Hydrogen Ion Activity Research Articles

Ионно-молекулярная модель памяти. Механизмы поиска информации в библиотеке памяти

In the context of developed by the authors of the memory model, the mechanism of information search in the library memory were studied. The authors proposed consistent variants of such mechanisms from the positions of biophysics, chemistry and mathematical logic. As in the previous articles, the role of the spectrum of the activity of hydrogen ions was highlighted. Events (facts) are placed in the library memory on the time axis - stratigraphy memory (according to V.V. Nabokov) and are subject to internal chronotype (according to A.A. Ukhtomsky). The authors touched a question upon vectorization time in biological systems in relation to the functioning of mechanisms of information search in the library memory. The authors note that the vectorization time in biosystems is the basis of the "speed" work of all mechanisms of memory. The mechanism of memory functions; for example, sleep, even deep - its essence is the work of the subconscious, i.e. the reference to the images and content of memory. Even taking into account the principle of anthropomorphism in the design of technical devices memory by individual, don’t associate characteristic for these devices "compression" of information from the brain work: here the information is not "shrinks" and recoded, contributing to lower power consumption of the process and reducing the overall entropy. The authors argue that the geometry of the drive with inherent spectra of the activity of hydrogen ions it is logically to consider the characteristics of similarity. This characteristic has fractal dimension that gives some level of comfort to compare patterns, which were discussed in this work. Identical and similar items patterns contribute equally to the formation of fractal dimension.

Effect of elevated Al and pH on the growth and root morphology of Al-tolerant and Al-sensitive wheat seedlings in an acid soil .

Aluminium ion (Al<sup>3+</sup>) toxicity and hydrogen ion (H<sup>+</sup>) activity are the major constraints for plant growth in acid soil. This study was undertaken to determine the effect of pH and Al on the growth response and changes in root morphology of Al-tolerant (ET8) and Al-sensitive (ES8) wheat seedlings. Different levels of AlCl<sub>3</sub> and CaCO<sub>3</sub> were added to the soils to manipulate soil pH and extractable Al. The results showed that the bulk soil pH remained constant at pH 4.1 with further applications of AlCl<sub>3</sub>, and that the seedlings died at the 200 mg AlCl<sub>3</sub>/kg treatments. The ET8 seedlings responded better than the ES8 seedlings in both low and high Al and pH. The ET8 seedlings had higher root surface areas and root tip numbers than the ES8 seedlings in the Al treatment. In contrast, the ES8 had higher root diameters than the ET8 seedlings due to the elevated Al supply. Apoplast Al increased with the increase of soil available extractable Al, and declined with the decrease of soil extractable Al. The ET8 seedlings accumulated more Al in their apoplast than the ES8 seedlings. This study concluded that accumulation of Al in the apoplast is also involved in Al tolerance mechanism with the addition of organic acid exudation.

Iridium oxide-based chemical sensor for in situ pH measurement of oilfield-produced water under subsurface conditions

The objective of this study was to develop a pH sensor for monitoring hydrogen ion activity in situ in oilfield-produced water under subsurface conditions. An iridium oxide film was prepared, and the performance of the iridium oxide film was evaluated in produced water using a series of high salinities, temperatures, and pressures. Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS) were employed to assess the morphology and oxidation state of the iridium oxide film. A series of open-circuit potential (OCP) measurements were performed to access the sensitivity and stability of the iridium oxide film electrode in produced water. Research results indicated that the pH sensor exhibits a very high sensitivity in the environment with varied interference ions, including Na+, Mg2+, Ca2+, Cl−, and SO42−, and the existence of sodium chloride in produced water made a slight positive shift of potential. The iridium oxide film-based pH sensor exhibited a near-Nernstian response at −57.9 mV/pH in produced water. An increase in temperature from 24 to 80 °C decreased the pH potential. Compared with the theoretical Nernst equation, the iridium oxide sensor showed fair Nernstian behavior at high temperatures. The iridium oxide pH sensor was accurately investigated under high pressure; the sensor seems independent of pressure, and the slope is the same at different potentials under a wide range of pressures.

