Ask the Expert Question:
How do you overcome the redox interferences of 7196A/3060A?
Expert Response:
When using Methods 3060A/7196A it is common to also test for several other general chemistry parameters that relate to the reduction / oxidation conditions present in site soil samples. These auxiliary results are then used to help interpret the Cr(VI) results. Including client specified matrix spike samples is also an excellent way to assess matrix interferences.
In reducing soil conditions Cr(VI) is converted to Cr(III). This is a common remediation process. However, in some instances the reducing agent is not evenly distributed and unreacted pockets of Cr(VI) remain. When the sample is brought to the lab and run through the 3060A alkaline leaching process, both the reducing agent and the Cr(VI) are solubilized and complete the reaction. It makes it look as if there was no Cr(VI) in the sample, even though there really was. Matrix spike recoveries usually alert the data user when this situation has occurred. Using the method of standard additions (MSA) with Methods 3060A/7196A to calculate the Cr(VI) concentration can compensate for moderate reduction/oxidation interferences. Both the auxiliary general chemistry tests and MSA options are available at TestAmerica.
Colorimetric interferences are also possible in 7196A. When these occur, the ion chromatographic Method 7199 is the best and most cost effective way to minimize these interferences. This is about 2-4X more expensive than method 7196A. Method 7199 doesn’t resolve the reduction / oxidation interferences described above.
Method 6800 is capable of resolving most reduction / oxidation interferences as well as the colorimetric interferences that Method 7196A experiences. Method 6800 testing is about 10X more expensive than Method 7199. It is usually quoted by project.
View Dr. Mark Bruce's expert profile
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