Ion-selective Electrode Sensor Research Articles

MXene-Coated Ion-Selective Electrode Sensors for Highly Stable and Selective Lithium Dynamics Monitoring.

Lithium holds immense significance in propelling sustainable energy and environmental systems forward. However, existing sensors used for lithium monitoring encounter issues concerning their selectivity and long-term durability. Addressing these challenges is crucial to ensure accurate and reliable lithium measurements during the lithium recovery processes. In response to these concerns, this study proposes a novel approach involving the use of an MXene composite membrane with incorporated poly(sodium 4-styrenesulfonate) (PSS) as an antibiofouling layer on the Li+ ion selective electrode (ISE) sensors. The resulting MXene-PSS Li+ ISE sensor demonstrates exceptional electrochemical performance, showcasing a superior slope (59.42 mV/dec), lower detection limit (10-7.2 M), quicker response time (∼10 s), higher selectivity to Na+ (-2.37) and K+ (-2.54), and reduced impedance (106.9 kΩ) when compared to conventional Li+ ISE sensors. These improvements are attributed to the unique electronic conductivity and layered structure of the MXene-PSS nanosheet coating layer. In addition, the study exhibits the long-term accuracy and durability of the MXene-PSS Li+ ISE sensor by subjecting it to real wastewater testing for 14 days, resulting in sensor reading errors of less than 10% when compared to laboratory validation results. This research highlights the great potential of MXene nanosheet coatings in advancing sensor technology, particularly in challenging applications, such as detecting emerging contaminants and developing implantable biosensors. The findings offer promising prospects for future advancements in sensor technology, particularly in the context of sustainable energy and environmental monitoring.

Design and Synthesis of the Poly Vinyl Chloride (PVC) Membrane for Fe(III) Ion Selective Electrode (ISE) Based on Ester Modified Humic Acid (EHA) as an Ionophore

Design and synthesis of the poly vinyl chloride (PVC) membrane as Fe(III) ion selective electrode (ISE) sensor based on EHA ionophore as an excellent sensory molecule for monitoring Fe(III) in the sample solution had been done. Simple and efficient EHA was made by condensation of carboxylic acid HA with ethanol and sulfuric acid as catalist. PVC membrane was prepared by mixed all component such as PVC powder, EHA, plasticizer (o-NPOE and DOP) and anion lipophilic (Sodium-TPB and oleic acid) with a total weight of 100 mg and dissolved in 1 mL of dry THF. The electrode membrane responses showed that the electrodes were highly selective to ion Fe3+ at pH 3 with response time of ±50 seconds. The dynamic range of the membrane ISE sensor was 10-1–10-7 M with the value of Nernst factor was ±19 mV per decade. The membrane electrode can be used for at least 5 weeks. The analytical results of Fe(III) measurement showed good confirmity with reference method (AAS), suggesting that the developed analytical tool could be used to measure and monitor Fe(III) in the real water samples.

Sample Temperature Affects Measurement of Nitrate with a Rapid Analysis Ion Selective Electrode System Used for N Management of Vegetable Crops

The practical value of portable hand-held ion selective electrode sensors (ISE) for on-farm [NO3−] measurement to assist with crop N management of vegetable crops has been demonstrated in numerous previous studies. They provide rapid, in-situ measurement of the nitrate concentration ([NO3−]) in nutrient and soil solutions, and in petiole sap. Sample temperatures, for on-farm measurements, vary appreciably. This study evaluated the effects of sample temperature on [NO3−] measurement using two different models of a commonly used, commercially available, portable ISE meter. The temperatures (5, 10, 15, 20, and 25 °C) examined were in the range likely to be encountered in practical on-farm work. Aqueous solutions of 6, 12, and 18 mmol NO3− L−1 were prepared from KNO3, Ca(NO3)2 and NaNO3. [NO3−] was measured in three replicate samples of each of the three concentrations, made from each NO3− compound, at each temperature. The results consistently and clearly demonstrated a strong negative linear relationship between temperature-induced errors and sample temperatures. The temperature-induced error was considerable for cooled samples, being +50% at 5 °C and +31% at 10 °C. At sample temperatures of 17–20 °C, the temperature effects were minimal. Above this range, the temperature effect caused underestimation. At 25 °C, the temperature-induced error was −24%. These results show that care must be taken to ensure that sample temperatures do not erroneously affect the measurement of [NO3−] with ISE meters. Particular care needs to be taken with both refrigerated and warmer samples.