Ионно-молекулярная модель памяти. Способы кодирования (формализации) и переноса информации

In previous papers, the authors examined the material elements of memory. This paper is devoted to the methods of coding and transfer information. Encoding mechanisms and spectrum of activity of hydrogen ions, as the dominant feature in the process of encoding information, were studied. In particular, the role of the tunneling effect - as the ability of a hydrogen ion H+ to change their position without loss of energy - was marked. The ability to relay proton transfer in the basic biochemical reactions was noted. The authors identified the main thing - compared with other ions of the material structure of the brain - the small mass of the proton makes it unique among them in this aspect encoding, and transfer information in realization of the mechanism of memory. The basic concept is the informational code memory. Herewith, the actual coding of the information is considered as far from a trivial task, but it is quite solvable (i.e. analyzed in memory model), given the large number of parameters of the electric neural signal. Another aspect is that whatever happened encoding when saving the information in memory should be recoding of parameters of electrical signals parameters of structure and energy of biopolymers or other drives. To retrieve information from memory must be implemented by its decoding images, verbal or other characteristics. The authors note that mechanisms decoding in-memory structure previously known to the authors of the works were not discussed at all. The article considers the basic notion of the spectrum of the activity of hydrogen ions (SАHI). It is SAHI as the general characteristics of the activity of proton, defines all of the “subtle” mechanisms the effectiveness of ion-molecular memory model. It is shown that only the uniqueness of the proton, as biochemical agent, and led him to the isolation of the other ion - as an important element of memory.

Hydrogen evolution reaction on spheroidal graphite cast iron with different pearlite areas in sulphuric acid solutions

The rate equation of hydrogen evolution reaction of spheroidal graphite cast iron with different pearlite area has been studied in sulphuric acid solutions at 298 K. The cathodic Tafel slope of −0·130 V/decade and the reaction order with respect to the activity of hydrogen ion of 1 are obtained by linear potential sweep technique. The rate equation of hydrogen evolution reaction does not depend on the area of pearlite. There is no difference in hydrogen evolution reaction mechanisms between pure iron and spheroidal graphite cast iron.

Modeling of the Extraction Equilibrium of Copper from Sulfate Solutions with Acorga M5640

The extraction equilibrium of copper from sulfate media with the aldoxime Acorga M5640 in ShellSol D70 has been investigated. The distribution results were interpreted by taking into account the nonideality of the aqueous phase. The activity of copper and hydrogen ions in the target systems CuSO4/H2SO4/Na2SO4 and CuSO4/H2SO4/Fe2(SO4)3/ZnSO4 were calculated through the speciation of the aqueous solutions and by applying the Pitzer model. The experimental pH values were found in good agreement with the predicted pH values. A model considering the dimerization of the aldoxime extractant was proposed to predict the distribution ratio and the copper loading isotherms. The extraction constant at infinite dilution and the apparent dimerization constant were evaluated from the experimental data and were found to be 103.06 ± 0.07 and 51 ± 9 M−1, respectively, at 25°C.

