Pseudohyperchloremia and Negative Anion Gap - Think Salicylate!

BACKGROUND: Pseudohyperchloremia results in a very low or negative anion gap. Historically, the most common cause of this artifact was bromide poisoning. Bromide salts have been removed from most medications and bromism has become very uncommon. More recently, the introduction of chloride ion selective sensing electrodes (Cl-ISE) has generated a new cause of pseudohyperchloremia-salicylate poisoning. We describe 5 such patients and quantitate the error generated by this measurement artifact. METHODS: The magnitude of artifactual hyperchloremia generated by high salicylate levels was quantified in 5 patients by measuring chloride concentration with several Cl-ISEs from different manufacturers and with Cl-ISEs of different "ages," and comparing these results to measurements with a chloridometer (coulometric titration), which is free of the salicylate artifact. RESULTS: Cl-ISEs from different manufacturers generated a wide range of artifactual chloride concentration elevation. Furthermore, the same Cl-ISE generated increasingly severe pseudohyperchloremia as it was repeatedly reused over time and "aged." CONCLUSIONS: Salicylate interferes with measurement of the blood chloride concentration when a Cl-ISE is used. The severity of this artifact is related to the salicylate level, the specific Cl-ISE, and the "age" of the electrode. Toxic blood salicylate levels can generate marked pseudohyperchloremia, and consequently, an artifactual very small or negative anion gap. The large anion gap metabolic acidosis typical of salicylate poisoning is masked by this artifact. Salicylate has become the most common cause of pseudohyperchloremia, and physicians should immediately consider salicylate poisoning whenever the combination of hyperchloremia and a very small or negative anion gap is reported by the laboratory.

An all-solid-state NO3- ion-selective electrode with gold nanoparticles solid contact layer and molecularly imprinted polymer membrane.

To improve the single-layer all-solid-state ion selective electrode' defects including poor conductivity of PVC sensitive membrane and interference of water layer between substrate electrode and sensitive membrane, a double-layer all-solid-state ion selective electrode with nanomaterial as the solid contact layer and conductive polymer as the ion sensitive membrane was developed. A gold nanoparticles solid contact layer and a nitrate-doped polypyrrole molecularly imprinted polymer membrane were prepared by electrodeposition. The optimal parameters obtained by electrochemical performance test were 2.5 mmol/L HAuCl4 electrolyte for solid contact layer and 1800s electrodeposition time for sensitive membrane. The new electrode exhibited a Nernstian response of -50.4 mV/decade and a low detection limit of 5.25×10-5mol/L. Potentiometric water layer test showed no water film formed between the gold nanoparticles solid contact layer and nitrate-doped polypyrrole molecularly imprinted polymer membrane. The contact angle between droplet and the surface of solid contact layer was 112.35° and showed good hydrophobic property. Furthermore, the developed electrode exhibited fast response, excellent potential stability and long lifetime. This electrode is suitable for the detection of nitrate concentration in water and liquid fertilizer.

Establishing Meaningful Limits of Detection for Ion-Selective Electrodes and Other Nonlinear Sensors.

Although IUPAC has recommended a probabilistic approach to determining limit of detection (LOD) based on false-positive and false-negative rates for more than 20 years, the LOD definition for ion-selective electrodes (ISEs) long predates these recommendations and conflicts substantively with them. Although it is well known that the ISE LOD definition does not follow best practice, it continues to be used due to simplicity and a lack of available methods for estimating LOD for nonlinear sensors. Here, we use ISEs as a model system for estimation of LOD for nonlinear sensors that is consistent with broad IUPAC recommendations and justified using statistical theory. Using freely available software, the new approach and updated definition is demonstrated through theory, simulation, and an environmental application. The results show that the current LOD definition for ISEs performs substantially worse than the proposed definition when assessed against IUPAC recommendations, including ignoring sensor noise and LOD uncertainty, leading to bias of an order of magnitude or more. Further, the environmental application shows that the new definition, which includes estimates of LOD uncertainty, allows more objective assessment of sensor response and fitness for purpose. The growing demand for ultrasensitive sensors that operate in complex matrices has pushed the boundaries of traditional calibration approaches. These sensors often operate near their limit of detection (LOD), with additional challenges created if their response is nonlinear. These challenges are amplified when assessing new sensors, since they may be less reproducible and noisier than benchmark techniques.

