An Overview of Caring For and Using a Cadmium Reduction Coil in the Determination of Nitrate.
Over the years, we’ve fielded a lot of questions from clients about how to operate and get the most out of their cadmium coils. Cadmium coils are fairly complicated and there are definitely areas for problems to occur. Sometimes we see red flags right away when customers have trouble with their cadmium coils out of the box and sometimes we have to do a little more digging, for instance, if we see our clients ordering cadmium coils sooner than we expect them to. Either way, whether you’re using cadmium reduction methods for the Flow 3000, Flow 3100, Flow IV or any autoanalyzer, cadmium coil activation and regeneration are extremely important techniques to master in the analysis of nitrate.
Over the next few weeks we are going to be dissecting this topic and providing a comprehensive overview of the cadmium coil including cadmium coil activation, cadmium coil regeneration and methods for the reduction of nitrate to nitrite with cadmium. Enjoy!
First, a little history of the cadmium coil…
Cadmium coils were designed to avoid the “messiness” of using granulated cadmium in packed columns. Packed columns were originally the only way that nitrate was reduced to nitrite. But because of the toxic nature of using them in the laboratory setting and the slowness of sample flow, many people began to use the much safer, easier, and faster coil version. All cadmium is toxic but since there is no preparation as in a packed bed column the coil is much easier. But the fact remains that the increased surface area, because of the many granules in packed columns, tends to make them hold their efficiency longer between activation’s then the tubular surface area of the coils. Of course the many granules slow down the progress through the column which makes the free flowing coils much faster. For the most part, though, this substitution is more than satisfactory, especially considering the toxic nature of handling cadmium granules/powder in the laboratory. The function of the coil is achieved by a reaction of copper in the cupric sulfate solution with the cadmium along the interior of the coil surface. The acid is first used to clean the surface of the cadmium and then the copper is added to create a copper complex, which is what actually causes the reduction of Nitrate to Nitrite. Because of this complex it is important for you to understand that the nature of the reduction is based mostly on the preparation and handling of the surface of the coil by the user not the quality of the cadmium metal itself. It so happens that a very pure cadmium is utilized but in general the reaction does not require the purest of metal as such but a good copper complex at the surface of the metal itself. This complex must be created during the activation process and then sustained over time by proper handling. So to wrap this point up we emphasize the point that the quality of the metal is not vital to sustain this copper complex as much as the handling of the coil and ultimately the surface of the cadmium by the user during its operation.
Variables such as pH, dissolved oxygen, trace metals, and chloride concentration affect the reduction efficiency of cadmium coils. The buffering reagent attempts to control pH rise to the presence of dissolved oxygen in reagents and samples. The pH of the reaction inside a cadmium coil (OTCR) must never exceed 8.5 or cadmium hydroxide can precipitate and adhere to the walls of the coil reducing its reduction efficiency. The addition of EDTA to ammonium chloride buffers used in EPA methods decreases, but does not completely prevent, cadmium hydroxide formation. Trace metals, such as iron, readily precipitate at pH 8.5 as hydroxides and also coat the inner walls of the coil reducing its efficiency.
The optimum pH for nitrate reduction to nitrite in OTCRs has been experimentally determined at pH 7.5. This obvious deviation from the EPA recommended pH of 8.5 is due to the formation of cadmium hydroxide precipitate and coating on the internal walls of the coil. It must be remembered that EPA data was collected using PBCRs and not coils. For better functionality it is recommended that an Imidazole solution buffered at pH 7.5 be used in place of the Ammonium Chloride pH 8.5 buffer reagents.
Dissolved oxygen reacts with the cadmium surface at a rate thirty times greater than nitrate reduction to nitrite, and may be present in reagents at more than 100 times the concentration of nitrate. Also, hydrogen ions are consumed during the reaction causing the pH to rise. Excessive dissolved oxygen (or oxygen in the segmentation gas) will react with cadmium instead of nitrate. If the pH rises above 8.5, cadmium hydroxide precipitates, coating the inner walls of the coil, and reducing its efficiency.
Chloride interferes with nitrate analysis by retarding the rate of conversion of nitrate to nitrite slowing reaction rates such that full conversion may not occur while the sample is flowing within the tubing. The rate of reaction decreases with increasing chloride content. Preparing standards at the same chloride concentration as samples, and increasing the time the samples spends in the reactor can compensate for this interference. Using two 24 inch OTCRs in series may be necessary for complete reduction in saline samples.