Hexavalent Chromium in Aqueous Samples Containing Color and Chemical Interferences
Today there are two common colorimetric techniques using a Diphenylcarbazide (DPC) solution for determining Hexavalent Chromium (Cr+6) in RCRA site water samples. EPA Method 7196, a colormetric method, analyzes the untreated or filtered sample as is. EPA Method 7199, an ion chromatographic method, separates the Cr+6 ion from interferences and is then followed by post column reaction with DPC. Therefore, colored matrices or matrices containing chemical interferences may be better analyzed by ion chromatography as shown in the following example.
Two highly colored samples were received from a treatability study. They had gone through an oxidative treatment process to remove VOCs. One of the requested analyses was Cr+6. The two samples were analyzed fi rst by EPA 7196. When an acidified sample is reacted with the DPC solution, a pink color (540 nm) appears. The absorbance is measured and its intensity is proportional to the concentration of Cr+6 in the sample. These samples were highly colored, so a background sample was also prepared which contained all of the analytical reagents except for the color reagent. The absorbance of the background sample is subtracted from the color-reacted sample to correct for any color interference. In these samples, after the color reagent was added to the sample, the sample became colorless. This indicated an interference with the normal coloring reaction. The DPC reagent is dissolved in acetone. To rule out a possible interference from the acetone, a third pair of samples were prepared adding acetone without DPC reagent. This experiment showed that the chemical interference in the sample was reacting with the DPC reagent. We suspect the chemical interference in the sample also produced the observed violet color and was from an oxidizing agent used in the treatment process, possibly potassium permanganate, a known method interferent.
In addition to the EPA 7196 method, the samples were then analyzed for Cr+6 using Ion Chromatography by EPA Method 7199. Using this method, good chromatography and QC data was obtained on these samples. The results have been compared below. A comparison of the results shows the impact of the interferences between the two methods. Because of interferences, the colorimetric method is prone to false negatives and matrix spike recovery failure. The IC method accurately quantitates Cr+6 results with good matrix spike recoveries.
The Ion Chromatography Method, EPA 7199, may be the better choice for the analysis for Cr+6, because it removes color and chemical interferences in samples of potentially diffi cult matrices. The chromatogram in the fi gure below demonstrates the effectiveness in the separation from interferences. In addition, Method 7199 can be performed rapidly, at least four samples per hour, which is very beneficial due to the short (24-hour) hold time specified by the method. In samples containing color and chemical interferences, the Cr+6 value is more accurately reported by method 7199.