New Approaches in Modeling Carbonate Alkalinity and Total Alkalinity

Alkaline measurements (carbonate alkalinity and total alkalinity) are of great importance in analyses of ocean, marine, lake and river waters, and other samples. Carbonate alkalinity (CA) refers to the cases in which only bicarbonate and carbonate species are considered. Total alkalinity (Alk) implies additional species having acid-base properties. Biochemical processes give rise to the organic alkalinity (OA) in natural waters. The value of these topics is obvious if we refer to the plethora of publications in this field. As a matter of fact, the approaches towards Alk provide ambiguous results and those for CA arouse some reservations as well. Having this in mind, a new approach for alkalinity determination, presented in recent review paper published in this journal (Crit. Rev. Anal. Chem. 2011, 41, 151), was developed in a more systematic and complete way, and new acronyms, TAL for total alkalinity and CAM for carbonate alkalinity, were introduced in that article. The methods proposed there and here are based on potentiometric pH titration, as the most common method applied for studying acid-base properties of humic substances (humic and fulvic acids). The parameters involved with CAM are resolved with the use of an Excel spreadsheet for the resolution of linear equations, whereas the parameters of a nonlinear regression equation referring to the TAL method are obtainable according to a more advanced, iterative computer program. A compact approach to CAM, based on the modified Gran II method, is carried out, but in a manner quite different and more thorough than the ones presented hitherto. Except for equivalence volumes, the CAM method enables evaluating the activity coefficient of hydrogen ions and values for (hybrid and concentration) dissociation constants of carbonic acid and ionic product of water. The possibilities and limitations of this method are tested by using a simulation procedure. The results obtained from simulated titrations provide the reference level related to error-free V and pH values at the points of titration curve ph = ph(V), ph = – log h, h – activity of hydrogen ions. The TAL model with Simms constants involved allows “homogenizing” the treatment of mono- and polyprotic acids and bases, especially those of undefined composition, i.e., carbonate species, fulvic acids, and their soluble complexes with metal ions. The TAL model may also be extended to the option where an approach based on a pre-dose addition is applied. pH and acid-base equilibria are very important topics and some aspects of them have been treated in this journal. Alkalinity determinations are of value in anaerobic digesters and anaerobic wastewater treatment, mine drainage and associated water, coastal seawater, marine aquaculture, estuarine water, and food and beverages, to point out a few recent examples.

Physiological Demands and Changes Resulting from Effort in Low Handicap Polo Horses

In order to understand the physiological demands and possible changes resulting from effort in horses and determine the type and intensity of exercise performed in equestrian polo of low handicap, we analyzed the negative logarithm of the activity of hydrogen ions (pH), carbon dioxide partial pressure (pCO2), oxygen partial pressure (pO2), the concentration of plasma bicarbonate ions (HCO3-) and serum concentrations of Na+, Cl- and K+ of seven healthy horses after a polo match of low handicap. Variables pCO2, pO2 and HCO3- were the only parameters that were significantly different before and after the match, and there were no significant hydroelectrolytic changes. It was possible to classify the low handicap polo as an exercise of short duration and varying intensity, considering the lack of change in the hydroelectrolytic balance. The exercise to which the animals were subjected is compatible with their athletic fitness.

Direct Optical Carbon Dioxide Sensing Based on a Polymeric Film Doped with a Selective Molecular Tweezer-Type Ionophore

A novel optical method for the determination of CO(2) concentration in aqueous and gaseous samples of plasticized PVC film is presented. The detection principle makes use of a direct molecular recognition of the carbonate ion by a molecular tweezer-type ionophore, which has previously been demonstrated to exhibit excellent carbonate selectivity. The carbonate ion is extracted together with hydrogen ions into a polymeric film that contains the anion exchanger tridodecylmethylammonium chloride, a lipophilic, electrically charged, and highly basic pH indicator, which is used for the readout in absorbance mode, in addition to the lipophilic carbonate ionophore. According to known bulk optode principles, such an optical sensor responds to the product of the carbonate ion activity and the square of hydrogen ion activity. This quantity is thermodynamically linked to the activity of carbon dioxide. This allows one to realize a direct carbon dioxide sensor that does not make use of the traditional Severinghaus sensing principle of measuring a pH change upon CO(2) equilibration across a membrane. A selectivity analysis shows that common ions such as chloride are sufficiently suppressed for direct PCO(2) measurements in freshwater samples at pH 8. Chloride interference, however, is too severe for direct seawater measurements at the same pH. This may be overcome by placing a gas-permeable membrane over the optode sensing film. This is conceptually confirmed by establishing that the sensor is equally useful for gas-phase PCO(2) measurements. As expected, humid air samples are required for proper sensor functioning, as dry CO(2) gas will not cause any signal change. The sensor showed acceptable response times and good reproducibility under both conditions.

Abstracts of the 17th International Symposium on Bioluminescence and Chemiluminescence ‐ (ISBC 2012)

WARNING : The light-emitting molecular structures responsible for the chemiluminescence and fluorescence phenomena are not necessarily the same!

