Accuracy Of pH Measurements Research Articles

In vivo measurement of cytosolic and mitochondrial pH using a pH-sensitive GFP derivative in Saccharomyces cerevisiae reveals a relation between intracellular pH and growth

The specific pH values of cellular compartments affect virtually all biochemical processes, including enzyme activity, protein folding and redox state. Accurate, sensitive and compartment-specific measurements of intracellular pH (pHi) dynamics in living cells are therefore crucial to the understanding of stress response and adaptation. We used the pH-sensitive GFP derivative 'ratiometric pHluorin' expressed in the cytosol and in the mitochondrial matrix of growing Saccharomyces cerevisiae to assess the variation in cytosolic pH (pHcyt) and mitochondrial pH (pHmit) in response to nutrient availability, respiratory chain activity, shifts in environmental pH and stress induced by addition of sorbic acid. The in vivo measurement allowed accurate determination of organelle-specific pH, determining a constant pHcyt of 7.2 and a constant pHmit of 7.5 in cells exponentially growing on glucose. We show that pHcyt and pHmit are differentially regulated by carbon source and respiratory chain inhibitors. Upon glucose starvation or sorbic acid stress, pHi decrease coincided with growth stasis. Additionally, pHi and growth coincided similarly in recovery after addition of glucose to glucose-starved cultures or after recovery from a sorbic acid pulse. We suggest a relation between pHi and cellular energy generation, and therefore a relation between pHi and growth.

Cystic Fibrosis Transmembrane Conductance Regulator-independent Phagosomal Acidification in Macrophages

It was reported recently that the cystic fibrosis transmembrane conductance regulator (CFTR) is required for acidification of phagosomes in alveolar macrophages (Di, A., Brown, M. E., Deriy, L. V., Li, C., Szeto, F. L., Chen, Y., Huang, P., Tong, J., Naren, A. P., Bindokas, V., Palfrey, H. C., and Nelson, D. J. (2006) Nat. Cell Biol. 8, 933-944). Here we determined whether the CFTR chloride channel is a generalized pathway for chloride entry into phagosomes in macrophages and whether mutations in CFTR could contribute to alveolar macrophage dysfunction. The pH of mature phagolysosomes in macrophages was measured by fluorescence ratio imaging using a zymosan conjugate containing Oregon Green(R) 488 and tetramethylrhodamine. Acidification of phagolysosomes in J774A.1 macrophages (pH approximately 5.1 at 45 min), murine alveolar macrophages (pH approximately 5.3), and human alveolar macrophages (pH approximately 5.3) was insensitive to CFTR inhibition by the thiazolidinone CFTR(inh)-172. Acidification of phagolysosomes in alveolar macrophages isolated from mice homozygous for DeltaF508-CFTR, the most common mutation in cystic fibrosis, was not different compared with that in alveolar macrophages isolated from wild-type mice. We also measured the kinetics of phagosomal acidification in J774A.1 and murine alveolar macrophages using a zymosan conjugate containing fluorescein and tetramethylrhodamine. Phagosomal acidification began within 3 min of zymosan binding and was complete within approximately 15 min of internalization. The rate of phagosomal acidification in J774A.1 cells was not slowed by CFTR(inh)-172 and was not different in alveolar macrophages from wild-type versus DeltaF508-CFTR mice. Our data indicate that phagolysosomal acidification in macrophages is not dependent on CFTR channel activity and do not support a proposed mechanism for cystic fibrosis lung disease involving defective phagosomal acidification and bacterial killing in alveolar macrophages.

