Combined Glass Electrode Research Articles

Composites: A novel alternative to construct solid state Ag/AgCl reference electrodes

An Ag/AgCl solid-state reference electrode is developed by means of a graphite–AgCl–Silver dag-epoxy resin composite. The response of the composite reference electrode (CRE) to chloride ions is evaluated; a linear non-nernstian response is observed associated to the following equation E = −15.15 (±1.10) − 44.05 (±0.38) log[Cl −]. Comparing the CRE's response potential versus a saturated commercial Ag/AgCl reference in KCl 0.1 M, a mean of 40.7 ± 0.4 mV of the distribution of potential versus time data is observed over a period of 1 h. The performance of the CRE as reference for a glass membrane electrode by means of direct pH measurements and quantitative determination of acids by acid–base titrations is evaluated obtaining statistically stable, precise and exact results compared with those obtained using a combined glass electrode. The typical cylindrical configuration of the CRE is changed to adapt it to a FIA system for the determination of ammonium ion, obtaining a sensitivity 50.30 ± 0.26 mV/log[NH 4 +] and a linear range 8.5 × 10 −5 to 0.1 M, which are analytical parameters statistically equivalent to those presented by the classical determination system.

A New Calibration Free pH‐Probe for In Situ Measurements of Soil pH

AbstractA new pH‐probe was developed for in situ determination of soil pH. It consists of a stainless steel tube with a plastic inset containing the indicator electrode, the reference electrode and a temperature sensor at the end of the tube. The indicator electrode is a quinhydrone composite electrode that does not need to be calibrated, because it acquires almost the theoretical predicted potential and has a constant formal potential and slope under all fabrication conditions. The pH‐probe has a low standard deviation (±4 mV or 0.07 pH units). The response time is short (5 s). To characterize its function the soil pH‐probe was used to analyse pond and arable soil samples. The results were compared with those obtained with a conventional combined glass electrode. To evaluate the results, measurements were performed (i) in natural wet soil samples (in situ conditions), (ii) after drying and moistening the soil samples (moistened samples) and (iii) after drying the soil samples and mixing with bidistilled water (soil solutions; generally accepted method in laboratories). The minimum water content required to obtain stable potentials in soil samples was 10%. The influence of S2−, NO$\rm{ {_{3}^{-}}}$ and Fe3+ as naturally available reducing and oxidising agents on the potential response of the pH‐probe was investigated. All the obtained results demonstrate that the developed pH‐probe is a powerful tool to measure the pH of a soil sample under in situ conditions without a calibration step.

Obtention of Silicate-Substituted Calcium Deficient Hydroxyapatite by Dry Mechanosynthesis