The challenges of sodium measurements: indirect versus direct ion-selective method.

BACKGROUND: Diagnosis and treatment of dysnatremia is challenging and further complicated by the pitfalls of different sodium measurement methods. Routinely used sodium measurements are the indirect (plasma/serum) and direct (whole blood) ion-selective electrode (ISE) method, showing discrepant results especially in the setting of acute illness. Few clinicians are aware of the differences between the methods in clinically stable patients or healthy volunteers. METHODS: Data of 140 patients and 91 healthy volunteers undergoing osmotic stimulation with hypertonic saline infusion were analyzed. Sodium levels were measured simultaneously by indirect and direct ISE method before and at different time points during osmotic stimulation up to a sodium threshold of ≥150 mmol/L. The primary outcome was the difference in sodium levels between the indirect and direct ISE method. RESULTS: 878 sodium measurements were analyzed. Mean (s.d.) sodium levels ranged from 141 mmol/L (2.9) to 151 mmol/L (2.1) by the indirect ISE compared to 140 mmol/L (3) to 149 mmol/L (2.8) by the direct ISE method. The interclass correlation coefficient between the two methods was 0.844 (95% CI: 0.823-0.863). On average, measurements by the indirect ISE were 1.9 mmol/L (95% CI limits: -3.2 to 6.9) higher than those by the direct ISE method (P < 0.001). The tendency of the indirect ISE method resulting in higher levels increased with increasing sodium levels. CONCLUSION: Intra-individual sodium levels differ significantly between the indirect and direct ISE method also in the absence of acute illness. It is therefore crucial to adhere to the same method in critical situations to avoid false decisions due to measurement differences.

A Simple and Accurate Measurement Procedure for Serum Chloride by Ion Chromatography and Bias Evaluation of Six Ion Selective Electrode Measurements in China.

BACKGROUND: Chloride is the main anion in human body. A simple and accurate method for serum chloride measurement was developed using ion chromatography (IC) in China. METHODS: In the measurement, serum samples were diluted 500 times with water, filtered and injected into ion chromatography column. A mixed eluent (2 mmol/L CO32- + 12 mmol/L OH-) was used and peak area signal was collected. Five calibrators made from Standard Reference Material (SRM) 919b were used in the bracketing method. The IC method was applied as the comparative method and six ion selective electrode (ISE) measurement systems were evaluated using 60 individual patient serums. RESULTS: The IC method was proven to be accurate. The precision was 0.18% - 0.30%, the recovery was 99.66% - 100.60%, the bias was 0.19% to -0.06%, and the related expanded uncertainty was 0.775% (k = 2). The precisions of the ISE systems were smaller than the 0.9% tolerable CV except for the Beckman DXC (0.91% - 1.16%). In comparison, the results of linear regression analysis showed that the correlation coefficients were 0.9876 to 0.9979. For all systems, the range of mean biases was -5.96 - 1.48 mmol/L (-5.57% - 1.36%); the expected biases at the medical decision levels were -4.97% - 0.84% at 90 mmol/L and -6.02% - 1.76% at 120 mmol/L. All biases of the Beckman AU met the requirement of within ± 1.5%. CONCLUSIONS: The IC measurement method is proven to be of high precision and trueness, and the quality of routine ISE measurement of serum chloride still needs significant improvement. The establishment of the IC method can improve the measurement quality and promote its standardization process in China.

Extracellular H+ fluxes from tiger salamander Müller (glial) cells measured using self-referencing H+-selective microelectrodes.