Spectrophotometric determination of pH in seawater off Taiaroa Head, Otago, New Zealand: Full-spectrum modelling and prediction of pCO 2 levels

Accurate measurement of seawater pH has long been sought by marine chemists (for example: [Dickson, A.G. 1993a. The measurement of sea water pH. Marine Chemistry, 44, 131–142, Dickson, A.G. 1993b. pH buffers for sea water media based on the total hydrogen ion concentration scale. Deep-Sea Research, 40, 107–118; Zhang, 1996; Tapp, M., Hunter, K.A., Currie, K. and Macaskill, B. 2000. Apparatus of continuous-flow underway spectrophotometric measurement of surface water pH. Marine Chemistry 72(2–4), 193–202; Friis, K., Koetzinger, A., Wallace, D.W.R. 2004. Spectrophotometric pH measurement in the ocean: Requirements, design and testing of an autonomous charge-coupled device detector system. Limnology and Oceanography: Methods 2, 126–136]. Recently, such attempts have taken on greater significance as anthropogenic carbon dioxide emissions may create rapidly changing oceanic pH. Spectrophotometric techniques have been accepted generally as the best for determination of seawater pH. Here we report a new technique using thymol blue as the indicator dye and fitting the entire spectrum from 400 to 900 nm rather than measuring the absorbance values at only two or three points in the spectrum. This full-spectrum modelling enables a reduction in signal to noise over other techniques. In the laboratory, we find with seawater samples a pH precision increase of five-fold “within” a sample and seven-fold “between” samples when comparing the full spectrum to the three-point method of analysis [Zhang, H., Byrne, R.H. 1996. Spectrophotometric pH measurements of surface seawater at in-situ conditions: absorbance and protonation behaviour of thymol blue. Marine Chemistry 52, 17–25]. We then use this analysis technique for a series of transects between July 2005 and March 2006 east of Taiaroa Head, New Zealand. The pH values determined at sea using the full-spectrum modelling technique along with independently measured alkalinity ( A T) are used to determine the total inorganic carbon concentration ( C T). C T in turn is used with A T to calculate pCO 2 levels in surface seawater. Comparing pCO 2 levels from independently measured pCO 2 acquired in parallel with pH, we show an average internal consistency agreement of 1.15% ( σ = 0.23%) when using the Mehrbach et al. [Mehrbach, C., Culberson, C.H., Hawley, J.E., Pykowicz, R.M. 1973. Measurements of the apparent dissociation constants of carbonic acid in seawater at atmospheric pressure. Limnology and Oceanography 18, 897–907] (as refit by Dickson and Millero[Dickson, A.G., Millero, F.J. 1987. A comparison of the equilibrium constants for the dissociation of carbonic acid in seawater media. Deep-Sea Research 34, 1733–1743] carbonate dissociation constants. The pCO 2 values predicted from pH are very close to the measured values of pCO 2. We indicate that while both pH and pCO 2 are of great interest in surface seawater, it may be possible to measure only pH and alkalinity to obtain an excellent predictor of pCO 2 levels. The instrument we use to measure pH combined with the full-spectrum modelling show that regular, real-time analysis of the carbonate system in surface seawater on a small boat (14 m) can be precise and consistent.

Real-Time Downhole pH Measurement Using Optical Spectroscopy

Summary A new downhole pH sensor has been developed to provide an in-situ pH measurement of formation water at reservoir conditions, and results are presented for two wells in the Norwegian Sea. The measurement technique, for use with wireline formation-sampling tools, uses pH-sensitive dyes that change color according to the pH of the formation water. To make a real-time pH measurement, the dye is injected into the formation fluid being pumped through the tool flowline, and the relevant visible wavelengths in an optical detector are used to record the dye signal and calculate pH with 0.1-unit accuracy. The pH of a formation fluid alters as the sample is brought to surface from the high-temperature and high-pressure conditions downhole, owing to acid gases and salts coming out of solution and changes in water-chemistry equilibria. To obtain an accurate pH, the measurement must be made downhole at reservoir conditions. Unlike potentiometric methods in which fouling of electrode surfaces by oil and mud is a potential problem, the dye technique is robust because the dye is isolated from the formation fluid and is injected into the sample only when a measurement is made. The technique has been applied successfully to both oil-based and water-based drilling muds, with successful measurements even in mixed oil/water flows. Multiple measurements of pH at a single sampling station demonstrate that the method is robust and repeatable. These measurements have been compared with numerical simulations using a multiphase chemical-equilibrium model that uses laboratory analysis of collected water samples as input. pH is a key parameter in water chemistry and is critical for corrosion and scale studies. Accurate downhole pH measurement allows a more-accurate selection of appropriate completion materials and more-effective planning for scale treatment and inhibition.