Analysis of Soil Nitrate Ion Selective Electrode (ISE) Sensor Using Arduino UNO

The ecological concern over soil and groundwater pollution caused by agricultural activities has led to the growing interestin precision agriculture. One of the most common types of fertilizer is the nitrogen fertilizer which needed in major amount for plant growth. Over fertilization will contaminate soil and groundwater which can have adverse effect on environment and human health.The main purpose of this research is to measure soil nitrate concentration using nitrate ion-selective electrode (ISE) sensor and Arduino programmable microcontroller. The optimum soil-to-water ratio and the effect of soil solution clarity will be investigated. Standard sampling procedures was conducted at oil palm plantation area, Felda Bukit Goh, Pahang, Malaysia. Validation of the results were carried out in the laboratory. The recorded data indicated accurate readings for Nitrate ISE Arduino wasR² = 0.84. The soil-to-water ratio of 1:2.5 was observed as an optimal proportion ISE analysis. A clear soil solution was crucial for maintaining the accuracy of ISE sensor, to avoid declining of 46.2% accuracy.These results could assist researchers and farmers to accurately monitor the concentrations of soil nitrate on the field effectively as well as an insight to ISE sensor with Arduino technologies.

Temperature Self-Calibration of Always-On, Field-Deployed Ion-Selective Electrodes Based on Differential Voltage Measurement.

Originally developed for use in controlled laboratory settings, potentiometric ion-selective electrode (ISE) sensors have recently been deployed for continuous, in situ measurement of analyte concentration in agricultural (e.g., nitrate), environmental (e.g., ocean acidification), industrial (e.g., wastewater), and health-care sectors (e.g., sweat sensors). However, due to uncontrolled temperature and lack of frequent calibration in these field applications, it has been difficult to achieve accuracy comparable to the laboratory setting. In this paper, we propose a novel temperature self-calibration method where the ISE sensors can serve as their own thermometer and therefore precisely measure the analyte concentration in the field condition by compensating for the temperature variations. We validate the method with controlled experiments using pH and nitrate ISEs, which use the Nernst principle for electrochemical sensing. We show that, using temperature self-calibration, pH and nitrate can be measured within 0.3% and 5% of the true concentration, respectively, under varying concentrations and temperature conditions. Moreover, we perform a field study to continuously monitor the nitrate concentration of an agricultural field over a period of 6 days. Our temperature self-calibration approach determines the nitrate concentration within 4% of the ground truth measured by laboratory-based high-precision nitrate sensors. Our approach is general and would allow battery-free temperature-corrected analyte measurement for all Nernst principle-based sensors being deployed as wearable or implantable sensors.

Steady-State and Transient Performance of Ion-Sensitive Electrodes Suitable for Wearable and Implantable Electro-Chemical Sensing.

Traditional Potentiometric Ion-selective Electrodes (ISE) are widely used in industrial and clinical settings. The simplicity and small footprint of ISE have encouraged their recent adoption as wearable/implantable sensors for personalized healthcare and precision agriculture, creating a new set of unique challenges absent in traditional ISE. In this paper, we develop a fundamental physics-based model to describe both steady-state and transient responses of ISE relevant for wearable/implantable sensors. The model is encapsulated in a "generalized Nernst formula" that explicitly accounts for the analyte density, time-dynamics of signal transduction, ion-selective membrane thickness, and other sensor parameters. The formula is validated numerically by self-consistent modeling of multispecies ion-transport and experimentally by interpreting the time dynamics and thickness dependence of thin-film solid-contact and graphene-based ISE sensors for measuring soil nitrate concentration. These fundamental results will support the accelerated development of ISE for wearable/implantable applications.