Self-referencing H+-selective electrodes were used to measure extracellular H+ fluxes from Müller (glial) cells isolated from the tiger salamander retina. A novel chamber enabled stable recordings using H+-selective microelectrodes in a self-referencing format using bicarbonate-based buffer solutions. A small basal H+ flux was observed from the end foot region of quiescent cells bathed in 24 mM bicarbonate-based solutions, and increasing extracellular potassium induced a dose-dependent increase in H+ flux. Barium at 6 mM also increased H+ flux. Potassium-induced extracellular acidifications were abolished when bicarbonate was replaced by 1 mM HEPES. The carbonic anhydrase antagonist benzolamide potentiated the potassium-induced extracellular acidification, while 300 µM DIDS, 300 µM SITS, and 30 µM S0859 significantly reduced the response. Potassium-induced extracellular acidifications persisted in solutions lacking extracellular calcium, although potassium-induced changes in intracellular calcium monitored with Oregon Green were abolished. Exchange of external sodium with choline also eliminated the potassium-induced extracellular acidification. Removal of extracellular sodium by itself induced a transient alkalinization, and replacement of sodium induced a transient acidification, both of which were blocked by 300 µM DIDS. Recordings at the apical portion of the cell showed smaller potassium-induced extracellular H+ fluxes, and removal of the end foot region further decreased the H+ flux, suggesting that the end foot was the major source of acidifications. These studies demonstrate that self-referencing H+-selective electrodes can be used to monitor H+ fluxes from retinal Müller cells in bicarbonate-based solutions and confirm the presence of a sodium-coupled bicarbonate transporter, the activity of which is largely restricted to the end foot of the cell.NEW & NOTEWORTHY The present study uses self-referencing H+-selective electrodes for the first time to measure H+ fluxes from Müller (glial) cells isolated from tiger salamander retina. These studies demonstrate bicarbonate transport as a potent regulator of extracellular levels of acidity around Müller cells and point toward a need for further studies aimed at addressing how such glial cell pH regulatory mechanisms may shape neuronal signaling.

PVDF-ErGO-GRC electrode: A single setup electrochemical system for separation, pre-concentration and detection of lead ions in complex aqueous samples.

An effective electrode was developed based on electromembrane extraction (EME) and square wave voltammetry (SWV) for simultaneous separation, pre-concentration and determination of lead (II) (Pb(II)) ions in complex aqueous samples. Electrochemically reduced graphene oxide-graphite reinforced carbon (ErGO-GRC) was utilized in conjunction with the SWV. Pb(II) ions were extracted from an aqueous sample solution into an acidic acceptor phase (1M HCl) in the lumen of the polyvinylidene fluoride (PVDF) membrane bag by the application of voltage of maximum 6 V across the supported liquid membrane (SLM), consisting of organic solvent and di-(2-ethylhexyl)phosphoric acid (D2EHPA). The parameters affecting the EME were optimized for Pb(II) ions. The optimum EME conditions were found to be 20% D2EHPA in 1-octanol impregnated in the wall of PVDF membrane (PVDF17) as the SLM, extraction time of 20 min, pH of sample solution of 8 and a voltage of 5 V. The PVDF-ErGO-GRC electrode system attained enrichment factors of 40 times and 80% of extraction with relative standard deviations (n=5) of 8.3%. Good linearity ranging from 0.25 to 2 nM with coefficients correlation of 0.999 was obtained. The Pb(II) ions detection limit of PVDF-ErGO-GRC electrode was found to be 0.09 nM. The newly developed single setup electrochemical system was applied to complex aqueous samples such as tap, river and sea water to evaluate the feasibility of the method for applications.

Neutral-carrier ion-selective electrodes assessed by the Nernst-Planck-Poisson model.