Evaluation of the reproducibility and accuracy of pH-determining devices used to measure urine pH in dogs

To evaluate the reproducibility and accuracy of 4 portable pH meters, a reagent strip, and pH paper for measuring urine pH in dogs. Prospective masked randomized study. 201 free-catch urine samples from 114 hospitalized dogs. Urine samples were divided into 2-mL aliquots. Measurements of urine pH were obtained by use of a laboratory benchtop pH meter, 4 portable pH meters, a urine reagent strip, and pH paper. The pH of each aliquot was measured within 4 hours of collection by an evaluator unaware of the aliquot's origin. To assess reproducibility, the coefficient of variation for each pH measurement device was calculated. To determine which device was most accurate, the degree of agreement among the different devices was assessed in comparison with the benchtop pH meter, which was considered the reference method. 3 of the 4 portable pH meters had nearly perfect agreement with the reference method. The reagent strip and pH paper had moderate to poor agreement with the reference method. Urine pH measurements should be made by use of a portable or benchtop pH meter when accurate measurements are crucial for diagnosis or treatment. Reagent strips and pH papers are useful in obtaining pH approximations but are not recommended when accurate measurements of urine pH are required.

The impact of ionic strength and background electrolyte on pH measurements in metal ion adsorption experiments

Our understanding of metal ion adsorption to clay minerals has progressed significantly over the past several decades, and theories have been promulgated to describe and predict the impacts of pH, ionic strength, and background solution composition on the extent of adsorption. Studies evaluating the effects of ionic strength on adsorption typically employ a broad range of background electrolyte concentrations. Measurement of pH in these systems can be inaccurate when pH values are measured with liquid junction pH probes calibrated with standard buffers due to changes in the liquid junction potential between standard, low ionic strength (0.05 M) buffers and high ionic strength solutions (>0.1 M). The objective of this research is to determine the extent of the error in pH values measured at high ionic strength, and to develop an approach for accurately measuring pH over a range of ionic strengths using a combined pH electrode. To achieve this objective, the adsorption of cobalt (10 −5 M) onto gibbsite (10 g/L) from various electrolyte solutions (0.01–1 M) was studied. The pH measurements were determined from calibrations with standard buffers and ionic strength corrected buffer calibrations. The results show a significant effect of the aqueous solution background electrolyte anion and ionic strength on pH measurement. The 0.5 and 1 M ionic strength metal ion adsorption edges shifted to lower pH with increasing ionic strength when pH was calibrated with standard buffers whereas no shift in the adsorption edges was observed when calibrated with ionic strength corrected buffers. Therefore, to obtain an accurate pH measurement, pH calibration should contain the same electrolyte and ionic strength as the samples.

Persistence and leaching of sulfonylurea herbicides over a 4-year period in the highly alkaline soils of south-eastern Australia

