Calcium-ion electrode

Schematic experimental procedure is shown in Figure 1. All the chemicals used were of analytical grade, and ion-exchanged distilled water was used for aU the procedure. Amberhte IRC-76 (Organo K.K.) was used for cation exchange reactions. Its cation exchange capacity for 1 dm of wet resin is 200 g of CaCOs. The resin was treated in the diluted HCl solution to displace Na by H , and then treated in saturated CaCOs solution to displace H+ by Ca . After washing with the distilled water, 1 cm of wet Ca +-resin was dispersed in the 300 cm of the distilled water. Pure CO2 gas was introduced into the resin-dispersed solution at the constant flow rate (10 cm min i). Time variation of the pH value and Cs concentration of the resin-dispersed solution was analyzed by using pH / ion meter (Horiba K.K. model F-23 with pH and calcium ion electrodes).

The following data were obtained in calibrating a calcium ion electrode for the determination of pCa. A linear relationship between the potential and pCa is known to exist. 

Figure 21-15 Comparison of a liquid-membrane calcium ion electrode with a glass pH electrode. (Courtesy of Thermo Orion, Beverly, MA.)...

The calcium ion liquid-membrane electrode is a valuable tool for physiological investigations because this ion plays important roles in such processes as nerve conduction, bone formation, muscle contraction, cardiac expansion and contraction, renal tubular function, and perhaps hypertension. Most of these processes are influenced more by the activity than the concentration of the calcium ion activity, of course, is the parameter measured by the membrane electrode. Thus, the calcium ion electrode (and the potassium ion electrode and others) is an important tool in studying physiological processes. 

The difference between activity and concentration is illustrated by Figure 21-19, in which the response of a calcium ion electrode is plotted against a logarithmic... 

The measurement of pH is further complicated by the effect of high concentrations of sucrose (e.g., 60 Brix or 60%w/w) on hydrogen ion activity. Clarke (1970) has discussed the effect of sucrose solution structure on pH and calcium ion electrode processes and shown a decreased response of these electrodes to changes in ionic activity in sucrose solutions at 60 Brix and 24 °C. This reduced electrode response can in part be explained by the structural order of the sucrose-water mixture (molecular association in sucrose-water systems has been reviewed by Allen et at. (1974). In a 60 Brix sucrose solution the ratio of water molecules to sucrose molecules is 12.7 1, with water molecules hydrogen-bonded to sucrose (/.e., in the solvation shell) in dynamic equilibrium with free water. Therefore, the concentration of free water molecules and dissociated ions is much less than in dilute sucrose solutions. The number of water molecules in the sucrose solvation... 

In these electrodes, the active material is a large organic molecule capable of interacting specifically with an anion or cation. Typical of these materials are the phosphate diesters (R0)2P02, used for calcium ion electrodes, metal complexes used for anion electrodes and the neutral macrocyclic crown ethers which are suitable for alkali-electrodes. The active organic molecule is adsorbed onto an inert porous support or dissolved in an organic solvent, and indeed their selectivity can be aflected by the choice of medium. Some typical electrodes are shown in Table 12.4. 

One example of a liquid-based ion-selective electrode is that for Ca +, which uses a porous plastic membrane saturated with di-(n-decyl) phosphate (Figure 11.13). As shown in Figure 11.14, the membrane is placed at the end of a nonconducting cylindrical tube and is in contact with two reservoirs. The outer reservoir contains di-(n-decyl) phosphate in di- -octylphenylphosphonate, which soaks into the porous membrane. The inner reservoir contains a standard aqueous solution of Ca + and a Ag/AgCl reference electrode. Calcium ion-selective electrodes are also available in which the di-(n-decyl) phosphate is immobilized in a polyvinyl chloride... 

Similar to the pH meter, gas meters employ specific ion electrodes. The electrodes generate a potential proportional to the activity of a specific ion in solution. The calibration is achieved in standard solution and results read in mV or concentration in mg/L or ppm on the meter. The water can be adapted to monitor the concentration of carbon dioxide, hydrogen sulfide, ammonia, chloride, calcium, potassium and sodium to name a few. 

FIGURE 5-9 Schematic diagram of a calcium ion-selective electrode. 

Assay of calcium and magnesium by specific ion electrodes (37) is not practicable on micro quantities today, although it is quite possible that these may become available in the near future, especially with the developments that are taking place in foreign countries in this area. 

Kessler, M., Hajek, K., Simon, W. Four-Barreled Microelectrode for the Measurement of Potassium, Sodium, and Calcium-Ion Activity, in Ion and Enzyme Electrodes in Biology and Medicine (Kessler, M., Clark, Jr, L. C., Lubbers, D, W., Silver, I. A., Simon, W., eds.) Munich Urban and Schwarzenberg, 1976, p. 136... 