Prototyping of a Low-Cost Ion-Selective Electrode Sensor Unit for Determination of Lead Ions in Aqueous Samples

Exposure to lead via drinking water is a substantial health hazard. The current common practice of measuring lead levels in drinking water is to use analytical chemical analysis that involves sophisticated laboratory instruments. As lead contamination in drinking water is often a close-to-home contamination caused by corroded lead service pipes connecting households to main lines or lead-based pluming within the households, a low-cost and portable device that can be conveniently deployed to individual households to monitor lead levels in drinking water would offer consumers the opportunity to obtain first-hand information about the lead levels in their drinking water. The main objective of this study is to develop and prototype a portable low-cost device for detecting lead levels in drinking water. The lead-detecting device consists of two primary components: lead ion-selective electrode (ISE) sensing membrane and a microcontroller-based measuring unit. ISE membrane has proven to show low detection limits, good selectivity, low cost and great potential to be integrated in a miniature device. The ISE membrane in this study utilized polymeric membrane as the matrix with lead-selective ionophores embedded in the matrix. Hydroxyl type Vinyl Chloride-vinyl Acetate Terpolymer was used to create polymeric membrane matrix and allowed for all components to be housed in the membrane. The selected ionophore was Tert-Butylcalix(4)Arene-Tetrakis with its primary function is to differentiate and transport a specific ion (in this case, Pb2+ ions) through a membrane. A limit of detection of 1.0 ´ 10-8 M was achieved along with a linear Nernstian response over a lead concentration range from 1.0 ´ 10-8 M to 1.0 ´ 10-4 M. Durability test of the membrane showed a lifetime close to 30 days. A low-cost printed circuit board (PCB) was custom designed and created as an amplification unit to measure the ISE’s electromotive force (EMF). The custom-built device was able to capture the limit of detection (LOD) of the ISE sensing membrane and showed reasonably good consistency in comparison to measurements from commercial potentiostat.

Reconstruction of corrupted datasets from ammonium-ISE sensors at WRRFs through merging with daily composite samples

Long-term, continuous datasets of high quality are important for instrumentation, control, and automation efforts of wastewater resources recovery facility (WRRFs). This study presents a methodology to increase the reliability of measurements from ammonium ion-selective electrodes (ISEs). This is done by correcting corrupted ISE data with a data source that often is available at WRRFs (volume-proportional composite samples). A yearlong measurement campaign showed that the existing standard protocols for sensor maintenance might still create corrupted dataset, with poor sensor recalibrations responsible for abrupt and unrealistic jumps in the measurements. The proposed automatic correction methodology removes both recalibration jumps and signal drift by using information from composite samples that already are taken for reporting to legal authorities. Results showed that the developed methodology provided a continuous, high-quality time series without the major data quality issues of the original signal. In fact, the signal was improved for 87% of days when a reference sample was available. The effect of correcting the data before use in a data-driven software sensor was also investigated. The corrected dataset led to noticeably smaller day-to-day variations in estimated NH4+ loads, and to large improvements on both median estimates and prediction bounds. The long time series allowed for an investigation of how much training data that is required to fit a software sensor, which provides estimates that are representative for the entire study period. The results showed that 8 weeks of data allowed for a good median estimate, while 16 weeks are required for obtaining good 80% prediction bounds. Overall, the proposed method can increase the applicability of relatively cheaper ISE sensors for ICA application within WRRFs.

Preliminary Study of Poly(Tetrahydrofurfuryl Acrylate) Thin Film as a Potential Material of Ion Selective Electrodes: The Case of Nitrate Ion-Selective Electrode

A preliminary study on the use of a photocurable poly-tetrahydrofurfuryl acrylate (pTHFA) has been successfully performed as an alternative membrane for application in the Ion-Selective Electrode (ISE) sensors such as Nitrate-ISE. The pTHFA membrane was synthesized using photopolymerization technique and further optimized by varying the concentration of the photo-initiator. The pTHFA photopolymer was characterized by C-NMR, H-NMR, FTIR, and DSC. The best sensing formulation comprising pTHFA photopolymer was obtained from composition II with Tg of -17.3 °C. In the Nitrate-ISE fabrication process, initially, the tetraoctylammonium nitrate (TOAN) ionophore was optimized. The optimum TOAN concentration of 4.2 mg was then immobilized onto composition II as a sensing matrix. Results showed that the pTHFA based ISE sensor exhibited a slope near the Nernstian number with a good linear response for detecting nitrate ion concentration between 10–1 to 10–4 M (r2 = 0.9994) and limit of detection as low as 3.47 × 10–5 M. Furthermore, the selectivity behavior of pTHFA based nitrate-ISE sensor was determined in various types of interfering ions such as SO42–, H2PO4–, HPO42–, Cl– and I–. The sensor has demonstrated selectivity coefficient of -2.27 ± 0.2, -2.49 ± 0.6, -2.18 ± 0.8, -1.31 ± 0.1 and 0.41 ± 0.2, respectively. The fabricated pTHFA ISE-Nitrate sensor was further tested in the fish ponds, soils, and also rivers. The sensors have shown excellent performance and is comparable to the standard method.