The sulfonylurea herbicides are commonly used in the cereal belt of south-eastern Australia and there is concern that their persistence in alkaline soils is long enough to damage subsequent rotational crops such as legumes and oilseeds. In this study, we investigated leaching and persistence of 3 commonly used herbicides (chlorsulfuron, triasulfuron and metsulfuron-methyl) in alkaline soils of south-eastern Australia (pH range 7.4–8.6) for at least 4 years after treatment. In general, chlorsulfuron was predicted to persist for 3–5 years [time to degrade to 1% (DT99) of 33–63 months after treatment depending on the field site], triasulfuron for 1–3 years (DT99 of 13–37 months after treatment), and metsulfuron-methyl for less than 1 year (although data were insufficient for degradation estimates) after its application. However, this varied between sites and years of application. Although, the majority of residues remained in the top 20 cm of the soil profile throughout the study, leaching of a small fraction of the residue to deeper layers of the profiles (up to 1 m) was observed. Despite their slow rate of degradation, the herbicides did eventually dissipate, even in soils with very high pH (8.5). In most cases, the current product labels provide an adequate safety period to protect sensitive rotational crops from potential damage due to excessive persistence. However, in particular years at 3 of the 5 field sites, metsulfuron-methyl and triasulfuron persisted beyond the recommended recropping period (9 months for metsulfuron-methyl and 22 months for triasulfuron in soils up to pH 7.5 or 24 months in soils pH 7.6 and above). An accurate measurement of soil pH and its variability within the paddock is essential to minimise any subsequent crop damage by these herbicides.

A New Device for Measuring Kinetics of Ruminal pH and Redox Potential in Dairy Cattle

A sampling and measuring device was set up to measure continuously the pH and the redox potential (Eh) of ruminal content in absence of any gaseous contamination (method 1). It was compared with a conventional suction device in which no precaution was taken to prevent air from coming into contact with the surface of collected samples (method 2). Two fistulated dry cows were used and fed a total mixed ration. Redox potential and pH measurements were performed repeatedly on collected samples using these 2 methods during a 9-h period; each period started 1h before feeding. The partial pressure of oxygen (log fO2) was calculated from Nernst's equation using pH and Eh values. Results indicated that pH, Eh, and log f(O2) were affected by sampling method. In method 1, pH values ranged from 6.7 to 6.37 and Eh from −173.5 to −216.8mV. In method 2, pH and Eh values varied, respectively, from 6.93 to 6.49 and from −111.3 to −139.5mV. The partial pressure of oxygen was 106 times lower in samples that were continuously collected than in hand-samples. As a result, method 1 could make accurate measurements of pH and Eh of ruminal content.

NON-ENDOSCOPIC VERSUS ENDOSCOPIC PLACEMENT OF THE BRAVO pH CAPSULE

Purpose: The BRAVO pH capsule system is a relatively new device that measures esophageal acid reflux. It is recommended that it be placed after upper endoscopy. However, endoscopy is expensive, carries some risk, and sedation may reduce oral intake on Day 1, leading to falsely low pH values. BRAVO capsule placement without sedation (Non-Endo) may increase patient access and provide more accurate pH measurements than placement after endoscopy with sedation (Endo). Methods: Oral placement of the BRAVO capsule was 5 cm above the LES, which was identified by manometry on the same day. Endoscopic placement of the BRAVO capsule was 6 cm above the z-line. These locations have previously been shown to be similar. Results: 252 consecutive patients were scheduled for BRAVO pH capsule placement over a 14 month period (9/02 to 11/03). Reflux symptoms were the predominant reason for referral (78.7%), followed by regurgitation, and chest pain. 210 patients were scheduled for Non-Endo BRAVO pH capsule placement; 185 (125F:60M) completed the study (88.1%). There were 8 equipment failures (3.8%), 13 premature capsule releases (6.2%), 1 incomplete study due to patient request (0.48%), and 3 patients who were intolerant of oral placement and underwent Endo placement. Mean time for premature capsule release was 12.6 hrs. 45 patients were scheduled for Endo BRAVO placement. 40 studies (25F:15M) were completed (88.9%). There were 3 equipment failures (6.7%), and 2 premature capsule releases (4.4%; mean = 2 hours). In the Non-Endo group, the mean Demeester scores on Day 1, Day 2, and Total were 25.60, 25.49, and 26.44, respectively. The mean percentage of time with distal esophageal acid exposure on Days 1, 2, and Total were 7.66, 7.58, and 7.50. In the Endo group, the mean Demeester scores on Day 1, Day 2, and Total were 24.14, 24.22, and 25.14, respectively. In the Endo group, the mean percentage of time with distal esophageal acid exposure on Day 1, 2, and Total were: 7.31, 6.86, and 7.09, respectively. No complications occurred in either group. Conclusions: Non-Endo placement of the BRAVO pH capsule is safe, well-tolerated, and easily accomplished in out-patients. No significant differences were found between endoscopic placement and non-endoscopic placement. Non-Endo BRAVO placement may reduce costs associated with the procedure and improve patient access.