Ion-selective electrodes (ISEs) containing neutral ionophores are used in clinical, industrial, and environmental analysis. The wide range of applications requires deep theoretical description. This work concentrates on the development of the general approach to the description of electro-diffusion processes, namely, Nernst-Planck-Poisson (NPP) model to allow the description of the time-dependent responses in the case of complexation reactions occurring in the ion-selective membranes. The impact of the chemical reaction on the calibration curves and apparent selectivity of ISE is discussed. Results obtained using NPP model with time-dependent reaction are compared with those obtained with the Phase Boundary Model (PBM), as well as with the previous solutions of NPP model, using the infinite reaction rates and constant ligand concentration assumption. The validity of these assumptions is investigated and the limitations of PBM in the description of neutral-carrier ISE are discussed.

Preliminary evaluation of an improved enzymatic assay method for measuring potassium concentrations in serum.

BACKGROUND: K(+) has important physiological functions. K(+) concentrations in serum are generally determined using ion-selective electrodes (ISEs), though measurement using reagents in aqueous medium is also useful. METHODS: K(+) concentrations were measured using recombinant inosine 5'-monophosphate dehydrogenase (IMPDH), which was activated only by K(+) and NH4(+). Exogenous NH4(+) and endogenous NH4(+) were eliminated using glutamine synthase. RESULTS: Regression analysis of the enzymatic assay (y) vs. the ISE method (x) gave the following relation: y=1.03x+0.09 (n=54, Sy,x=0.06 mmol/l). The linear range was up to 12 mmol/l when 1 U/ml IMPDH was used. CONCLUSION: Advantages of the proposed assay method are: (i) the measured range is wider than that of existing enzymatic methods; (ii) the conditions for K(+) determination can be maintained constant, regardless of the amount of NH4(+) in the analyte and reagents; and (iii) the elimination system is simpler because the recombinant IMPDH is stimulated by only K(+) and NH4(+) and is unaffected by biological materials.

Non-Severinghaus potentiometric dissolved CO2 sensor with improved characteristics.

A new type of carbon dioxide sensor comprising a pH glass electrode measured against a carbonate-selective membrane electrode based on a tweezer type carbonate ionophore is presented here for the first time. No cumbersome liquid junction based reference element is utilized in this measurement. The sensor shows an expected nernstian divalent response slope to dissolved CO(2) over a wide range covering the routine environmental and physiological PCO(2) levels. Unlike the conventional Severinghaus CO(2) probe for which the response is substantially delayed to up to 10 min due to diffusion of carbon dioxide into the internal compartment, the ion-selective CO(2) sensor proposed here shows a response time (t(95%)) of 5 s. When used together with a traditional reference electrode, the sensor system is confirmed to also monitor sample pH and carbonate along with carbon dioxide. A selectivity analysis suggests that Cl(-) does not interfere even at high concentrations, allowing one to explore this type of sensor probe for use in seawater or undiluted blood samples. The CO(2) probe has been used in an aquarium to monitor the CO(2) levels caused by the diurnal cycles caused by the metabolism of the aquatic plants and shows stable and reproducible results.

Calibration of potentiometric sensor arrays with a reduced number of standards.

In this paper a novel calibration procedure for the parameter determination of ion-selective electrodes used in an array is described. Commonly used procedures require a large number of standards to determine the parameters based on the Nicolsky-Eisenman model. The elaborated procedure reduces the number of standards to a minimum by using a standard containing a mixture of ions instead of a couple of pure standards. This paper presents a complete calibration procedure, which consists of designing the composition of the standards, parameter determination and verification of the calibration results. Comparison of the results obtained by the procedure presented with results obtained by the Two-Point Calibration and Separate Solution methods proves that the accuracies of both procedures are comparable. The outlined procedure can be applied in multicomponent analysers.

Calibration of equipment for analysis of drinking water fluoride: a comparison study.