POTENTIOMETRIC DETERMINATION OF GLYCOPYRRONIUM BROMIDE IN BINARY MIXTURE WITH INDACATEROL MALEATE USING POLYVINYL CHLORIDE (PVC) MEMBRANE, COATED GRAPHITE AND CARBON PASTE SENSORS

A novel approach was conducted in this article for the determination of glycopyrronium bromide (GLY) in binary mixture with indacaterol maleate using ion selective electrodes for its selective determination in pure form and pharmaceutical formulation. Four ion selective electrode sensors were designed for determination of glycopyrronium bromide namely; glycopyrronium bromide-polyvinyl chloride (GLY-PVC) membrane, GLY-Coated graphite sensor and GLY-modified carbon paste sensors. The sensors are based on the ion association complexes of GLY cation with sodium tetra phenyl borate (GLY-TPB) and phosphomolybidic acid (GLY-PMA) counter anions as ion exchange sites using dioctylphthalate (DOP) as plasticizer. Validation of the method shows suitability of the proposed sensors for use in the quality control assessment of GLY and for its routine analysis. The results obtained were statistically compared to reported method and there was no significant difference between the proposed methods and the reported method regarding the accuracy and precision. The method was validated according to International Conference on Harmonization (ICH) guidelines and the results were satisfactory.

Exploration of ISE Sensor Embedded in Garden Weed Block Fabric for Continuous Soil Nitrate Sensing

The objective of this research was to explore novel sensing apparatus for quantifying spatial variation of leached nitrate in soil as well as serve as a method for site specific crop management. There is a need to develop sensors that can more accurately and successfully facilitate site specific soil management for monitoring nutrients and pollutants. The dominantly used sensing methods, both in field and in lab, can require lengthy time when analyzing samples and are costly. Therefore, the objective of this research was to develop a sensor on a water permeable weed barrier fabric used in garden bed soil to allow continuous monitoring of nitrate levels. An electrically conductive fabric material was prepared by applying conductive coatings onto the porous material. Nitrate doped polypyrrole (PPy(NO3 -)) electrodes were used as sensing membranes and proved to have high selectivity towards nitrate ions. The electropolymerization protocols and thickness of the polymer layer were optimized (monomer concentration, deposition time, and scan rate) to synthesize the ion selective electrode (ISE) for nitrate determination. The ISE sensor provides a wide dynamic range from 0.008 mg/L to 849.9 mg/L for nitrate ions in real soil solution samples. Overall, the sensing apparatus proves to successfully detect nitrate ions in soil, sand, and solution.

Applied Technology in Diaper-Based UTI Testing for Elder People by Using Nitrite Ion Selective Electrode

Urinary tract infections (UTI) are common infection diseases in the hospitals, especially for the elder people. Detecting UTI for the elder people is difficult and time-consuming procedure. Since measuring nitrite directly in urine is often employed in the diagnosis of UTI, a nitrite ion selective electrode (ISE) was investigated in this study in order to detect UTI samples directly on used diapers of patients. With a commercial ISE sensor for nitrite, the detecting system was built to quickly and precisely quantify different concentrations of nitrite ions in both water solutions and real human urine samples. Good results were shown in the experiment that the nitrite can be precisely detected in the fresh urine by the ISE with the relations as Ε=224.7-21.259×Lg (C+13.476), where E is the electrode potential and C is the concentration of nitrite ions. Besides, the method of extracting urine samples from used diapers was also studied for further research. The investigation suggested that a sampling module for used diapers could be simply realized if necessary.

INCREASING ION SELECTIVE ELECTRODES PERFORMANCE USING NEURAL NETWORKS

This paper reports the implementation of a neural processing structure as a component of an intelligent measuring system that uses ion selective electrodes (ISEs) as sensing elements of heavy metal ions (Pb+2, Cd+2) concentration. The neural network (NN), designed and implemented to reduce errors due to ion interference and to pH and temperature variations, is of the multiple-input multiple-output Multilayer Percepton (MLP-NN) type. The NN is a component of a virtual instrument that includes a PC laptop, a PCMCI data acquisition board with associated conditioning circuits and the specific ISE sensors. A practical approach concerning the optimal neural processing solution (number of NN structures, number of neurons, neuron transfer functions) to increase the performance of low cost ISEs is presented. Results are presented to evaluate the performance of the NN intelligent ISE system and to discuss the possibility of transferring the acquisition and processing task to a low cost acquisition and control unit such as a microcontroller.

