Direct potentiometry standard additions

Potentiometric measurements with ISEs can be approached by direct potentiometry, standard addition and titrations. The determination of an ionic species by direct potentiometry is rapid and simple since it only requires pretreatment and electrode calibration. Here, the ion-selective and reference electrodes are placed in the sample solution and the change in the cell potential is plotted against the activity of the target ion. This method requires that the matrix of the calibration solutions and sample solutions be well matched so that the only changing parameter allowed is the activity of the target ion. 

The trade-offs between direct calibration and standard addition are treated in Ref 103. The same recovery as is found for the native analyte has to be obtained for the spiked analyte (see Section 3.2). The application of spiking to potentiometry is reviewed in Refs. 104 and 105. A worked example for the application of standard addition methodology to FIA/AAS is found in Ref 106. Reference 70 discusses the optimization of the standard addition method. 

Most measurements include the determination of ions in aqueous solution, but electrodes that employ selective membranes also allow the determination of molecules. The sensitivity is high for certain ions. When specificity causes a problem, more precise complexometric or titri-metric measurements must replace direct potentiometry. According to the Nernst equation, the measured potential difference is a measure of the activity (rather than concentration) of certain ions. Since the concentration is related to the activity through an appropriate activity coefficient, calibration of the electrode with known solution(s) should be carried out under conditions of reasonable agreement of ionic strengths. For quantitation, the standard addition method is used. 

Direct potentiometric measurements are used to complete chemical analyses of species for which an indicator electrode is available. The technique is simple, requiring only a comparison of the voltage developed by the measuring cell in the test solution with its voltage when immersed in a standard solution of the analyte. If the electrode response is specific for the analyte and independent of the matrix, no preliminary steps are required. In addition, although discontinuous measurements are mainly carried out, direct potentiometry is readily adapted to continuous and automatic monitoring. 

The determination of an ion or molecule by direct po-tentiomctric measurement is rapid and simple, requiring only a comparison of the potential developed by the indicator electrode in the test solution with its potential when immersed in one or more standard solutions of the analyte. Because most indicator electrodes are selective, preliminary separation steps are seldom required. In addition, direct potentiometrie mea.sure-ments are rapid and readily adapted to the continuous and automatic monitoring of ion activities. 

Fluoride samples of high concentration can be titrated potentiometrically [60] with lanthanum nitrate or thorium nitrate reagent. The potentiometric standard addition technique with NaF standard solution [61] also was found applicable however, direct potentiometry using calibration curves is most often relied on in water analysis. 

The matrix is complicated and the ionic strength in it is high. In order to avoid errors from these uncertainties, the use of standard addition or sample addition is recommended instead of direct potentiometry based on the calibration data. 

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