EPA Method 218.6

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EPA Method 218.6:
Hexavalent Chromium in Water by Ion Chromatography. Official Name: Method 218.6: Determination of Dissolved Hexavalent Chromium in Drinking Water, Groundwater, and Industrial Wastewater Effluents by Ion Chromatography

An aqueous sample is filtered through a 0.45 um filter and the filtrate is adjusted to a pH of 9-9.5 with a concentrated buffer solution. A measured volume of the sample (50-250 uL) is introduced into the ion chromatograph. A guard column removes organics from the sample before the Cr (VI), as CrO4 2- , is separated on a high capacity anion exchange separator column. Post-column derivatization of the Cr (VI) with diphenylcarbazide is followed by detection of the colored complex at 530 nm.

This method provides procedures for determination of dissolved hexavalent chromium (as CrO4 2-) in drinking water, groundwater, and industrial wastewater effluents.

Methods for the Determination of Metals in Environmental Samples, Supplement 1 (EPA/600/R-94/111)

Interferences which affect the accurate determination of Cr (VI) may come from several sources. 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. Reduction of Cr (VI) to Cr (III) can occur in the presence of reducing species in an acidic medium. At pH 6.5 or greater, however, CrO4 2-which is less reactive than HCrO4 is the predominant species. Overloading of the analytical column capacity with high concentrations of anionic species, especially chloride and sulphate, will cause a loss of Cr (VI). The column specified in this method can handle samples containing up to 5% sodium sulphate or 2% sodium chloride2. Poor recoveries from fortified samples and tailing peaks are typical manifestations of column overload.

QC Requirements:
The Linear Dynamic Range should be determined by analyzing a minimum of seven calibration standards ranging in concentration from 1-5,000 ug/L across all sensitivity settings of the spectrophotometer. Laboratories are required to analyze a Laboratory Reagent Blank (LRB), Laboratory Fortified Blankb (LFB) with each batch. After each ten samples an Instrument Performance Check standard (midpoint of calibration) and LRB should be analyzed. Each quarter the laboratory should analyze a quality controls sample that is obtained from an outside source.

Maximum Holding Time:
24 hours, though this can be extended through the use of the buffer solution as specified in the Methods Update Rule (March 12, 2007).



1-10,000 ug/L

Single-analyst precision and accuracy data is provided for five matrix waters, drinking water, deionized water, groundwater, treated municipal sewage wastewater, and treated electroplating wastewater.

The MDL was computed from analyses by a single laboratory in drinking water, ground water, and wastewater spiked at 2 ug/L.

Revision Number:
Revision 3.3, 1994

Test Description:
Chromium, Dissolved

Instrument used for this test: