An improved computer program for calculating formation constants of ligand complexes from pH data

Several improvements are made to available computer programs for calculating formation constants from pH data. The most important features are wide portability of program among computers, ability to optimize any parameter in the model equations and to fix any parameters (including stepwise formation constants) to a constant, and options to allow for variations in thermodynamic activity of ions and to fit pH after fitting the volume of added reagent. The latter feature was used with synthetic data to explore the effects of random errors in either reagent volume or pH or both. Whereas fits to reagent volume will usually be more accurate for systems with pK_a values near 7, this is not the case for systems with pK_a values which differ by > 2.0 from 7.     

Calculation of reference pH values for standard solutions from the corresponding acid dissociation constants

Making the simplest possible assumption about the activity coefficient of the charged species, pH values of standard buffer solutions have been evaluated from the thermodynamic acidity constants, K, of the weak acids involved. A general equation is given for a triprotic acid, H₃A, as it can be simplified to derive the equations for other systems. A computer program for the solution of the equation was written giving m_H values, species distribution coefficients, α, buffer capacities, β, species activity coefficients, γ, and ionic strength, I. Iteration was continued until agreement between successive values to within 1 ± 10^?6 was reached.The activity coefficients of singly charged ions were taken as equal to γ_Cl, where log γ_Cl=?AI^1/2 (1 + 1.5I^1/2), which is the Bates-Guggenheim convention, and those of doubly and triply charged ions were given by the valence relations of the Debye-Hückel theory as γ⁴_Cl and γ⁹_Cl, respectively.     

Development of a colorimetric microfluidic pH sensor for autonomous seawater measurements

High quality carbonate chemistry measurements are required in order to fully understand the dynamics of the oceanic carbonate system. Seawater pH data with good spatial and temporal coverage are particularly critical to apprehend ocean acidification phenomena and their consequences. There is a growing need for autonomous in situ instruments that measure pH on remote platforms. Our aim is to develop an accurate and precise autonomous in situ pH sensor for long term deployment on remote platforms. The widely used spectrophotometric pH technique is capable of the required high-quality measurements. We report a key step towards the miniaturization of a colorimetric pH sensor with the successful implementation of a simple microfluidic design with low reagent consumption. The system is particularly adapted to shipboard deployment: high quality data was obtained over a period of more than a month during a shipboard deployment in northwest European shelf waters, and less than 30 mL of indicator was consumed. The system featured a short term precision of 0.001 pH (n = 20) and an accuracy within the range of a certified Tris buffer (0.004 pH). The quality of the pH system measurements have been checked using various approaches: measurements of certified Tris buffer, measurement of certified seawater for DIC and TA, comparison of measured pH against calculated pH from pCO₂, DIC and TA during the cruise in northwest European shelf waters. All showed that our measurements were of high quality. The measurements were made close to in situ temperature (+0.2 ?C) in a sampling chamber which had a continuous flow of the ship’s underway seawater supply. The optical set up was robust and relatively small due to the use of an USB mini-spectrometer, a custom made polymeric flow cell and an LED light source. The use of a three wavelength LED with detection that integrated power across the whole of each LED output spectrum indicated that low wavelength resolution detectors can be used instead of the current USB mini spectrophotometer. Artefacts due to the polychromatic light source and inhomogeneity in the absorption cell are shown to have a negligible impact on the data quality. The next step in the miniaturization of the sensor will be the incorporation of a photodiode as detector to replace the spectrophotometer.     

Dynamic method as a simple approach for wide range pH measurements using optodes

In this paper, a flow system equipped with an optode has been suggested for wide range pH measurements. Triacetyl cellulose was used as the optode membrane in which different pH indicators were immobilized. For extending the pH range, the dynamic response rather than the steady-state response of the optode was measured. Since diffusion is the main process governing the system response, different parameters having influence on diffusion of the analyte into the membrane were optimized. Under the optimum conditions, wide range pH determination (up to 11 pH units) is simply achieved regardless of the pK_a of the pH indicator immobilized in the membrane. To validate the application of the method different indicators with different structures and pK_a values were tested and the results were all confirming the precision and accuracy of the method. The suggested method also has combined advantages of flow systems together with inherent advantages of kinetic systems.     

