Many chemical plants need colorimetric/photometric analysis of various compounds in their process and wastewater systems. Historically, processors relied upon lab-based bench top systems to perform this type of analysis. Today however, improved designs and digital technologies in field-based process analyzers can produce lab-quality results automatically, more frequently and with much less required labor.
Colorimetric/photometric analysis determines the concentration of a chemical element or compound in a solution by combining a reagent with a sample. This results in a specific sample color; its intensity is proportional to the concentration of the compound of interest.
The color intensity is measured photometrically based on the Beer-Lambert law. This law states that a linear relationship exists between a sample’s absorbance of light and the concentration of the absorbing species, in this case, color.
Lab-based systems rely upon hand-delivered and manually prepared samples for analysis. Field-based systems get samples via independent sample systems operating in concert with the analyzer.
Proper colorimetric/photometric analysis involves three elements: sample preparation and delivery; colorimetry (combining the sample and reagent to produce color); and photometry (measurement of the resulting color intensity). Improper or mediocre performance in any of these three areas will lead to measurement error. Therefore, it’s important to thoroughly understand the potential limitations of any given system.
Colorimetric/photometric analyzers can handle a range of compounds and suit a diverse range of applications. For instance, analyses of ammonium, phosphorus, iron and nitrite are crucial for water and wastewater. Ammonium measurement is critical in wastewater for monitoring ammonia reduction. Ortho-phosphate measurement is used in process control of phosphate removal, either biologically or through flocculation.
Measurements of total phosphorus now are vital to ensure effluents meet discharge permit levels, which are becoming increasingly stringent as state environmental agencies and the U.S. Environmental Protection Agency (EPA) clamp down on nutrient levels in effluent.
Drinking water plants need iron analysis, which falls under the non-mandatory standards with secondary maximum contamination levels (SMCLs); the EPA secondary standard for iron is 0.3 ppm. In industrial processes, iron can build up in tanks, storage vessels, water heaters and pipelines. Iron deposits can decrease capacity, reduce pressure and increase maintenance. They also can cause self-closing valves to stick.
Plant steam and cooling-water systems require monitoring and control of hardness to improve the quality of the water. Hardness isn’t a health hazard but the calcium, magnesium and other metals causing water hardness can lead to scaling in boilers, cooling towers and other equipment, reducing the efficiency of heat transfer. In steam systems, lower levels of impurities decrease corrosion rates in boilers and lower the frequency of blow downs, which cuts fuel costs
Benchtop spectrophotometers are a common tool for colorimetric/photometric analysis in industrial processes. These spectrometers use a small vial of sample, with colorimetric reagents added to achieve the desired color reaction. Because they operate over a range of light wavelengths, these devices can measure a range of colors — and thus can provide results for various compounds. It’s not uncommon to measure more than 35 different parameters with one spectrometer by using a variety of reagents. Newer benchtop systems incorporate barcode readers and RFID [radio frequency identification] technology to auto-identify the appropriate method and calibration adjustments.
However, benchtop systems are relegated to the lab, with testing performed by lab technicians. This necessitates sampling at the process and transferring those samples to the lab for analysis. These systems can produce an accurate result — but one only indicative of the sample at the time of analysis, not when it was taken.
Moreover, sample collection and transfer can require very specific criteria to ensure sample integrity prior to analysis. Samples may need filtering or treating and may demand analysis within a certain limited time. What may appear on the surface as a straightforward process—sample collection and preparation—can be time-consuming and prone to error.
For example, when analyzing for iron, dissolved iron requires a well-filtered sample while less filtering is necessary for total iron that includes undissolved iron. Turbidity, iron oxide, high iron content and other factors can interfere with the results and all must be taken into account.
In addition, the demand on lab technicians’ time severely limits the frequency at which they can perform the analyses. As with any analysis, cost also must be considered. Benchtop systems can run more than $4,000. To this you must add the cost of reagents and sample preparation, as well as labor to collect and deliver the samples to the lab.
Because lab results don’t represent what’s happening in the process in realtime, they at best enable manual adjustments to operations, with the impact of these adjustments not known until the next sample is analyzed.