Measurement Uncertainty Estimation in Amperometric Sensors: A Tutorial Review

This tutorial focuses on measurement uncertainty estimation in amperometric sensors (both for liquid and gas-phase measurements). The main uncertainty sources are reviewed and their contributions are discussed with relation to the principles of operation of the sensors, measurement conditions and properties of the measured samples. The discussion is illustrated by case studies based on the two major approaches for uncertainty evaluation–the ISO GUM modeling approach and the Nordtest approach. This tutorial is expected to be of interest to workers in different fields of science who use measurements with amperometric sensors and need to evaluate the uncertainty of the obtained results but are new to the concept of measurement uncertainty. The tutorial is also expected to be educative in order to make measurement results more accurate.

Related papers:

title: Amperometric sensing in the gas-phase
abstract: The design of amperometric sensors for gaseous species presents a challenge as the gas phase has to be brought into contact with a solid electrode as well as an electrolyte phase which usually is liquid. However, many species of analytical interest are electroactive, such as SO 2 , NO x , O 3 , CO, formaldehyde or ethanol, and electrochemical means are always attractive in designing sensors because the electronic signal is obtained directly. Therefore, different approaches have been implemented and some types of such sensors have been available commercially for quite some time. Nevertheless, many new developments with regard to an improvement of sensitivity, selectivity and in the construction of these devices have been reported over the last few years, as well as approaches to miniaturization. In this review it is attempted to give an overview of the state of the art of this field, highlighting recent developments.

title: Amperometric Gas Sensors of High Sensitivity
abstract: Amperometric gas sensors for the detection of ethylene, ethanol, acetaldehyde, and sulfur dioxide down to very low ppb concentrations are described. The design is based on porous Au electrodes supported on solid polymer electrolytes (SPE). Nafion as cation exchange membrane and a chemically grafted perfluorinated anion exchange membrane were employed as SPE. The sensors may be tailored to the highly sensitive detection of a specific gas by carefully selecting the component materials and the experimental conditions. The influence of gas flow rate, relative humidity of the gas streams and real surface area of the SPE electrodes are discussed.

title: Microfabricated amperometric gas sensors
abstract: A series of Nafion-based amperometric gas sensors have been constructed using microelectronic fabrication technology. These sensors use a gold sensing-electrode in the shape of a square grid. Three different grid densities were used. To ascertain the optimal design parameters for such sensors, the response characteristics of the various microfabricated sensors were compared to each other and to a conventional 'macro' amperometric sensor. The critical performance criteria considered were sensor response time, structural integrity under operating conditions, selectivity, and sensitivity to the analyte vapor. The response time of the miniaturized sensors was over an order of magnitude faster, although they had a lower sensitivity than conventional sensors. The sensitivity of these sensors was dependent on the total length of the gold-Nafion interface. A model that rationalizes this dependence is presented.

title: Measurement uncertainty revisited : Alternative approaches to uncertainty evaluation
abstract: Editorial notes This report was prepared by a Eurolab expert group on measurement uncertainty, affiliated to the Eurolab Technical Committee on Quality Assurance in Testing (TCQA), that was established for this purpose and disbanded after completing the work. The group held a series of meetings in Berlin Example 4: Comparing and combining calculated uncertainty and experimental variability – a study of sample preparation uncertainty in chemical analysis..

title: Amperometric sensor for selective and stable hydrogen measurement
abstract: Amperometric gas sensors in a three-electrode configuration were investigated for hydrogen measurements in atmospheres containing carbon monoxide. The hydrogen-sensing working electrode was based on a platinum black electrode deposited on a porous Teflon® substrate. To achieve selectivity, an FEP membrane was mounted at the gas inlet of the sensor. Hydrogen readily diffused through the FEP membrane and was detected on the Pt working electrode. The cross-sensitivity to gases such as carbon monoxide, ammonia and hydrogen sulfide was eliminated with nearly 100% efficiency in the short term. In addition, the poisoning effect of the carbon monoxide was virtually eliminated. Sensor stability to high levels of hydrogen was improved by the development of a hydrogen-insensitive reference electrode.

title: Metrological parameters of sulphur dioxide amperometric sensor containing addition of aprotic solvents
abstract: The paper presents the results of investigation on influence of aprotic solvents contained in an internal electrolyte on metrological parameters of a prototype of sulphur dioxide sensor with Nafion membrane. Five sensors differing in composition of the internal electrolyte have been compared. Gold electrode vacuum sublimation deposited on the Nafion membrane surface was a working electrode. The electrolytes were aqueous solutions of sulphuric acid of the summary concentration 5 mol dm−3 (electrolyte A). The electrolyte B contained an addition of dimethylsulphoxide (DMSO), the electrolyte C contained an addition of N,N-dimethylformamide (DMF), the electrolyte D contained an addition of acetonitrile (AC) and the electrolyte E contained an addition of propylene carbonate (PC); water/solvent molar ratio in these solutions was 2:1. Based on a proposed equation an influence of the analyte flow rate on the sensor signal and response time to changes of SO2 concentration was investigated.