Stibnite (Sb2S3) oxidative dissolution kinetics from pH 1 to 11

The kinetics of oxidative dissolution of natural stibnite (Sb2S3) under environmentally relevant conditions is investigated. The effects of hydrogen ion activity (pH 1.08–10.63), dissolved oxygen saturation (p(O2) from 0 to 80% of atmospheric pressure), and temperature (25–48°C) on the reaction rate were studied using a mixed flow reactor. Trivalent cations (Fe3+, Al3+, Ce3+) in acidic solution, and divalent cations (Mg2+, Ca2+) in basic solution were found to accelerate the rates, with the effect being related to ionic radius. In both acid and basic domains, the stibnite dissolution rate was found to be proportional to fractional powers of hydrogen ion and dissolved oxygen activities. The rate dependence on dissolved oxygen could also be described in terms of Langmuir adsorption, with saturation predicted to occur at oxygen partial pressures in excess of 3.5atm in acid solution and 15.5atm in basic solution. The empirical rate equations are given by:r=k′a(H+)αθ=k′a(H+)αKa(O2)1+Ka(O2)where k′=1.25×10−10±0.02×10−10; α=0.133±0.005; K=2.22×104±0.16×104mol−1l in acid solution (pH 3.0) and k′=8.88×10−12±0.62×10−12; α=−0.133±0.008; K=5.01×103±1.24×103mol−1l in basic solution (pH 9.8), both at 25°C. Activation energies are 7.4±0.5kJmol−1 (pH 3.0, between 25°C and 40°C) and 56.1±5.0kJmol−1 (pH 9.9, between 25°C and 48°C), suggesting diffusion control in acidic solution and surface control in basic solution. Ferric iron in acidic solution strongly catalyzes the dissolution, with an acceleration of the rate by approximately an order of magnitude for a(Fe3+)=4.03×10−4M at pH 2.0. Aluminum ion, which has an ionic radius (54pm) almost identical to that of ferric iron (55pm), produces a similar rate enhancement. The effect of cationic radius on the dissolution rate of stibnite in the acid domain was evaluated using Ce3+ (114pm), which increased the rate even more. In basic solution, Mg2+ and Ca2+ increased the dissolution rate; the effect was stronger with the larger of the two cations (Ca2+). Stibnite dissolution was incongruent to varying degrees, with elemental sulfur forming a residual surface layer, in both acid and basic solution. In the presence of high concentrations of ferric iron, the dissolution became highly incongruent, with a substantial accumulation (7–13 atomic layers) of elemental sulfur. In contrast to Fe3+, neither Ce3+ nor Al3+ had an effect on the formation of elemental sulfur, clearly demonstrating the important role played by Fe3+ in the oxidation of sulfide to elemental sulfur. In the case of iron catalyzed dissolution, the elemental sulfur formed at the reacting surface accounts for a very significant proportion of the stoichiometric total reactive sulfur, so that reaction rates may not be assessed by measuring the concentration of dissolved sulfur species.

Liquid-Liquid Extraction of Ruthenium from Chloride Media by N,N'-Dimethyl- N,N'-Dicyclohexylmalonamide

Research on the solvent extraction of ruthenium from hydrochloric acid media has been carried out using N,N'-dimethyl-N,N'-dicyclohexylmalonamide (DMDCHMA) dissolved in 1,2-dichloroethane. Ruthenium extraction percentages (%E) ranging from 50% to 80% have been achieved for HCl concentrations between 5 M and 7 M. Extraction curves exhibiting the dependence of the %E ruthenium on HCl concentration in the aqueous phases are presented, the latter solutions being obtained by dissolution of either Ru(III) or Ru(IV) salts. The influence of some experimental parameters on the %E Ru, such as the equilibration time, extractant concentrations, and hydrogen-ion activities, has been thoroughly investigated. Additionally, DMDCHMA is also adequate for extracting Pd(II) from 5 M to 7 M HCl solutions and under similar experimental conditions, %E Rh(III) is below 5%, and Pt(IV), Ir(III), and Ir(IV) cause the formation of third phases. Both Ru and Pd(II) can be successfully stripped from the loaded organic phases with water. A partition scheme to isolate Ru from a number of some associated elements has also been attempted.

