Lead selective electrodes

Figure 4- Response of an lead-selective electrode based on a calix[6]arene hexaphosphene oxide to sequential 10-fold dilutions of a sample solution demonstrating a very rapid Nernstian response down to sub-nanomolar concentrations of lead. The inset shows a linear Nernstian plot is obtained with almost theoretical slope (25.7 mV per decade) down to 10-10 M.

The extremely low solubility of lead phosphate in water (about 6 x 10 15m) again suggests potentiometric analysis. Selig57,59 determined micro amounts of phosphate by precipitation with lead perchlorate in aqueous medium. The sample was buffered at pH 8.25-8.75 and a lead-selective electrode was used to establish the end-point. The detection limit is about 10 pg of phosphorus. Anions which form insoluble lead salts, such as molybdate, tungstate or chromate, interfere with the procedure. Similar direct potentiometric titrations of phosphate by precipitation as insoluble salts of lanthanum(III), copper(II) or cadmium(II) are suggested, the corresponding ion-selective electrodes being used to detect the end-point. 

L. K. Shpigun, I. D. Eremina, and Y. A. Zolotov, Flow-Injection Analysis. Determination of Lead and Sulfate Ions by Using a Lead-Selective Electrode [in Russian]. Zh. Anal. Khim., 41 (1986) 1557. 

L2. Lai, S., and Christian, G. D., Potentiometric studies with a liquid ion-exchange lead-selective electrode. And. Chim. Acta 52, 41-46 (1970). 

Fig> 6 Effect of pH on a lead-selective electrode, pH varied with HCIOi and NaOH at the indicated background level of Pb(CI0i )2t (ref. 9). 

This procedure allows the determination of species which are not directly indicated by an electrode, but which react quantitatively with some indicated ion. The comparison solution contains an ion indicated by an ion-selective electrode in position C The ion to be measured quantitatively combines (precipitate, complex) with this indicator ion. A lower activity of this indicator ion results at point B, Examples of this are the determination of Al " ions as already mentioned, of sulfate ions by means of two lead-selective electrodes and a Pb(C104)2 standard solution or phosphate ions with two fluoride-selective electrodes and a L2l(NOs)s standard solution. The calculation corresponds to that of the subtraction method. 

Lead Telluride. Lead teUuride [1314-91 -6] PbTe, forms white cubic crystals, mol wt 334.79, sp gr 8.16, and has a hardness of 3 on the Mohs scale. It is very slightly soluble in water, melts at 917°C, and is prepared by melting lead and tellurium together. Lead teUuride has semiconductive and photoconductive properties. It is used in pyrometry, in heat-sensing instmments such as bolometers and infrared spectroscopes (see Infrared technology AND RAMAN SPECTROSCOPY), and in thermoelectric elements to convert heat directly to electricity (33,34,83). Lead teUuride is also used in catalysts for oxygen reduction in fuel ceUs (qv) (84), as cathodes in primary batteries with lithium anodes (85), in electrical contacts for vacuum switches (86), in lead-ion selective electrodes (87), in tunable lasers (qv) (88), and in thermistors (89). 

Ion probes. Determining the level of ions in solution also helps to control corrosion. An increase in concentration of specific ions can contribute to scale formation, which can lead to a corrosion-related failure. Ion-selective electrode measurements can be included, just as pH measurements can, along with other more typical corrosion measurements. Especially in a complete monitoring system, this can add information about the effect of these ions on the material of interest at the process plant conditions. 

The methods most commonly used to detect hydrogen sulfide in environmental samples include GC/FPD, gas chromatography with electrochemical detection (GC/ECD), iodometric methods, the methylene blue colorimetric or spectrophotometric method, the spot method using paper or tiles impregnated with lead acetate or mercuric chloride, ion chromatography with conductivity, and potentiometric titration with a sulfide ion-selective electrode. Details of commonly used analytical methods for several types of environmental samples are presented in Table 6-2. 

Controlled potential methods have been successfully applied to ion-selective electrodes. The term voltammetric ion-selective electrode (VISE) was suggested by Cammann [60], Senda and coworkers called electrodes placed under constant potential conditions amperometric ion-selective electrodes (AISE) [61, 62], Similarly to controlled current methods potentiostatic techniques help to overcome two major drawbacks of classic potentiometry. First, ISEs have a logarithmic response function, which makes them less sensitive to the small change in activity of the detected analyte. Second, an increased charge of the detected ions leads to the reduction of the response slope and, therefore, to the loss of sensitivity, especially in the case of large polyionic molecules. Due to the underlying response mechanism voltammetric ISEs yield a linear response function that is not as sensitive to the charge of the ion. 