title: New functional possibilities for amperometric dissolved oxygen sensors
abstract: This papers deals with the study of new functional possibilities for amperometric sensors and problems in developing sensors capable of measuring oxygen transport characteristics (OTC) in liquids. The development of such sensors is carried out on the basis of a proposed scheme which differs from the Clark electrode in that the liquid to be analyzed is held on the external surface of the gas-permeable membrane. The functional possibilities of these sensors were studied by analysis of a three-layer dynamic model and by experimental investigations. It is demonstrated that these sensors may be applied in three areas: • partial oxygen measurement; • the measurement of oxygen transport characteristics, such as: dissolved oxygen concentration, oxygen specific flow in liquid, its coefficients of solubility, permeability and diffusion in liquid; • a combination of these. From this model conditions were formulated for choosing from a range of elementray parameters, to ensure the correct functioning of amperometric sensors in each of the above applications, with the required degree of measurement accuracy. An analysis of sensor behaviour was carried out, based on a model of diffusion-kinetic processes, permitting the explanation of the nature of systematic errors in sensor operation occuring in each of the application areas/modes mentioned. This model was also used for developing the algorithm for calculating the oxygen diffusion coefficient from the dynamic curve. The results of these theoretical investigations were confirmed satisfactorily by experimental investigations into the design of amperometric sensors.

title: Amperometric monitoring of sulphur dioxide in liquid and air samples of low conductivity by electrodes supported on ion-exchange membranes
abstract: An amperometric sensor is described for the determination of sulphur dioxide in both gaseous atmospheres and solutions of low conductivity. It consists of a porous Pt electrode (facing the sample) supported on one face of an ion-exchange membrane (Nafion 417) which serves as a solid polymer electrolyte. The other side of the membrane faces an internal electrolyte solution (1 mol dm–3 aqueous perchloric acid) containing the counter and reference electrodes. This sensor is inserted into a flow cell in which gaseous or electrolyte-free aqueous samples are fed by a peristaltic pump placed in a closed-loop path and SO2 is oxidized at an applied potential of 0.65 V versus Ag–AgCl. The device is found to be characterized by a high current sensitivity and a short response time, 24 A cm–2 mol–1 dm3 and 1 s respectively for gaseous samples; (0.4 A cm–2 mol–1 dm–3 and 4 s, respectively, for water solutions), and by good stability and low background noise. The dynamic range extends up to 2 × 10–4 mol dm–3(gaseous samples) and 1 × 10–3 mol dm–3(water samples) with good linearity, and detection limits of 8 × 10–9 mol dm–3(gaseous samples) and 4 × 10–7 mol dm–3(water samples) are predicted for a signal-to-noise ratio of 3. The advantages offered by this type of sensor over conventional gas-permeation membrane electrodes are discussed.

title: Solid‐state amperometric sensors for gas phase analytes: A review of recent advances
abstract: A review is geven on solid-state amperometric sensors designed for the detection of gaseous analytes. Attention is geven primarily to sensors used in the absence of a bulk liquid phase that have been reported in the literature since 1992, although a historical perspective is presented. The review is limited to sensors employing solid polymer electrolytes, transition metal powders, and inorganic materials prepared by sol-gel processes as the solid-state electrylyte. The wide applicability of solid atate sensors to gas phase species is demonstrated through numerous examples. Analytes determined include O2, H2, CO, NO2, NH3, and SO2.

title: Chemical sensors based on nanostructured materials
abstract: This article provides a comprehensive review of current research activities that concentrate on chemical sensors based on nanotubes, nanorods, nanobelts, and nanowires. We devote the most attention on the experimental principle, design of sensing devices, sensing mechanism, and some important conclusions. We elaborate on development of chemical sensors based on nanostructured materials in the following four sections: (1) nanotube sensors; (2) nanorod sensors; (3) nanobelt sensors; (4) nanowire sensors. We conclude this review with personal perspectives on the directions towards which future research on nanostructured sensors might be directed.

