Accuracy Of pH Measurements Research Articles

Preferential solvation in the THF–water mixtures. Dissociation constants of acid components of pH reference materials

Dissociation constants of acid components of pH reference materials in tetrahydrofuran–water mixtures containing 0, 10, 20, 30, 40, 50, 60, 70 and 100% (w/w) tetrahydrofuran, THF, were obtained. Dissociation constant values determined were: pK1 and pK2 for tartaric and phthalic acid; pK1, pK2 and pK3 for citric acid; pKa for acetic and boric acid and pK2 for phosphoric acid. These values are essential to the determination of the reference pHS values of the standard buffer solutions, which are needed for accurate pH measurements in THF–water mixtures. In order to explain the variation of the pKa values obtained over the whole composition range studied, the quasi-lattice quasi chemical (QLQC) theory of preferential solvation was applied. To identify the solvent characteristics affecting the pKa values, the results are discussed in terms of average macroscopic properties of the mixed solvent and in terms of the solvation shells around the solute. Thus, the methodology of linear solvation energy relationships (LSER) was used to relate pKa values with parameters of THF–water mixtures. The equations obtained allow calculation of the pKa values of substances in THF–water mixtures.

Development of novel 19F NMR pH indicators: synthesis and evaluation of a series of fluorinated vitamin B6 analogues

We have synthesized a series of novel fluorinated vitamin B6 analogues (6-fluoropyridoxol derivatives) as potential 19F NMR pH indicators for use in vivo. Modifications included addition of aldehyde, carboxyl or aminomethyl groups at the 4- or 5-ring position, and examination of a trifluoromethyl moiety as an internal chemical shift standard. The variation in chemical shift with respect to acid–base titration showed pKa values in the range 7.05–9.5 with a chemical shift sensitivity in the range 7.4–12ppm. Several of the molecules readily cross cell membranes providing estimates of both intra- and extra-cellular pH in whole blood. 6-Fluoropyridoxamine (6-FPAM) exhibits a pKa=7.05, which is closer to normal physiological pH than the parent molecule 6-fluoropyridoxol (6-FPOL) (pKa=8.2), and should thus, be useful for precise and accurate measurements of pH in vivo. Enhanced spectral resolution for 6-FPAM over 6-FPOL is demonstrated in whole blood and the perfused rat heart.

ZnO Solubility and Zn 2 Complexation by Chloride and Sulfate in Acidic Solutions to 290°C with In-Situ pH Measurement

The solubility of zincite in mildly to strongly acidic aqueous solutions, according to the reaction ZnO + 2H + ⇔ Zn 2+ + H 2O, has been measured at ionic strengths of 0.03–1.0 (stoichiometric molal basis) from 50 to 290°C at saturation vapor pressure in sodium trifluoromethanesulfonate solutions (NaTriflate, a noncomplexing 1:1 electrolyte). The hydrogen-electrode concentration cells employed in this study permit continuous and highly accurate pH measurement at elevated temperatures, and periodic sampling to determine the dissolved metal content of the experimental solution. The solubility of zincite is shown to be reversible at 200°C by addition of acidic and basic titrants, at constant ionic strength. The equilibrium constant is precisely described (±0.05 log units) by the function log K = −4.0168 + 4527.66/T. One additional adjustable parameter, together with an extended Debye-Hückel function, is sufficient to model the ionic strength dependence of the reaction. The solubility product at infinite dilution obtained from this study is in quantitative agreement with the thermodynamic model of Ziemniak 1992. This experimental approach is demonstrated to be advantageous in studying the complexation of Zn 2+ with Cl − and SO 4 2−, by titrations involving the appropriate anion into NaTriflate solutions pre-equilibrated with zincite at constant temperature and ionic strength. Formation constants in 0.1 molal NaTriflate for the reaction Zn 2+ + yL z− ⇔ Zn(L) y 2−yz are reported for ZnCl +, ZnCl 2° and ZnSO 4° at 200°C (log Q = 1.7 ± 0.1, 3.0 ± 0.1, and 2.6 ± 0.1, respectively). Estimates of the equilibrium constants for the chloride species at infinite dilution and 200°C are log K = 2.5 ± 0.1 (ZnCl +), and 4.2 ± 0.1 (ZnCl 2°). This value for the dichlorozinc complex agrees quantitatively with values reported by Bourcier and Barnes 1987 and Ruaya and Seward 1986. However, the latter authors give a value for the monochlorozinc complex (log K = 4.01 ± 0.02) that is markedly different from our result and that of Bourcier and Barnes 1987 (log K = 3.1 ± 0.3).