Multiple approaches for enhancing all-organic electronics photoluminescent sensors: Simultaneous oxygen and pH monitoring

Key issues in using organic light emitting diodes (OLEDs) as excitation sources in structurally integrated photoluminescence (PL)-based sensors are the low forward light outcoupling, the OLEDs’ broad electroluminescence (EL) bands, and the long-lived remnant EL that follows an EL pulse. The outcoupling issue limits the detection sensitivity (S) as only ~20% of the light generated within standard OLEDs can be forward outcoupled and used for sensor probe excitation. The EL broad band interferes with the analyte-sensitive PL, leading to a background that reduces S and dynamic range. In particular, these issues hinder designing compact sensors, potentially miniaturizable, that are devoid of optical filters and couplers. We address these shortcomings by introducing easy-to-employ multiple approaches for outcoupling improvement, PL enhancement, and background EL reduction leading to novel, compact all-organic device architectures demonstrated for simultaneous monitoring of oxygen and pH. The sensor comprises simply-fabricated, directionally-emitting, narrower-band, multicolor microcavity OLED excitation and small molecule- and polymer-based organic photodetectors (OPDs) with a more selective spectral response. Additionally, S and PL intensity for oxygen are enhanced by using polystyrene (PS):polyethylene glycol (PEG) blends as the sensing film matrix. By utilizing higher molecular weight PS, the ratio τ₀/τ₁₀₀ (PL decay time τ at 0% O₂/τ at 100% O₂) that is often used to express S increases ×1.9 to 20.7 relative to the lower molecular weight PS, where this ratio is 11.0. This increase reduces to ×1.7 when the PEG is added (τ₀/τ₁₀₀ = 18.2), but the latter results in an increase ×2.7 in the PL intensity. The sensor's response time is <10 s in all cases. The microporous structure of these blended films, with PEG decorating PS pores, serves a dual purpose. It results in light scattering that reduces the EL that is waveguided in the substrate of the OLEDs and consequently enhances light outcoupling from the OLEDs by ~60%, and it increases the PL directed toward the OPD. The multiple functional structures of multicolor microcavity OLED pixels/microporous scattering films/OPDs enable generation of enhanced individually addressable sensor arrays, devoid of interfering issues, for O₂ and pH as well as for other analytes and biochemical parameters.     

Effect of pH on the electrochemical reduction of some heterocyclic quinones

The reduction of 1,10-phenanthroline-5,6-quinone(I), 5,8-quinolinequinone(II) and 6,7-dichloro-5,8-quinolinequinone(III) was investigated using cyclic voltammetry and coulometry at mercury electrodes and 50% dimethylsulfoxide+water solvent. Each compound is reduced to the corresponding hydroquinone in a diffusion-controlled, reversible two-electron process. The pH-dependence of the reversible potential indicated that the quinone forms were unprotonated, but the hydroquinones could be protonated at the heterocyclic nitrogen atom with pK_a = 5.3 for I and 3.5 for III. Careful analysis of the cyclic voltammetric peak shape revealed that the difference between the standard potentials for the introduction of successive electrons, E₂⁰ ? E₁⁰, was 70 ±20, >100 and 80 ± 20 mV for I–III. Investigation of the pH-dependence of E₁⁰ and E₂⁰ showed that the pK_a of the semiquinone of I was about 8.     

A high-performance fluorosensor for pH measurements between 6 and 9

This study presents a high-performance ratiometric pH optode based on the fluorophore 6,8-dihydroxypyrene-1,3-disulfonic acid (DHPDS). The two pH-sensitive terminal hydroxy groups of DHPDS facilitated dual excitation/dual emission (F₁: λ_1,ex = 420 nm, λ_1,em = 462 nm; F₂: λ_2,ex = 470 nm, λ_2,em = 498 nm) properties for ratiometric (R_F1,F2 = F₁/F₂) normalization of sensor signal. The sensor demonstrated an exponentially decreasing ratiometric response with increasing pH, with a linear correlation (R² = 0.9936) between ¹⁰log(R_F1,F2) and pH within the pH interval 6-9. Precision determined as the IUPAC pooled standard deviation for the pH values 6.00, 7.01 and 9.01, was 0.0057 pH units for the fluorosensor and 0.0054 for a commercially available pH electrode used for comparison. Between the end-points of calibration at pH 7.01, the precision of the sensor was 0.0037 pH units. Effects from changes in ionic strength (I_tot, 10-700 mM) were more pronounced for the electrode, with a linear (R² = 0.9976) increase in response (δE/δpH) with increasing I_tot. The DHPDS-based fluorosensor, however, retained sensitivity (δ¹⁰log(R_F1,F2)/δpH = 0.8024 ± 0.0145), though with an overall increase in ratiometric signal with increasing I_tot. The preserved sensitivity despite changes in ionic strength was possibly a consequence from the dual photo-acidic properties of DHPDS. Analytical characteristics of immobilized DHPDS therefore not only facilitated high-performance measurements over a wide pH range, but also opened for straightforward simultaneous measurements of pH and ionic strength.     

A new fluorescent pH probe for extremely acidic conditions

A novel turn-off fluorescent probe based on coumarin and imidazole moiety for extremely acidic conditions was designed and developed. The probe with pK_a = 2.1 is able to respond to very low pH value (below 3.5) with high sensitivity relying on fluorescence quenching at 460 nm in fluorescence spectra or the ratios of absorbance maximum at 380 nm to that at 450 nm in UV–vis spectra. It can quantitatively detect pH value based on equilibrium equation, pH = pK_a − log[(I_x − I_b)/(I_a − I_x)]. It had very short response time that was less than 1 min, good reversibility and nearly no interference from common metal ions. Moreover, using ¹H NMR analysis and theoretical calculation of molecular orbital, we verified that a two-step protonation process of two N atoms of the probe leaded to photoinduced electron transfer (PET), which was actually the mechanism of the fluorescence quenching phenomenon under strongly acidic conditions. Furthermore, the probe was also applied to imaging strong acidity in bacteria, E.coli and had good effect. This work illustrates that the new probe could be a practical and ideal pH indicator for strongly acidic conditions with good biological significance.     

Phase diagrams of systems SrF2(Y,Ln)F3. II. Fusibility of systems and thermal behavior of phases

The phase diagrams of 14 SrF₂(Y,Ln)F₃ systems are given, where Ln are all the lanthanides except Pm and Eu. The diagrams have been constructed for temperature intervals from 850°C to the melting points according to the thermal and X-ray analysis. The fusibility diagrams for 12 systems have been obtained for the first time. The oxygen content in the specimens before and after thermal treatment was checked. The thermal behavior of the three types of solid solutions has been studied: (1) with the fluorite-type defective structure and its derivatives; (2) with the defective structure of the lanthanum fluoride, and (3) α-YF₃(α-UO₃) types. Maxima reflecting a noticeable effect of thermal stabilization on the fluorite-type structure by the heterovalent isomorphous substitution have been found for the majority of systems (with Ln = LaHo). The Sr_1?xLn_xF_2+x nonstoichiometric fluorite phases are formed in all the systems. Similar maxima corresponding generally to irrational compositions are present on the fusibility curves of the Ln_1?ySr_yF_3?y nonstoichiometric phases with the LaF₃-type structure (tysonite). Tysonite solid solutions are in all the systems, too. Nonstoichiometric phases with the α-YF₃-type structure are formed in the systems with Ln = ErLu. They are decomposed in the process of cooling and are the most unstable. The structure of the phase diagrams in the regions adjacent to lanthanide trifluorides are determined by polymorphism and morphotropy of the above-named compounds. Changes in the thermal stability of the nonstoichiometric phases and double chemical compounds in the series of lanthanides have been observed. The SrF₂(Y,Ln)F₃ systems studied give examples of the formation of phases with the highest concentrations of point defects among all the known binary fluoride systems (up to 50 at.%). The thermal stabilization effect of the nonstoichiometric phases with the fluorite structure results in the fact that the series of the two-component compositions is melted at considerably higher temperatures as compared with scandium fluoride, the most refractory single-component fluoride compound. This effect leads to formation of tysonite-type solid solutions with melting points exceeding 1500°C (mp of LaF₃—the most refractory fluoride material with tysonite-type structure).     

Simultaneous spectrophotometric flow-through measurements of pH, carbon dioxide fugacity, and total inorganic carbon in seawater

An autonomous multi-parameter flow-through CO₂ system has been developed to simultaneously measure surface seawater pH, carbon dioxide fugacity (fCO₂), and total dissolved inorganic carbon (DIC). All three measurements are based on spectrophotometric determinations of solution pH at multiple wavelengths using sulfonephthalein indicators. The pH optical cell is machined from a PEEK polymer rod bearing a bore-hole with an optical pathlength of ∼15 cm. The fCO₂ optical cell consists of Teflon AF 2400 (DuPont) capillary tubing sealed within the bore-hole of a PEEK rod. This Teflon AF tubing is filled with a standard indicator solution with a fixed total alkalinity, and forms a liquid core waveguide (LCW). The LCW functions as both a long pathlength (∼15 cm) optical cell and a membrane that equilibrates the internal standard solution with external seawater. fCO₂ is then determined by measuring the pH of the internal solution. DIC is measured by determining the pH of standard internal solutions in equilibrium with seawater that has been acidified to convert all forms of DIC to CO₂. The system runs repetitive measurement cycles with a sampling frequency of ∼7 samples (21 measurements) per hour. The system was used for underway measurements of sea surface pH, fCO₂, and DIC during the CLIVAR/CO₂ A16S cruise in the South Atlantic Ocean in 2005. The field precisions were evaluated to be 0.0008 units for pH, 0.9 μatm for fCO₂, and 2.4 μmol kg⁻¹ for DIC. These field precisions are close to those obtained in the laboratory. Direct comparison of our measurements and measurements obtained using established standard methods revealed that the system achieved field agreements of 0.0012 ± 0.0042 units for pH, 1.0 ± 2.5 μatm for fCO₂, and 2.2 ± 6.0 μmol kg⁻¹ for DIC. This system integrates spectrophotometric measurements of multiple CO₂ parameters into a single package suitable for observations of both seawater and freshwater.     

Die Berechnung komplexer Absorptionsgleichgewichte ionogener Substanzen in wäßrigen Lösungsmitteln mit Hilfe eines Reaktionsmodells

The solubility of ionogen substances in water and aqueous ionic solutions is important for calculation of absorption processes. Aqueous solutions with complex reaction systems behave themselves extremely nonideal. In simple cases equilibria can be determined with the concept of nonideal thermodynamics. The model used in this work is based on ideal calculation of reaction equilibria and gas solubility. The model parameters (equilibrium constants andHenry constants) for the systems SO₂-H₂O,MEA-H₂S-H₂O,DEA-H₂S-H₂O andMEA-CO₂-H₂O are computed by regression of experimental data. Equilibrium reactions are selected according toBrinkley's method. The selection of the reacting species has decisive influence on the accuracy of the data fitting. Data regression is done numerically and leads to the formulation of nonlinear systems of equations, which have to be solved for each data point. This solutions are performed in an inner loop. By using the maximum-likelihood-principle the model parameters are optimized in the superior regression loop. Experimental data for the regression are the partial pressure and the total concentration of gas in the liquid phase. The used model is able to fit these data satisfactoryly. The model parameters, which are calculated from simultaneous data regression for different temperatures, ensure a simple correlation ofvan't Hoff. However, for similar reactions equilibria in different reaction systems, it is impossible to compute the same values for the equilibrium constants.     

High performance liquid chromatographic analysis of phytoplankton pigments using a C16-amide column

In this study, a reverse-phase HPLC method incorporating a ternary solvent system was developed to analyze most polar and non-polar chlorophylls and carotenoids present in phytoplankton. The method is based on an RP-C₁₆-Amide column and provided excellent peak resolution of most taxonomically important pigments and an elution profile different than C₈ or C₁₈ columns provide. Analysis of mixed pigment standards, extracts of phytoplankton monocultures, and field samples showed that this method was able to resolve more than sixty pigments, ranging from very polar acidic chlorophylls to the non-polar hydrocarbon carotenes in less than 36 min. This included chlorophylls c₁, c₂ and c₃, divinyl chlorophylls a and b, the carotenoids lutein and zeaxanthin and some recently discovered pigments. The ability of this method to resolve divinyl chl b from monovinyl chl b and divinyl chl a from monovinyl chl a is particularly important for the quantification and identification of the marine cyanobacteria Prochlorococcus spp. in oceanic waters. The described protocol is sensitive and reproducible and can be used to assess the distribution and dynamics of major phytoplankton groups in marine and freshwater ecosystems.     