This paper describes a new method to obtain silicate-substituted calcium deficient hydroxyapatite (Si-CDHA): mechanosynthesis. In previous studies mechanosynthesis has been showed to be an efficient method to obtain nanosized calcium deficient hydroxyapatite (CDHA). In this study, the silicate ions stemed from calcium silicate hydrate (CSH). Both, CSH and Si-CDHA were obtained by mechanichal activation in a planetary Retsch mill. Titration with 0.01M HCl of the samples of Si-CDHA obtained at different milling times demonstrated that a 9.8% w/w of silicate had been incorporated into the apatite. Moreover, XRD showed that the silicate incorporation in the apatite increased the crystallinity and the stability of the apatitic phase. These results indicate that mechanosynthesis is an effective method for incorporating silicate into CDHA. As a result a hydroxyapatite nanosized powder is obtained, with promising characteristics such as an improved bioactivity due to its calcium deficiency and silicate substitution. Introduction It is well-known that hydroxyapatite can incorporate different types of ionic substitutions, with the subsequent changes in its structural parameters and physico-chemical properties. Some recent studies have shown that bioactivity of hydroxyapatite is significantly enhanced by the incorporation of silicate ions into its lattice [1,2]. In previous studies mechanosynthesis has been presented as a new method to obtain nanosized hydroxyapatite with an improved control of the stoichiometry [3,4]. Even more, this technique can allow the incorporation of different ions in the apatitic structure. In this study, the introduction of silicate in a calcium deficient hydroxyapatite (CDHA) by mechanosynthesis is presented. Materials and Methods Two mechanochemical reactions were performed in a planetary Retsch mill. The first one to obtain a calcium silicate hydrated (CSH, CaSiO3.0.25H2O) and the second one to obtain the silicatesubstituted hydroxyapatite. For the first one, the reactants used were calcium hydroxide (Ca(OH)2; Fluka 21181) and silicic acid hydrate(SiO2.0.24H2O; Fluka ref 60780). In the second one, the reactants were adjusted to obtain a substitution of one phosphate group for one silicate group in the CDHA, having a Ca/P molar ratio of 1.6 and a Ca/(P+Si) of 1.5. For this purpose, a mixture of dicalcium phosphate dihydrate (DCPD, CaHPO4.2H2O; Fluka 21184 ), calcium oxide (CaO; Aldrich 24,856-8) and calcium silicate hydrate (CSH), previously obtained from the first mechanochemical reaction, were milled at 350 rpm. 2 Title of Publication (to be inserted by the publisher) The total milled mass was 10 grams and the amount of silicate in the mixture represented a 9.8 % w/w. Powder samples were obtained after different grinding times (2, 4, 8, 12, 20, 26 and 30 h). These samples were titrated with 0.01M HCl and analysed by X-ray diffraction before and after heating at 950oC. Titration analysis were performed with a PH-meter Tacussel PHM210 combined glass electrode Bioblock and standardized with NBS buffer PH 4, 7 and 10 (Fluka Biochemica color-coded). XRD patterns were obtained by an automatic diffractometer Philips PW 3830X with horizontal goniometer CGR, using an anticatode Cu (Kα1= 1,5405A) and Ni filter. Results and Discussion CSH obtention by mechanosynthesis. CSH was obtained after 33h of grinding calcium hydroxide and silicon oxide. The XRD patterns of the samples milled at different times (0,1,4,6,12,16,20,23 and 33h) showed the disappearance of the Ca(OH)2 peaks and the appearance of an amorphous profile (Fig.1). After heating at 950oC for 2h a crystalline CaSiO3 was formed (JCPDS 43-1460). 0 400 80

Solubility of Alkali and Alkali Earth Salts of Dihydrogen Ethylenediaminetetraacetate in Aqueous Solutions

The solid metal−EDTA compounds K2H2Y·2H2O, MgH2Y·6H2O, CaH2Y·2H2O, SrH2Y·2H2O, and BaH2Y·3H2O (H2Y2- is dihydrogen ethylenediaminetetraacetate anion) prepared from aqueous media were characterized by elemental analysis, IR spectrometry, and thermogravimetry (TG). The solubilities of these EDTA salts and reagent grade disodium salt (Na2H2Y·2H2O) in water at 25.0 °C were determined by potentiometric titrations with NaOH using a combined glass electrode. The infrared spectra showed that metal ions do not form chelates with EDTA in the studied salts.

Estimation of uncertainty in routine pH measurement

A procedure for estimation of measurement uncertainty of routine pH measurement (pH meter with two-point calibration, with or without automatic temperature compensation, combination glass electrode) based on the ISO method is presented. It is based on a mathematical model of pH measurement that involves nine input parameters. Altogether 14 components of uncertainty are identified and quantified. No single uncertainty estimate can be ascribed to a pH measurement procedure: the uncertainty of pH strongly depends on changes in experimental details and on the pH value itself. The uncertainty is the lowest near the isopotential point and in the center of the calibration line and can increase by a factor of 2 (depending on the details of the measurement procedure) when moving from around pH 7 to around pH 2 or 11. Therefore it is necessary to estimate the uncertainty separately for each measurement. For routine pH measurement the uncertainty cannot be significantly reduced by using more accurate standard solutions than ±0.02 pH units – the uncertainty improvement is small. A major problem in estimating the uncertainty of pH is the residual junction potential, which is almost impossible to take rigorously into account in the framework of a routine pH measurement.1

Retention of ionizable compounds on HPLC. 6. pH measurements with the glass electrode in methanol–water mixtures