Determination of the Activity of Hydrogen Ions in Dilute Sulfuric Acids by Use of an Ionic Liquid Salt Bridge Sandwiched by Two Hydrogen Electrodes

The activities of hydrogen ions in 20-200 μmol dm(-3) H(2)SO(4) solution were estimated by use of an ionic liquid salt bridge (ILSB), made of tributyl(2-methoxyethyl)phosphonium bis(pentafluoroethanesulfonyl)amide (TBMOEPC(2)C(2)N), sandwiched by two hydrogen electrodes. The experimental pH values (pH = -log a(H), where a(H) is the activity of hydrogen ions) were in good agreement, within 0.01 pH unit, with those calculated using the Pitzer model. The difference between the experimental and theoretical pH values at 50 μmol dm(-3) H(2)SO(4) solution was much smaller than that obtained by use of a glass electrode in combination with a reference electrode with a concentrated KCl salt bridge. The source of the small deviation can be explained by the residual diffusion potential due to the dissolution of TBMOEPC(2)C(2)N in the H(2)SO(4) solution (W) and the resultant increase in the ionic strength of W. The use of a reference electrode equipped with an ILSB opens the way to accurately estimate the pH in dilute aqueous solutions, for which we have not had effective means.

Arsenopyrite dissolution rates in O2-bearing solutions

Abstract Arsenopyrite dissolution was studied by means of long-term, stirred and non-stirred flow-through experiments in the pH range of 1 to 9 at 25, 50 and 70 °C and at different input dissolved-O2 concentrations (from 0.2 to 8.7 mg L− 1). At pH lower than 4, aqueous iron, which is mainly in the ferrous form, and arsenic are stoichiometrically released. Sulphur concentrations released were lower than stoichiometrically expected (S/As In the light of these results, the steady-state dissolution rates were estimated by the release of arsenic at pH R arsenopyrite mol m − 2 s − 1 25 ° C = 10 − 7.41 ± 0.47 ⋅ a O 2 0.76 ± 0.11 ⋅ a H + − 0.12 ± 0.07 where aO2 and aH+ are the activities of hydrogen ions and dissolved oxygen, respectively and their exponents were estimated from multiple linear regression of the dissolution rates. Temperature increase from 25 to 70 °C yields an apparent activation energy for the arsenopyrite oxidation by dissolved oxygen of 18.5 ± 1.6 kJ mol− 1. At pH > 6, aqueous iron is mainly in the ferric form and is depleted as it precipitates as Fe-oxyhydroxide onto arsenopyrite surfaces, yielding Fe/As and Fe/S less than one; between pHs 7 and 9, iron depletion is complete, and sulphur released is more abundant than arsenic released, which is precipitated as As–O phases, as confirmed by MicroRaman spectroscopy. At pHs 6–9, iron-oxyhydroxide phases and arsenic oxide phases upon the arsenopyrite surface provide an effective layer that reduces diffusion of dissolved oxygen and arsenopyrite dissolution. As coating on the arsenopyrite surface becomes the rate-limiting step, the Shrinking Core Model (SCM) allows quantification of the surface dissolution rate, especially from data obtained where the effect of coating was still negligible. The SCM also allowed us to calculate the effective coefficient for oxygen diffusion through the coating, which can vary from 10− 17 to 1.5 · 10− 16 m2 s− 1. The formation of such a coating produced a decrease in arsenic and sulphur release over time and a final surface passivation.