Current American Dental Association evidence-based recommendations for prescription of dietary fluoride supplements are based in part on the fluoride concentration of a pediatric patient's drinking water. With these recommendations in mind, this study compared the relative accuracy of fluoride concentration analysis when a common apparatus is calibrated with different combinations of standard values. Fluoride solutions in increments of 0.1 ppm, from a range of 0.1 to 1.0 ppm fluoride, as well as 2.0 and 4.0 ppm, were gravimetrically prepared and fluoride concentration measured in pentad, using a fluoride ion-specific electrode and millivolt meter. Fluoride concentrations of these solutions were recorded after calibration with the following 3 different combinations of standard fluoride solutions: 0.1 ppm and 0.5 ppm, 0.1 ppm and 1.0 ppm, 0.5 ppm and 1.0 ppm. Statistical analysis showed significant differences in the fluoride content of water samples obtained with different two-standard fluoride solutions. Among the two-standard fluoride solutions tested, using 0.5 ppm and 1.0 ppm as two-standard fluoride solutions provided the most accurate fluoride measurement of water samples containing fluoride in the range of 0.1 ppm to 4.0 ppm. This information should be valuable to dental clinics or laboratories in fluoride analysis of drinking water samples.

Effect of fluoride sustained slow-releasing device on fluoride, phosphate and calcium levels in plaque biofilms over time measured using ion chromatography.

OBJECTIVES: To determine whether there are any differences in fluoride (F), calcium (Ca) or phosphate (PO(4)) concentrations in natural plaque biofilms between the upper right and left quadrants using a fluoride sustained slow-releasing device (FSSRD) placed in the upper right quadrant after 7 and 21 days. To report and validate a new methodology in measuring very low concentrations of F in dental plaque and saliva using ion chromatography. METHODS: Twenty-one participants were divided into two groups with 11 participants in group one and 10 in group two. Each participant had a FSSRD attached to the upper right second permanent molar and two plaque generating devices (PGDs) attached to the upper right and left first permanent molars. The PGDs were recovered after 7 days in group one and 21 days in group two. RESULTS: At both 7 and 21 days (right, left), F (1.081±1.517 ppm, 0.736±0.840 ppm) and (0.459±0.888 ppm, 0.203±0.139 ppm), PO(4) (1053±533 ppm, 654±246 ppm) and (865±1099 ppm, 474±304 ppm) and Ca (136±132 ppm, 74±36 ppm) and (130±109 ppm, 77±24 ppm), were higher in the quadrant containing the FSSRD but not significantly so (p>0.05). Fluoride and PO(4) fell in both quadrants between 7 and 21 days, though not significantly. CONCLUSIONS: Intriguingly while not statistically significant, 21 day plaque contained less fluoride than those investigated after 7 days. While the data was not statistically significant, it seems possible that F, Ca and PO(4) accumulated around the device to a limited extent but were washed away fairly quickly and distributed around the oral cavity. CLINICAL IMPORTANCE: The FSSRD was found to reduce dmfs/DMFS by 76% and raise salivary F levels by ∼10 folds. This device is very helpful in reducing dental decay where compliance is impaired such as in patients with special needs. This study further investigates the anti-cariogenic effect of this device.

New tetradecyltrimethylammonium-selective electrodes: surface composition and topography as correlated with electrode's life span.

Two conventional plastic membrane electrodes that are selective for the tetradecyltrimethylammonium cation (TTA) have been prepared. The ion exchangers of these sensors were the ion associate, TTA-PT, and the ion aggregate, TTA-PSS, where PT and PSS are phosphotungstate and polystyrene sulfonate, respectively. The following performance characteristics of the TTA-PT- and TTA-PSS-containing electrodes were found: conditioning time of 30 and 20 min; potential response of 58.2 and 61.1 mV/TTA concentration decade; rectilinear concentration ranges of 2.0 × 10(-5)-5.0 × 10(-2) and 1.5 × 10(-5)-7.9 × 10(-2) mol L(-1); average working pH ranges of 4.0-10.5 and 3.8-10.7; life spans of 20 and 28 weeks, and isothermal temperature coefficients of 4.44 × 10(-4) and 6.10 × 10(-4)V/°C, respectively. Both electrodes exhibited high selectivity for TTA with an increasing number of inorganic and quaternary ammonium surfactant cations. These electrodes have been successfully applied to assay an antiseptic formulation containing TTA. Surface analyses using electron microscopy and X-ray photoelectron spectroscopy were used to determine the cause of the limited life span of plastic membrane electrodes.