Surface plasmon resonance enhanced ellipsometric analysis for monitoring of cobalt electrochemical reaction in solution

Cobalt-based ion selective electrode (ISE) is one of the key methods for phosphate detection. In this paper, surface plasmon resonance (SPR) enhanced ellipsometric analysis method has been used to monitor the electrochemical reaction process at Co ISE sensor surface and the mass consumption of sensitive electrode membranes. An Au–Co bilayer film is designed as optical detection interface. In order to obtain detectable reflection intensity variation with Co thickness, Au film and Co film thickness are optimized at TM polarization state. According to the calculation results, 50nm Au film is formed on glass substrate by electron beam evaporation. Then cobalt film with thickness of about 20nm is grown on gold film by electrochemical deposition method. Ellipsometric measurements were performed at SPR angle 59°. The consuming process of Co film reacting with dissolved oxygen in deionized water has been obtained by monitoring the change of reflection intensity.

Tattoo-based potentiometric ion-selective sensors for epidermal pH monitoring

This article presents the fabrication and characterization of novel tattoo-based solid-contact ion-selective electrodes (ISEs) for non-invasive potentiometric monitoring of epidermal pH levels. The new fabrication approach combines commercially available temporary transfer tattoo paper with conventional screen printing and solid-contact polymer ISE methodologies. The resulting tattoo-based potentiometric sensors exhibit rapid and sensitive response to a wide range of pH changes with no carry-over effects. Furthermore, the tattoo ISE sensors endure repetitive mechanical deformation, which is a key requirement of wearable and epidermal sensors. The flexible and conformal nature of the tattoo sensors enable them to be mounted on nearly any exposed skin surface for real-time pH monitoring of the human perspiration, as illustrated from the response during a strenuous physical activity. The resulting tattoo-based ISE sensors offer considerable promise as wearable potentiometric sensors suitable for diverse applications.

Reduced Graphene Oxide Films as Solid Transducers in Potentiometric All-Solid-State Ion-Selective Electrodes

The development of ion-selective electrodes (ISEs) using solid-state transducer materials is of great interest for advanced potentiometric detection systems. At present, conducting polymers are the most used solid-state transducing materials. However, their reliability is strongly related to their chemical stability and the formation of internal water films. Here we report on the use of reduced graphene oxide (RGO) films of different thicknesses as transducer materials in potentiometric all-solid-state ISEs. First, the transduction mechanism is fully analyzed, revealing that RGO films act as asymmetric capacitors where their electron density is in contact with ions of the electrolyte solution, creating a capacitance due to the constant phase elements present in the system. Second, as a proof of concept, RGO films are used in a calcium ISE showing highly reproducible sensing responses and outstanding increased signal-to-noise ratios with drifts of only 10 μV/h. These performance parameters are among the best compared to those of other ISE transducer materials so far. With its ease of fabrication and processing into reproducible films of controlled thickness and ease for further tailoring chemical composition and tailoring electrical properties, RGO offers great promise as a reliable high-performance transducer material for solid-state ISE sensors.

A Microfluidic Long-Period Fiber Grating Sensor Platform for Chloride Ion Concentration Measurement