A signal processing ASIC for ISFET-based chemical sensors

With the advantages of small size, reliability, rapid response, compatibility to standard CMOS technology and on-chip signal processing, Ion-Sensitive Field Effect Transistor (ISFET)-based transducers are increasingly being applied in physiological data acquisition and environment monitoring. This paper presents a signal processing Application Specific Integrated Circuit (ASIC) and a discrete temperature compensation chip design of a potentiometric ISFET-based chemical sensor. To assure the measurement of a correct pH value, the two-point calibration circuitry based on the response of standard pH 4 and 7 buffer solution has been implemented by using Algorithmic State Machine hardware algorithms. For battery power consideration, the proposed signal processing ASIC consisting of low voltage (3 V) mixed signal modules has been developed and fabricated in a 0.5 μm CMOS technology. The results demonstrate small differences of pH response of ISFET's operating with and without ASIC device, i.e. only the order of 0.1 mV/pH with respect to sensitivity. Furthermore, using the temperature compensating circuitry reduces the ISFET temperature coefficient to +0.15 mV/°C, and hence results in a lower pH value variation of −0.0023pH/°C. It was shown that more accurate pH measurement can be concluded with the proposed ASIC and VT extractor circuitry.

Collection under paraffin is not necessary for stability of urine pH over 24�h

Collection of urine under paraffin is routinely used in clinical practice for urine pH evaluation. To examine whether the accuracy of pH measurement is adversely influenced by the method of collection and storage, we divided each urine specimen and sent one aliquot to the laboratory under paraffin and another in a sealed 5-ml plastic syringe. In another experiment we evaluated the stability of urine pH measurement in a clinical setting. Urine collected under paraffin was tested immediately for pH; an aliquot from the same specimen was stored in a capped 5-ml syringe at 4 degrees C and tested after 24 h. We found no appreciable difference in measured pH between samples sent to the laboratory under paraffin or in a capped plastic syringe.

Rapid, quantitative and spatial field measurements of soil pH using an Ion Sensitive Field Effect Transistor

The objectives of this work are threefold. Firstly, to determine adequate times for rapid and accurate field-based measurements of soil pH (field pH). Secondly, to develop a simple protocol for quantitative spatial field measurements of soil pH using an Ion Sensitive Field Effect Transistor (ISFET) and thirdly, to demonstrate and validate its implementation. In order to determine adequate times, the kinetics of soil pH reactions in 1:5 soil/0.01 M CaCl 2 (pH Ca) and soil/H 2O (pH w) suspensions were quantified for 17 representative agricultural soil samples from Brittany, France. The mean and variance of pH deviations at various times, from the long-term equilibrium pH as well as the accuracy of the measurements, were calculated for both pH Ca and pH w. These data together with the required accuracy of the measurements may be used to determine an adequate time for field pH measurements. The mean and variance of the deviations decreased and the accuracy of measurements increased with longer reaction times. For example, the expected accuracy of 10 s pH Ca and pH w measurements was 0.15 and 0.33 pH units, respectively. The expected accuracy of 20 s pH Ca and pH w measurements was 0.11 and 0.15 units, respectively. Based on these data, a simple protocol for rapid field pH measurements was developed. This protocol was implemented by making 10 s pH w measurements in a 4-ha agricultural field near Rennes, France. A total of 476 pH w data were collected in 6 h. At 57 validation sites, measurements were obtained using conventional grid sampling and laboratory analysis (laboratory pH w). The observed accuracy of the 10 s field pH w measurements was 0.34 pH units. The efficiency of the field and laboratory pH w measurements was also compared. Field measurements were more efficient in terms of both the time and cost involved in obtaining the measurements. Semivariograms and kriged maps of both laboratory and field pH w measurements were compared, the latter appearing to more truthfully depict the spatial structure of soil pH w in the field. A cross-semivariogram of the laboratory and the 10 s field pH w measurements was also derived and co-kriging performed. Where time and/or economic restrictions prevent the use of sampling and laboratory analysis or the collection of more accurate field pH measurements, a co-kriging of a smaller set of these data as the primary variable along with a more exhaustive data set of more rapid but less accurate field pH measurements as the secondary variable may be practical and advantageous. Temporal measurements of field pH w at the 57 validation sites over 2 years showed good agreement.