The relationship, δ values, between the two rigorous pH scales, s spH (pH measured in a methanol–water mixture and referred to the same mixture as standard state) and s wpH (pH measured in a methanol–water mixture but referred to water as standard state), in several methanol–water mixtures was determined ( δ= s wpH− s spH). δ values were measured using a combined glass electrode and a wide set of buffer solutions. The results are consistent with those obtained with the hydrogen electrode. This confirms the aptness of the glass electrode to achieve rigorous pH measurements in methanol–water mixtures. An equation that relates δ and composition of methanol–water mixtures, and allows δ computation at any composition by interpolation, is proposed. Therefore, s spH can be achieved from the experimental s wpH value and δ at any mobile phase composition. s spH (or s wpH) values are related to the chromatographic retention of ionizable compounds through their thermodynamic acid–base constants in the methanol–water mixture used as mobile phase. These relationships were tested for the retention variation of several acids and bases with the pH of the mobile phase. Therefore, the optimization of the mobile phase acidity for any analyte can be easily reached avoiding the disturbances observed when w wpH is used.

Electrochemical Study of the Chelation of Cu2+ Cation by 18-Crown-6-Ether and Hydroxybenzoic Acids in Aqueous Solution

The chelation of Cu2+ cation by salicylic acid, sodium salicylate, m- and p-hydroxybenzoic acids, and a macrocyclic receptor, such as 18-crown-6-ether, have been studied by means of pH and emf measurements with a Cu2+/Cu ion-selective electrode (ISE). The latest permit the determination of thermodynamic stability constants based on activity quotients, which is unusual in potentiometric studies. An experimental procedure has been set up to measure simultaneously both properties by using the combined glass electrode as the reference electrode of the ISE, with clear advantages with respect to the conventional double-junction electrodes with sleeve diaphragms. The studies have been carried out at 25°C as a function of ligand concentration at constant [Cu2+], and as a function of [Cu2+] at constant ligand concentration. Several models have been proposed to determine the stability constants of the chelates, proving different stoichiometries.

Performance of Low Output Impedance Composite pH Glass Electrodes

Low output impedance composite pH sensors were constructed by direct attachment of an impedance converter to laboratory purpose combined pH glass electrodes. The signal was transmitted in analog form by unshielded electric cable. The performance of new and aged composite pH sensors was determined by the multiple-point calibration method. In case of new electrodes, the slope and the response time, as well as the reproducibility, were insignificantly influenced by the converter attached (the mean slope values calculated for the six electrode group studied were 57.80 mV/pH for unmodified electrodes and 57.97 mV/pH for modified electrodes). The electrode response was not affected by the presence of various electromagnetic noise sources or by the input impedance value of the measuring instrument. The slope and the response time of aged sensors were considerably improved using the impedance converter. The response time decreased from about 150–180 sec to about 30 sec and the average slope value increased from 5...

Performance of Low Output Impedance Composite pH Glass Electrodes

Low output impedance composite pH sensors were constructed by direct attachment of an impedance converter to laboratory purpose combined pH glass electrodes. The signal was transmitted in analog form by unshielded electric cable. The performance of new and aged composite pH sensors was determined by the multiple-point calibration method. In case of new electrodes, the slope and the response time, as well as the reproducibility, were insignificantly influenced by the converter attached (the mean slope values calculated for the six electrode group studied were 57.80 mV/pH for unmodified electrodes and 57.97 mV/pH for modified electrodes). The electrode response was not affected by the presence of various electromagnetic noise sources or by the input impedance value of the measuring instrument. The slope and the response time of aged sensors were considerably improved using the impedance converter. The response time decreased from about 150–180 sec to about 30 sec and the average slope value increased from 54.94 mV/pH, calculated for unmodified electrodes, to 56.96 mV/pH, for modified electrodes.

Potencialidades do dióxido de chumbo eletrodepositado como sensor potenciométrico

This paper proposes an experiment to be performed in both instrumental analysis and experimental physical-chemistry curricular disciplines in order to open options to develop challenging basic research activities. Thus the experimental procedures and the results obtained in the preparation of electrodeposited lead dioxide onto graphite and its evaluation as potentiometric sensor for H3O+ and Pb2+ ions, are presented. The data obtained in acid-base titrations were compared with those of the traditional combination glass electrode at the same conditions. Although a linear sub-Nernstian response to free hydrogen ions was observed for the electrodeposited PbO2 electrode, a good agreement was obtained between them. Working as lead(II) sensing electrode, the PbO2 showed a linear sub-Nernstian behavior at total Pb2+ concentrations ranging from 3,5 x 10-4 to 3,0 x 10-2 mol/L in nitrate media. For the redox couple PbO2/Pb(II) the operational slope converges to the theoretical one, as the acidity of the working solution increases.