Optical fiber sensors based on waveguide technology are promising and attractive in chemical, biotechnological, agronomy, and civil engineering applications. A microfluidic system equipped with a long-period fiber grating (LPFG) capable of measuring chloride ion concentrations of several sample materials is presented. The LPFG-based microfluidic platform was shown to be effective in sensing very small quantities of samples and its transmitted light signal could easily be used as a measurand. The investigated sample materials included reverse osmosis (RO) water, tap water, dilute aqueous sample of sea sand soaked in RO water, aqueous sample of sea sand soaked in RO water, dilute seawater, and seawater. By employing additionally a chloride ion-selective electrode sensor for the calibration of chloride-ion concentration, a useful correlation (R2 = 0.975) was found between the separately-measured chloride concentration and the light intensity transmitted through the LPFG at a wavelength of 1,550 nm. Experimental results show that the sensitivity of the LPFG sensor by light intensity interrogation was determined to be 5.0 × 10−6 mW/mg/L for chloride ion concentrations below 2,400 mg/L. The results obtained from the analysis of data variations in time-series measurements for all sample materials show that standard deviations of output power were relatively small and found in the range of 7.413 × 10−5−2.769 × 10−3 mW. In addition, a fairly small coefficients of variations were also obtained, which were in the range of 0.03%–1.29% and decreased with the decrease of chloride ion concentrations of sample materials. Moreover, the analysis of stability performance of the LPFG sensor indicated that the random walk coefficient decreased with the increase of the chloride ion concentration, illustrating that measurement stability using the microfluidic platform was capable of measuring transmitted optical power with accuracy in the range of −0.8569 mW/ to −0.5169 mW/ . Furthermore, the bias stability was determined to be in the range of less than 6.134 × 10−8 mW/h with 600 s time cluster to less than 1.412 × 10−6 mW/h with 600 s time cluster. Thus, the proposed LPFG-based microfluidic platform has the potential for civil, chemical, biological, and biochemical sensing with aqueous solutions. The compact (3.5 × 4.2 cm), low-cost, real-time, small-volume (∼70 μL), low-noise, and high-sensitive chloride ion sensing system reported here could hopefully benefit the development and applications in the field of chemical, biotechnical, soil and geotechnical, and civil engineering.

Determination of the ionic composition of perfluorinated sulfocationite membranes based on Donnan potential estimates

An express method for determination of the membrane ionic composition is developed based on the possibility of estimating the Donnan potential difference on the individual studied-solution/membrane interface. The Donnan potential is assessed using a new device one end of which serves as the ion-selective electrode sensor. The ionic composition of perfluorinated sulfocationite membranes is determined in systems with mixed inorganic electrolyte solutions by comparing the Donnan potentials in a given system and the reference system. As the reference, a system with perfluorinated sulfocationic membrane and individual potassium chloride solutions is used. The time of one measurement does not exceed 10–15 min. The accuracy and sensitivity of determination were 3% and 0.02 mmol/g, respectively.

Potential of in-situ sensors with ion-selective electrodes for aeration control at wastewater treatment plants

A pilot-scale activated sludge wastewater treatment plant (WWTP) operated with nitrification and pre-denitrification was monitored with a set of on-line sensors for over 3 years. Wet-chemistry ex-situ analyzers, UV and UV-Visible in-situ sensors and in-situ sensors based on ion-selective electrodes (ISE) were used. New ISE sensors for ammonium, nitrate and nitrite, adapted to water and wastewater matrices, have been released in recent years, With adequate quality control they proved to be highly accurate and reliable in WWTP influents and activated sludge (AS) reactors even at the end of the biological treatment zone, working at low ammonium concentrations (1-2 mgN/l). The ammonium measurement was used to test several feed-forward and feed-back aeration control strategies. The first aim was to keep inorganic nitrogen compounds, i.e. ammonium, nitrate and particularly nitrite, as low as possible in the effluent, and within Swiss national standards (<2.0 mgNH(4)-N/l, <0.3 mgNO(2)-N/l, 24 h average). All the strategies were successful at keeping ammonium low and subsequently at gaining denitrification capacity to significantly reduce the total nitrogen discharge. Some control strategies however generated temporary peaks of ammonium or even accumulation of nitrite.

수경재배용 배양액의 이온성분 분석을 위한 고체형 센서 모듈 개발

A solid state ion sensor module has been developed and evaluated for hydroponic nutrients analysis. The sensor module consisted of five ion-selective electrodes (ISE) fabricated by screen-printing technology. The electrochemical responses of ion sensors for nitrate, ammonium, potassium, calcium, and pH were measured with specially designed 7-channel low voltage signal transducers. The analytical characteristics of the sensors were comparable with those of conventional ISE sensors. The solid state ion sensors exhibit linear relationships over five concentration decades. Detection limit of the sensors were depends on ions. Performance test results showed that relative errors of measured ion concentrations were less than 5% for ion, and pH. The concentration of , and pH ion in standard solution and nutrient solutions could be determined by direct potentiometric measurements without any conditioning before measurements.