Uncertainties electrode calibration

Major problems could be encountered due to errors associated with the liquid junction. It is recommended that either a free diffusion junction is used or it is verified that the junction is working correctly using dilute solutions as follows. For commercial electrodes calibrated with lUPAC aqueous RVS or PS standards, the pH(X) of dilute solutions should be within 0.02 of those given in Table 1. The difference in determined pH(X) between a stirred and unstirred dilute solution should be < 0.02. The characteristics of glass electrodes are such that below pH 5 the readings should be stable within 2 min, but for pH 5 to 8.8 or so minutes may be necessary to attain stability. Interpretation of pH(X) measured in this way in terms of activity of hydrogen ion, is subject to an uncertainty of 0.02 in pH. 

The direct electrode calibration method offers the advantages of simplicity, speed, and applicability to the continuous monitoring of pX or pA. The method has two important disadvantages, however. One of these is that the accuracy of a measurement obtained by this procedure is limited by the inherent uncertainty caused by the junction potential and unfortunately, this uncertainly can never be totally eliminated. The second disadvantage of this procedure is that results of an analysis are activities rather than concentrations (for some applications this is an advantage rather than a disadvantage). 

A serious disadvantage of the electrode calibration method is the inherent uncertainty that results from the assumption that K in Equation 23-26 or 23-28 remains constant between calibration and analyte deleriiiina-... 

For a measurement of pH with cell (I) to be traceable to the SI, an uncertainty for the Bates-Guggenheim convention must be estimated. One possibility is to estimate a reasonable uncertainty contribution due to a variation of the ion size parameter. An uncertainty contribution of 0.01 in pH should cover the entire variation. When this contribution is included in the uncertainty budget, the uncertainty at the top of the traceability chain is too high to derive secondary standards as used to calibrate pH meter-electrode assemblies. 

Multi point calibration will be recommended if minimum uncertainty and maximum consistency are required over a wide range of pH(X) values [21, 22]. The calibration function of the electrode is then calculated by linear regression of the difference in cell voltage results from the standard pH values. This calibration procedure is also recommended for characterising the performance of electrode systems. 

Figure 19.6 Typical ccdibration curve of an ISE by direct potentiometry. The calibration curve of the specific electrode for the chloride ion has almost an ideal slope value. The range of linear response for the different ISE extends over 4 to 6 orders of magnitude depending on the ion. Expression 19.5 leads to the estimation that an uncertainty of 0.2 mV on E leads to an inexactness of 0.8 per cent in the concentration (for a monovalent ion). Here, the TISAB consists of NaCl 1 M for adjusting the ionic force, a complexing agent for metals and a hufler mixture of acetic acid/sodium acetate.

Much of the uncertainty concerning activity coefficients and liquid junctions can be minimised by calibrating the ion-selective electrode with calcium chloride standards (approx. 0.5—0.2 mmol dm ) in 0.150 mol dm sodium chloride solutions. This corresponds to an emf span of just approx. 18 mV, thus emphasising the need for using precise emf-measuring instruments and attention to electrode care and solution details. 

The response of ion-selective electrodes in biological fluids may be affected by a wide variety of physical and chemical factors. These may influence the indicator electrode directly or may affect the liquid junction of the reference electrode. A brief discussion is presented of the various sources of error and uncertainty in electrode measurements in biologic media especially with microelectrodes. A serious need exists for the development of practical standards for the calibration of ion-selective electrodes in physiologic media in order to ensure the consistency of interlaboratory measurements. 

As can be seen from Table 7.9, the potential difference between the CcICc" couple and the TMPD first redox process is constant for the ILs tested. The approximate 20 mV shifts observed are most likely due to experimental uncertainties. Thus, the TMPD complex offers an alternative to the ferrocene or cobaltocene couples for reference potential calibration. If the potential of the TMPD couple is close to that of the electroactive species, then use of the bis (phenyl)chromium(l) tetraphenylborate (BCr) redox couple with ILs has also been proposed by Lewandowski et al. [49]. Studies of this redox reference compound in [EMIm][BF4] show that it has a stable potential at +0.236 to +0.248 V vs. CcICc" (range dependent on electrode material) [49]. 

In potentiometric analysis, the indicator electrode is intrinsically responsive to single ion activities. It is the potential at the reference electrode that introduces a non-thermodynamic assumption (liquid junction potential) to the measurements. This term can be kept small and corrected by calculation if needed. In pH measurements, one uses the practical determination of pH, which uses the output of a calibrated combination pH electrode (normally with the bridge electrol34 e at the reference electrode containing 3 M KCl) as the accepted pH value of the solution. In essence, any uncertainty in potential arising at the reference electrode is ascribed to... 

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