Passivating ions

More details of other factors that affect the critical pitting potential have been discussed by Uhlig and his co-workers" . They indicated that for stainless steel the critical pitting potential decreased with increasing concentration of chloride ion. At a fixed chloride level, passivating ions in solution, such as sulphate and nitrate, etc., cause the pitting potential to become more positive at a sufficient concentration these ions totally inhibited pitting, as shown in Fig. 19.40 for SO and CIO . 

E Fromter. (1972). The route of passive ion movement through the epithelium of Necturus gallbladder. J Membr Biol 8 259-301. 

A relatively new cell line that has not to date been characterised for its use in biopharmaceutics is based on primary airway epithelial cells infected with retroviruses expressing hTERT and HPV-16 E6/E7 (NuLi-1) [54], NuLi-1 cells were cultured on plastic up to passage 30. When grown on collagen-coated, semi-permeable membranes (Millicell-PCF), NuLi-1 TEER decreased only slightly over the 30 passages from 685 31 to 389 21 ohm.cm2. The TEER of NuLi-1 is similar to that observed with the primary bronchial cultures of 532 147 ohm.cm2. Thus, NuLi-1 cells can form an electrically tight airway epithelial barrier that mimics active and passive ion transport properties of primary human bronchial epithelial cells [54],... 

Some extractants have been used in the development of selective sensors for radionuclides. These have been noted in Tables 9.1 through 9.3 and briefly mentioned above. They were recently reviewed in detail.99 In addition to column sensor formats, extractants have been coated onto passivated ion-implanted planar silicon (PIPS) diodes to create selective radionuclide sensors.152153 Though not a focus of this review, it is worth noting the such sensors, combining separation and detection in a single functional unit, have potential for use in process-monitoring applications. 

Egorov, O. B., Addleman, R. S., O Hara, M. J., Marks, T., and Grate, J. W., Direct measurement of alpha emitters in liquids using passivated ion implanted planar silicon (PIPS) diode detectors, Nucl. Instrum. Methods Phys. Res., Sect. A, 537, 600-609, 2005. 

A number of factors influence formation of proteoliposomes with a low passive ion permeability. Lin et al. (1990) have reported that polyethylene glycol (PEG) treatment of proteoliposomes lowers passive permeability to small molecules, enabling the ion-gradient to be maintained for longer periods. In several ion-coupled carrier reconstitution procedures (Fafoumoux et al., 1989 Tamarappoo and Kilberg, 1991 Ramamoorthy et al., 1992 Ramamoorthy et al., 1993), solubilized proteins have been precipitated with PEG before incoiporation into lipid vesicles. Trace amounts of PEG associated with the proteoliposomes in these procedures may reduce passive permeability and lead to high levels of ion-coupled transport (Ramamoorthy et al., 1993). 

A He(KCl)-jet transportation system was used for the transfer of the activities. The KC1 aerosol with the reaction products was collected by impaction on a Pt or Teflon slip for 60 or 90 s, was picked up with lOpL of the aqueous phase and was transferred to a 1 mL centrifuge cone containing 20 iL of the organic phase. The phases were mixed ultrasonically for 5 s and were centrifuged for 10 s for phase separation. The organic phase was transferred to a glass cover slip, was evaporated to dryness on a hot plate, and was placed over a passivated ion-implanted planar silicon detector (PIPS). This procedure took about 1 min. 

Figure 9.29 Some mammalian (left) and microbial (right) membrane transport systems. (A) Primary electrogenic mechanisms (pumps) creating either a Na+ or a H+ gradient. (B) Secondary active transport systems of the symport type, in which the entry of a nutrient S into the cell is coupled with the entry of either the sodium ions or protons. (D) Various passive ion movements, possibly via channels or uniports. (Reproduced by permission from Serrano R. Plasma Membrane ATPase of Plants and Fungi. Boca Raton CRC Press, 1985, p. 59.)...

Whereas SECM usually uses tips that pass a current in amperometric or voltammetric mode, an important related application uses passive ion-selective sensor tips which can be used for mapping two-dimensional and even three-dimensional ion distributions and concentration profiles over the surface, of species such as protons and zinc ions [53]. 

Sulphur(IV) Atmospheric water Dialysis UV-Vis 1.6 x 10-7 mol L 1 Flow injection system passive ion-exchange tubular membrane for reagent addition [553]... 

Cruzeiri-Hansson L and Mouritsen OG (1988) Passive ion permeability of lipid membranes model via hpid-domain interfacial area. Biochim Biophys Acta 944 63-72 Edge R, McGarvey DJ and Trascott TG (1997) The carotenoids as anti-oxidants—a review. Photochem Photobiol B Biol 41 189-200... 

To study the effects of electrochemical properties on passive ion transport processes, we developed a model that focuses on ionic processes at membrane and channel surfaces (14). The surface compartment model (SCM) is based on a Helmholtz electrical double layer, where the enhanced concentration of counterions and the depletion of co-ions at charged surfaces is described by straight line gradients. Treatment of the electrical double layer as a compartment greatly simplifies the calculation of ion transport. 

Passivator ions acting as oxidizers are adsorbed on the substrate and reduced easily, thus enlarging the cathode surface area. An optimum passivator solution concentration should exceed some critical value and, the higher the passivator concentration, the easier it is adsorbed, and the smaller the anodic areas on the substrate will be. This promotes an increase in anodic polarization and total passivation of the substrate. If the passivator concentration is lower than some critical value, it initiates local corrosion of the substrate. 

Not much more is known about the expeditiousness which, in this context, may be defined as the rate at which the protonmotive force rises after the onset of the pump, but some general predictions can be made. The expeditiousness is the higher the fester the pumping rate of protons is relative to the conductance of passive ions, which tend to shunt the electrogenic pump effect, in other words, to maintain electroneutrality. We see that the two components of the protonmotive force, the electrical and the chemical PD, are controlled by different factors and may therefore develop at different rates, as is illustrated by the following two extreme conditions ... 

If the passive conductance is extremely low, the electrical potential will be generated purely electrogenically, i.e. without appreciable net movement of passive ions. As the electric capacity of the membrane is rather low, l F/cm, electrogenic expulsion of 300-400 protons per cell, passive shunting effect being negligible at this stage, should suffice to raise the electrical PD by 1 mvolt. 

On the other hand, if the passive conductance is very high, it will keep electrical effects small but allow instead an equivalent rise of the chemical PD of protons. This rise will depend on the buffer capacity of the medium, which is normally much higher than the e lectrical capacity. From its value In the mitochondria we would estimate that about a million protons have to be expelled per 1 mvolt rise of the chemical PD, and hence of the protonmotive force. Obviously, the rise in chemical PD in this case is thousand or more times slower than the electrogenic one in the former case. At the rate assumed for the mitochondrial proton pump, we would predict that the critical protonmotive force, i. e. about 200 mvolts, could be reached within a few hundred milliseconds in the first case, but would take a few himdred seconds in the last case. It follows that the expeditiousness of the coupling, i.e. the speed at which this critical protonmotive force is reached after the initiation of the pump depends rai the relative contribution of the initial pumping rate relative to the shunting pathways for passive ions. 

Passive ion transfer across cell membranes refers to ion movement without the expenditure of metabolic energy by the cell. The driving force for such movements are the existing electrochemical gradients across the membrane, even though these may well have arisen from the prior expenditure of metabolic energy. 

---