Measurement of phagosomal pH of normal and CGD-like human neutrophils by dual fluorescence flow cytometry.

Phagosomal pH is thought to play an important role in the antimicrobial activity of polymorphonuclear leukocytes (PMNs). In this study, we set up a method for a rapid and accurate measurement of phagosomal pH in PMNs with the use of Candida albicans doubly labeled with a pH-insensitive and a pH-sensitive probe and flow cytometry. Heat-killed, serum-opsonized C. albicans were doubly labeled with fluorescein, a pH-sensitive probe, and rhodamine, a pH-insensitive probe, and incubated with human PMNs. Flow cytometric readings of PMN-associated Candida were then taken, and the intraphagosomal pH was calculated on the basis of the ratio of fluorescein:rhodamine fluorescence by using a calibration curve obtained after equilibration of phagosomal pH with different external pH values after addition of digitonin. A rapid rise in phagosomal pH, which reached pH 7.8, was observed 2 min after initiation of phagocytosis and progressively declined to pH 6.9 after 15 min. Such a rise was not observed in PMNs with defective microbicidal activity (deficient in nicotinamide adenine dinucleotide phosphate oxidase), where phagosomal pH dropped to pH 6.6, 2 min after phagocytosis. The abnormal initial acidification in PMNs deficient in nicotinamide adenine dinucleotide phosphate oxidase was prevented by using lysosomotropic weak bases or the vacuolar-type H(+) pump inhibitor concanamycin A. Phagosomal pH of PMNs can be easily and accurately measured by dual fluorescence flow cytometry. The method can be applied to assess phagosomal pH in PMNs with defective microbicidal activity and to monitor the outcome of pharmacologic interventions aimed at correcting its abnormalities.

Removal of deadspace volume from arterial catheter: How muchis enough?

OBJECTIVE: To evaluate the amount of volume needed to be removed from arterial catheter systems to compensate for "deadspace" and to allow an accurate measurement of pH and hemoglobin (Hb). DESIGN: Twenty patients undergoing heart surgery were evaluated in a steady state after the induction of anesthesia before surgery. Six blood samples were removed from the arterial catheter, the total volume of which was 1.5 mL at 30-sec intervals and measured for pH and Hb. The first sample was then taken after removing 1.5 mL from the tubing. In subsequent samples, the volume removed before sampling increased by 0.5-mL intervals. All other samples were compared with sample number 6, in which 4 mL of volume were removed before measurements. RESULTS: The first three samples with volumes of 1.5, 2.0, and 2.5 mL before measurement were inaccurate compared with sample number 6 (p <.000), giving artificially low values for both pH and Hb. There was no significant difference between the values measured in sample numbers 4, 5, and 6 (3.0, 3.5, and 4.0 mL, respectively). CONCLUSION: The amount of volume needed to be removed before measurement from an arterial catheter system, the volume of which is 1.5 mL, is 3 mL to achieve accurate measurements of pH and Hb. Removal of less volume results in an artificially low measurement.

Intracellular pH homeostasis during cell-cycle progression and growth state transition in Schizosaccharomyces pombe.

