Ion-selective electrode measurement

Instmmental methods are useful for the determination of the total silver ia a sample, but such methods do not differentiate the various species of silver that may be present. A silver ion-selective electrode measures the activity of the silver ions present ia a solution. These activity values can be related to the concentration of the free silver ion ia the solution. Commercially available silver ion-selective electrodes measure Ag+ down to 10 flg/L, and special silver ion electrodes can measure free silver ion at 1 ng/L (27) (see Electro analytical techniques). 

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. 

C. Rundle, A Beginner s Guide to Ion-Selective Electrode Measurements, http //www.nico2000.net/ Book/Guidel.html. (2005). 

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. 

EN62 Payne, R.B., Buckley, B.M. and Rawson, K.M. (1991). Protein interference with ion-selective electrode measurement depends on reference electrode composition and design. Ann. Clin. Biochem. 28, 68-72. 

Other Instrumentation. - An Orion Model 901 microprocessor ion analyzer was used for pH and for ion selective electrode measurements. A Norelco PW-1212 was used for X-ray... 

A total ionic strength adjustment buffer (TISAB) is used to control the ionic strength and the pH of samples and standards in ion-selective electrode measurements. 

Note In principle, the ion activity coefficient of a salt (y+) can be determined, but not those of individual ions (y and y,), because their concentrations cannot be varied independently. Nevertheless, the activity coefficients of individual ions are very useful, and as mentioned, one tries to calculate them from theory. Ion-selective electrodes measure chemical potentials (which depend on activities, not concentrations), but the standard potential is unknown. For the measurement of pH, which is the negative logarithm of the hydrogen ion activity, one has therefore arbitrarily chosen a reference potential for a certain buffer, which potential is of course as close to the real one as theory permits it to be calculated. 

Determination of the sodium level in separate portions of a blood sample by ion-selective electrode measurement gave the following results 139.2, 139.8, 140.1, and 139.4 meqTL. What is the range within which the true value falls, assuming no determinate error (a) at the 90% confidence level, (b) at the 95% confidence level, and (c) at the 99% confidence level ... 

Potentiometric electrodes measure activity rather than concentration, a unique feature, and we will use activities in this chapter in describing electrode potentials. An understanding of activity and the factors that affect it are important for direct potentiometric measurements, as in pH or ion-selective electrode measurements. You should, therefore, review the material on activity and activity coefficients in Chapter 6. 

Reference 9 gives a review of applications of atomic absorption spectrophotometry to biological samples. Tiiis technique is widely used for metal analysis in biological fluids and tissues, in environmental samples such as air and water, and in occupational health and safety areas. Routine applications of flame emission spectrometry to biological samples are generally limited to the alkali and alkaline earth metals. Ion-selective electrode measurements (Chapter 13) have largely replaced the flame emission measurements in the clinical chemistry laboratory. 

This idea is the essence of ion-selective electrodes. Measurements with these devices are essentially determinations of membrane potentials, which themselves comprise junction potentials between electrolyte phases. The performance of any single system is determined largely by the degree to which the species of interest can be made to dominate charge transport in part of the membrane. We will see below that real devices are fairly complicated, and that selectivity in charge transport throughout the membrane is both rarely achieved and actually unnecessary. 

Rundle, C.C. A beginners guide to ion-selective electrode measurements. Nice, www.nico2000. net/Book/Guidel.html (2005). 

Ion selective electrodes measure an interface potential at the surface of the electrode The specifier is either generated by a specific property of the surface (glass electrode) or by a selective carrier transporting the ion across a membrane The main problem here as with poten-tiometry in general is the stability of the output signal. Consequently the production of a stable reference electrode junction potential is very critical. 

Inconspicuous instrumental, environmental, or chemical effects often cause a loss of instrument response. In atomic emission spectroscopy, for example, sensitivity is affected by such instrumental factors as flame temperature, aspiration rate, and slit width. In amperometric measurements, diffusion currents vary with temperature, and a significant loss in sensitivity may occur with a drop in sample temperature. In ion-selective electrode measurements, sensitivity may be affected by chemical effects, such as changes in ionic strength or pH. 

Some important automated instruments based on ion-selective electrodes measure sodium in boiler feedwaters, fluoride in public water-supplies, and water hardness (Ca " and Mg +) in water-conditioning systems. The sulfide electrode is extensively used in the paper industry for monitoring and control of sulfidity in paper pulping liquors (for instance, the Kraft process), for waste-treatment control of excess sulfide using a process that oxidizes sulfide with air, and for monitoring... 

Sauve, S., McBride, M.B., Hendershot, W.H., 1995. Ion-selective electrode measurements of copper (n) activity in contaminated soils. Arch. Environ. Contam. Toxicol. 29, 373-379. 

The ORION 701/Digital pH meter was used for all ion selective electrode measurements in order to employ an expanded scale, whereas pH measurements were recorded on an ORION 601/digital ionalyzer. The Model 476050 Corning single pH reference combination glass electrode was used for the measurement of the hydrogen ion activity. 

A.B.T.J. Boink et al.. Recommendations on the expression of results of ion-selective electrode measurement of sodium and potassium ion activities in undiluted serum, plasma or whole blood in clincal practice. In Methodology and Clinical Applications of Ion-Selective Electrodes , A.H.J. Maas et al., (Ed.), Vol. 8, p. 137, Graz, 1987. 

Apart from true interference, all ions to various degrees affect the ionic strength of solutions and hence the activity of the primary ions,. A, to which the appropriate ion-selective electrode respond. The difficulties of developing ion-activity scales for ion-selective electrode measurements to match the success of the pH scale have formed a barrier to the adoption for routine measurements of these devices and particularly for the complex mixtures found in biological fluids. The problem has been recognised [28] and attention has been given to calibrating... 

Ion chromatography can be used for the quantitation of a mixture of anions collected as salts, which might include such substances as fluoride or nitrate ion. Specific ion-selective electrode measurements are also used for the analysis of numerous anions and cations. Special attention should be given to elimina-tion/minimization of interferences from the matrix. 

Graggs A, Moody GJ, and Thomas JDR (1979) Calcium ion-selective electrode measurements in the presence of complexing ligands. The Analyst 104 961-972. 

Ion selective electrodes Measurement of the free metal ion is an important goal of metal speciation studies in natural waters, because of the tenet of the free ion activity model, that bioavailability is proportional to the free metal ion concentration. Ion selective electrodes respond to the free metal ion activity. A range of ion selective electrodes are available, but only the copper electrode has sufficient sensitivity for use in measurement at realistic environmental concentrations in natural waters. The correct use of the copper ion selective electrode remains a matter of debate, especially its calibration at the very low free metal ion concentrations using metal ion buffers and the applicability of this procedure to natural samples. 

Ion-selective electrodes measure activities rather than concentrations. They require a buffer to maintain the... 

In a series of papers, Hurlen (13,14) has reported convenient" single ion activities derived from cells with transference. The liquid-junction potentials involved were estimated by the Henderson equation. In addition, Shatkay s ion-selective electrode measurements of the activities of Na" " and Ca2+ ions in NaCl and CaCl2 solutions, based on the Henderson equation, appear eminently reasonable in comparison with other estimates (15,16). 

Electrochemical methods (ISE). Ion-selective electrode measurements provide information about a number of anions/cations through potentiometric measurements against a suitable reference electrode in water solutions (requires polymer digestion/ash-ing). Detection limits vary for different ions but are usually in the upper parts-per-million range. Interference by similar ions (Br, F when measuring d ) is one of the major drawbacks of this techique, which limits its applicability. 

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