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Hexavalent Chromium (Cr(VI))  by Ion Chromatography (IC), more specific for Cr(VI) and up to 1,000 times more sensitive than other methods

Introduction

Chromium exists primarily in trivalent (Cr(III)) or hexavalent (Cr(VI)) oxidation states. Cr(VI) is a notorious environmental pollutant because it is a strong oxidant and much more toxic than Cr(III). Cr(VI) exists as the

chromate ion in basic solutions and as dichromate in acidic solutions.

One of the traditional methods for determining Cr(VI) uses diphenylcarbohydrazide (DPC) to form an intensely colored complex with Cr(VI). The complex is measured quantitatively by its visible absorption at 520 nm. However, as in any colorimetric analysis, this test is subject to positive interferences from other colored materials in the sample as well as from other elements that form colored complexes with DPC.


Update 12-26-00.  WCAS has been granted interim certification by California ELAP for hexavalent chromium in drinking water by EPA 218.6.


Cr(VI) by Ion Chromatography

 A method was developed by others using ion chromatography (IC) to separate Cr(VI) from other positive interferences, followed by a post column reaction with DPC. This method has appeared as EPA Method 218.6 for water, EPA 3060A and 7199 for wastes, EPA 425 for stack samples,  OSHA 215 for industrial hygiene samples, and South Coast Air Quality Management 

District Method 205.1. Not only is it more specific for Cr(VI), but it can be 1000 times more sensitive. Detection limits in solution can be as low as 0.01 ug/L in dilute bicarbonate solutions. This method has been used in ambient air risk assessments to measure Cr(VI) to 1 ng/m3.  Example reports with detection limits and quality control limits can be viewed (pdf files) at Cr(VI) by EPA 218.6 (waters) and Cr(VI) by EPA 3060A/7199 (soils and wastes).

Cr(VI) - Hexavalent Chromium by IC


Cr(VI) Stability

EPA has reported that soil samples were found to be stable for at least 30 days. However the holding time for wastewater samples is still 24 hours to our knowledge.

Once Cr(VI) is in an alkaline solution, such as sodium bicarbonate or sodium hydroxide, it appears to be very stable. In one study, Cr(VI) air samples in sodium bicarbonate were found to be stable for more than three months at room temperature.

Cr(VI) Extraction

EPA published Method 3060, an alkaline digestion, in the second edition of SW846, but withdrew it in the third edition.  An updated version 3060A now

 appears in Update III. A similar extraction method is used by NIOSH for air filters. EPA has noted that the 3060A extracts are stable for 7 days.

For a quotation...


Permanganate Interference

Using EPA 7199 (ion chromatography coupled with post column colorimetric detection), we have seen an interference when high levels of permanganate are present in a sample. Of course almost any type of colored species may interfere with the more traditional colorimetric analysis such as EPA 7196 offered by most other labs. With the addition of ion chromatography in 7199, such interferences are minimized. Even so large concentrations of permanganate do interfere in 7199. This interference appears to be due to a reaction of permanganate with the post-column reagent that gives a colored species with much less absorbance at 540 nm than either permanganate itself or the complex of chromium species with the post-column reagent. Thus

permanganate causes a problem in EPA 7199 when permanganate is present in great excess over the chromate species.

We now have found conditions which further reduce this interference. Using a different anion column, such as that used for EPA 300.0 to separate common inorganic anions, we have been able to separate permanganate and chromate. Detection with suppressed conductivity gives a detection limit of 0.1 ppm. An actual field sample rich in permanganate which gave a result of over 300 ppm Cr+6 by EPA 7199 gave a result of 2 ppm by this method with an average recovery of a matrix spike of 96%. Further work is ongoing to achieve lower detection limits using post column colorimetry and to document interference levels.

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