Effects of Storage on Serum Ionized Calcium and pH From Horses With Normal and Abnormal Ionized Calcium Concentrations

It has been previously shown that Ca(I) concentration is stable in serum collected from healthy horses for 10 days if stored at 40 degrees C. This may not be true for horses with abnormal Ca(I) concentrations. Thus the stability of ionized calcium (Ca(I)) concentration and pH measurement in serum from horses with both normal and abnormal Ca(I) concentrations stored for various times at 40 degrees C and -10 degrees C was evaluated. Our results indicated that serum Ca(I) concentration was stable throughout 7 days of cold or frozen storage, after being received by the Clinical Chemistry Laboratory. Serum Ca(I) concentration showed a significant decrease by 14 days of frozen storage (-10 degrees C). Serum pH showed a statistically significant increase by 7 days of cold storage, and within 3 days of frozen storage. If equine serum is collected, handled and stored anaerobically, and kept cold or frozen, Ca(I) concentration can be accurately measured for approximately 7 days after collection, regardless of the health status of the animal. An accurate measurement of pH may be made within 3 days of cold or 1 day of frozen storage.

Measurement of Fe2+ ion by coulometry method at incubation of Thiobacillus ferrooxidans.

Thiobacillus ferrooxidans is a chemoautotrophic bacterium that is capable of using Fe2+ oxidation by O2 as the sole source of energy for growth and CO2 fixation. The idea of the solar bacterial biomass farm by using of this bacterium is proposed. The incubation experiment of these bacteria was carried out, and the 9K culture medium as the standard medium for T. ferrooxidans was used. The measurement of Fe2+ in the growth stage was carried out as the first step of the experiments to clarify the possibility of this system. The items of measurement were Fe2+ ion density, pH of the medium, bacterium density and quantity of total organic carbon (TOC). The density of Fe2+ ion in the medium was measured by coulometry method. This method has the following advantage, high accuracy (<1%), easy operation, short measurement time (a few minutes) and small sample quantity (about 0.1 ml). The experimental results show that the Fe 2+ ion density is measured as same as the accuracy of pH measurement. At the final stage of the growth, the pH decreased due to the generation of the iron hydroxide (Fe(OH)3). The bacterium density and TOC slightly increased after that Fe2+ runs short. This result shows that the CO2 fixation speed is slower than Fe2+ oxidation speed. It is shown by the experiment that the growth limit of T. ferrooxidans is caused by the disappearance of the Fe2+ ion. It may be possible that the bacterium density increases by the continuous supply of Fe2+ ion.

Estimation of the thermal liquid junction potential of an external pressure balanced reference electrode

The accurate measurement of pH and potential in high-temperature high-pressure aqueous solutions using an external pressure balanced reference electrode (EPBRE) requires the development of accurate methods for calculating the thermal liquid junction potential (TLJP). Using well-developed irreversible thermodynamic techniques, as well as Agar's hydrodynamic theory to calculate the ionic entropies of transport, we show that the TLJP makes a very significant contribution to the measured potential and can vary by more than ± 150 mV depending on the temperature and composition of the non-isothermal electrolyte bridge. Our calculations of the TLJP are compared with experimentally measured data and good agreement is obtained. The development of an advanced EPBRE, in which the composition of the non-isothermal liquid junction is maintained accurately at a well-defined concentration, is considered to be an appropriate way to solve the problem of performing accurate potentiometric measurements in high-temperature high-pressure aqueous systems.