Novel screen-printed electrode for the determination of dodecyltrimethylammonium bromide in water samples.

The construction and electrochemical response characteristics of a screen-printed electrode (SPE) for the determination of dodecyltrimethylammonium bromide (DTAB) are described. The sensor was based on the use of DTA-tetraphenylborate ion association complex as an electroactive material in screen-printed electrode with dioctylsebacate (DOS) as a solvent mediator. In aqueous solution of pH 3, the sensor displayed a stable response for six months with reproducible potential and linear response for surfactant over the concentration range 1.20 × 10(-2) -5.6 × 10(-7) mol L(-1) at 25 °C with Nernstian slope of 55.95 ± 0.58 mV decade(-1) for detection limit of 6.8 x 10(-6) mol L(-1) . The response time was 6-10 s. The selectivity coefficients indicate excellent selectivity for DTAB over many common cations (e.g. Mg(2+), Na(+), K+, Co(2+), Ni(2+), Ca(2+), Cl(-), I(-), SO(4)(-2) and cetylpyridinium chloride (CPC). The sensor was used successfully for the determining of DTAB in pure form and water samples with average recoveries of 99.98, 98.78, and 99.99%.

[Evaluating the uncertainty in urinary fluoride measurement by ion selective electrode method].

OBJECTIVE: To evaluate the uncertainty of measurement result of urinary fluoride and to provide quality assurance for determinations. METHOD: The investigation was conducted, according with principles and methods for uncertainty evaluation. RESULTS: The uncertainty of the combined standard of present method was 2.86 %. For the sample containing 4.47 mg/L urinary fluoride, the expanded uncertainty was 0.26 mg/L. CONCLUSION: The uncertainty of the present method was mainly from the sample repeatability, the preparation of standard solution, the linearity of the calibration curve and instruments and so on.

Development of gold standard ion-selective electrode-based methods for fluoride analysis.

BACKGROUND/AIMS: Currently available techniques for fluoride analysis are not standardized. Therefore, this study was designed to develop standardized methods for analyzing fluoride in biological and nonbiological samples used for dental research. METHODS: A group of nine laboratories analyzed a set of standardized samples for fluoride concentration using their own methods. The group then reviewed existing analytical techniques for fluoride analysis, identified inconsistencies in the use of these techniques and conducted testing to resolve differences. Based on the results of the testing undertaken to define the best approaches for the analysis, the group developed recommendations for direct and microdiffusion methods using the fluoride ion-selective electrode. RESULTS: Initial results demonstrated that there was no consensus regarding the choice of analytical techniques for different types of samples. Although for several types of samples, the results of the fluoride analyses were similar among some laboratories, greater differences were observed for saliva, food and beverage samples. In spite of these initial differences, precise and true values of fluoride concentration, as well as smaller differences between laboratories, were obtained once the standardized methodologies were used. Intraclass correlation coefficients ranged from 0.90 to 0.93, for the analysis of a certified reference material, using the standardized methodologies. CONCLUSION: The results of this study demonstrate that the development and use of standardized protocols for F analysis significantly decreased differences among laboratories and resulted in more precise and true values.

New selenite ion-selective electrodes based on 5,10,15,20-tetrakis-(4-methoxyphenyl)-21H,23H-porphyrin-Co(II).

