Salinity is a key variable in the modelling and observation of ocean circulation and ocean-atmosphere fluxes of heat and water. In this paper, we examine the climatological relevance of ocean salinity, noting fundamental deficiencies in the definition of this key observable, and its lack of a secure foundation in the International System of Units, the SI. The metrological history of salinity is reviewed, problems with its current definitions and measurement practices are analysed, and options for future improvements are discussed in conjunction with the recent seawater standard TEOS-10.

https://iopscience.iop.org/article/10.1088/0026-1394/53/1/R12/pdf

Metrological challenges for measurements of key climatological observables Part 2: oceanic salinity

A tetrapolar probe to measure the electrical properties of electrolyte solutions was implemented with gold electrodes according to the van der Pauw method. Electrical impedance spectroscopy (EIS) measurements of different concentrations of phosphate buffer saline (PBS) solution and an oral mucosal tissue sample dispersed in PBS were performed in the galvanostatic mode using a four-electrode cell (tetrapolar probe). Taking advantage of using a potentiostat/galvanostat for carrying out the electrical measurements, a simple and rapid method using a three-electrode electrochemical cell is described for: a) cleaning of electrodes, b) verification of surface chemical state of electrode material and c) choice of current supplied to electrodes for EIS measurements. Results of this research shown a depolarization effect due to the addition of oral mucosa tissue cells into the PBS solution.

https://iopscience.iop.org/article/10.1088/1742-6596/687/1/012101/pdf

Considerations on electrical impedance measurements of electrolyte solutions in a four-electrode cell

We propose a theoretical approach to determine the conductivity of electrolytic solution using sine waveform voltage as a triggering signal. The approach is based on the acquisition of the complex impedance in two-electrode conductance cell by analyzing the equivalent circuit. We deduce the formula of resistance in electrolytic solution by doing a series of calculations in transformation, simplification, and replacement for the complex impedance and solving the quadratic equation with unknown solution resistance. Even though there exists electrode polarization or the distributed capacitance which may affect the measurement, we can still obtain the electrolytic conductivity. In this paper, we focus on deducing the expression for the value of resistance in electrolytic solution. To our knowledge, this is the first report on directly deducing the resistance’s value. In particular, the extent of the electrode polarization is also analyzed and mathematically formulated. The experimental results show that the method proposed in this paper can achieve high precision.

A theoretical study on resistance of electrolytic solution: Measurement of electrolytic conductivity

This paper reports a closed-flow-loop electrolytic conductivity measurement system enabling calibration of conductivity sensors from the conductivity level of ultrapure water (UPW) at $5.5~\mu \text{S}$ /m up till the range of 10 to 140 mS/m, where stable certified reference materials (CRMs) for conductivity are available. The system is traceable to the SI unit system. A comparison made with another electrolytic conductivity measurement system, which is based on Jones-type conductivity cells designed to be used primarily for conductivities larger than ~10 mS/m, shows an agreement better than 0.1%. The most commonly used calibration method of conductivity sensors is based on CRMs and the extrapolation of the data to low conductivity values ( $ /m). In contrast, the present closed-flow-loop electrolytic conductivity measurement system is based on the measurements made at conductivity levels where the sensor is used; it is, therefore, a better alternative.

Bridging the Gap Between Ultrapure Water and Reference Materials in Electrolytic Conductivity Measurements

This paper addresses the challenges of making accurate and traceable electrolytic conductivity cell constant calibrations and sensor system calibration measurements over a wide range of conductivity, from 5.5 µS m−1 (ultrapure water) up to 140 000 µS m−1, where stable and reliable commercial certified reference materials (CRMs) are available. The challenges of making electrolytic conductivity measurements in the time domain versus the frequency domain are discussed and the relationship between the two domains is analysed in terms of a Gaver–Stehfest algorithm for inverse Laplace transform. Data from a commercial current-step-pulse-based sensor reader operating in the time domain are compared with accurate and traceable data from a closed-flow-loop electrolytic conductivity measurement system operating in the frequency domain and traceable to the SI length unit. In the high conductivity range, the readouts of the sensor reader are observed to deviate from traceable conductivity values by up to 10 times the estimated standard measurement uncertainty, which is in the range 0.07%–0.17%. If accurate conductivity calibrations are required (<1%), the usual method of calibrating a conductivity sensor system at a high conductivity level, where CRMs are available, and relying on the same calibration for low conductivity measurements is, therefore, risky.

https://iopscience.iop.org/article/10.1088/1361-6501/aa875d/pdf

Addressing the challenges of traceable electrolytic conductivity measurements in water

Based on an innovative application of van der Pauw's theory, a system was developed for the absolute measurement of electrolytic conductivity in aqueous solutions. An electrolytic conductivity meter was designed that uses a four-electrode system with an axial–radial two-dimensional adjustment structure coupled to an ac voltage excitation source and signal collecting circuit. The measurement accuracy, resolution and repeatability of the measurement system were examined through a series of experiments. Moreover, the measurement system and a high-precision electrolytic conductivity meter were compared using some actual water samples.

https://iopscience.iop.org/article/10.1088/0957-0233/25/5/055005/pdf

System for absolute measurement of electrolytic conductivity in aqueous solutions based on van der Pauw's theory

The invention provides a pure water conductivity measurement system. The system comprises a pure water reservoir which is connected with a Van Der Pauw method conductivity measurement system through a circulation pipeline. The Van Der Pauw method conductivity measurement system comprises a Van Der Pauw method closed conductivity pool, a micro current excitation source and a signal sampling and processing circuit. According to the system, the Van Der Pauw method is introduced into the measurement of pure water conductivity, and the continuous measurement of the pure water conductivity can be realized. The electrode constant of the Van Der Pauw method closed conductivity pool only depends on the height of a measured solution (the length of an electrode), the electrode constant solution setting is not needed, and thus a problem that the electrode constant can not be accurately set due to the lack of low value conductivity standard material is effectively overcome with an absolute measuring method. An AC micro current source is employed as an excitation signal, and through adjusting the frequency and current intensity, the accurate measurement of the low value and high value conductivity by using the same device is realized.

Pure water conductivity measurement system

A system was designed for measuring the low electrolytic conductivity of aqueous solutions based on an innovative application of van der Pauw’s theory. A four-electrode system with an axial–radial two-dimensional adjustment structure was used. A flow-through cell and a sealed system were built to avoid the CO2 contamination at low-conductivity values. In this paper, the new cell and the measurement system with flowing solution are described. A series of experiments were conducted to determine the appropriate frequency and flow rate. Moreover, static and flowing cell systems were compared for their efficacy to measure an ultrapure water sample. The second cell in the system can also be used to calibrate other conductivity devices.

Zhen Lin

Papers

System for absolute measurement of electrolytic conductivity in aqueous solutions based on van der Pauw's theory

Pure water conductivity measurement system

Absolute measurement system for low electrolytic conductivity of aqueous solutions based on van der Pauw’s theory using a flow-through cell