Ion activity measurement

Counterion specificity has been observed to be markedly more pronounced for cationic surfactants than for anionic ones. This can certainly be mainly referred to a weaker hydration of typical counter-anions. From the variation of CMC with counterion and from ion activity measurements it can be inferred that the binding to -N(CH3)3 and -NH3 headgroups follows the sequence P>NOj >Br > CP. (As an example a recent study223-1 of decylammonium salts shows the CMC to decrease from 0.064 M for the chloride to 0.038 M for the iodide). The counterion specific effects on micellar shape for -N(CH3)3 surfactants were discussed above. For cationic (as well as some anionic) amphiphiles, a marked counterion specificity is also indicated in the phase diagrams8 but systematic studies of the counterion dependence have not yet been reported. 

For the specific case of a standard for fluoride ion activity KF rather than NaF has been suggested. KF is a better choice because ion pairing is much less. Further, the average hydration number of the fluoride ion is almost the same as that for potassium ion, so that activity coefficients of the two ions are similar. Suggested reference activity values (pM or pAJ for use in the operational definitions for ion-activity measurements [Equations (13-26) or (13-27)] are shown in Table 13-2. For the case of fluoride ion, measurements of its activity in NaF-NaCl mixtures up to 1 m and KF-KX mixtures up to 4 m yielded the same values as pure NaF or KF at the same ionic strength. 

In principle these can also be used for ion activity measurement. Life scientists first tried to take advantage of electrodes of the second kind as early as the 1950s. Mauro (36) measured chloride ion concentrations inside the squid giant axon with a micro chloride electrode that he prepared by cementing a 15 gm diameter silver chloride-coated silver wire into the end of a glass capillary. 

In his classic work [17], Lux used a gas gold oxygen electrode for oxide-ion activity measurements in the K2S04-Na2S04 equimolar mixture at 950 °C. 

As typically observed for polyelectrolytes, a large proportion of small co-ions and counterions is held within the domain of the macro-ion. Activity measurements made with ion-specific electrodes have shown that 66% of the Na" " ions of sodium heparinate are bound to the polyanionic chain. Binding of Na" " by heparin was also studied by Na-n.m.r. spectroscopy. Ion-activity data have been interpreted in the framework of the Manning theory, providing intercharge distances ranging from 0.24 to 0.47 nm, depending on the heparin preparation and the experimental conditions (see Ref. 7, and references cited therein). 

Early support for the role of polymer hydrophobicity came from a study (158) of the interaction of a series of nonionizable polypeptides (poly-DL-alanine and three derivatives of poly-L-glutamine) with SDS. Strongest interaction, as judged from the lowest T l value (from dye solubilization and electrical conductivity measurements) and highest ionic dissociation (a, from sodium ion activity measurements) of the complex, was obtained with the most hydrophobic member, viz., poly-DL-alanine, and vice versa. 

In 1963, an important development was the zirconia membrane electrode showing ionic conductivity due to oxide ion (10). This electrode, initially composed of calcium oxide and zirconia, later has appeared in other forms, notably zirconia-yttria and zirconia-thoria. It has proven effective for oxide ion activity measurements over an extremely wide range at temperature of 1000 or higher, but it is of limited use at temperatures below 500 because of excessive resistance. 

In these bio-related fields, it is primarily the pH, pNa, pK, pCa, pCl or pF electrodes which are employed. Depending upon whether extra- or intracellular ion activities are to be determined, different electrode constructions are utilized. Measurements in extracellular fluids (whole blood, serum, etc.) are usually carried out with the conventional macroelectrodes. Anaerobic conditions are harder to maintain with this method.is expedient to thermostat the entire sample beaker and electrode pair set-up. For more accurate pH and pCa determinations the air space above the sample must also be equilibrated with an O2-CO2 mixture, to establish the same partial pressure of CO2 as in the original sample environment. This expense can be spared by working with microprobes under anaerobic conditions with small surface areas (low CO2 loss). In such cases the commercially available flow-thru cells or capillary cells are used. For ion activity measurements within the cell special microelectrodes with tip diameters about one micron are employed, so that the cell membrane is not damaged too much. To date, such electrodes are available only for pH and pNa measurements (Transidyne, U.S.A.). 

With the help of ion-selective electrodes and the proper calibration, the ion activity at the indicating electrode position can be monitored directly and continuously. According to Friedman [249], who was the first to carry out continuous blood pNa and pK measurements, one can distinguish between static and dynamic flow measurements. Static measurements are those such as ion activity measurements in the stomach or on the surface of the skin. In vivo flow measurements are primarily ion activity measurements in bloodstreams, either in the flow circuit of a heart-lung machine, or also post-operatively by means of vein shunts. 

The Ag/AgCl micropipette reference electrodes usually employed in physiological measurements of cell membrane potentials can be used as reference electrodes for intracellular ion activity measurements as well. The salt bridge electrolyte must be chosen with care so that the intracellular measured ion concentration is not influenced. 

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