations are 

an indication that Cr(III) is not being 

retained by the resin.

Accuracy of the method: standards 

and spike recoveries

Demonstrating accuracy of the cationexchange 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 

figure 3, demonstrate that recoveries 

were variable for the Cr(VI) solutions, 

with an average recovery over the 

concentration range tested of about 91 

percent. Retention of Cr(III) appears 

to be excellent, with an average Cr(III) 

retention of about 99.8 percent.

Four Cr-containing samples, one 

each from the United Arab Emirates, 

North Carolina, Arizona, and the state 

of Washington, were prepared for 

analysis by spiking separate aliquots 

in the laboratory with 10, 20, or 30 

μg/L Cr(III). Each solution was then 

divided into two subsamples. One 

set of 12 subsamples was separated 

by the cation-exchange method and 

analyzed by GFAAS. The second set 

of 12 subsamples was preserved, and 

then analyzed by ion chromatography 

according to USEPA method 

218.6. Results, shown in 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 was 98.0 

percent. The anomalous result for 

sample UAE-30 from the cationexchange method was not included 

in the recovery calculations. This 

sample is saline water with a specific 

conductance of more than 55,000 

microSiemens per centimeter (μS/cm), 

for which procedural modifications 

may be needed to achieve good cationexchange separation of the Cr redox 

species.
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Figure 5. Comparison of Cr(IV) determinations by USEPA method 218.6 

with determinations by the new cation-exchange method.

Comparison of results with those 

from alternative methods

Several groundwater samples from 

the Mojave Desert in California 

were analyzed for Cr(VI) using a 

modification of the Hach Company 

(1992) diphenylcarbazide direct 

colorimetric method, and for Cr(VI) 

and total Cr using the new cationexchange method. Because some of 

the water samples may not have been 

preserved for optimal stabilization 

of Cr redox species, Cr redox 

species distributions may not always 

represent concentrations that were 

present at the time of collection. 

However, comparisons between the 

two analytical approaches are valid 

and may be illustrative. The results 

(data not shown) demonstrated 

favorable comparability with the 

modified diphenylcarbazide method, 

suggesting that, to the extent the 

diphenylcarbazide method is 

accurate, the new cation-exchange 

method is at least as accurate and is at 

least 10 times as sensitive. 

Many samples from diverse sources 

were analyzed using the new 

cation-exchange method and using 

USEPA Method 218.6 (Arar and 

others, 1991). The results (fig. 5) 

illustrate that, for these samples and 

over the Cr(VI) range covered, the 

proposed method produces Cr(VI) 

concentrations virtually identical to 

EPA method 218.6.

James W. Ball (jwball@usgs.gov)

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.

Hach Company, 1992, Chromium, hexavalent, 

for water and wastewater, Method 

8023, in DR/2000 Spectrophotometer 

Procedures Manual, 7th Edition: 

Loveland, Colo., 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.

Additional details on the development 

of this method can be found in:

Ball, J.W., McCleskey, R.B., 2003, A 

New Cation-Exchange Method 

for Accurate Field Speciation 

of Hexavalent Chromium: 

U.S. Geological Survey WaterResources Investigations Report 

03-4018, 17 p.

Acrobat pdf versions of this fact sheet 

and the above report can be found at 

the Project web site:

http://wwwbrr.cr.usgs.gov/projects/

GWC_chemtherm/

For more information, contact:

Chief, Branch of Regional Research 

U.S. Geological Survey

Mail Stop 418, Box 25046

Denver Federal Center,

Denver, CO 80225-0046

For reprints, contact:

U.S. Geological Survey

Branch of Information Services

Box 25286

Denver, CO 80225-0286

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