title: Carbon Nanotubes as Active Components for Gas Sensors
abstract: The unique structure of carbon nanotubes endows them with fantastic physical and chemical characteristics. Carbon nanotubes have been widely studied due to their potential applications in many fields including conductive and high-strength composites, energy storage and energy conversion devices, sensors, field emission displays and radiation sources, hydrogen storage media, and nanometer-sized semiconductor devices, probes, and quantum wires. Some of these applications have been realized in products, while others show great potentials. The development of carbon nanotubes-based sensors has attracted intensive interest in the last several years because of their excellent sensing properties such as high selectivity and prompt response. Carbon nanotube-based gas sensors are summarized in this paper. Sensors based on single-walled, multiwalled, and well-aligned carbon nanotubes arrays are introduced. Modification of carbon nanotubes with functional groups, metals, oxides, polymers, or doping carbon nanotubes with other elements to enhance the response and selectivity of the sensors is also discussed.

title: Electrochemistry and electroanalytical applications of carbon nanotubes: a review.
abstract: This review addresses recent developments in electrochemistry and electroanalytical chemistry of carbon nanotubes (CNTs). CNTs have been proved to possess unique electronic, chemical and structural features that make them very attractive for electrochemical studies and electrochemical applications. For example, the structural and electronic properties of the CNTs endow them with distinct electrocatalytic activities and capabilities for facilitating direct electrochemistry of proteins and enzymes from other kinds of carbon materials. These striking electrochemical properties of the CNTs pave the way to CNT-based bioelectrochemistry and to bioelectronic nanodevices, such as electrochemical sensors and biosensors. The electrochemistry and bioelectrochemistry of the CNTs are summarized and discussed, along with some common methods for CNT electrode preparation and some recent advances in the rational functionalization of the CNTs for electroanalytical applications.

title: Enhanced sensing performance of the amperometric gas sensor by laser-patterning of the polymer membrane electrode
abstract: Ultraviolet laser patterning has been employed for enhancing the sensing performance of the electrochemical gas sensors. Two kinds of catalyst layers (platinum for CO and gold for SO 2 ) coated on the polytetrafluoloethylene film have been selectively removed by KrF laser irradiation. The laser fluence, repetition rate and number of shots were carefully controlled to avoid the damage of the membrane substrate. The threshold fluences for complete removal of gold film and PTFE-bonded platinum layer are about 60 and 500 mJ/cm 2 , respectively. The sensors (especially the SO 2 sensors) with patterned catalyst electrodes exhibit excellent sensing capabilities in sensitivity as well as the response and recovery times. The sensitivity of the SO 2 sensor with patterned sensing electrode is about 0.63 μA/ppm, 14 times improved as compared with the unpatterned sensor. The response and recovery times are shortened from 11 and 17 s to 6 and 9 s, respectively. The signal-to-noise ratio is improved by two times. Due to the original good porous microstructure of the PTFE-bonded platinum layer, the improvement of the patterned CO sensor is not as much as that of the patterned SO 2 sensor. However, the sensitivity is doubled. The great enhancement in the sensing performance is attributed to the increase of the electrochemically active surface area and the higher diffusion efficiency.

title: Oxygen sensors based on a new design concept for amperometric solid state devices
abstract: A new design for all-solid amperometric detectors was tested as an oxygen sensor in the 1%–25% v/v concentration range. The design consisted of both the working (WEs) and counter electrodes (CEs) being vacuum-deposited as non-porous Au layers on the same face of a Nafion® membrane and in contact with the gas sample. Both a three-electrode device (the reference electrode being a strip of Ag/AgCl inserted between the two Au layers) and a two-electrode one were tested. In the former case, the sensor exhibited good linearity with oxygen concentration, response times comparable to a commercial sensor but a strong dependence on humidity. The origin of the latter is not due to ohmic losses but rather to the loss of catalytic activity with decreasing water contact in the polymer. The two-electrode device exhibits signal saturation at high oxygen concentrations, which is interpreted by limitations imposed by the CE reactivity. In both cases, oxygen reduction led to an exponential current rise over a wide potential range indicating very high mass transport rates and implying that the electroactive gas reacts at the line formed by the gas/solid electrolyte/metal layer interface.

title: A Nafion®-based co-planar electrode amperometric sensor for methanol determination in the gas phase
abstract: A co-planar electrode device, fabricated with all electrodes (working, counter and reference) on the same face of a Nafion® polymer electrolyte membrane, is proposed for the amperometric detection of gaseous methanol using Pt as the working electrode. Clear voltammetry is obtained for methanol oxidation from its vapours in equilibrium with methanol aqueous solutions, both in the absence and presence of oxygen in the gas stream. Using an appropriate pulse sequence to keep the indicator electrode active, methanol vapours in the 1–13 Torr partial pressure range (in equilibrium with methanol aqueous solutions in the 1–10% w/w concentration range) could be determined, in a constant potential, amperometric mode. The methanol detector could be operated both in a nitrogen stream and (in what is essential for practical applications) in an air atmosphere too, with estimated detection limits of 1·2 and 1·4 Torr respectively.

title: A new approach to the design of amperometric gas sensors
abstract: Preliminary experiments are described in order to demonstrate the principle of a new design of amperometric gas sensors. In this design, non-porous electrodes are all fabricated onto the gas side of a proton conducting medium so that the oxidation/reduction of the gas to be sensed occurs at the three phase interface between the atmosphere, the electrode and the proton conducting medium, i.e. the system approximates to a line electrode. Such a cell is described which gives good quality responses for the oxidation of SO2 from the gas phase.

title: Indirect measurement of oxygen solubility
abstract: This paper introduces an experimental methodology for estimating saturation concentrations of oxygen in surface waters and wastewater that employs commonly available analytic techniques (polarographic probe and Winkler titration) for dissolved oxygen (DO) analysis. Because DO analysis techniques can contain significant measurement bias, a protocol intended to minimize the estimate uncertainty is proposed. The protocol's chief attribute is its ability to estimate solubility indirectly, i.e. it avoids the complication of equilibrating samples with the atmosphere and uses measurements with unsaturated water to project a value of solubility. Using this protocol, saturation concentrations in Mississippi River water collected in Minnesota were measured. The ratio of the saturation concentration of Mississippi River water to pure distilled water varied seasonally in 1993 and 1994, with slightly higher ratios in the winter months and lower values in the late summer and fall. The lowest measured ratio was more than 8% below the equivalent clean water value.

title: Amperometric gas sensor response times.
abstract: This work examines the relative importance of diffusion and the electrical time constant of the working electrode on the response time of amperometric gas sensors. The response curve of an electrochemical carbon monoxide sensor to a 200 ppm CO test gas was measured as a function of the added resistance that is in series with the working electrode to increase the time constant. It has been found that the response time increases with additional resistance. The experimental behavior was compared to the values predicted on the basis of Fick's second law of diffusion and to the values expected on the basis of the electrical properties of the sensor. The behavior of the sensor was explained using an equivalent electrical circuit, describing the time constants of the working electrode. Values obtained from this model were further compared to experimental values obtained from electrochemical impedance spectra. It has been found that the response time of the sensors depends on both the rate of diffusion and the electrical time constants of the working electrode.

title: Amperometric determination of ethanol in beverages at copper electrodes in alkaline medium
abstract: The anodic oxidation of ethanol at copper surfaces in alkaline medium has been reported. The participation of electrogenerated Cu(III) species in the electrocatalysis of the oxidation of ethanol at copper surfaces was investigated by rotating ring-disc voltammetry and a mechanism involving the participation of radicals was proposed. The copper oxide sensor was assembled in a wall-jet electrochemical cell and a FIA method that incorporates flow extraction of ethanol through a PTFE membrane at room temperature was developed. Under optimal experimental conditions the peak current response (measured at 0.5 V versus Ag/AgCl) increases linearly with ethanol concentration over the range 2–10% (v/v). The repeatability of the electrode response in the FIA configuration was evaluated as 3% ( n =20) and the limit of detection of the method was 0.4% ( S / N =3). The analytical frequency was estimated to be 120 h −1 . Experiments to investigate the selective of the PTFE membrane were carried out and they demonstrated that only small molecules such as methanol may interfere in the determination of ethanol. For some samples of different alcoholic beverages, the results showed excellent agreement ( r =0.998) with the ones obtained by using gas chromatography.

title: A Novel Amperometric Nitric Oxide Sensor Based on Polythionine / Nation Modified Glassy Carbon Electrode
abstract: A novel amperometric sensor for the determination of nitric oxide was developed by coating polythionine/nafion on a glassy carbon electrode. This sensor exhibited a great enhancement to the oxidation of nitric oxide. The oxidation peak currents were linear to the concentration of nitric oxide over the wide range from 3.6×10^(-7) to 6.8×10^(-5)mol.L^(-1), and the detection limit was 7.2×l0^(-8)mol.L^(-1). Experimental results showed that this nitric oxide sensor possessed excellent selectivity and longer stability. NO releasing from rat kidney was monitored by this sensor.

