Electrochemical Analysis for Total Alkalinity of Water by the Measurement of Cathodic Prepeak of Quinone Caused by Surplus Acid

In this study, an electrochemical analysis, coupled with the concept of back neutralization titration and the voltammetric determination of surplus acid, is proposed for determining the total alkalinity of water samples. When linear sweep voltammetry of 3,5-di-tert-butyl-1,2-benzoquinone (DBBQ) with H2SO4 in a water and ethanol (44 : 56, v/v) mixture was carried out using a bare glassy carbon working electrode, a cathodic prepeak of DBBQ caused by H2SO4 was observed on the voltammogram at a more positive potential than when compared with the original cathodic peak of DBBQ. When similar voltammetry was carried out in the presence of Na2CO3 and H2SO4, the cathodic prepeak height of DBBQ was decreased with an increase in the Na2CO3 concentration. The decrease of the cathodic prepeak height of DBBQ was found to be linearly related to the Na2CO3 concentration ranging from 0.025 to 2.5 mM (r2 = 0.998). The total equivalent concentrations of inorganic bases in samples of mineral water and tap water were determined, and then the results were converted to the total alkalinities of the water samples (mg/L CaCO3). The total alkalinities of the water samples determined by the present electrochemical analysis were essentially the same compared with those by the neutralization titration method. From these results, we were able to demonstrate that the present electrochemical analysis with accuracy and precision could be applied to determine the total alkalinity, which is one of the indicators to examine water quality. The present electrochemical analysis would contribute to achieving the sustainable development goals (SDGs) of #6 and #14.

Imaging of CO2 and Dissolved Inorganic Carbon via Electrochemical Acidification-Optode Tandem

Dissolved inorganic carbon (DIC) is a key component of the global carbon cycle and plays a critical role in ocean acidification and proliferation of phototrophs. Its quantification at a high spatial resolution is essential for understanding various biogeochemical processes. We present an analytical method for 2D chemical imaging of DIC by combining a conventional CO2 optode with localized electrochemical acidification from a polyaniline (PANI)-coated stainless-steel mesh electrode. Initially, the optode response is governed by local concentrations of free CO2 in the sample, corresponding to the established carbonate equilibrium at the (unmodified) sample pH. Upon applying a mild potential-based polarization to the PANI mesh, protons are released into the sample, shifting the carbonate equilibrium toward CO2 conversion (>99%), which corresponds to the sample DIC. It is herein demonstrated that the CO2 optode-PANI tandem enables the mapping of free CO2 (before PANI activation) and DIC (after PANI activation) in complex samples, providing high 2D spatial resolution (approx. 400 μm). The significance of this method was proven by inspecting the carbonate chemistry of complex environmental systems, including the freshwater plant Vallisneria spiralis and lime-amended waterlogged soil. This work is expected to pave the way for new analytical strategies that combine chemical imaging with electrochemical actuators, aiming to enhance classical sensing approaches via in situ (and reagentless) sample treatment. Such tools may provide a better understanding of environmentally relevant pH-dependent analytes related to the carbon, nitrogen, and sulfur cycles.

An Automated Microfluidic Analyzer for In Situ Monitoring of Total Alkalinity

We have designed, built, tested, and deployed an autonomous in situ analyzer for seawater total alkalinity. Such analyzers are required to understand the ocean carbon cycle, including anthropogenic carbon dioxide (CO2) uptake and for mitigation efforts via monitoring, reporting, and verification of carbon dioxide removal through ocean alkalinity enhancement. The microfluidic nature of our instrument makes it relatively lightweight, reagent efficient, and amenable for use on platforms that would carry it on long-term deployments. Our analyzer performs a series of onboard closed-cell titrations with three independent stepper-motor driven syringe pumps, providing highly accurate mixing ratios that can be systematically swept through a range of pH values. Temperature effects are characterized over the range 5-25 °C allowing for field use in most ocean environments. Each titration point requires approximately 170 μL of titrant, 830 μL of sample, 460 J of energy, and a total of 105 s for pumping and optical measurement. The analyzer performance is demonstrated through field data acquired at two sites, representing a cumulative 25 days of operation, and is evaluated against laboratory measurements of discrete water samples. Once calibrated against onboard certified reference material, the analyzer showed an accuracy of -0.17 ± 24 μmol kg-1. We further report a precision of 16 μmol kg-1, evaluated on repeated in situ measurements of the aforementioned certified reference material. The total alkalinity analyzer presented here will allow measurements to take place in remote areas over extended periods of time, facilitating affordable observations of a key parameter of the ocean carbon system with high spatial and temporal resolution.

