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Why test for metals?
Testing for metals like iron and copper is crucial to maintaining a healthy water treatment program. Elevated residual metal concentrations detected in a system can be an early indication of metal corrosion taking place or that there is a change in the municipal water supply.
Sample preparation for total metals analysis
Many procedures for total metal analysis, including iron and copper, require a sample pretreatment by digestion. The digestion uses acid and heat to breakdown any organo-metallic bonds and/or free ions in the sample for accurate analysis.
For USEPA/EPA reporting, this digestion pretreatment is required. The following is quoted from the USEPA Code of Federal Regulations (CFR), Title 40, Chapter 1, Subchapter D, Part 136.3, comment 4 on Table IB – List of Approved Inorganic Test Procedures:
For the determination of total metals (which are equivalent to total recoverable metals) the sample is not filtered before processing. A digestion procedure is required to solubilize analytes in suspended material and to break down organic-metal complexes (to convert the analyte to a detectable form for colorimetric analysis) ... Regardless of the digestion procedure, the results of the analysis after digestion procedure are reported as "total" metals.
The Chem Inc analytical lab follows the EPA approved pretreatment method for total metal analysis. For a complete overview of our testing procedure, please contact a member of our analytical team.
Testing Methods for Iron
The preferred method for total iron analysis at Chem Inc is the Hach FerroZine method, including the sample pretreatment. However, different water sources and interferences may require alternate methods. Table 1 summarizes various methods for iron analysis and Table 2 summarizes the possible interferences associated with each method. All the following test methods and reagents are supplied by Hach.
Each method works to convert all soluble and some insoluble forms of iron to ferrous iron (Fe+2) for measurement. Performing these methods without the heat and acid digestion may result in lower measurable iron.
Testing Methods for Copper
The preferred method for total copper analysis at Chem Inc is the Hach CuVer2 method including the sample pretreatment. However, different water sources and interferences may require alternate methods. Table 1 summarizes various methods for copper analysis and Table 2 summarizes the possible interferences associated with each method. All the following test methods and reagents are supplied by Hach.
Similar to the iron methods, the copper methods work to measure both complexed and free forms of copper. To break down all complexed forms of copper, a heat and acid digestion is required. Without this step, lower levels of copper may be measured.
Free vs. Complexed vs. Dissolved Copper
There is another method for differentiating free versus complexed copper in a sample using the Free Copper Reagent. Copper may bind to EDTA or other complexing agents, which must be broken down to be measured. However, free copper is not to be confused with total dissolved copper, which uses hydrosulfite to break the complexes down further. Complexed copper is measured indirectly by taking the difference of total dissolved copper and free copper:
Complexed Copper = Total Dissolved Copper – Free Copper
Conclusion
With all metal testing, it is important to perform an acid and heat digestion of the sample prior to testing (unless otherwise stated by the specific method being used). When testing in the field, without access to proper lab equipment for digestion, always be aware that metal levels measured may be underestimated. Therefore, it is crucial to periodically confirm your testing results with an analytical lab.
In addition, there are many interferences to be aware of, and proper method modification may be required depending on the contamination levels of your water sample. Please refer to the standard Hach methods or contact a member of the Chem Inc analytical team if any assistance is needed.