title: Study of nano-Au-assembled amperometric CO gas sensor
abstract: A novel electrochemical sensor based on nano-Au colloid modified on Pt disk microelectrode was developed for determination of carbon monoxide (CO). The catalytic activation of nano-Au colloid modified on Pt disk microelectrode as substrate was investigated in different supporting electrolyte solutions by cyclic voltammetry and constant potential transient method. The result shows that nano-Au colloid modified on Pt disk microelectrode exhibited strong catalytic effect toward the electrochemical oxidation of CO in 0.5 mol l −1 HClO 4 . As compared with the bare Pt disk microelectrode, the nano-Au colloid modified on Pt disk microelectrode can greatly decrease the overpotential and obviously increase the current of CO oxidation. The current–time curve recorded under conditions of constant potential and various CO concentrations suggests that current response depends linearly on CO concentration. A linear equation I (μA) = 0.00917 (μg ml −1 ) + 0.151 with a correlation coefficient of 0.9938 is obtained over the concentration range 0.756 μg ml −1 at potential +800 mV relative to Ag/AgCl reference electrode. The detection limit is 0.65 μg ml −1 and the relative standard deviation is 5.3% ( n  = 5) at room temperature.

title: An amperometric microsensor for the determination of H2S in aquatic environments
abstract: A new amperometric microsensor for detection of dissolved H2S in aquatic environments was developed. The design of the microsensor is based on the same principle as the Clark-type oxygen microsensor. The sensor is equipped with a glass-coated platinum working electrode and a platinum guard electrode positioned in an outer glass casing (tip diameter 20−100 μm). Both working electrode and guard electrode were polarized at a fixed value in the range from +85 to + 150 mV with respect to a counter electrode. The outer casing is sealed with a thin silicone membrane and filled with a buffered electrolyte solution containing ferricyanide (K3[Fe(CN)6]) as redox mediator. Hydrogen sulfide penetrates the silicone membrane and is oxidized by K3[Fe(CN)6], resulting in the formation of elemental sulfur and ferrocyanide (K4[Fe(CN)6]). The latter is electrochemically reoxidized at the exposed end of the platinum working electrode, thereby creating a current that is directly proportional to the dissolved H2S concentration...

title: Flow injection, amperometric determination of ethanol in wines after solid‐phase extraction
abstract: Ethanol in wines was determined by flow injection analysis with an amperometric detector using an oxidized nickel wire. Solid-phase extraction with a strong anion exchanger was used to remove interferences such as organic acids from the matrix, and the residue of the extraction was injected directly into the FIA system. The recoveries of ethanol from wines spiked with standards ranged from 101% to 103%. The response of the nickel electrode to ethanol is dependent on the applied potential and the pH of the carrier. The optimal conditions for the detection of ethanol were an applied potential of +0.60 V (vs. Ag/AgCl) in a carrier of 100 mM sodium hydroxide solution. The electrode exhibited a linear response from 10−5 to 10−3 M, with a detection limit of 1 × 10−6 M. The method was demonstrated by the determination of ethanol in wines.

title: Electrochemical determination of sulphide at nickel electrodes in alkaline media: a new electrochemical sensor
abstract: The electrochemistry of a nickel hydroxide electrode has been studied both in the presence and absence of sulphide at microdisc and macroelectrodes. With sulphide present a new oxidative wave is observed. At a macroelectrode, this response produced a linear range from 20 to 200 μM with a corresponding limit of detection of 19 μM. Under the microelectrode regime, the response was found to be linear from 20 to 200 μM with a detection limit of 10 μM. The protocol has been developed into the design of a simple and cheap electrochemical sensing cell for the detection of sulphide in aqueous media.

title: A novel amperometric sensor for the determination of nitric oxide, and its application in rat liver cells
abstract: A novel amperometric nitric oxide sensor with a wide linear range, low detection limit and fast response time was developed. The sensor was fabricated using a poly-brilliant cresyl blue (PBCB)/Nafion film modified glassy carbon electrode (GCE). The PBCB on the working GCE surface dramatically improves the oxidation response of nitric oxide and lowers the required potential for the two-step oxidation of NO. Otherwise, Nafion coated onto the film electrode surface does not only improve the selectivity of the sensor, but also further lowers the active energy of the direct three-electron oxidation of NO to nitrate. The effect of the preanodic time and the film thickness of the electropolymerization on the nitric oxide response as well as the volume of Nafion coated onto the surface and the potential for amperometric detection were optimized. This novel sensor has been applied to the determination of NO released from rat liver cells, and the result is satisfactory.

