Magdalena Maj-Żurawska

DETERMINATION OF TRUE SELECTIVITY COEFFICIENTS OF NEUTRAL CARRIER CALCIUM SELECTIVE ELECTRODE

Literature data concerning selectivity coefficients of neutral carrier calcium selective electrode show large discrepancies up to several orders of magnitude for sensors containing the same active component. It can be shown theoretically that determination of selectivity coefficients in non-complexing media gives too high values because of additional sources of the main ion in the membrane vicinity. Determination of selectivity coefficients in solutions containing a complexing agent gives the possibility to evaluate true selectivity coefficients not influenced by additional factors. Measurements performed in complexing as well as in non-complexing media and calculation of the detection limit indicate a good agreement between expectations and experimental results. This has been confirmed using the calcium selective electrode with the neutral carrier ETH 1001 witho-nitrophenyl octyl ether or dioctylsebacate as mediators. As interferents sodium, potassium and magnesium ions were tested.

Effect of a plasticizer on the detection limit of calcium-selective electrodes

Abstract The effect of the plasticizer on the extended linear calibration curve and on the selectivity of a calcium selective electrode with ETH 1001 ionophore was studied as a function of calcium activity in the internal solution. (2-Ethylhexyl)sebacate (DOS) and o -nitrophenyloctyl ether ( o -NPOE) were used as plasticizers. The poly(vinylchloride) membrane also contained potassium tetrakis(4-chlorophenyl)borate. The linear part of the calibration curve of the electrode with o -NPOE is longer and the detection limit is lower compared to values for the electrode containing DOS as the plasticizer. The optimal activity of free Ca 2+ and Na + in the internal reference solution was 10 −4 and 10 −1 for the membrane with DOS and 10 −6 and 10 −1 for the membrane with o -NPOE, respectively. The repeatability of the response for electrodes with the lowest detection limit is similar in the case of both plasticizers. The selectivity coefficients were determined for electrodes having activities of calcium ion in the internal solution in the range from 10 −2 to 10 −10 . The properties of the electrodes can be correlated with the transport properties of their membranes.

Observations on the behaviour of some trifluoroacetophenone derivatives as neutral carriers for carbonate ion-selective electrodes

Properties of three derivatives of trifluoacetophenone [4-(n-butyl)-1-trifluoroacetylbenzene (BTFAB), 4-(n-hexadecyl)-1-trifluoroacetylbenzene (HDTFAB), p-dodecyloxytrifluoroacetylbenzene (DDTFAB)] as neutral carriers for carbonate were examined. The best electrode characteristic (slope, S= 29 mV decade–1, lowest detection limit) was for HDTFAB. The sensitivity towards carbonates was for DDTFAB pH dependent. This eliminates DDTFAB from practical applications. A theoretical treatment concerning carbonate and hydrogencarbonate selectivity under investigated conditions has been performed. Preliminary determination of carbonate in bovine serum was made using electrodes with HDTFAB.

Study of time changes of anion exchanger electrode characteristics

The elution of the anion exchanger from the membranes of ion-selective electrodes and its influence on the electrode characteristics are studied. The decrease of the exchanger concentration in the PVC membrane depends on the exchanger lipophilicity and the concentration of the counter ion in the external solution. On this basis it is possible to estimate the electrode life time and limit of detection.

Dissociation and complexation constants of some β-diketone ionophores

The dissociation constants of dibenzoylmethane (DBM), 1-phenyl-3-(p-chlorophenyl)-1,3-propanedion (ETH 245) and 1,13-bis-[4′-(3″-phenyl-1″,3″-dioxopropyl)-phenyl]-tridecan (ETH 1224) were potentiometrically evaluated in 80% dioxane medium at 30 °C. The stability constants of DBM complexes with calcium, magnesium and barium as well as those of ETH 245 with magnesium were determined and correlated with the selectivity coefficients of ion-selective electrodes containing DBM as ionophore.