The properties of a pH electrode are characterized by parameters like linear response slope, response time, sensitivity, selectivity, reproducibility/accuracy, stability and biocompatibility. Most of these properties are related to each other, and an optimization process of sensor properties often leads to a compromised result. For the development of pH sensors for in-vivo measurements or implantable applications, both reproducibility and biocompatibility are crucial. Recommendations about using ion-selective electrodes for blood electrolyte analysis have been made by the International Federation of Clinical Chemistry and Laboratory Medicine (IFCC) [37], IUPAC working party on pH has published IUPAC s recommendations on the definition, standards, and procedures... 

Prior to the introduction of ion-selective electrode techniques, in situ monitoring of free copper (II) in seawater was not possible due to the practical limitations of existing techniques (e.g., ligand competition and bacterial reactions). Ex situ analysis of free copper (II) is prone to experimental error, as the removal of seawater from the ocean can lead to speciation of copper (II). Potentially, a copper (II) ion electrode is capable of rapid in situ monitoring of environmental free copper (II). Unfortunately, copper (II) has not been used widely for the analysis of seawater due to chloride interference that is alleged to render the copper nonfunctional in this matrix [288]. 

Another aspect of tin as a constituent of electrode material is shown by tin(IV)TPP complexes incorporated into PVC membrane electrodes. These increase the selectivity to salicylate over anions such as Cl-, Br- I-, I()4, Cl()4, citrate, lactate and acetate. The specificity is attributed to the oxophilic character of the Sn ion in TPP at the axial coordination sites. Indeed, carboxyl groups incorporated into the membrane polymer compete for these binding sites. The complete complex structure is important. Substitution of TPP with octaethylporphirine results in loss of salicylate selectivity231. Preparation and analytical evaluation of a lead-selective membrane electrode, containing lead diethyldithiocarbamate chelate, has also been described232. 

They lead to the formation of a membrane potential at BLM and thick membranes [30,49, 122, 156, 193], forming a basis for their use in ion-selective electrodes, as was first demonstrated by Stefanac and Simon [202]. 

Another method used for nitrate determination on dried and milled herbage employs the nitrate selective electrode. One of the first published methods was that of Paul and Carlson (1968). Other anions, especially chloride, can interfere. These authors removed chloride with silver resin, but Barker ef al. (1971) omitted the resin because it tended to foul the electrode and cause excessive drift. Normally the Cl N03 ratio is so low as not to interfere, but saline precipitation from coastal plots could affect this. The method was further modified to allow storage of extracts for up to 64 h by adding a preservative of phenyl-mercuric acetate and dioxane, both very toxic (Baker and Smith, 1969). This paper mentions the need to change the electrode s membrane, filling solution and liquid ion exchanger every 2 months to minimize chloride interference. It is easy to overlook electrode maintenance between batches of nitrate analyses, and this can lead to errors and sluggish performance. 

Figure 4.20.A shows a more recent cell reported by Cobben et al. It consists of three Perspex blocks, of which two (A) are identical and the third (B) different. Part A is a Perspex block (1) furnished with two pairs of resilient hooks (3) for electrical contact. With the aid of a spring, the hooks press at the surface of the sensor contact pads (4), the back side of which rests on the Perspex siuface, so the sensor gate is positioned in the centre of the block, which is marked by an engraved cross as in the above-described wall-jet cell. Part B is a prismatic Perspex block (2) (85 x 24 x 10 mm ) into which a Z-shaped flow channel of 0.5 mm diameter is drilled. Each of the wedges of the Z reaches the outside of the block. The Z-shaped flow-cell thus built has a zero dead volume. As a result, the solution volume held between the two CHEMFETs is very small (3 pL). The cell is sealed by gently pushing block A to B with a lever. The inherent plasticity of the PVC membrane ensures water-tight closure of the cell. The closeness between the two electrodes enables differential measurements with no interference from the liquid junction potential. The differential signal provided by a potassium-selective and a sodium-selective CHEMFET exhibits a Nemstian behaviour and is selective towards potassium in the presence of a (fixed) excess concentration of sodium. The combined use of a highly lead-selective CHEMFET and a potassium-selective CHEMFET in this type of cell also provides excellent results.