title: A novel nano-Au-assembled amperometric SO2 gas sensor: preparation, characterization and sensing behavior
abstract: In this paper, a novel nano-Au-assembled gas sensor was prepared and characterized with transmission electron microscope (TEM) and electrochemical methods, and its sensing behavior to SO 2 was also investigated. Studies showed that when nano-Au particles were assembled on Pt working electrodes, the electrochemical behavior of the Pt electrode was changed in 1.00×10 −3  mol l −1 K 3 Fe(CN) 6 +0.10 mol l −1 KNO 3 , which indicated that the adsorption of Au colloid occurred on the Pt electrodes. Current responses of the nano-Au-assembled gas sensor to SO 2 in different electrolytes, such as 0.1 M HCl, 1 M H 2 SO 4 , 0.1 M HClO 4 and 1 M NaOH, were investigated with cyclic voltammetry. It was found that the optimal conditions for determining SO 2 were 1 M NaOH as internal electrolyte and +0.6 V as applied potential, while other copresent gases, such as CO, NO, NH 3 and CO 2 , did not cause interference under these conditions. It was also found that the linear scope was from 5 to 500 ppm, the sensitivity was 5.4×10 −11  A ppm −1 and the detection limit was 2.6 ppm (S/N) when the nano-Au assembled gas sensor was used to determine SO 2 under the optimal conditions.

title: Nitric oxide determination by amperometric carbon fiber microelectrode.
abstract: Nitric oxide (NO) amperometric microsensor was prepared by the modification of bare carbon fiber electrode by Nafion and cellulose acetate (CA). Detection limit, response time, reproducibility and influence of some possible interferences (nitrite, nitrate, arginine) were tested and evaluated. This sensor was used for in vitro determination of NO release from fresh porcine aorta induced by calcium ionophore A23187 (CI).

title: Study on response time of SPE carbon monoxide sensor
abstract: The influence of structural design and the parameters of the working electrode on the response time of a solid polymer electrolyte (SPE) carbon monoxide sensor has been studied. Results show that the response time is mainly determined by the RC time constant of the catalyst layer and also related with the working electrode potential.

title: Amperometric Determination of Hydrogen Peroxide With a Manganese Dioxide-modified Carbon Paste Electrode Using Flow Injection Analysis
abstract: A carbon paste electrode bulk modified with MnO 
 2 
 was 
investigated as an amperometric detector for hydrogen peroxide in flow 
injection analysis (FIA). With an operating potential of +0.46 V 
versus Ag/AgCl, H 
 2 
O 
 2 
 produces catalytic 
oxidation currents which can be exploited for quantitative determinations. 
Factors influencing the analytical performance of the electrode, such as 
paste composition and pH, were studied both for batch voltammetry and for 
FIA. For the flow system the influence of the injection volume and flow 
rate were examined. The amperometric signals are linearly proportional to 
H 
 2 
O 
 2 
 concentrations in the range 0.5–350 mg 
l 
 - 
 
 1 
, showing a detection limit (three times 
the signal-to-noise ratio) of 45 µg 
l 
 - 
 
 1 
.

title: Assessment of the uncertainty budget for the amperometric measurement of dissolved oxygen.
abstract: This work aimed at identifying the main sources of uncertainty for the measurement of dissolved oxygen concentration in aqueous solutions. The experimental apparatus consists of an amperometric cell based on the Clark-type sensor. The corresponding uncertainty budget was assessed, this being a fundamental step for the validation of a measurement method. The principle of the measurement, as well as the procedure for the set-up and the characterisation of the cell, are described. The measurement equation was defined as a combination of Faraday's and Fick's laws, and a method was worked out for the empirical determination of the diffusivity parameter. In this connection, the solutions of oxygen were standardised by way of the Winkler's titration, as suggested by the ISO Guide 5813 and 5814. With this approach we aimed at contributing to the development of a potential primary method of measurement. A discussion of all the contributions to the overall uncertainty is reported, allowing operators to locate the largest ones and plan specific improvements.

title: Novel poly(arylene ether) as membranes for gas separation
abstract: Fluorine containing polymers are of special interest because of their possible use as gas selective membranes, their remarkably low water absorption and their low dielectric constant and enhanced flame resistance. In this work, the gas transport and separation properties of H2, CO2, O2, N2 and CH4 were studied for four novel poly(arylene ether)s (PAEs) membranes, two containing the 2,6-bis(trifluoromethylphenylene)pyridine unit (6FPPy series) and the others containing 2,5-bis(3-trifluoromethylphenylene)thiophene unit (6FPT series) in the polymer backbone. The preparation of this new class of semifluorinated polymers relied on the aromatic substitution polymerization of trifluoromethyl activated bishalo monomers with different bis(phenol)s. It was found that the introduction of CF3 groups into the bisphenol unit increases the gas permeabilities and decreases slightly the selectivities for both series of membranes. The gas permeability coefficients of 6FPPy membranes are higher than those of 6FPT membranes while their selectivities are comparable. These results were discussed based on the gas diffusion and solubility in the membranes and could be ascribed to the polymer structure. The effect of temperature on the gas transport properties of these membranes was measured and the corresponding activation energies for gas permeation and diffusion were calculated.

