Determination of cupric ion concentrations in marine waters: an improved procedure and comparison with other speciation methods

Abstract

Environmental context A Cu ion-selective electrode is potentially an excellent tool for cupric ion measurements in salt water, but it is prone to poor reproducibility. We show that dramatic improvements can be obtained by using a Cu ion-selective electrode and a one-point calibration method that corrects for electrode fouling. The method shows promise to be used to collect data on toxic cupric ion concentrations in saltwater environments. Abstract Free Cu is often used as an indicator for potential Cu toxicity. Free ionic Cu2+ was measured using a flow-through ion-selective electrode (ISE). Four different marine samples were collected from various locations and analysed during a fixed-pH Cu titration using an external standard calibration ISE method. Free cupric determinations in the range 10–12 to 10–7molL–1 were consistent with published literature but replicate measures showed up to four orders of magnitude variability. To improve reproducibility, an internal calibration method was developed. The new method was validated using artificial seawater with added tryptophan as the model ligand. The free Cu measured using the improved method showed the same trends as the external calibration data but reproducibility increased to an order of magnitude or better. The potential applicability of this new method was also highlighted in that it matches, in the environmentally and regulatory significant range of total Cu, with a fluorescence quenching method applied to one of the four samples. The ISE data do not agree with free ion concentrations estimated from anodic stripping voltammetry (ASV) though. This suggests that, at least for these samples, ASV responded to a larger fraction of total Cu than simply the inorganic complexes. Caution should be exercised when using ASV as a proxy for bioavailability because the trends in ASV-derived free Cu did not match the free Cu as estimated by ISE. This ability to more reliably measure free Cu is significant for predicting and measuring toxicity on Cu exposure.

Influence of dissolved organic matter (DOM) source on copper speciation and toxicity to Brachionus plicatilis

“…Free copper was measured using the internal calibration flow-through method described in Tait et al [46] All measurements were conducted in a flow-through system using an Orion copper electrode (Model 94-29, Boston, MA) contained within an ion selective electrode (ISE) micro-Flowcell (FIAlab, Bellevue, WA). The flow cell was contained within a Faraday cage made from wire wrapped to electrical ground (a water tap).…”

“…A one-point internal calibration was performed using the lower pH measurement to determine free Cu as detailed in Tait et al [46] The electrode is assumed to have a linear Nerstian response with a slope of 29.6 mV per decade. [46] For the low-pH sample, it is assumed that the total copper equals the free copper, corrected for chloride complexation using the National Institute of Standards and Technology (NIST) formation constant (K) log K CuCl of 0.3.…”

“…[46] For the low-pH sample, it is assumed that the total copper equals the free copper, corrected for chloride complexation using the National Institute of Standards and Technology (NIST) formation constant (K) log K CuCl of 0.3. [47] This allows the free copper to be determined at pH 8.0.…”

Chronic effects of lead exposure on topsmelt fish (Atherinops affinis): Influence of salinity, organism age, and relative sensitivity to other marine species

“…Lead speciation calculations were performed with all stability constants taken as concentration constants as opposed to activity constants. This method was previously reported for Cu by Tait et al (2015). This method avoids the need to specify an activity model for seawater (Angel et al 2016).…”

Testing the Underlying Chemical Principles of the Biotic Ligand Model (BLM) to Marine Copper Systems: Measuring Copper Speciation Using Fluorescence Quenching

“…The Jimbo site (JB) was previously measured using fluorescence quenching techniques in Tait et al (2015). In this case, the humic-like fraction had a log K of 9.20 and a binding capacity of 890 nmole mg C −1 .…”