The Calcium Electrode

Electrical connection is made via a salt bridge which is a small-diameter glass tube filled with 0.1m CaCl2 aqueous 2% agar gel. A chloridized silver wire (Ag-AgCl electrode) is used for external connection. 

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In addition to solid-state electrodes, other ISEs operate with a polymer membrane, with a good example being the calcium electrode described below in Section 3.5.2.3. 

With tables of ion entropies available, it is possible to estimate a Gibbs function change without the necessity of carrying out an experiment or seeking specific experimental data. For example, without seeking data for the potential of calcium electrodes, it is possible to calculate the calcium electrode potential or the Gibbs function change in the reaction... 

Before the appearance of analytically useful ISEs with liquid membranes, Sollner and Shean [200] obtained a liquid ion-exchange membrane with marked selectivity for anions. The first ISE with a liquid membrane was the calcium electrode described by Ross [179] with Ca " -dialkylphenylphosphate in dioctylphenylphosphonate. 

The most important electrode of the ion-exchange type is the calcium electrode. A water-immiscible hydrophobic solvent is used—for example... 

Limited dispersion has also been used for the potentiometric determination of the calcium activity in serum or the so-called ionized fraction of calcium [21]. The calcium electrode was placed in a specially designed flowthrough cell (Fig. 4.4), which also accommodated the reference electrode, and by incorporating a pH glass capillary flowthrough electrode into the system, it was possible to measure simultaneously the pH and pCa of the injected sample at a rate of approximately 100 samples/h with high reproducibility. Inherent for the latter is that the design of the FIA... 

Calculate the relative error incurred in the direct potentiometric determination of the calcium concentration of sea water due to magnesium ion interference. A typical magnesium level is 1300 parts per million (ppm), and calcium level is 400 ppm. The selectivity coefficient of the calcium electrode for magnesium is 0.014. 

Solutions 1, 2 and 3, as given in Table 1, are required to establish the relative sensitivity of the calcium electrode and should be used to establish secondary calibration solutions. Solutions 2 and 3 differ by a decade. In principle the primary reference solutions should not contain buffer. 

Table 4. Data referring to the calcium electrode potential in solutions ...

In confirmation of this hypothesis, we determined the concentration of calcium on the calcium electrode in a series of solutions with a constant concentration of sodium ion, using the sodium electrode as reference electrode. The calibration is carried out with 1 and 3 mM... 

Some ionic species will interfere with other ionic species thus distorting the voltage reading produced by the electrode. Zinc interferes with the calcium electrode. Hydroxide interferes with the fluoride electrode. Sugar solutions produce low readings from pH and calcium electrodes that have been calibrated in aqueous standard solutions (Clarke, 1970). 

Experimental evidence (Ross, 1967) indicates that the calcium electrode can be used to measure free-calcium-ion activity in solutions which contain as high as 10 times as many sodium or potassium ions. In typical biological saline solutions, other monovalent cations exceed the uncomplexed-calcium-ion concentration by a factor of 10 at most. Figure 6.8 indicates electrode response as a function of pH. 

After stress, the calcium electrodes were washed off with deionized water and blown dry in the room with yellow lights, and packaged under A1 foil for transportation to the TOF-SIMS system (Charles Evans Associates), where they were inserted with minimal (but not zero) exposure to room light into the vacuum systenL A Ga ion beam at 7.5 keV generated the signal. The images were acquired fi om an 80 /xm x 80 /xm raster scan the beam size is about 1 xm for the spatial maps. The Tof-Pak software firom Phi was used for analysis. To gain access to the ITO surface, the device was immersed in a beaker of toluene for -30-60 sec. the films came off readily. 

Liquid membrane electrodes are shown in Figure 3(a). Typically an organophilic porous plastic membrane separates the internal reference from the sample solution. Another reservoir contains a liquid ion-exchanger, which saturates the pores in the membrane. The calcium electrode uses calcium dodecyl phosphate [(ROlj POjJj Ca where R is the dodecyl radical, dissolved in dioctyl phenylphosphonate. 

The term selectivity constant has been used for the coefficient although strictly speaking it is not constant, and with ion-exchange and ion-solvating electrodes is a function of the absolute ionic strengths of the measured and interfering ions. With the calcium-electrode in dilute solution (< O.IM) a selectivity coefficient fc a-Na ... 

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