title: Polymer membranes for gas separation
abstract: In the past several decades, membrane separation has gone from a laboratory curiosity to commercial reality. These membranes often have a separating layer thickness

title: Electrochemical oxidation and the quantitative determination of phosphine on a SPE-based sensor.
abstract: A new type of solid polymer electrolyte (SPE) PH(3) electrochemical sensor is reported. The oxidation of PH(3) on the SPE-Pt electrode is studied by electrochemical and XPS methods. At the same time, the linearity, the stability and response time of the sensor is investigated and discussed. The results obtained indicate the SPE-based PH(3) sensor possesses a number of attractive properties.

title: Failure prediction for a galvanic oxygen sensor
abstract: Electrochemical oxygen sensors are widely used for industrial hygiene applications to monitor potentially dangerous conditions due to oxygen deficiency or excess. Oxygen sensors of the galvanic type operate by consumption of an internal, easily oxidizable anode. Once the entire anode has been consumed, the sensor will no longer respond to oxygen and this failure of the sensor is often rapid and with little warning. This paper describes an electrical test, consisting of measuring the ratio of the steady state current at an applied potential of ∼0.4 V vs. the steady state current at 0 V. A ratio close to unity indicates that the sensor is functioning normally, whereas a ratio much less than unity indicates that the sensor will fail soon. This test may be performed in situ on the oxygen sensor by a gas detection instrument, which provides a means for the detection of an imminent failure, while the sensor is still operational.

title: Improved planar amperometric nitric oxide sensor based on platinized platinum anode. 1. Experimental results and theory when applied for monitoring NO release from diazeniumdiolate-doped polymeric films.
abstract: An improved miniature amperometric nitric oxide sensor design with a planar sensing tip (ranging from 150 microm to 2 mm in diameter) is reported. The sensor is fabricated using a platinized platinum anode and a Ag/AgCl cathode housed behind a microporous poly(tetrafluoroethylene) (PTFE; Gore-tex) gas-permeable membrane. Platinization of the working platinum electrode surface dramatically improves the analytical performance of the sensor by providing approximately 10-fold higher sensitivity (0.8-1.3 pA/nM), approximately 10-fold lower detection limit (< or =1 nM), and extended (at least 3-fold) stability (>3 d) compared to sensors prepared with bare Pt electrodes. These improvements in performance arise from increasing the kinetics and lowering the required potential for the 3-electron oxidation of NO to nitrate, relative to that observed using a nonplatinized working electrode. The outer porous PTFE membrane provides complete selectivity for NO over nitrite ions (up to 10 mM nitrite). The new sensor is applied for surface measurements of NO released from diazeniumdiolate-loaded silicone rubber films (SR-DACA-6/N(2)O(2)). The effects of sensor size (for sensor dimensions of 0.15-, 1-, and 2-mm o.d.) and the distance of the sensor from the surface of the NO-emitting polymer film are investigated via experiments as well as theoretical calculations. A significant analyte trapping effect is demonstrated, the degree of which depends on the sensor size and its distance from the surface. It is further demonstrated that surface NO concentrations for fresh SR-DACA-6/N(2)O(2) loaded films are also influenced by the polymer film thickness, with thicker films generating higher surface concentrations of NO.

title: The electrochemical oxidation and the quantitative determination of hydrogen sulfide on a solid polymer electrolyte-based system
abstract: A study has been made of the adsorbed products of H 2 S on a solid polymer electrolyte (SPE)-Pt electrode by electrochemical and XPS methods. The adsorbed products depend on the potential. On the basis of an investigation of the oxidation of hydrogen sulfide, a quantitative determination of H 2 S concentration in the gas phase was carried out on the SPE-based electrode system in a controlled potential mode. The electrode system has a number of attractive properties, including a low detection limit, good reproducibility, fast response and satisfactory linearity.

title: Mackereth Oxygen Sensor: Measurement Uncertainty
abstract: The measurement of oxygen concentrations in natural and waste water is a major requirement of environmental monitoring. Since 1953 amperometric membrane-covered sensors are commonly used for dissolved oxygen measurements. The device consists of a two-electrode electrochemical cell with a thin gas-permeable membrane separating the electrodes and the electrolyte solution from the analyzed media. The reliability of dissolved oxygen analysis has been extensively discussed in the relevant literature. There is a remarkable disagreement between the values of measurement uncertainty of dissolved oxygen measurement reported by different authors. We draw out the main uncertainty sources and show the potential measures for increasing the reliability of continuous electrochemical oxygen measurements, carried out in environmental monitoring and waste water treatment.