A computerized artificial stomach model to assess sodium alginate-induced pH gradient

A computerized `artificial stomach' model capable 1) of measuring the pH both in gastric contents, using a combined glass electrode, and at its surface, using a surface microelectrode, 2) of simulating the gastric emptying regulation in regard to the intragastric pH in physiological situation and 3) of controlling the position of the surface microelectrode in regard to gastric volume variations by means of a surface detector system, has been used to analyze the antacid activity and the pH gradient induced by both alginate-containing drugs, different in their carbonate composition. When added to 0.1 N HCl solution, both drugs developed a floating `raft' at the surface supporting a pH gradient (close to 4 pH units) simultaneously with a moderate or weak antacid activity in gastric contents. Both studied drugs are capable of exerting a protective effect in gastro-esophageal reflux disease mainly by the induction of a pH gradient.

Trace Determination of Strong Acids in NitroPlasticizers

A titration method was applied to the trace determination of strong acids in a variety of nitro plasticizers. The analyses were performed in acetonitrile using an automatic titrator and a calomel combined glass electrode for non-aqueous solutions. A dilute solution of sodium hydroxide (5.00 × 10 - 4 M) in methanol was used as the titrant. The sensitivity and precision of the method were greatly increased by the addition of a known volume of dilute nitric acid to the solvent in which the titrations were performed. Very low acid content values were detected in the plasticizers with the experimental conditions used. The detection limit was found to be 0.001 mg g - 1 of NaOH in the sample (0.000025 m equiv. g - 1 ) and the measured acid content values were reproducible.

The ion product of H2O, dissociation constants of H2CO3 and pitzer parameters in the system Na+/H+/OH?/HCO 3 ? /CO 3 2- /ClO 4 ? /H2O at 25�C

The ion product of water and the dissociation constants of carbonic acid have been determined in 0.1, 1.0, 3.0, and 5.0M NaClO4 at 25°C. The ion product of water Kw' has been evaluated by emf measurements with a combined glass electrode in NaClO4 solutions containing 0.001–0.1M HCLO4 or NaOH. The product KH'Kl'K2' of the Henry constant for CO2 and the dissociation constants for H2CO3 have been determined by titration of carbonate solutions equilibrated with pCO2 =10−3.52 atm, and K2' has been evaluated by potentiometric titration and by measuring the H+ concentration at fixed HCO3− and CO32- concentrations. The ion interaction (Pitzer) equations are applied to describe the constants Kw', K2' and KH'H1'K2' as a function of the NaClO4 concentration. The experimental data are used to evaluate the mixing parametersΘi/ClO4 andΨi/ClO4-/Na+ fori = OH-,HCO3- andCO32-

Potentiometric Biosensing of Penicillins Using a Flow-Through Reactor with Penicillinase or Penicillin Amidase Immobilized by Gamma-Irradiation

Abstract Penicillinase and penicillin amidase were immobilized, with similar yield, by physical entrapment in porous beads of poly-2-hydroxyethyl methacrylate matrix formed by gamma-irradiation. Flow-injection analysis of penicillins was performed by using a column reactor filled with enzyme-loaded polymer beads and a wall-jet pH-metric micro-cell with a combined flat-membrane glass electrode as the sensing element. With both enzymes it was possible to have either a linear response dynamic range or a logarithmic one by appropriate selection of buffer capacity and flow rate of the carrier solution.

