The influence of ionic sites on the behavior of charged carrierbased ion-selective liquid membrane electrodes is described by theory and experiments for cationand for anion-selective electrodes. The cation exchanger potassium tetrakis [3,5-bis-(trifluoromethyl)phenyl]borate proved to be a beneficial additive for nitrite-selective electrodes. On the other hand, the anion exchanger tridodecylmethylammonium chloride improved the potentiometric properties of cation-selective electrodes. By the incorporation of a defined amount of these sites, the selectivity was enhanced, the emf functions became theoretical, the electrode resistance was lowered, and the long-term stability improved. The optimal molar ratio of additive to ionophore was in the range of 0.3-0.6 for the ionophores studied. The theory and the experiments show clearly that ionic sites should be used not only with neutral ionophores but also with charged ones. ISE membranes without ionic additives should be avoided, since otherwise inherent ionic impurities could have a decisive influence on the response characteristics.
The behavior of cation-selective solvent polymeric membrane electrodes based on acidic ionophores is investigated by studying the selectivity coefficients as a function of cationic or anionic additive concentration. This technique allows discrimination between neutral and charged carrier-related mechanisms, resulting in the following findings. Ionophores with a carboxylic acid group act as neutral carriers and sulfonic acid derivatives behave as charged carriers irrespective of the plasticizer used. The Ca2~-selective organophosphate ionophore examined (bis-[4-( 1,1,3,3-tetramethylbutyl)phenyl]phosphate
An improved protamine-sensitive electrode based on a polymeric membrane doped with the charged ion exchanger dinonylnaphthalenesulfonate (DNNS) is used for monitoring heparin concentrations in whole blood. The electrode exhibits significant nonequilibrium potentiometric response to polycationic protamine over the concentration range of 0.5–20 mg/L in undiluted whole-blood samples. The sensor can serve as a simple end point detector for the determination of heparin via potentiometric titrations with protamine. Whole-blood heparin concentrations determined by the electrode method (n ≥157) correlate well with other protamine titration-based methods, including the commercial Hepcon HMS assay (r = 0.934) and a previously reported potentiometric heparin sensor-based method (r = 0.973). Reasonable correlation was also found with a commercial chromogenic anti-Xa heparin assay (r = 0.891) with corresponding plasma samples and appropriate correction for whole-blood hematocrit levels. Whereas a significant positive bias (0.62 kU/L; P <0.001) is observed between the anti-Xa assay and the protamine sensor methods, insignificant bias is observed between the protamine sensor and the Hepcon HMS tests (0.08 kU/L; P = 0.02). The possibility of fully automating these titrations offers a potentially simple, inexpensive, and accurate method for monitoring heparin concentrations in whole blood.
With the evolution of ion-selective ionophore-based liquid/polymer membrane electrodes (ISEs) over the past 25 years, many cations of physiological and industrial significance can be measured effectively by direct potentiometry. However, there is a noticeable lack of analogous electrodes for many common anions. 1 Although commercial electrodes based on anionexchangers such as quaternary ammonium salts can be analytically useful, their selectivity patterns are always correlated solely with anion lipophilicity, resulting in the classical Hofmeister series (ClO 4 ->SCN -> salicyl a t eTherefore, highly hydrated anions such as fluoride, bicarbonate, chloride, and nitrite are difficult to monitor due to significant interference from more lipophilic anion species that may be present in the sample. Recently, examination of a variety of compounds that have strong yet reversible interactions with target anions has resulted in new ionophores with decidedly non-Hofmeister selectivity toward anions. 3,4 Many research groups are studying a wide variety of new ionophores that demonstrate these characteristics such as mercury 5 , silver 6 , and tin 7 organometallic compounds, fluorinated (poly)ethers 8 , metallophthalocyanines 9 , metallocenes 10 , and trifluoroacetyl derivatives. 11 The pioneering work of Simon et al., in which the behavior of lipophilic vitamin B 12 derivatives as ionophores was examined, led to one of the first nitrite selective liquid/polymer membrane electrodes. 12 Metalloporphyrins, which are one carbon larger but structurally analogous to the cobyrinate ring of vitamin B 12 , also exhibit unique anion ionophore properties when incorporated into plasticized polymer membranes. Mn(III) [13][14][15][16][17][18][19][20] , Co(III) 13,16,[21][22][23][24][25][26] , Ru(II) 16 , and Sn(IV) 25,27-29 porphyrin-based ISEs have been shown to exhibit useful selectivity for thiocyanate, nitrite/thiocyanate, thiocyanate, and salicylate, respectively. More recently, the nature of the ionophore-anion interaction mechanism has been correlated to the charge of the central metal ion.25 Metal(II) porphyrins were found to serve as neutral carriers, metal(IV) as charged carriers and metal(III) porphyrins as either neutral or charged depending on the existance and number of bound axial ligands. The use of lipophilic ionic additives, either cationic (quaternary ammonium salts for neutral carriers) or anionic (tetraphenylborate salts for charged carriers) can help to determine the dominant mechanism within the polymer membrane responsible for EMF response. 30 These ionic sites have also been found to improve general electrode performance by reducing the membrane resistance, decreasing counter-ion interference, and enhancing selectivity. 25,[30][31][32] In this work, porphyrins containing group XIII ions as metal centers are examined as unique ionophores in the preparation of anion ISEs. Electrodes based on indium(III) porphyrins in poly(vinyl chloride) (PVC) membranes have been shown previously to exhibit enhanced responses toward...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.