title: Improved Nafion-based amperometric sensor for hydrogen in argon
abstract: An improved polymer electrolyte membrane fuel cell-based amperometric hydrogen sensor has been developed. The sensor operates at room temperature, and the electrolyte used in the sensor is Nafion which is a proton-conducting solid polymer electrolyte. Platinum black is used as both anode and cathode. The sensor functions as a fuel cell, H2/Pt//Nafion//Pt/O2, and a mechanical barrier limits the supply of hydrogen to the sensing side electrode. The limiting current is found to be linearly related to the hydrogen concentration. The sensor can be used to measure hydrogen in argon in parts per million and percentage levels. The basic principle, details of assembly, and response behavior of the sensor are discussed.

title: Estimation of uncertainty in pKa values determined by potentiometric titration
abstract: A procedure is presented for estimation of uncertainty in measurement of the pKa of a weak acid by potentiometric titration. The procedure is based on the ISO GUM. The core of the procedure is a mathematical model that involves 40 input parameters. A novel approach is used for taking into account the purity of the acid, the impurities are not treated as inert compounds only, their possible acidic dissociation is also taken into account. Application to an example of practical pKa determination is presented. Altogether 67 different sources of uncertainty are identified and quantified within the example. The relative importance of different uncertainty sources is discussed. The most important source of uncertainty (with the experimental set-up of the example) is the uncertainty of pH measurement followed by the accuracy of the burette and the uncertainty of weighing. The procedure gives uncertainty separately for each point of the titration curve. The uncertainty depends on the amount of titrant added, being lowest in the central part of the titration curve. The possibilities of reducing the uncertainty and interpreting the drift of the pKa values obtained from the same curve are discussed.

title: Interferences in a polypyrrole-based amperometric ammonia sensor.
abstract: The main focus of this article is on the interferences encountered when an amperometric ammonia sensor is used in matrices that are typical in clinical samples. The sensor is tested in presence of such interferences and ways to suppress them by sensor modification are presented. Two modification procedures are evaluated by comparing the stabilities, detection limits and signal-to-noise ratios of such sensors. Further, the issue of the ammonia-sensing mechanism of the polypyrrole electrode is addressed. Evidence is shown that a mobile counterion may be required for proper sensor operation. Such evidence supports the idea that polypyrrole undergoes a reversible redox reaction when ammonia is detected at submillimolar concentrations.

title: Uncertainty sources in UV-Vis spectrophotometric measurement
abstract: An overview is given of the most important uncertainty sources that affect analytical UV-Vis spectrophotometric measurements. Altogether, eight uncertainty sources are discussed that are expected to have influence in chemical analysis. It is demonstrated that the well-known intrinsic (or “physical”) sources of uncertainty that originate from the instrument itself (repeatability of spectrophotometer reading, spectrophotometer drift, stray light, etc.) often have significantly lower contributions to the combined uncertainty of the result than the “chemical” sources of uncertainty that originate from the object under study (interference from the constituents of the matrix, decomposition of the photometric complex, etc.). Although selectivity of a photometric procedure is often considered more a validation topic than an uncertainty topic, it is very often important to include it also in the uncertainty budget.Usually the most difficult part of uncertainty estimation of a chemical measurement result is to evaluate the magnitude of the actual uncertainty components, especially the chemical ones. For most of the uncertainty sources discussed in this paper, approaches for their evaluation are given. A generic uncertainty budget for absorbance is presented.

title: Determination of alcohols using a Ni–Pt alloy amperometric sensor
abstract: An amperometric sensor for monitoring ethanol concentration using a Ni–Pt alloy electrode has been developed. The films of Ni–Pt alloy were electrodeposited under various potentials on an Au/Al2O3 substrate. To examine the effects of Ni–Pt alloy films on the sensing performance, the electrodes with various Ni:Pt atomic proportions of 100:0, 25:75, 70:30, 82:18 and 0:100 were prepared and tested. X-ray diffraction and scanning electron microscope analyses indicate that Ni atoms of Ni–Pt alloy electrodes are inserted in the Pt lattice and a pyramid-like structure is formed with an increase of Pt content in the film. All the prepared electrodes have a linear relationship between response current and ethanol concentration for the detection range of 50 to 300 ppm ethanol in alkaline solutions. With an increase of Pt content in the film, the response time of the Ni–Pt alloy electrodes was reduced whereas the sensitivity was decreased. The sensor with 70 at.% Pt in the film was most stable with the duration over a 63-day-period. The sensitivity of the Ni–Pt alloy electrodes for detecting glucose, various alcohols and acids have also been studied.