Accurate whole-spectrum measurements of intracellular pH and [Na+

Fluorescent measurements of intracellular H(+) and Na(+) are improved by using whole spectra of the fluorescent indicators BCECF and SBFI, respectively. The extra data in whole spectra enable both an accurate calibration and a ready detection of artifacts which are not possible to identify using a more conventional data analysis that relies upon only two wavelength "windows" in the fluorescence spectra. The whole-spectrum technique is applicable to cell suspensions in a conventional fluorimeter (as is reported here with SBFI), as well as to attached cells using a fluorimeter combined with an inverted epifluorescence microscope. The spectral method was highly reproducible in that pairs of successive pH measurements differed, on average, by only 0.01±0.02 U. Random uncertainty from sample to sample was estimated numerically from the standard deviation of measurements on ionophore-treated cells. When full-spectrum analysis was employed, this scatter showed a two-fold improvement over results obtained using the two-wavelength ratio method. Because SBFI has a relatively narrow dynamic range, whole-spectrum analysis has been applied to improve the accuracy of sodium determinations. The calibrated system measured [Na(+)]i with excellent linearity over the range 2-150 mM and with an accuracy of approximately 5 mM.

Influence of the electrolyte composition and pH on the anodic dissolution of p-type Si in aqueous HF solutions

The influence of the electrolyte composition and pH on the anodic currents obtained during electrochemical etching of p-type silicon in aqueous HF solutions has been investigated. Original and accurate pH measurements were performed to characterize the exact composition of the HF + H 2O electrolytes commonly used. It is shown that for these very acid solutions (pH < 2) almost all fluoride is in the form of the non-dissociated HF species which appears to play a preponderant role in the silicon dissolution reaction kinetics. The effect of pH can be restricted to its influence on the modification of the different concentrations by shifting the equilibria.

Chapter 9 Measurement of Intracellular pH

Publisher Summary This chapter discusses measurement of intracellular pH using various fluorescent probes. The measurement of pHi using fluorescent probes generated in situ was first carried out by Thomas et al., using fluorescein and carboxyfluorescein diacetate. These early pHi probes suffered from considerable limitations including difficulties with cellular uptake, rapid leakage out of cells, and pKa that was too acidic to make accurate measurements of pH, at values close to neutrality. In addition to the flourescein derivatives, 2,3-dicyanohydroquinone (DCH) has been used to measure pHi using a ratiometric technique. New probes have become available for the measurement of pHi. The carboxyseminaphthofluoresceins (SNAFLs) and the carboxyseminaphthorhodafluors (SNARFs) were designed to be long wavelength, dual-emission pH indicators with a pKa near neutrality. The most promising of these for flow cytometry is SNARF-1, which has a pKa of 7.4. These probes have dual emission wavelengths, one that is maximal under acidic conditions and the other that is maximal under alkaline conditions. This provides the theoretical advantage of increased sensitivity.

Measurement of pH in subcritical and supercritical aqueous systems

The use of yttria-stabilized zirconia (YSZ) electrodes of the type Hg/HgO/ZrO2(Y2O3)/H+, for measuring pH in aqueous solutions at high subcritical (150 374°C) is reviewed. The construction and operation of the YSZ and reference electrodes employed in these studies are described and their application in measuring the pH of a variety of technologically-important aqueous system is discussed. We show that the YSZ electrode is thermodynamically viable at temperatures into the supercritical region, and that it is a primary pH sensor in that calibration is not necessary, provided that the activity of water is known. However, highly accurate pH measurements at high subcritical and supercritical temperatures will require the development of more accurate reference electrodes.

The Viability of pH Measurements in Supercritical Aqueous Systems

The measurement of pH of aqueous systems at high temperature is becoming increasingly important because of the need to monitor the chemistry of nuclear power plant cooling water systems and the cycle chemistry of fossil‐fueled power reactors. We have previously shown that the yttria‐stabilized zirconia pH sensor can be used for accurate pH measurements of aqueous systems up to 320°C. This paper describes the viability of pH measurement of aqueous systems up to and above the critical temperature of water.

The pH of gingival crevices and periodontal pockets in children, teenagers and adults

Gingival crevice and periodontal pocket pH, measured directly with glass micro-electrodes, was near neutral at most sites in most individuals (mean pH 6.92 ± 0.03 SEM, 69 subjects). Periodontal state ranged from healthy to periodontitis but neither clinical evidence of gingivitis at a site nor pocket depth were associated with crevicular pH different from that at healthy sites. This finding contradicts earlier reports that gingivitis is associated with a crevicular pH as alkaline as pH 9.06. Metallic antimony electrodes as used by earlier investigators were found to give pH readings that were too high by as much as 1.5 pH units in the presence of organic reducing agents of the type produced by oral bacteria within gingival crevices. In contrast, glass micro-electrodes respond only to hydrogen ions and thereby provided accurate measurements of pH even in the presence of organic reducing agents. Loss of CO 2 to the atmosphere from biological fluids that are bicarbonate buffered resulted in a shift to alkaline pH by as much as 1 pH unit. As a result, only measurements taken within gingival crevices or periodontal pockets can provide accurate measurements of crevice or pocket pH.

Progress in the development of a fluorescent intravascular blood gas system in man

In vitro and in vivo animal studies have shown accurate measurements of arterial blood pH (pHa), carbon dioxide tension (PaCO2), and oxygen tension (PaO2) with small intravascular fluorescent probes. Initial human clinical studies showed unexplained intermittent large drops in sensor oxygen tension (PiO2). Normal volunteers were studied to elucidate this problem. In the first part of this study, the probe and cannula were manipulated and the probe configuration and its position within the cannula were varied. The decreases in PiO2 were judged to be primarily due to the sensor touching the arterial wall. Retraction of the sensor tip within the cannula eliminated the problem. In the second part of this study, the accuracy of the retracted probe was evaluated in 4 subjects who breathed varying fractions of inspired oxygen and carbon dioxide. The arterial ranges achieved were 7.20 to 7.59 for pH, 22 to 70 mm Hg for PaCO2, and 46 to 633 mm Hg for PaO2. Linear regression of 48 paired sensor (i) versus arterial values showed pHi = 0.896 pHa + 0.773 (r = 0.98, SEE = 0.017); PiCO2 = 1.05 PaCO2 - 1.33 (r = 0.98, SEE = 2.4 mm Hg); and PiO2 = 1.09 PaO2 - 20.6 (r = 0.99, SEE = 21.2 mm Hg). Bias (defined as the mean differences between sensor and arterial values) and precision (SD of differences) were, respectively, -0.003 and 0.02 for pHi, 0.77 and 2.44 mm Hg for PiCO2, and -2.9 and 25.4 mm Hg for PiO2. The mean in vivo 90% response times for step changes in inspired gas were 2.64, 3.88, and 2.60 minutes, respectively, for pHi, PiCO2, and PiO2.

Monitoring nutrient film solutions using ion-selective electrodes

An automated measuring system was developed for monitoring the concentration of individual chemical ions in nutrient film solutions. This used ion-selective electrodes to measure pH, nitrate, potassium, calcium, sodium and chloride in solutions used for growing greenhouse tomatoes. The use of frequent and regular calibrations with two solutions which encompassed the concentrations existing in the nutrient solution enabled both short and long term drift of the electrodes to be accommodated, and electrode life to be extended. The accuracy of pH measurement was shown to be ± 0·4 pH units, while the concentrations of nitrate and potassium could be obtained with uncertainties of ± 10 and ± 20% respectively. The repeatability of measurements made on the same samples were within 10 p.p.m. or 0·05 pH units, with the exception of a value of 27 p.p.m. obtained with calcium. The life of the glass and solid state electrodes used for pH, sodium and chloride exceeded 2 years, but that of the p.v.c. electrodes used for nitrate, potassium and calcium, were typically 4, 2 and less than one month, respectively.