Upgrading a low-cost physicochemical wastewater treatment plant to solve operational problems

A comprehensive programme of jar tests utilizing lime to treat municipal wastewater in northeast Brazil showed that a supernatant could be achieved that attained the WHO (1989) guidelines for effluent reuse for unrestricted irrigation (≤ 1000 faecal coliforms per 100 ml; ≤ 1 intestinal nematode egg per litre). In the light of these findings a pilot-plant was built to investigate the continuous treatment of wastewater with lime for effluent reuse in irrigation. Bearing in mind the problems often encountered in less-developed countries with the operation and maintenance of complex wastewater treatment systems, it was decided to base the lime treatment technology on that used successfully throughout the world (in both industrialised and non-industrialised countries) to treat potable water; namely non-mechanised coagulation and flocculation, and settlement in a horizontal-flow sedimentation tank. Although the prototype pilot-plant achieved a final effluent that attained the WHO (1989) guidelines for unrestricted irrigation, a number of operational and maintenance problems were encountered. Effluent quality data has been reported previously. This paper summarises the data and describes the practical problems associated with the operation and maintenance of the lime treatment plant, and the upgrading techniques used to overcome these difficulties. The prototype plant consisted of a rapid-mix basin in which raw municipal wastewater was combined with lime slurry at a ratio that achieved a pH of greater than 11.0 after mixing; a sedimentation tank, for solid-liquid separation; and a recarbonation basin for tertiary treatment. Constant monitoring of the lime-wastewater mix pH, using a pH controller to regulate the flow of lime slurry, proved difficult. Frequent cleaning of pipework was needed to avoid clogging by the lime slurry. In a second phase of the research work, the pilot-plant was upgraded to tackle these and other operational and maintenance problems. Plant geometry was improved to prevent the sedimentation of lime solids within the delivery pipework. All pipework carrying lime slurry was set up in duplicate so that plant operation could continue during cleaning operations. An improved pH control system, which linked lime slurry flow to the pH of the lime-wastewater mix, was also installed. The design of the sedimentation tank was altered to aid the removal of sludge and the tank inlet and outlet were upgraded in order to minimise short-circuiting. Plant operation and maintenance were simplified by the design improvements and the frequency of mechanical breakdown was reduced. The pH control system was difficult to operate effectively because the pH probe became rapidly fouled by lime solids. However, gel-filled, low sodium-error pH electrodes calibrated in pH 11.0 buffer proved significantly more robust in the aggressive high-lime environment than did conventional combined glass electrodes. Options for further improving the design of a lime treatment plant are considered.

Pattern of 72-hour intragastric acidity in a homogeneous group of intensive care unit patients.

To gain insight into the variation over time of gastric acidity in postoperative ICU patients, intragastric pH was prospectively studied in patients undergoing elective abdominal aortic reconstructive surgery during a 72-hr intra- and postoperative period. Intragastric pH was monitored continuously in 14 patients with combined glass electrodes. During the day of surgery (day 1), the median 24-hr pH for all patients was 6.25 (5.8-7.0, IQR). However, three of the 14 studied patients had a median 24-hr pH of 1.8. The median 24-hr pH throughout day 2 for all was 2.45 (1.6-4.7, P = 0.001). The median 24-hr pH on day 3 was 1.6 (1.5-2.1, P = 0.001). Median 8-hr pH values demonstrate a remarkable interpatient and intraindividual variation in the course of the postoperative period. A progressive lowering of the intragastric pH was observed in the first 40 hr. From the 40- to 48-hr interval until the end of the study, no further significant decrease was found. The intragastric pH was above 4, 74% of the time during day 1, 39% during day 2 (P = 0.006) and 16% during day 3 (P = 0.003). Percentage of time above 4 on day 2 was significantly higher than on day 3 (P = 0.04). In conclusion, since gastric acid and pepsin seem to play a role in stress ulceration, this study suggests some patients are at risk of stress ulceration from the beginning of surgery, but most patients become at risk of stress ulceration in the course of the postoperative period.

Comments on potentiometric pH titrations and the relationship between pH-meter reading and hydrogen ion concentration