Accurate measurement of intracellular pH in unperturbed cells is fraught with difficulty. Nevertheless, using a variety of methods, intracellular pH oscillations have been reported to play a regulatory role in the control of the cell cycle in several eukaryotic systems. Here, we examine pH homeostasis in Schizosaccharomyces pombe using a non-perturbing ratiometric pH sensitive GFP reporter. This method allows for accurate intracellular pH measurements in living, entirely undisturbed, logarithmically growing cells. In addition, the use of a flow cell allows internal pH to be monitored in real time during nutritional, or growth state transition. We can find no evidence for cell-cycle-related changes in intracellular pH. By contrast, all data are consistent with a very tight homeostatic regulation of intracellular pH near 7.3 at all points in the cell cycle. Interestingly, pH set point changes are associated with growth state. Spores, as well as vegetative cells starved of either nitrogen, or a carbon source, show a marked reduction in their internal pH compared with logarithmically growing vegetative cells. However, in both cases, homeostatic regulation is maintained.

Spectrophotometric determination of freshwater pH using bromocresol purple and phenol red.

The dissociation constants (KI = [H+][I2-]/[HI-]) of two sulfonephthalein indicators (bromocresol purple and phenol red) were determined as function of temperature (10-30 degrees C) at zero ionic strength. Freshwater pH, on the free hydrogen ion concentration scale (molal units), can be precisely calculated from measurements of indicator absorbance ratios (lambda2A/lambda1A) using the following equations: pH = pKI + log((R - e1)/(e2 - Re3)) and pKI = pKI(degrees) - AdeltaZ2(mu1/2 /(1 + mu1/2) - 0.3 mu), where R = lambda2A/lambda1A, pKI = -log KI, mu is the ionic strength, deltaZ2 = 4, and values of A for 283 < or = T < or = 303 can be estimated from the equation: A = 0.5092 + (T-298.15) x 8.5 x 10(-4). For bromocresol purple (lambda1 = 432 nm, lambda2 = 589 nm), pKI(degrees) = 5.226 + 378.1/T, e1 = 0.00387, e2 = 2.858, and e3 = 0.0181. For phenol red (lambda1 = 433 nm, lambda2 = 558 nm), pKI(degrees) = 5.798 + 666.7/T, e1 = 0.00244, e2= 2.734, and e3 = 0.1075. These two indicators can be used to make accurate pH measurements of freshwaters (river water, lake water, groundwater, rainwater, etc) within the range 4.5 < or =pH < or =8.5. The precision of pH measurements using phenol red in well-buffered freshwaters is on the order of +/-0.001 or better.

Reference systems for assessing viability and accuracy of pH sensors in high temperature subcritical and supercritical aqueous solutions

This paper describes an effort in developing reference systems to be used in high temperature subcritical and supercritical aqueous solutions for assessing the viability and accuracy of high-temperature pH sensors. Each of these reference systems consists of a couple of three-component aqueous solutions in which the concentration of NaCl is greater than the concentration of another solute, such as HCl and NaOH. The viability of the reference systems was tested using the recently published experimental data employing a flow-through electrochemical cell at temperatures from 25°C to 400°C and densities from 0.17 to 1 g cm −3 [S.N. Lvov, X.Y. Zhou, and A.V. Bandura, 1999a, Estimation of Isothermal Potential for HCl/NaCl and NaOH/NaCl Systems in High Temperatures Supercritical Aqueous Solutions, J. Supercrit. Fluids, in preparation; S.N. Lvov, X.Y. Zhou, and D.D. Macdonald, 1999, Flow-Through Electrochemical Cell for Accurate pH Measurements at Temperature up to 400°C, J. Electroanal. Chem., 463, 146–156]. The applicability of the method was tested for dilute (0.01 and 0.001 mol kg −1) HCl aqueous solutions having background NaCl electrolyte in the amount of 0.1 mol kg −1. The most significant property of the reference systems is that the pH difference, and hence, the cell potential, can easily be estimated with sufficient accuracy using only the analytical concentrations of the electrolytes without considering the speciation calculations, given the fact that the association and dissociation constants at supercritical temperatures have not been accurately determined yet.