An Inexpensive Flow‐Through Cell and Measurement System for Monitoring Selected Chemical Parameters in Ground Water

AbstractAn inexpensive flow‐through cell was designed and constructed to enable the accurate measurement of redox potential (Eh), conductivity and pH from shallow monitoring wells. The cell accepts sample water from inert gas‐operated bladder pumps. The electrodes are monitored by meters mounted in a portable rugged waterproof carrying case. The cell is useful for monitoring the purging of stagnant water from the well prior to sample collection for chemical analysis, as well as to provide pH, Eh and conductivity data under conditions as close to in situ as presently practical. In conjunction with the development of the flow‐through cell and measuring system, a portable field kit was constructed and equipped to determine the alkalinity of the water via modified Gran plot titration, immediately upon sample collection.In addition to the measurement cell system, an in‐line membrane filtration system was developed, that included valves to sequentially switch to different filter units, in case of membrane plugging.

Methods for the validiation of precipitation pH: Applications to Texas data

Abstract Methods for the validation of reported pH values are developed for incomplete precipitation chemistry data sets. The most stringent tests are based on a comparison between the net strong acidity calculated from the the conservative inorganic cations and anions and that determined from the measured pH. The strong acidity of a sample in equilibrium with atmospheric carbon dioxide can be determined from an accurate pH measurement. Less severe tests are based on charge and conductivity balances. The proposed methods are applied to Texas precipitation chemistry data to identify unreliable measurements and to determine the spatial distribution of the mean pH for validated data. Reported mean pH values, especially those of pH > 5, are lower than can be validated. For data sets with a complete analysis of the major ionic species, the comparison of net acidities is generally a stronger quality assurance test for the validation of pH measurements than the charge balance.

Effect of Variables on pH Measurement in Acid-Rain-Like Solutions as Determined by Ruggedness Tests.

Ruggedness Test (RT) experiments were performed to assess the significance of the various main factors which affect pH measurements in low ionic strength aqueous solutions, as well as to establish the presence of interactions between the main factors. Stirring has an adverse effect on the measurement of pH, since it not only increases the random noise but also biases the measured value. Temperature control to the nearest 0.5 °C is sufficient for maintaining measurements accurate to 0.01 pH. Addition of NaNO3 or KCl can not be tolerated in accurate pH measurements. Three small two-factor interactions were also revealed.

Field and laboratory measurement of pH in low-conductivity natural waters

The accurate measurement of pH is fundamental to most environmental and hydrogeological studies. Many mineral and ion exchange equilibria are controlled by acidity levels and establishment of saturation indices requires accuracies to within 0.1 pH units. Correspondingly, in studies of the impact of acidic deposition, accurate pH measurement of rainfall, surface runoff and unsaturated zone/groundwater it is essential to establish the proton sources and sinks in the various hydrological pathways. Measurement of pH both in the field and laboratory almost universally involves electrode systems based on silver-silver chloride and calomel cells; these devices, although convenient, inexpensive and portable, are in many instances not sufficiently accurate for low-conductivity waters such as rainfall and many upland streams. Numerous studies for biological and inorganic systems as well as inter-laboratory comparisons have demonstrated this problem (Illingworth, 1981; Tyree, 1981; Mason, 1984; Covington et al., 1985a); hydrogen ion activity/concentration discrepancies of up to an order of magnitude are observed even under laboratory conditions (pH = −log H + activity and for very low ionic strengths H + activity ⋍ H + concentration). Such inaccuracies will be exacerbated in the more testing environment of field measurement. Improved electrodes based on a free-diffusion liquid junction and flowing sample are being designed (Covington et al., 1983, 1985b), however, these are not readily available and may not be suitable for field use. A description of the sources of these errors is given and suggestions are made for pragmatic solutions to the problems encountered. It is concluded that a fresh approach to field and laboratory measurement of the pH of low-conductivity waters is urgently needed.