It is emphasized and shown that the concept of pH is more complicated than might be thought (and to some extent also unsatisfactory); there are (at least) three pH scales in general use and it is the aim to make this fact recognized. These scales are: (1) an activity scale, where the hydrogen ion activity is measured based on NBS or similar standards by carefully eliminating the liquid-junction potentials of the electrode system via experimental determinations; (2) a practical scale, which has unintentionally developed by convenience over the past ca. 30 years, is based on now generally available combined glass electrodes together with NBS (or related) buffers used for calibration; and (3) a concentration scale which uses strong acids and/or bases for calibration and defines the pH-meter reading in terms of −log[H +]. Scale (2) is clearly the one least well defined, yet it is also the one most widely used. If a ‘pH’ is measured for a given constant H + concentration in the three scales, its value decreases in the order (1) > (2) > (3). Scales (1) and (3) may be converted into each other by using the single ion activity coefficient of H +, e.g., at 25°C and at ionic strengths of 0.1 and 0.5 M the differences correspond to 0.11 and 0.15 log unit, respectively. The conversion term from scale (2) to (3) corresponds at 25°C and an ionic strength between 0.1 and 0.5 M to about 0.03 log unit. It is evident that any acidity constant, i.e. p K a value, determined for a given system (HA ⇌| A − + H +) is affected to the same extent; hence, the mentioned conversion factors have to be applied if P K a values determined in different scales are to be compared or used. It may be added that many workers believe that combined glass electrodes measure the hydrogen ion activity and that they are working in sale (1), yet this is not the case, they are actually working in scale (2). Moreover it is also barely (or not at all) recognized that the values in scale (2) are in fact closer to those of scale (3) and not to those of scale (1), as is often assumed. Some general comments regarding potentiometric pH titrations and the determination of equilibrium constants (i.e., p K a values and stability constants of metal ion complexes) are also made, and the advantages of different titration procedures are discussed and pitfalls are pointed out.

Circadian pattern of intragastric acidity in duodenal ulcer patients: a study of variations in relation to ulcer activity.

The relation between intragastric acidity and duodenal ulcer activity was studied prospectively in 21 patients with endoscopically proved duodenal ulcers. The 24 hour intragastric acidity was measured on four separate occasions by continuous recording using combined glass electrodes: (a) in the presence of an ulcer crater without treatment; (b) during active ulceration being treated with ranitidine; (c) during early healing after a six week course of ranitidine; (d) during late healing six months after acute ulceration. Intragastric acidity was also monitored in 20 healthy subjects. At all stages of ulcer activity and during all predefined time periods, duodenal ulcer patients had significantly higher gastric acidity than healthy control subjects. Duodenal ulcer patients showed a similar circadian pattern of intragastric acidity during exacerbation of ulcer disease and in remission during the early and late ulcer healing periods. These results argue against a direct relation between the activity of duodenal ulcer disease and gastric acidity. It is concluded that the chronic recurrent course of duodenal ulcer disease does not result from a fluctuation in intragastric acidity.

Repeated Intravenous Bolus Injections of Omeprazole: Effects on 24-Hour Intragastric pH, Serum Gastrin, and Serum Pepsinogen A and C

We determined the effect of four times daily dosing with intravenous omeprazole on 24-h intragastric acidity, serum gastrin, and serum pepsinogen A and C in 10 fasting subjects (median age, 23.5 years). Two dose regimens (80-20-20-20 mg and 80-40-40-40 mg) were compared in a randomized placebo-controlled crossover study. Intragastric pH was continuously monitored during 24 h, using combined glass electrodes, and blood samples were taken every 6 h. Repeated boluses of omeprazole every 6 h significantly increased (p less than 0.01) the median 24-h intragastric pH compared with placebo (median pH, 3.7 (140 mg/day), 4.3 (200 mg/day), and 1.4 (placebo)) but failed to continuously raise pH levels above 4.0 in 8 (140 mg/day) or 9 (200 mg/day) of 10 subjects. No advantage of the 200-mg dose over the 140-mg dose was demonstrated. A cumulative effect of intravenous omeprazole was shown after repeated boluses but also a marked interindividual variation in response, which was correlated with the omeprazole area under the plasma concentration time curve. A significant elevation of serum gastrin coincided with the increased pH levels, which was followed after 12-18 h by a significant increase of both serum pepsinogens.

Semimicrotitration device for cholinesterase assay

A semimicrotitration device for reaction volumes of 0.5 to 2 ml and effective stirring of reaction solution or suspension is described. The principal part of the device is a combined glass electrode functioning as a reaction vessel (combined “cup”-glass electrode). Characteristics of the device are small volumes and great reliability. Nonspecific cholinesterase activities measured in rat plasma and liver tissue homogenates and compared with the activities of the same tissue and plasma samples measured radiometrically and spectrophotometrically illustrate the applicability of the device. The method is best suited for kinetic studies with multiple substrates of esterases, either in the soluble form or in tissue homogenates.