In Vivo Application of Intestinal pH Measurement Using 2,7‘‐Bis(carboxyethyl)‐5,6‐carboxyfluorescein (BCECF) Fluorescence Imaging

Measurement of gastrointestinal intramucosal pH (pHim) has been recognized as an important factor in the detection of hypoxia-induced dysfunctions. However, current pH measurement techniques are limited in terms of time and spatial resolutions. A major advance in accurate pH measurement was the development of the ratiometric fluorescent indicator dye, 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein (BCECF). This study aimed to set up and validate a fluorescence imaging technique to measure in vivo the intramucosal pH (pHim) of the intestine. The intestine was inserted into an optical chamber placed under a microscope. Animals were injected intravenously with the pH-sensitive fluorescent dye BCECF. Fluorescence was visualized by illuminating the intestine alternately at 490 and 470 nm. The emitted fluorescence was directed to an intensified camera. The ratio of emitted fluorescence at excitation wavelengths of 490 and 470 nm was measured, corrected and converted to pHim by constructing a calibration curve. The pHim controls were performed with a pH microelectrode and were correlated with venous blood gas sampling. Results show that pHim is determined with an accuracy of +/- 0.07 pH units and a response time of 1 min. In conclusion pHim mapping of rat intestine can be obtained by fluorescence imaging using BCECF. This technology could be easily adapted for endoscopic pH measurements.

The application of factor analysis to solvatochromic parameters and pH s values for the standardization of potentiometric sensors in mobile phases used in liquid chromatography

The pH s values for reference buffer solutions in aqueous acetonitrile, methanol, and tetrahydrofuran mixtures used as mobile phases in LC were subjected to factor analysis in order to discover the number of factors which affect the variation with the percentage of organic co-solvent of the whole data sets. Afterwards, target factor analysis was used to identify these factors. The pH s values were correlated with the solvatochromic parameters (π*, α, β, and E T) of the hydro-organic mixtures according to the results obtained using these chemometric techniques. An equation is proposed which permits the calculation of pH s values in any aqueous acetonitrile, methanol, or tetrahydrofuran mixtures, and thus accurate pH measurements can be made in these media as well as in water. An overview of the effects on pH s values of preferential solvation in these media is also presented.

Flow-through electrochemical cell for accurate pH measurements at temperatures up to 400°C

Abstract We have developed a flow-through electrochemical cell (FTEC) and demonstrated that it operates successfully in both subcritical and supercritical aqueous environments at temperatures up to 400°C. The FTEC consists of a previously developed flow-through external (Ag ∣ AgCl) pressure-balanced reference electrode (FTEPBRE) (Lvov et al., J. Electroanal. Chem. 444 (1998) 186) and a modified flow-through platinum hydrogen electrode (FTPHE). The experimental measurements have been carried out for two solutions: (1) 10−3 mol kg−1 HCl+10−1 mol kg−1 NaCl and (2) 10−2 mol kg−1 HCl+10−1 mol kg−1 NaCl at flow rates between 0.1 and 1.4 cm3 min−1 and at temperatures of 25°C (18.7 MPa), 200°C (23.8 MPa), 350°C (24.2MPa), and 400°C (25.3 MPa). We have also carried out a detailed irreversible thermodynamic analysis of the FTEC and estimated different significant contributions to the measured potentials. Comparison of experimentally observed and theoretically calculated pH values shows an ability to use the FTEC for high precision potentiometric and pH measurements. We conclude that high temperature pH measurements with accuracy better than ±0.03 logarithmic units can now be accomplished. Also, we assume that valuable streaming potential data can be obtained using the FTEC for estimating the surface charge (zeta potential) on the oxide material capillary tube in hydrothermal solutions at temperatures up to 400°C.