pH and molar ratio dependent formation of monomeric,dimeric and tetrameric cobalt malate complexes

The reactions of cobalt(II) chloride with racemic malic acid (H₃mal = C₄H₆O₅) result in the isolation of monomeric, dimeric and tetrameric cobalt malato complexes: (NH₄)₂[Co(R-Hmal)(S-Hmal)] · 2H₂O (1), [Co₂(R-Hmal)(S-Hmal)(H₂O)₄]_n · 2nH₂O (2), K₄[Co₄(OH)₂(R-mal)₂(S-mal)₂(H₂O)₄] · 10H₂O (3) and trans-[Co(R-H₂mal)(S-H₂mal)(H₂O)₂] · 2H₂O (4). The formations of the malato complexes are dependent on the pH value, the molar ratio of the solutions, the reaction temperature and the counterions. In the water-soluble compound 1, the Co^II ion is octahedrally coordinated by two tridentate malates via their α-hydroxy, α-carboxy and β-carboxy groups. The malate ligands in 2 coordinate with the cobalt ion via their α-hydroxy and α-carboxy groups, while the β-carboxy group acts as a bridging ligand for the other two cobalt ions, forming a novel dimeric unit [Co₂(R-Hmal)(S-Hmal)(H₂O)₄], which further connects into a layered structure through links from the oxygen atoms of the β-carboxy groups. Complex 3 is a tetranuclear mixed-valence species. Both of the Co^II ions exist in trans-[Co(R-mal)(S-mal)(H₂O)₂] units, which are linked by a Co^III₂(OH)₂ unit with bridging α-alkoxy and β-carboxy groups. Compound 4 is the main product of reaction between cobalt chloride and excess malate under weakly acidic conditions.     

pH Monitoring: a review

Glass pH electrodes are being utilized for the measurement of pH values using liquid internal reference systems, which had been introduced on principle nearly hundred years ago and are still existing. To avoid several drawbacks in the practical usage of these kinds of chemical sensors, every effort has been made to develop an all-solid-state electrode with properties that are comparable to those of the conventional glass electrode. Metal oxide electrodes like RuO₂ or IrO _x are a low-priced alternative. Different concepts for substituting the conventional (aqueous) reference system by solid systems and also for changing the classical bulb shape design to a planar structured one have been proposed. A suitable reference system can be achieved by means of modification of classic reference electrodes by employing a new type of mixed conducting oxides. Both metal oxide and glass electrodes can be screen-printed on substrate materials like ceramics and plastics etc. to get miniaturized all-solid-state electrodes. pH sensors based on field effect transistors (FET) become more important. However, up to now an equivalent FET compatible reference electrode is not available.     

Liquid fragility of the unsaturated polyester resin

Curing behaviors of the unsaturated polyester resin (UPR) containing 1–1.8 wt% methyl ethyl ketone peroxide (MEKP) initiator are investigated. The viscosity, gelation and vitrification transition of the UPR-MEKP systems are examined using the rotating viscometer and differential scanning calorimetry (DSC). A liquid fragility parameter, M _c, defined as the viscosity variation rate of the liquids towards the curing temperature is presented. It is found that M _c has a good negative relation with the glass transition temperature (T _g) in the systems. M _c can be used for predicting the stability of the cured amorphous systems. The relationship between the liquid and cured thermoset polymer systems is studied from both the thermodynamic and kinetic point of view.     

A new equation for the correlation of surface tension–composition data of solvent–solvent and solvent–fused salt mixtures

An attempt has been made to propose accurate equations for correlating the surface tension of binary liquid mixtures. The method is applicable to the systems comprising of components with widely different molecular sizes. Two adjustable parameters, δ_p and δ_m obtained from the least squares analyses of the surface tension–composition data are reported for a number of systems. Temperature dependence of δ_p and δ_m is demonstrated for a few systems. The framework of operational equations has later been applied to cover multi-component systems comprising of fused salts with a single liquid component in full mole fraction range. Excellent fits of the surface tension for binary, ternary and multi-component ionic systems in aqueous or non-aqueous media have been obtained from the proposed method. The surface tension–composition data of 59 different types of systems with about 400 data points can be correlated by the equation with an average percent deviation of about 0.61. In contrast to previous equations from literature to calculate surface tension data, the proposed correlation is noted to be more accurate in different situations.     