Chalcopyrite dissolution rate law from pH 1 to 3

Chalcopyrite dissolution kinetics in the pH range of 1 to 3 were studied by means of long-term flow-through experiments to obtain a dissolution rate law which can be coupled with reactive transport models to forecast Acid Rock Drainage. In the range of conditions under study, the rate of chalcopyrite dissolution is only slightly dependent on hydrogen ion activity, increasing with decreasing pH. The steady-state dissolution rates obtained in the present study were combined with earlier results presented by Acero et al. (2007a) to obtain the following expression for chalcopyrite dissolution rate law: where Rchalcopyrite is the chalcopyrite dissolution rate (mol m-2 s-1), aH+ is the activity of hydrogen ion in solution, R is the gas constant (kJ mol-1 K-1) and T is the temperature (K). This expression can applied through a wide range of environmental conditions similar to the ones found in systems affected by acid drainage. In agreement with earlier chalcopyrite kinetic studies, iron was released to solution preferentially over copper and sulfur, compared with the stoichiometry of the pristine mineral. Consistently, XPS examination of the samples showed that reacted surfaces were enriched in sulfur and copper (relative to iron) compared with the initial, pristine chalcopyrite surface. However, this surface layer does not exert any passivating effect on chalcolpyrite dissolution and the kinetics of the overall process in the long term seems to be surface-controlled.

Mechanistic Characterization of the HDV Genomic Ribozyme: Solvent Isotope Effects and Proton Inventories in the Absence of Divalent Metal Ions Support C75 as the General Acid

The hepatitis delta virus (HDV) ribozyme uses the nucleobase C75 and a hydrated Mg(2+) ion as the general acid-base catalysts in phosphodiester bond cleavage at physiological salt. A mechanistic framework has been advanced that involves one Mg(2+)-independent and two Mg(2+)-dependent channels. The rate-pH profile for wild-type (WT) ribozyme in the Mg(2+)-free channel is inverted relative to the fully Mg(2+)-dependent channel, with each having a near-neutral pKa. Inversion of the rate-pH profile was used as the crux of a mechanistic argument that C75 serves as general acid both in the presence and absence of Mg(2+). However, subsequent studies on a double mutant (DM) ribozyme suggested that the pKa observed for WT in the absence of Mg(2+) arises from ionization of C41, a structural nucleobase. To investigate this further, we acquired rate-pH/pD profiles and proton inventories for WT and DM in the absence of Mg(2+). Corrections were made for effects of ionic strength on hydrogen ion activity and pH meter readings. Results are accommodated by a model wherein the Mg(2+)-free pKa observed for WT arises from ionization of C75, and DM reactivity is compromised by protonation of C41. The Brønsted base appears to be water or hydroxide ion depending on pH. The observed pKa's are related to salt-dependent pH titrations of a model oligonucleotide, as well as electrostatic calculations, which support the local environment for C75 in the absence of Mg(2+) being similar to that in the presence of Mg(2+) and impervious to bulk ions. Accordingly, the catalytic role of C75 as the general acid does not appear to depend on divalent ions or the identity of the Brønsted base.

Acute effects of antacids on gastric juice components in duodenal ulcer patients

In 10 duodenal ulcer patients gastric juice was aspirated every 10 min for 20 min before and 200 min after sham feeding. One antacid tablet or placebo was given 80 min after the sham feeding. Analyses of the aspirates showed that antacids reduced mean hydrogen ion activity and pepsin concentration significantly for 40 and 60 min, respectively, and increased phospholipid concentration for 30 min, compared with placebo. Highest mean pH was 2.52. The percentage of pH readings at or above pH levels of 2, 3, and 4 during the 2 h period following antacid administration was 29, 10, and 2%, respectively. No significant differences between antacid and placebo were found regarding intragastric concentrations of bile acids and prostaglandin E2. When one antacid tablet was administered 80 min after a real meal, the effect on intragastric pH was similarly weak, but lasted slightly longer. Acid neutralization' alone can hardly explain the ulcer-healing effect of low-dose antacids. Antacid-induced increase in intragastric concentration of phospholipids is a new and potentially important observation.