Elemental composition Pb 84.50%, F 15.50%. The compound can be identified from its physical properties and x-ray measurement. Lead may be analyzed by various instrumental techniques (See Lead). Fluoride ion may be determined by dissolving a minute quantity of the compound in water (the compound is slightly soluble in water) and analyzing the solution by ion chromatography or hy fluoride-ion selective electrode. 

Elemental composition Pb 62.55%, N 8.46%, O 28.98%. The aqueous solution may be analyzed for lead by various instrumental techniques (See Lead). The nitrate ion may be identified by a nitrate ion-selective electrode or by ion chromatography following appropriate dilution of the solution. The compound may be identified in crystalline forms by x-ray and by its physical properties. 

Total sulfate may be determined in a 50 50 water-methanolic formaldehyde solution by titration with standardized 0.1//lead perchlorate. Endpoint detection is effected using a combination lead ion-selective electrode, and the level of sulfate is typically 13.8 wt % [14]. 

For example, the aluminium ion, for which there is no selective electrode, can be determined in the following way. Sodium fluoride can be added as a reagent to lead to insoluble AIF3. The end point of the reaction can be determined by the presence of an excess of fluoride ions in solution, easily measured with a fluoride selective electrode. 

A difference of 59.16 mV (at 25°C) builds up across a glass pH electrode for every factor-of-10 change in activity of H+ in the analyte solution. Because a factor-of-10 difference in activity of H+ is 1 pH unit, a difference of, say, 4.00 pH units would lead to a potential difference of 4.00 X 59.16 = 237 mV. The charge of a calcium ion is n = 2, so a potential difference of 59.16/2 = 29.58 mV is expected for every factor-of-10 change in activity of Ca2+ in the analyte measured with a calcium ion-selective electrode. 

An ion-selective electrode responds to the activity of free analyte, not complexed analyte. For example, when the Pb2+ in tap water at pH 8 was measured with a sensitive ion-selective electrode, the result was [Pb2+] = 2 X 10 10 M.25 When lead in the same tap water was measured by inductively coupled plasma-mass spectrometry (Section 21-6), the result was more than 10 times greater 3 X 10-9M. The discrepancy arose because the inductively coupled plasma measures all lead and the ion-selective electrode measures free Pb2+. In tap water at pH 8, much of the lead is complexed by CO -, OH, and other anions. When the pH of tap water was adjusted to 4, Pb2+ dissociated from its complexes and the concentration indicated by the ion-selective electrode was 3 X 10-9M—equal to that measured by inductively coupled plasma. 

N. A. Chaniotakis, Thick Membrane, Solid Contact Ion Selective Electrode for the Detection of Lead at Picomolar Levels, Anal. Chem. 2005, 77, 1780. 

Table 5), and several are now being used, or are potentially useful, for measuring key ocean elements. The most common use of direct potentiometry (as compared with potentiometric titrations) is for measurement of pH (Culberson, 1981). Most other cation electrodes are subject to some degree of interference from other major ions. Electrodes for sodium, potassium, calcium, and magnesium have been used successfully. Copper, cadmium, and lead electrodes in seawater have been tested, with variable success. Anion-selective electrodes for chloride, bromide, fluoride, sulfate, sulfide, and silver ions have also been tested but have not yet found wide application. 

Fig. 2.4. Speciation analysis of drinking water spiked with 10 ppb Pb2+ as a function of sample pH performed with Pb2+-selective electrode described in Ceresa et al. [17]. The dotted line represents the behaviour calculated on the basis of lead-carbonate complexation [12]. Reprinted from Ref. [12] (p. 205), with permission from Elsevier.

The GECE sensors were used for lead determination in real water samples suspected to be contaminated with lead obtained from water suppliers. The same samples were previously measured by three other methods a potentiometric FIA system with a lead ion-selective-electrode as detector (Pb-ISE) graphite furnace atomic absorption spectrophotometry (AAS) inductively coupled plasma spectroscopy (ICP). The results obtained for lead determination are presented in Table 7.1. The accumulation times are given for each measured sample in the case of DPASV. Calibration plots were used to determine the lead concentration. GEC electrode results were compared with each of the above methods by using paired -Test. The results obtained show that the differences between the results of GECE compared to other methods were not significant. The improvement of the reproducibility of the methods is one of the most important issues in the future research of these materials. 

Buffle, 3., Greter, F.L. and Haerdi, W., 1977. Measurement of complexation properties of humic and fulvic acids in natural waters with lead and copper ion selective electrodes. Anal. Chem., 49 216-222. 

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