Reagentless Acid-Base Titration for Alkalinity Detection in Seawater

Herein, we report on a reagentless electroanalytical methodology for automatized acid–base titrations of water samples that are confined into very thin spatial domains. The concept is based on the recent discovery from our group (WiorekA.Anal. Chem.2019, 91, 14951−1495931691565), in which polyaniline (PANI) films were found to be an excellent material to release a massive charge of protons in a short time, achieving hence the efficient (and controlled) acidification of a sample. We now demonstrate and validate the analytical usefulness of this approach with samples collected from the Baltic Sea: the titration protocol indeed acts as an alkalinity sensor via the calculation of the proton charge needed to reach pH 4.0 in the sample, as per the formal definition of the alkalinity parameter. In essence, the alkalinity sensor is based on the linear relationship found between the released charge from the PANI film and the bicarbonate concentration in the sample (i.e., the way to express alkalinity measurements). The observed alkalinity in the samples presented a good agreement with the values obtained by manual (classical) acid–base titrations (discrepancies <10%). Some crucial advantages of the new methodology are that titrations are completed in less than 1 min (end point), the PANI film can be reused at least 74 times over a 2 week period (<5% of decrease in the released charge), and the utility of the PANI film to even more decrease the final pH of the sample (pH ∼2) toward applications different from alkalinity detection. Furthermore, the acidification can be accomplished in a discrete or continuous mode depending on the application demands. The new methodology is expected to impact the future digitalization of in situ acid–base titrations to obtain high-resolution data on alkalinity in water resources.

Total Dissolved Inorganic Carbon Sensor Based on Amperometric CO2 Microsensor and Local Acidification

We present a dipping probe total dissolved inorganic carbon (DIC) microsensor based on a localized acidic microenvironment in front of an amperometric CO₂ microsensor. The acidic milieu facilitates conversion of bicarbonate and carbonate to CO₂, which in turn is reduced at a silver cathode. Interfering oxygen is removed by an acidic CrCl₂ oxygen trap. Theoretical simulations of microsensor functioning were performed to find a suitable compromise between response time and near-complete conversion of bicarbonate to CO₂. The sensor exhibited a linear response over a wide range of 0-8 mM DIC, with a calculated LOD of 5 μM and a 90% response time of 150 s. The sensor was successfully tested in measuring DIC in bottled mineral water and seawater. This DIC microsensor holds the potential to become an important tool in environmental sensing and beyond for measurements of DIC at high spatial and temporal resolution.

From Molecular and Emulsified Ion Sensors to Membrane Electrodes: Molecular and Mechanistic Sensor Design

Selective molecular ion probes are often insoluble in water and require a hydrophobic solvent environment for strong and selective binding, which runs counter to the desire of utilizing them in a homogeneous solution. This Account aims to guide the reader on how such molecules, often coined ionophores, can be harnessed to design exceptionally useful optical and electrochemical sensors. We start here with some historical context on the design of such ionophores and continue with the explanation of the response mechanism of optical and potentiometric sensors and the role of combined components to build a robust ion sensor. This Account is addressed to nonspecialist readers and for this reason avoids extensive use of equations or theoretical considerations. The interested reader should turn to the original literature for further reading. Emulsified optical sensors are introduced as an initial example. Here, multiple reagents are confined in an attoliter sensing nanodroplet of the organic phase, immiscible with the aqueous sample phase. In this case, the ionophore molecules may retain their high affinity and selectivity to the target ion and the aqueous sample phase does not have to be modified. Emulsified optical sensors allow one to achieve the selective chemical sensing of ions, even with optically silent ionophores. Such ionophore-based nanodroplets are also discussed as a useful novel class of complexometric titration reagents and optical end point indicators with unique selectivities. We then turn our attention to potentiometric sensing probes and briefly discuss the unique opportunity of a direct characterization of ion-ionophore complexation properties offered by membrane electrodes. A carbonate-selective membrane electrode containing a highly selective tweezer-type ionophore with trifluoroacetophenone functional groups is then used as an example for the construction of a robust all-solid-state sensor. This potentiometric probe, in combination with a pH electrode, can directly measure PCO₂ in freshwater lakes, demonstrating a dramatically improved response time relative to traditional sensors equipped with a gas-permeable membrane. In recent years, new sensing modes and electrode designs have been introduced to expand the application scope of ionophore-based potentiometric sensors. Membrane electrodes containing ionophores are placed under dynamic electrochemistry control to give important progress in the field. We specifically highlight our recent works by membranes that are controlled by chronopotentiometry (controlled current) for speciation analysis, by ion transfer voltammetry on thin sensing films for multianalyte detection, by exhaustive coulometry for potentially calibration-free sensors and with coulometric membrane pumps for the selective delivery of reagents.

