Why Measure Chromium Concentration in Liquids?
HEXAVALENT CHROMIUM CONTAMINATION
In 1952, Pacific Gas & Electric (PG&E) started adding hexavalent chromium (Cr6+) to cooling water in order to suppress rust in a Hinkley, California compressor station. The toxic metal was stored in unlined pools, allowed to percolate into the ground and contaminate the water supply. Unexplained illnesses (including respiratory cancer and organ damage) in the town sparked an investigation (as dramatized in the film Erin Brockovich), ultimately resulting in a blockbuster settlement of $333M in 1996.
The Cr6+ levels in Hinkley groundwater were reported at 0.58 ppm in 1993, high above the 0.1 ppm legal limit of the time; due to widespread violations exposed since the Hinkley case, some states are planning to implement Cr6+ limits as low as 0.06 ppb. A 2010 study found that 21 US cities suffered from chromium-contaminated groundwater. There is currently no enforced contamination limit for Cr6+ in drinking water, but legislation for this purpose is in progress.
HOW IS CHROMIUM MEASURED?
There are various methods for measuring chromium, but most are difficult to operate because they require titrations, pulling samples, and/or extensive training. US EPA Method 218.6 calls for ion chromatography to separate the chromate out from the sample and optical analysis under very strict sample conditions.
Using a high-resolution UV-Vis spectrophotometer, the OMA-300 Metal Ion Analyzer provides an automated chromium measurement in real time. Much easier to operate than ion chromatography and colorimetric systems, the OMA defeats cross-interference between metals by using data-driven multi-component analysis. Water backgrounds are transparent in the UV wavelength domain, making the OMA ideal for environmental applications.
Absorbance Curve of Hexavalent Chromium (Source: OMA-300 Process Analyzer)
Due to the strong UV-Vis absorbance curve of Cr6+, the OMA can be used to monitor either low contamination (i.e. continuous groundwater analysis) or higher concentrations. High chromium measurement is particularly useful for electroplating applications where resource loss in the plating bath translates directly to financial losses; in these cases, the bath composition can be kept constant using vigilant chemical monitoring.