Calcium ion-selective electrodes have recently been commercialized for the measurement of either total or ionized calcium Approximately 45 % of the calcium present in serum is bound to proteins, 5% is complexed to simple anions and 50% exists as the free ion. Traditionally, total calcium measurements have been made by releasing the protein bound fraction. An ion-selective electrode has now allowed the free (ionized) calcium to be measured directly. There has been much debate on the clinical significance of these measurements. The dependence of ionized calcium on pH must be considered. Samples must be either treated anaerobically, tonometered to a constant pH or have a correction factor applied. 

Fig. 5.17 shows the curves for the potentiometric titration of Ca2f in the range 5 10 3-5 10 2 M with a titrant carrier stream of 5 10 4 M EDTA using a calcium ion-selective electrode each titration is initiated by an abrupt increase in the potential, followed by an S-shaped decrease in which the inflection point marks the end of titration. According to eqn. 5.12, where the titration product is AB , the mixing volume V, the original concentration of A in the sample Cl and the titrant concentration CB, can be calculated. In the experiments in Fig. 5.17 the sample volume was 200/d and/ = 0.84 ml min-1 by... 

Calcium-selective electrodes have long been in use for the estimation of calcium concentrations - early applications included their use in complexometric titrations, especially of calcium in the presence of magnesium (42). Subsequently they have found use in a variety of systems, particularly for determining stability constants. Examples include determinations for ligands such as chloride, nitrate, acetate, and malonate (mal) (43), several diazacrown ethers (44,45), and methyl aldofuranosides (46). Other applications have included the estimation of Ca2+ levels in blood plasma (47) and in human hair (where the results compared satisfactorily with those from neutron activation analysis) (48). Ion-selective electrodes based on carboxylic polyether ionophores are mentioned in Section IV.B below. Though calcium-selective electrodes are convenient they are not particularly sensitive, and have slow response times. 

Electrodes with liquid ion-exchange membranes are typified by a calcium-sensitive electrode (Figure 6.4). The membrane-liquid consists of the calcium form of a di-alkyl phosphoric acid, [(RO)2POO ] 2Ca2+, which is prepared by repeated treatment of the acid with a calcium salt. The internal solution is calcium chloride and the membrane potential, which is determined by the extent of ion-exchange reactions at the interfaces between the membrane and the internal and sample solutions, is given by... 

Blood samples were centrifuged at 1000 x g for 20 min at 0-4°. Ionized calcium levels were immediately determined in serum and urine samples using a calcium ion-selective electrode (Ionetics, Inc., Costa Mesa, CA) urine volumes were recorded. The remaining serum and urine were aliquoted for various analyses and stored at -40°. Serum insulin was analysed by radioimmunoassay (Amersham Corp., Arlington Heights, IL). Serum levels of total calcium, phosphorus and creatinine as well as urine creatinine were determined by colorimetric procedures using an automated analyzer (Centrifichem, Baker Instruments Corp., Pleasantville, NY). Glomerular filtration rates (GFR) were calculated from serum and urine creatinine data GFR = urine creatinine/serum creatinine. 

When ionic or free urinary calcium was evaluated, high protein meals resulted in equal or slightly depressed calciuric response. The levels of ionic calcium in the urine, as determined by a calcium ion-selective electrode, suggest that a considerable amount of urinary calcium was complexed to anions or compounds with anionic groups. 

Figure 16.7 A Calcium-ion Polymer (Liquid) Membrane Electrode.

One of the most important examples of a liquid-membrane ISE is the calcium-selective electrode. The salt utilized as the ion exchanger is the calcium salt of dodecylphosphoric acid, e.g. dissolved in di-( -acetylphenyl) phosphonate. The sensitivity of the electrode depends on the solubility of the ion exchanger in the test solution. The electrode response is generally nemstian down to a concentration of 10 mol dm . In the preferred pH range of 5.5-11, the selectivity of... 

Elemental composition Ca 24.42%, N 17.07%, 0 58.50%. Calcium ion in its aqueous solution may be measured by various instrumental techniques or titrimetry (see Calcium). Nitrate ion can be measured by ion-chromatography or using a nitrate ion-selective electrode. The aqueous solutions must be diluted appropriately for such measurements. 

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. 

Figure 15-20 Calcium ion-selective electrode with a liquid Ion exchanger.

A calcium ion-selective electrode gave a calibration curve similar to Figure B-2 in Appendix B, with a slope of 29.58 mV. When the electrode was immersed in a solution having = 1.00 X 10-3, the... 

J. Bobacka. T. Lindfors, A. Lewenstam. and A. Ivaska, All-Solid-State Ion Sensors Using Conducting Polymers as Ion-to-Electron Transducers, Am. Lab., February 2004, 13 A. Konopka, T. Sokalski, A. Michalska, A. Lewenstam, and M. Maj-Zurawska, Factors Affecting the Potentiometric Response of All-Solid-State Solvent Polymeric Membrane Calcium-Selective Electrode for Low-Level Measurement, Anal. Chem. 2004, 76, 6410 M. Fouskaki and... 

Kramer, R. and Lagoni, H. 1969. Calcium selective electrode for the measurement of calcium ion activity in milk and milk products. Milchwissenschaft 24, 68-70 (German). 

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. 

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