Observing Changes in Ocean Carbonate Chemistry: Our Autonomous Future

PURPOSE OF REVIEW

We summarize recent progress on autonomous observations of ocean carbonate chemistry and the development of a network of sensors capable of observing carbonate processes at multiple temporal and spatial scales.

RECENT FINDINGS

The development of versatile pH sensors suitable for both deployment on autonomous vehicles and in compact, fixed ecosystem observatories has been a major development in the field. The initial large-scale deployment of profiling floats equipped with these new pH sensors in the Southern Ocean has demonstrated the feasibility of a global autonomous open-ocean carbonate observing system.

SUMMARY

Our developing network of autonomous carbonate observations is currently targeted at surface ocean CO₂ fluxes and compact ecosystem observatories. New integration of developed sensors on gliders and surface vehicles will increase our coastal and regional observational capability. Most autonomous platforms observe a single carbonate parameter, which leaves us reliant on the use of empirical relationships to constrain the rest of the carbonate system. Sensors now in development promise the ability to observe multiple carbonate system parameters from a range of vehicles in the near future.

Distance and Color Change Based Hydrogel Sensor for Visual Quantitative Determination of Buffer Concentrations

We present here an innovative platform for the determination of pH buffer capacity based on FITC-dextran loaded hydrogels. Optical signals from the pH-sensitive hydrogels were analyzed by simple parameters including distance and color change. The methodology was validated on five different buffer systems and exhibited wide linearity (0.1 to 100 mM), good batch-to-batch reproducibility, high versatility, and resistance to background ionic strength changes. Experimental results also fit well with a theoretical model based on numerical simulation. Preliminary application in carbonate alkalinity determination of seawater proved very successful. This hydrogel buffer concentration sensor is fundamentally different from conventional acid-base titrations, brings minimum perturbation to samples, and shows great potential in real applications.

Bioinspired smart asymmetric nanochannel membranes

Bioinspired smart asymmetric nanochannel membranes (BSANM) have been explored extensively to achieve the delicate ionic transport functions comparable to those of living organisms. The abiotic system exhibits superior stability and robustness, allowing for promising applications in many fields. In view of the abundance of research concerning BSANM in the past decade, herein, we present a systematic overview of the development of the state-of-the-art BSANM system. The discussion is focused on the construction methodologies based on raw materials with diverse dimensions (i.e. 0D, 1D, 2D, and bulk). A generic strategy for the design and construction of the BSANM system is proposed first and put into context with recent developments from homogeneous to heterogeneous nanochannel membranes. Then, the basic properties of the BSANM are introduced including selectivity, gating, and rectification, which are associated with the particular chemical and physical structures. Moreover, we summarized the practical applications of BSANM in energy conversion, biochemical sensing and other areas. In the end, some personal opinions on the future development of the BSANM are briefly illustrated. This review covers most of the related literature reported since 2010 and is intended to build up a broad and deep knowledge base that can provide a solid information source for the scientific community.

Local Acidification of Membrane Surfaces for Potentiometric Sensing of Anions in Environmental Samples

The work dramatically improves the lower detection limit of anion selective membranes at environmental pH by using local acidification to suppress hydroxide interference at the membrane surface. Three separate localized acidification strategies are explored to achieve this, with ionophore-based membrane electrodes selective for nitrite and dihydrogen phosphate as guiding examples. In a first approach, a concentrated acetic acid solution (ca. 1 M) is placed in the inner filling solution of the PVC-based membrane electrode, forcing a significant acid gradient across the membrane. A second strategy achieves the same type of passive acidification by using an external proton source (fast diffusive doped polypropylene membrane) placed in front of a potentiometric solid contact anion selective electrode where the thin layer gap allows one to observe spontaneous acidification at the opposing detection electrode. The third approach shares the same configuration, but protons are here released by electrochemical control from the selective proton source into the thin layer sample. All three protocols improve the limit of detection by more than 2 orders of magnitude at environmental pH. Nitrite and dihydrogen phosphate determinations in artificial and natural samples are demonstrated.

Manipulation and measurement of pH sensitive metal–ligand binding using electrochemical proton generation and metal detection

Boron doped diamond generator-detector electrodes can both change and monitor the binding state of the pH sensitive metal–ligand complex [Cu²⁺:TETA] by locally varying pH and measuring the free metal concentration.