Calculating pH in pig manure taking into account ionic strength

Models such as the Anaerobic Digestion Model No. 1 (ADM1) assume that pH can be calculated directly from the concentration of hydrogen ions. However because pH is, by definition, the negative logarithm of the hydrogen ion activity, and thus pH measurements represent hydrogen ion activities, this approach may lead to a bias between measured and predicted pH values. Implementing ionic strength effects into the charge balance equation and the calculation of pH is a theoretical improvement to this. In this study a model, implementing a procedure for calculating pH, was developed to analyse the effect of ionic strength on pH in a pig manure. By adding KCl to samples of pig manure, experimental results could be analysed with help from the model. A modified form of the Davies equation was found to give the most accurate prediction of pH in the pig manure studied in this paper with changing ionic strength.

Zr/ZrO2 Sensors for in Situ Measurement of pH in High-Temperature and -Pressure Aqueous Solutions

The aim of this study is to develop new pH sensors that can be used to test and monitor hydrogen ion activity in hydrothermal conditions. A Zr/ZrO2 oxidation electrode is fabricated for in situ pH measurement of high-temperature aqueous solutions. This sensor responds rapidly and precisely to pH over a wide range of temperature and pressure. The Zr/ZrO2 electrode was made by oxidizing zirconium metal wire with Na2CO3 melt, which produced a thin film of ZrO2 on its surface. Thus, an oxidation-reduction electrode was produced. The Zr/ZrO2 electrode has a good electrochemical stability over a wide range of pH in high-temperature aqueous solutions when used with a Ag/AgCl reference electrode. Measurements of the Zr/ZrO2 sensor potential against a Ag/AgCl reference electrode is shown to vary linearly with pH between temperatures 20 and 200 degrees C. The slope of the potential versus pH at high temperature is slightly below the theoretical value indicated by the Nernst equation; such deviation is attributed to the fact that the sensor is not strictly at equilibrium with the solution to be tested in a short period of time. The Zr/ZrO2 sensor can be calibrated over the conditions that exist in the natural deep-seawater. Our studies showed that the Zr/ZrO2 electrode is a suitable pH sensor for the hydrothermal systems at midocean ridge or other geothermal systems with the high-temperature environment. Yttria-stabilized zirconia sensors have also been used to investigate the pH of hydrothermal fluids in hot springs vents at midocean ridge. These sensors, however, are not sensitive below 200 degrees C. Zr/ZrO2 sensors have wider temperature range and can be severed as good alternative sensors for measuring the pH of hydrothermal fluids.

Oxalate-promoted forsterite dissolution at low pH

We ran a series of 124 semi-batch reactor experiments to measure the dissolution rate of forsterite in solutions of nitric and oxalic acid solutions over a pH range of 0–7 and total oxalate concentrations between 0 and 0.35 m at 25 °C. We found that the empirical rate law for the dissolution of forsterite in these solutions is r = 10 - 7.03 a H + 0.46 + 10 - 5.44 m ox 2 - 0.40 a H + 0.47 where a H + is the activity of hydrogen ions, m ox is the concentration of oxalate in molal, and the rate is given in (mol/m 2 s). The first term of the rate law expresses the dissolution rate for solutions with no oxalate and the second term expresses the additional rate produced by the presence of oxalate ions. Our experiments show that oxalate-promoted dissolution rates depend upon both oxalate concentration and pH. Based on this, we propose a reaction mechanism in which a hydrogen ion and an oxalate ion are simultaneously present in the activated complex for the reaction that releases H 4SiO 4 into solution. By analogy, we propose that water acts as a ligand in the absence of oxalate, so the first term of our rate law has a H 2 O m (=1) as a hidden term. Thus, our results suggest that both a proton and a ligand (either an anion or a water molecule) must be present in the activated complex for forsterite dissolution, suggesting that proton-promoted and ligand-promoted dissolution are simply two aspects of the same process, and that the distinction between the two is artificial. The increased dissolution rate of forsterite produced by oxalate ions at a concentration of 0.001 m, a concentration comparable to the amount of organic acids in typical soils, translates into an approximately a 6-fold increase in forsterite dissolution rates at pH > 4.2. This suggests that organic acids have a measurable, but not profound, effect on chemical weathering.