A fully automatic system for underway N2O measurements based on cavity ring-down spectroscopy

N₂O is one of the most important greenhouse and ozone-depleting gases and has been the source of considerable concern in recent years. The oceans account for ~ 1/4 of the global N₂O emission budget; however, the oceanic N₂O source/sink characteristics are not well understood. To enhance the study of oceanic N₂O source/sink characteristics, our laboratory developed a fully automatic underway system for surface water N₂O concentration and atmospheric N₂O mole fraction measurements consisting of a cavity ring-down spectroscopy (CRDS) instrument and an upstream device. The developed device can be programmed to switch the CRDS measurements from the equilibrator headspace to the atmospheric sample and the reference gas sample. The surface water N₂O concentration is calculated from the equilibrium headspace N₂O mole fraction in the equilibrator. The response time of this equilibrator is ~ 3.4 min, and the estimated precision of this method for surface water N₂O measurements is better than 0.5% (relative standard deviation, RSD), which is one order of magnitude better than that of traditional gas chromatographic methods and can be further optimised. Data are acquired every 20 s, and the calibration frequency requirement of this system is approximately 7–10 days. This labor-saving underway system is a powerful tool for high-precision and high-resolution measurements of atmospheric and oceanic N₂O and can significantly improve the study of the characteristics of oceanic N₂O sources/sinks and their response to climate change.     

The effect of pH on some outer-sphere electrode reactions at carbon electrodes

The apparent heterogeneous rate constant, k°_app, for three couples known to undergo outer-sphere electron transfer reactions has been investigated at carbon, gold, and platinum electrodes as a function of solution pH. The rates are found to be pH dependent at two different types of glassy carbon and at carbon paste but not at metallic electrodes. The rate is found to decrease with increasing pH for Fe(CN)^{$\text{3?}\text{4?}$}₆ and IrCl^{$\text{2?}\text{3?}$}₆, and the opposite effect is observed for the Ru(NH₃)^{$\text{3+}\text{2+}$}₆ couple. For the three compounds examined, the magnitude of the pH dependent rate changes can be associated with a change in the magnitude of the potential of the reaction layer induced by a change in surface charge of ～ 2 × 10^?7 C cm^?2. The pH dependence of the observed rate constant at glassy carbon can be attenuated by heat treatment, chemical reduction, or esterification of the surface. This behaviour is consistent with the theory that the pH dependence of k°_app arises from the presence of functional groups on the carbon surface.     

A critical analysis of the hand and Othmer-Tobias correlations

Carniti, P., Cori, L. and Ragaini, V., 1978. A Critical Analysis of the Hand and Othmer-Tobias Correlations. Fluid Phase Equilibria, 2: 39–47.The Hand (H) and Othmer-Tobias (OT) correlations are extensively analyzed on 83 aqueous and 26 non-aqueous ternary systems. Twenty solutropic systems are also included. 65% and 58% of the systems give a linear correlation coefficient greater than 0.99 for H and OT respectively.An analysis of the sensitivity to systematic or random errors demonstrates the H correlation to be highly insensitive. A remarkable insensitivity, although not completly for some systematic errors, is also demonstrated for the OT correlation. However there is sufficient proof that they are both unsuitable as a check of experimental data, even though they are used for this purpose.The relation, not necessarily linear, derived from the H correlation, log(x₃₂/x₂₂) vs. log(x₃₁/x₁₁) is useful in locating the plait point.     

Electrochemically modulated liquid chromatographic separation of triazines and the effect of pH on retention

Electrochemically modulated liquid chromatography (EMLC) manipulates analyte retention by changing the potential applied (E_app) to a conductive stationary phase. This paper applies EMLC to the separation of a set of seven triazines which are commonly used but environmentally hazardous herbicides. Experiments herein examine the influence of E_app and the pH of the mobile phase on triazine retention. The results are discussed in terms of: (1) retention of triazines of dissimilar acid strengths and by correlations with the pH of the mobile phase; (2) how changes in E_app and acid–base equilibria modulate elution; (3) qualitative insights into EMLC-based retention; and (4) potential merits of EMLC in realizing the rapid separation of the seven-component triazine mixture.