New polymeric membrane (PME), modified carbon paste (MCPE), and coated wire (CWE) selenite ion-selective electrodes based on 5,10,15,20-tetrakis-(4-methoxyphenyl)-21H,23H-porphyrin-Co(II) (CoTMeOPP) are reported. The best composition was the electrode containing 2% CoTMeOPP as the active material and 49% TCP as plasticizer. The electrodes reveal a Nernstian behavior over a concentration range of 5.5x10(-5) to 1.1x10(-2) M for PME, 5.2x10(-5) to 1.2x10(-2) M for MCPE and 1.2x10(-4) to 4.4x10(-3) M for CWE. The potentiometric response is pH dependent, since selenous acid is a diprotic acid. The slope of the selenite PVC electrode was -57.0 mV for the monovalent anion at pH 6.47, and -26.0 mV for the divalent anion at pH 11.00. The detection limits were 3.4x10(-5) and 4.7x10(-5) M at pH values 6.47 and 11.00, respectively. The electrodes manifest advantages of low resistance, very short response time (15 s), and most importantly good selectivities relative to a wide variety of other anions. In fact, the proposed selenite ion-selective electrodes show a great improvement compared to previously reported electrodes for selenite ion. The electrode was used for the determination of selenite in selenite/selenate mixture, in sodium selenite raw material powder, and in VitaFit Selenium ACE antioxidant tablets with recovery ranges of 90.0-103.3%.

Lead(II) ion selective electrodes with PVC membranes based on two bis-thioureas as ionophores: 1,3-bis(N'-benzoylthioureido)benzene and 1,3-bis(N'-furoylthioureido)benzene.

Two PVC membrane ion selective electrodes for Pb(II) ion based on two bis-thioureas: 1,3-bis(N'-benzoylthioureido)benzene and 1,3-bis(N'-furoylthioureido)benzene as ionophores, are reported. A first membrane formulated using 1,3-bis(N'-benzoylthioureido)benzene as carrier exhibited a Nernstian response to Pb(II) over a wide concentration range (4.0x10(-6) to 1.0x10(-2)M) with a slope of 31.5+/-1.6 mV/dec. It showed a fast response time (t(90%)=14 s) and could be used for 10 weeks without any divergence in potentials. The membrane formulated using 1,3-bis(N'-furoylthioureido)benzene as carrier exhibited a Nernstian response in the concentration range (5.0x10(-6) to 1.0x10(-2) M), with a slope of 30.0+/-1.3 mV/dec. Its response time was t(90%)=14 s, and it could be used for 14 weeks without any divergence in potentials. The two proposed potentiometric sensors revealed acceptable selectivities for Pb(II) over a wide variety of other metal ions and could be used in a pH range of 2.2-6.0. Both electrodes were assayed in direct potentiometric determination of lead in soils (10-30 mg/kg range) with very good performance (0.99935 correlation coefficient in the comparison against ICP-MS method).

Comparative evaluation of Dy(III) selective poly(vinyl) chloride based membrane electrodes of macrocyclic tetraimine Schiff's bases.

Three different derivatives of macrocyclic tetraimine Schiff's base have been synthesized and explored as a neutral ionophores for preparing poly(vinyl chloride) based membrane sensors selective to Dy(3+). The addition of sodium tetraphenyl borate and various plasticizers, viz., o-NPOE, DBP, DBBP, DOP and CN has been found to substantially improve the performance of the sensors. The best performance was obtained with the sensor no. 1 having membrane of Schiff's base (SL-1) with composition (w/w) SL-1 (4.5%): PVC (30.5%): o-NPOE (59.5%): NaTPB (5.5%). This sensor exhibits Nernstian response with slope 19.4 mV/decade of activity in the concentration range of 10(-8) to 1.0x10(-2)M Dy(3+), performs satisfactorily over wide pH range of (2.8-7.2) with a fast response time (10s). The sensor was also found to work satisfactorily in partially non-aqueous media up to 20% (v/v) content of acetonitrile, methanol or ethanol. The proposed sensor can be used over a period of 1.5 months without significant drift in potentials. The sensor has been also utilized for the determination of Dy(3+) level in different soil samples.