Calcium ISE

Calcium activities as low as 5 x 10 7 M can be measured, with selectivity coefficients ACaMg and ACaK of 0.02 and 0.001, respectively. Such potential response is independent of the pH over the pH range from 5.5 to 11.0. Above pH 11, Ca(OH)+ is formed, while below pH 5.5, protons interfere. Because of its attractive response characteristics, the calcium ISE has proved to be a valuable tool for the determination of calcium ion activity in various biological fluids. 

Explain clearly how the presence of magnesium ion can influence the response of the calcium ISE. 

By how many millivolts will the potential of a calcium ISE change if the electrode is removed from a 1 x 10 3M CaCU solution and placed in... 

Since the evaluation of the first PTC calcium ISE in 1970, hundreds of other viable PTC models have been employed for diverse analytical purposes. Pd tentianetrie sensing has also gained in popularity by the introduction of flow injection analysis (FIA) techniques and advances in electronics. 

Alternative Fblymer Matrices. The prospect for alternative matrix materials (Table II) for calcium ISEs has been examined in conjunction with the model liquid calciun ion-exchanger/DOPP cocktail (2). Evidently functional electrodes can be fabricated with poly(2-methyl propyl methacrylate) (P2-MFMA) but not with the corresponding poly(butyl methacrylate) (PBMA), poly(methyl methacrylate ) (PMMA) or... 

Table I. Some Characteristics of Calcium ISEs Based on Different PVCs...

ISEs for the measurement of total calcium were introduced more recently than photometric methods. The specimen is acidified to convert protein-bound and complexed calcium to free calcium before measurement of calcium by ISE. Calcium ISEs are discussed later in this chapter. 

Calcium ISEs contain a calcium-selective membrane, which encloses an inner reference solution of calcium chloride often containing saturated silver chloride (AgCl) and physiological concentrations of sodium chloride and potassium chloride (KCl) and an internal reference electrode. The reference electrode, usually of Ag/AgCl, is immersed in this inner reference solution. 

A calcium ISE is calibrated in a 0.5 M buffer solution at 25°C. This solution contains a constant activity of magnesium ion (0.2 M), the principal inter-ferant. Use the following data to determine the standard potential for the cell, containing the ISE and the selectivity coefficient for Mg ... 

The two most well known liquid membrane electrodes are cation-selective the potassium and calcium electrodes. For the potassium ISE, the soft plastic membrane has the neutral hydrophobic ionophore valinomycin immobilised in it. This electrode has a 10000-fold selectivity for potassium ions over sodium ions. Eor the calcium ISE, the liquid ion exchanger membrane is a water-immiscible calcium chelator. Calcium ions are... 

Calcium and potassium selective electrodes have a sensing membrane made of poly(vinyl chloride) (PVC) containing a ligand that forms a complex with the cation. The membrane also contains some additional components. Calcium ISEs are mainly... 

Precision. The calcium ISE instruments overall show good precision (CV < 1%). This translates into at least a fourfold improvement in precision over total calcium measurements in blood. 

The fact that phosphate and polyphosphate ions formed relatively stable complexes with the Group II, but not Group I, cations led to the first commercial calcium ISE (2). This was based on a liquid ion-exchanger comprising at least calcium bis(didecylphosphate) (1) and di-octylphenyl-phosphonate (2). 

Table 3.2 Characteristics of PVC calcium ISEs based on calcium bis[di(4-alkylphenyl) phosphate] sensors and various mediators (16)... 

Nature of cocktail materials in calcium ISEs. The electrochemical performances of calcium ISEs (and all other ISEs based on organic-type sensors) depend on the structure of the sensor as well as mediator. Thus, calcium ISEs fabricated from calcium salts of dialkylphosphoric acids depend on the solvent mediator, e.g. di-octylphenyl phosphonate (DOPP) yields a calcium ISE whereas decan-l-ol produces the so-called water hardness (or calcium/ magnesium) ISE. Several fundamental studies have been made on the molecular variation of these two critical constituents (13-21). 

The overall improvement in performance of PVC calcium ISEs based on either (3) or (4) compared with the original didecylphosphoric acid sensor (1), is evident from Tables 3.2-3.4, although the dramatic (unexpected) improvement in sodium selectivity could not have been predicted. There is little difference in the performances of these aryl type phosphate sensors (membranes 9v. 13, Table 3.2) but the isomer (4) was the easier to synthesize since the branched chain 4-(l, 1,3,3-tetramethylbutyl)phenol starting compound needed for its synthesis (22) was much more readily available... 

