Donnan failure

Conversely, the fundamentals for the UDL he on the coextraction of counterions into the membrane therefore, the membrane is no longer permselective (Donnan failure) [9]. Ideally, when the ionophores are saturated by ions, the ion-ionophore complex functions as an ion-exchanger and the membrane shows an anion Nernstian response. The UDL can be estimated from the membrane composition, formation constant and coextraction coefficients obtained from the so-called sandwich membrane method [73]. 

These two limiting values are shown in Figure 1 points A and E. Likewise, Donnan Failure upon addition of excess aq. NY gives a constant negative limit of... 

K+ complex will favor Donnan failure and will thereby lower (i.e., worsen) the upper detection limit of the ISE... 

There are two main factors that influence the selectivity of a sensor limits in discrimination of an interfering ion and upper limits in stability constant of an analyte-ionophore complex. While an ideal ionophore does not form complexes with interfering ions, too strong complexation with the primary ion leads to a massive extraction of analyte into membrane phase coupled with a coextraction of sample counter-ions, known as Donnan exclusion failure. In such cases, at high activities and lipophilicities of sample electrolytes, fli(org) increases and a breakdown of membrane permselectivity prevents the Nemst equation to hold. 

To interpret the mechanisms related to the proper response at different PH levels, impedance studies were conducted on TDDA and DOBP membranes in various solutions. In both types of membranes, with and without TPB, the bulk semicircle decreased in size when the ion-selective membranes were soaked in low pH bathing solutions (Figure 6). This can be related to the previous observation (5) that at very acidic media, i.e. at pH less than 3, amine compounds, such as TDDA, tend to be protonated in the membrane. Consequently, chloride ions from contacting test solution enter the membrane to satisfy electroneutrality. This failure of the Donnan exclusion principle implies the presence of more ions in the membrane, and therefore a higher conductivity or a reduced real impedance ("size of the semicircle). Obviously, under such circumstances the membranes cease to be selective to H+. 

If, however, a carrier-mediated transport membrane containing charged species — in the form of either mobile ions or fixed sites — were placed between two electrolytic mixtures, significant Donnan effects could be expected. For example, consider a membrane in which the carrier is a counterion to the permeant. The permeant would be expected to be preferentially included in the membrane phase. If significant inclusion were to occur, the use of simple first-kind boundary conditions would be inappropriate and could lead to underestimation of flux. On the other hand, if the permeant and carrier were coions, the permeant could be excluded and failure to account for exclusion could lead to overprediction of flux. Further complications would arise if the complex were charged or if other charged species were present, since the net charge density inside the membrane defines Donnan equilibrium conditions. 

In the case of the sodium-calcium ferrocyamde solutions a somewhat unexpected result was obtained Whereas equation (b), which refers to the concentrations of the two salts, holds within the limit of experimental error, it was found that equation (a) does not accurately represent the relationship between the ionic concentrations of the calcium and sodium salts on the two sides of the membrane The activities of the ions in this case appear to be more closely related to the molar than to the ionic concentrations The difficulty here encountered is not to be regarded as a failure of Donnan s theory of distributional equilibrium, but a failure m the means possessed at the present time for determining with accuracy the true activities of ions... 

Yajima, S. Tohda, K. Buhlmann. P. Umezawa. Y. Donnan exclusion failure of neutral ionophore-based ion-selective electrodes studied by optical second-harmonic generation. Anal. Chem. 1997. 69. 1919-1924. 

While selective complexation makes the membrane more permeable to the analyte ion than the co-ions, selectivity against counter ions in neutral-ionophore-based membranes is achieved by ionic sites, not by the ionophore. In fact, the ionophore-analyte complexation decreases the free analyte activity in tlie membrane to enhance the salt extraction into the membrane phase, which may result in counter-ion interference due to Donnan exclusion failure (7). Although the PVC matrix has inherent negative sites as an impurity (11), the concentration is so low that the anionic sites were initially introduced in cation-selective ISEs based on neutral ionophores to suppress the counter-ion interference (12). 

ISEs are operated under open circuit condition, where no significant current flows. When a Nemstian response is obtained, only one ion is exchanged significantly between the membrane and sample phases so that there is no net current carried by the ion at the interface. In contrast, when co-extraction of the analyte and its aqueous counter ion (Donnan exclusion failure) or exchange of the analyte and its interfering co-ion occurs at the interface, the partial current carried by each ion, / , may become significant The partial currents, however, cancel each other so that the total current at the interface, /, is zero... 

Buck, R.P., K. Toth, E. Graf, G. Horvai, and E. Pungor. 1987. Donnan exclusion failure in low anion site density membranes containing vahnomycin. J. Electroanal. Chem. 223 51-66. 

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