Rapid acidification of endocytic vesicles containing asialoglycoprotein in cells of a human hepatoma line.

Acidification of endocytic vesicles has been implicated as a necessary step in various processes including receptor recycling, virus penetration, and the entry of diphtheria toxin into cells. However, there have been few accurate pH measurements in morphologically and biochemically defined endocytic compartments. In this paper, we show that prelysosomal endocytic vesicles in HepG2 human hepatoma cells have an internal pH of approximately 5.4. (We previously reported that similar vesicles in mouse fibroblasts have a pH of 5.0.) The pH values were obtained from the fluorescence excitation profile after internalization of fluorescein labeled asialo-orosomucoid (ASOR). To make fluorescence measurements against the high autofluorescence background, we developed digital image analysis methods for estimating the pH within individual endocytic vesicles or lysosomes. Ultrastructural localization with colloidal gold ASOR demonstrated that the pH measurements were made when ligand was in tubulovesicular structures lacking acid phosphatase activity. Biochemical studies with 125I-ASOR demonstrated that acidification precedes degradation by more than 30 min at 37 degrees C. At 23 degrees C ligand degradation ceases almost entirely, but endocytic vesicle acidification and receptor recycling continue. These results demonstrate that acidification of endocytic vesicles, which causes ligand dissociation, occurs without fusion of endocytic vesicles with lysosomes. Methylamine and monensin raise the pH of endocytic vesicles and cause a ligand-independent loss of receptors. The effects on endocytic vesicle pH are rapidly reversible upon removal of the perturbant, but the effects on cell surface receptors are slowly reversible with methylamine and essentially irreversible with monensin. This suggests that monensin can block receptor recycling at a highly sensitive step beyond the acidification of endocytic vesicles. Taken together with other direct and indirect estimates of endocytic vesicle pH, these studies indicate that endocytic vesicles in many cell types rapidly acidify below pH 5.5, a pH sufficiently acidic to allow receptor-ligand dissociation and the penetration of some toxin chains and enveloped virus nucleocapsids into the cytoplasm.

The role of pH∗ and buffer capacity in the recovery of function of smooth muscle cooled to −13 °C in unfrozen media

It has often been suggested that pH changes may be implicated in the injury sustained by biological systems during cooling. This particular mechanism of cryoinjury, however, has received little attention undoubtedly because of the difficulties encountered in making accurate pH measurements at low temperatures. New pH∗ scales established for some mixtures of dimethyl sulfoxide and water at low temperatures are used in this study to assess the effect of pH∗ and buffering ability upon the integrity of mammalian smooth muscle stored at −13 °C in a variety of unfrozen solutions containing 30% ( w v ) Me 2SO. Smooth muscle, as a component of every organ, is a good model tissue intermediate between cells and organs. Furthermore, its overall function is conveniently tested by measuring isometric contractile responses to the drug histamine. In this way the function of strips of guinea pig taenia coli were examined at 37 °C before and after storage at −13 °C in potassiumrich media containing a variety of zwitterionic buffers. Functional recovery depends markedly on the pH∗ with a welldefined optimum at the surprisingly high pH∗ −13 of 9.2. In medium containing TES buffer, which has a maximum buffer capacity at pH∗ −13= 8.6, the cooled muscles recover 50% of their control contractility but in medium containing the buffer Tricine, which has a maximum capacity at the optimum pH∗ for recovery, the contractile response upon rewarming improves to 70%. These data are the first to quantify the effect of pH in cryopreservation on a sound theoretical basis and some of the possible underlying mechanisms are discussed.