The extent of ion permeation in such tracer studies obviously relates to selectivity, and membranes with calcium ion-sensors and solvent mediators which promote good quality calcium ISEs in turn show high d(C"/C )/dt values. The largest value of 59 x 10" s has been reported for a tripentyl phosphate/calcium bis di[4-(l, 1,3,3-tetramethylbutyl)phenyl] phosphate based sensor [12]. 

Unusual interferences. PVC calcium ISEs based on the same di[4-(l, 1,3,3-tetramethylbutyl)phenyl]phosphate sensor but with trioctyl phosphate solvent mediator are superior to any others examined, including DOPP, in... 

ORGANIC SENSOR MATERIALS FOR ION-SELECTIVE ELECTRODES Table 3.9 Selectivity coefficients of some PVC calcium ISEs (15)... 

Table 3.11 Potential changes caused by biochemical components, in calcium chloride in 0.15 M sodium chloride solutions, on calcium ISE response (21)...

To complement the observations on anionic surfactants, the effects of various biochemical components on PVC calcium ISEs have been investigated (21). The electrodes with trioctyl phosphate mediator are again the most resistant (Table 3.11). 

High-quality calcium ISEs can also be fabricated with either acylic (6, 7, 23, 28-31) or cyclic (32-34) neutral carrier compounds (Table 3.12). Thus N, N -di[-(ll-ethoxycarbonyl)undecyl]N, N -4, 5-tetramethyl 3, 6-dioxa-octane diamide (ETH 1001) (5) now constitutes the basis of many commercial cocktails (Table 3.12). As demonstrated for a new pH-buffered calcium ion calibrant based on a mixed ligand calcium ion-buffered system, it may be... 

Macrocyclic polyethers (or crown ethers) also possess attractive cation complexing abilities resulting in the formation of lipophilic species. Not surprisingly, such ionophores have been studied with respect to calcium sensors (32-34), and two examples are shown in Table 3.12. The macrobicyclic polyether-amide (7) provides a better calcium ISE than its monocyclic analogue (8). 

Again, the classical pH-emf dips reported for phosphate-type calcium ISEs were absent (40). 

After the successful fabrication of PVC-calcium ISEs, viable PVC nitrate sensors were similarly constructed with commercial nitrate cocktails (66). Their selectivity for some parameters is quite similar, despite their different compositions—a tris(substituted 2-phenanthroline)nickel(II) exchanger in the Orion 92-07-02 cocktail and tridodecylhexadecylammonium nitrate sensor in the Corning 477316 cocktail (Table 3.18). Their respective lifetimes were 3 and 11 weeks (66). 

Therefore, the behaviour of eight calcium ISEs with membranes comprising ionophore (4), DOPP and seven standard lUPAC PVCs plus the widely-used reference PVC, Breon III EP, have been examined (Table 3.19). All the master membranes were clear and flexible and (except for Nos. 2, 3, 4 and 6 with M values <40000) non-sticky. In terms of the parameters listed in Table 3.19 there is little to recommend any one PVC type (70). 

Despite the universal adoption of PVC, some investigations have centred on the prospect of alternative matrix materials. Except for poly(vinylisobutyl ether) (71), poly(methyl methyl acrylate) (72), poly(methyl acrylate) (39) and poly(vinyl chloride/alcohol) copolymer (73), practically no functional calcium ISEs were realized with poly(vinylidene chloride) (19), cellulose acetate, ethyl cellulose, collodion or pyroxylin (11). Photocured sensor membranes based on acrylates are considered in section 3.4.4. 

The stiffening of calcium ISE membranes based on neutral carrier (4) and DOPP after use in 0.1 M sodium dodecyl sulphate indicates a similar loss of... 

Table 3.19 Some characteristics of lUPAC PVCs and associated calcium ISEs (70)... 

Several grafted calcium ISEs have been reported (73,76,77). In one instance membranes were synthesized by cross-linking the styrene-b-butadiene-b-styrene (SBS) triblock elastomer with triallylphosphate, followed by alkaline hydrolysis of the resultant covalently bound trialkyl phosphate to give a pendant dialkyl phosphate. Good calcium electrodes with lifetimes greater than 6 months were obtained, but the limited selectivity for magnesium and sodium could relate to the absence of a solvent mediator (76). 

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