Inorganic phosphorus (orthophosphate) determination is crucial within environmental applications. Conventional accredited measurement methods of orthophosphate provide accurate measurements for a limited number of samples due to cost, time, and labor involved with laboratory analysis and are insufficient to characterize phosphate variability within environmental applications. Precise electrochemical sensing has the potential to provide accurate phosphate measurements and has the advantage of being inexpensive to produce and portable. Cobalt is a robust metal that has shown a unique selectivity towards phosphate in potentiometric sensors. In this manuscript, we reviewed the cobalt phosphate ion-selective electrodes with cobalt matrices in the form of pure metal, microelectrode, thin-film, and heterogeneous metal membrane in building integrated probes for determining phosphate concentrations in aqueous solutions. We reviewed different proposals of the cobalt-phosphate chemical reactions on the electrode surface, the factors affecting the stability of the phosphate measurement, and the success stories in the form of the limit of detection, linear range, and sensitivity. With strong progress in recent decades, we restricted ourselves at the time between 1995 and 2018. We discussed future opportunities of cobalt sensors towards more reliable phosphate sensing using novel approaches like cobalt alloys, three in one cobalt phosphate sensors, and external interference elimination methods.

Phosphorous (P) is critical for food production and is vital to both plant and animal life. Measurement and management of soil P are crucial for soil fertility maintenance and optimum plant growth while reducing P losses from agricultural fields and improving downstream water quality. Several costly and time-consuming analytical methods provide offsite P analysis, while onsite sensor-based P analysis method shows the potential to provide fast and cost-effective P measurements. This study presented a portable electrochemical adaptation of the Environmental Protection Agency (EPA) recommended colorimetric method for measuring inorganic P. In this research, cyclic voltammetry was used to quantify inorganic P in the range between 0.25 and 3.08 mg · l−1 (typical soil P range). The limit of detection achieved was 0.18 mg · l−1. Other common ions did not interfere with P detection and confirmed P selectivity. Unlike the EPA recommended method, this method only required molybdate ions as the complexing agent. Processed soil samples in the laboratory were used to validate this method against inductively coupled plasma optical emission spectroscopy. This method showed an average recovery of 98.27% of P, highlighting its suitability for field P measurements. The proposed electrochemical approach is promising for low-cost, simple, and portable infield soil P tests.

Phosphorus (P) is ubiquitous in the environment, but its measurement is costly and time‐consuming. Sensor‐based measurement shows potential, but selection of right metal remains the major challenge due to strong P species dependence on pH. This study examined the feasibility of pure cobalt, molybdenum, and their electrodeposited alloy, Co63Mo42 (wt %), as phosphate sensors. The cobalt, molybdenum and alloy exhibited mixed potential, Nernst potential, and oxidation‐reduction (red‐ox) mechanisms, respectively. Alloy showed good selectivity over a wide pH range, but high limit of detection and long response time (8–14 minutes). Yet, alloy provides a new opportunity for improving electrochemical phosphate sensors.

The soil water retention curve (SWRC) shows the relationship between soil water (θ) and water potential (ψ) and provides fundamental information for quantifying and modeling soil water entry, storage, flow, and groundwater recharge processes. While traditionally it is measured in a laboratory through cumbersome and time-intensive methods, soil sensors measuring in-situ θ and ψ show strong potential to estimate in-situ SWRC. The objective of this study was to estimate in-situ SWRC at different depths under two different soil types by integrating measured θ and ψ using two commercial sensors: time-domain reflectometer (TDR) and dielectric field water potential (e.g., MPS-6) principles. Parametric models were used to quantify θ—ψ relationships at various depths and were compared to laboratory-measured SWRC. The results of the study show that combining TDR and MPS-6 sensors can be used to estimate plant-available water and SWRC, with a mean difference of −0.03 to 0.23 m3m−3 between the modeled data and laboratory data, which could be caused by the sensors’ lack of site-specific calibration or possible air entrapment of field soil. However, consistent trends (with magnitude differences) indicated the potential to use these sensors in estimating in-situ and dynamic SWRC at depths and provided a way forward in overcoming resource-intensive laboratory measurements.

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