Cations, bivalent

The rearranging entity has been shown to be the bivalent cation the adjacent charges may so weaken the N—N link that charges of nearly integral size may be built up in the 4 and 4 positions. In the bent, but strainless, cation the minimum separation of the two p-positions would suffice for the establishment of a lai ely electrostatic bond, which could pass smoothly into the covalent rearrangement product (benzidine). 

In addition, most devices provide operator control of settings for temperature and/or response slope, isopotential point, zero or standardization, and function (pH, mV, or monovalent—bivalent cation—anion). Microprocessors are incorporated in advanced-design meters to faciHtate caHbration, calculation of measurement parameters, and automatic temperature compensation. Furthermore, pH meters are provided with output connectors for continuous readout via a strip-chart recorder and often with binary-coded decimal output for computer interconnections or connection to a printer. Although the accuracy of the measurement is not increased by the use of a recorder, the readabiHty of the displayed pH (on analogue models) can be expanded, and recording provides a permanent record and also information on response and equiHbrium times during measurement (5). 

Table 19. Cell parameters of heptafluorotantalates and heptafluoroniobates containing bivalent cations. 

The Krafft point, i.e., the aqueous solubility, also depends greatly on the type of counterion [30,60,61]. For example, potassium salts have higher Krafft points than sodium or ammonium salts. Bivalent cations, like calcium and magnesium, raise the Krafft point. The rise is smaller for ester sulfonates than for alkyl sulfates [61]. 

Trivalent Oxides.—It is probable that the rhombohedral crystal corundum, AI2O3, consists of ions that is, the bonds are probably electrostatic. The oxygen ions are arranged in layers parallel to the base (111) within each layer the distances between adjacent ions are 2.50 and 2.89 A., the values being somewhat uncertain because of the difficulty in the accurate determination of the parameter. These values are probably compatible with the oxygen radius 1.40 A., calculated for crystals with bivalent cations. The aluminum-oxygen distances are 1.85 and 1.99 A., in satisfactory agreement with the calculated radius sum 1.90 A. 

In absence of any selectivity, the amount of each cation adsorbed in the cell wall must be exactly proportional to the amount of each cation present in the treatment solution. For example, the proportions of two bivalent cations adsorbed in the cell wall must be the same as in the equilibrium solution. In other words, the ion exchange isotherm must be diagonal. 

Only PAE, a novel acetyl esterase, could remove acetyl from beet pectin, to a maximum of 30%. This was shown to be one specific acetyl group in theJiomogalacturonan chain of pectin (smooth region) by NMR spectroscopy. PAE activity was influenced by buffer salts and the addition of bivalent cations. PAE worked cooperatively with pectolytic enzymes. 

This novel enzyme was the only esterase able to release acetyl from sugar beet pectin and removed about 30% of the total acetyl groups present. It also caused the release of acetyl groups from a range of other acetylated substrates, either synthetic or extracted from plants, in small amounts. PAE had an apparent molecular weight of 60 kDa and showed optimal activity at pH 5.5 and a temperature of 50 C. The enzyme is sensitive to buffer composition and requires a bivalent cation for optimal activity and stability. In purified form this enzyme proved unstable, especially in phosphate buffers. 

The transmittance IR spectra of propene adsorbed by the Ca and Mg forms of Y zeolite correspond to a superposition of the bands of weakly adsorbed molecules with those of molecules that are more strongly specifically adsorbed by the bivalent cations. Upon reducing the propene pressure, the bands from the weaker form were eliminated, while the stronger forms of adsorption predominate. Therefore, to separate the spectral contributions of the species strongly adsorbed by Ca2+ or Mg2+ cations from those of the weakly adsorbed species, we recorded spectra at very low propene pressures of 0.5 mbar. For propene adsorption by NaY the adsorbed species were homogeneous and all of the bands in the IR spectra increased in intensity with increasing pressure up to 15 mbar without changing their positions. 

A similar effect was observed earlier in [5] for ethene adsorption by X zeolite modified with bivalent cations of Cd and Ca. The C-H stretching bands, which are intense for free ethene, are not detectable at low pressure, while the normally forbidden C-H deformation and C=C stretching bands are the strongest in the spectrum. Further, ethene is weakly adsorbed by monovalent cations such as K, Na or Li and the relative intensities of C-H stretching bands are very strong. 

The presence of salts of univalent and bivalent cations increases, by several-fold, the activity of pectinesterases from higher plants, which is minimal in the absence of salts.38,50,57,6, 63,64 66,69,7° The activating effect of salts on pectinesterases of microbial origin is not so great, an increase by 1.5- to 2-fold being reported.51 56,63 76-78,80 Table III shows the concentrations of sodium chloride and calcium chloride that caused the maximal activation of pectinesterases of plant and microbial origin. The mechanism of activation has not yet been satisfactorily explained. 

Characteristic properties of endopectate lyases are the high pH optimum, and a requirement for Ca2+ ions in order to maintain catalytic activity. The pH optimum of various endopectate lyases ranges from 8.0 to 9.5 (Refs. 4, 178, 234, 236, 243). Besides activation by Ca2+ ions, the optimal concentration of which is 1 mM,234,236,244 strontium salts were also considered in the case of Bacillus sp. lyase.234 The enzyme from Pseudomonas sp. was also partly activated by magnesium chloride,178 and for the lyase of Clostridium felsineum, salts of other bivalent cations had an activating effect as well.245 (Ethylenedinitrilo)tetraacetic acid completely inactivated all of the lyases mentioned. The activity of endopectate lyase from Pseudomonas was also lessened in the presence of sodium chloride, potassium chloride, and dipotassium hydrogen phosphate (K2HP04). 

The activity of exopectate lyase of Clostridium multifermentans depends on the presence of bivalent cations of Ca, Ba, Sr, Mg, and Mn. The activation constant with calcium chloride is 0.06 mM. In a later stage of substrate degradation, an inhibition by Ca2+ was observed at concentrations above 0.5 mM this was attributed by Macmillan and Vaughn105 to a binding of calcium to the carboxyl group of two different molecules of substrate and to the inability of exopectate lyase to split the substrate as this barrier is approached. 

A common problem in offshore petroleum production is that sulfate scale may form when seawater is injected into the formation during waterflooding operations. The scale forms when seawater, which is rich in sulfate but relatively poor in Ca++ and nearly depleted in Sr++ and Ba++, mixes with formation fluids, many of which contain bivalent cations in relative abundance but little sulfate. The mixing causes minerals such as gypsum (CaSC>4 2H2O), anhydrite (CaSC>4), celestite (SrSOzO, and barite (BaS04, an almost insoluble salt) to become saturated and precipitate as scale. 

The distribution of metals between solution and the ferric hydroxide surface varies strongly with pH (Fig. 31.5). As discussed in Sections 10.4 and 14.3, pH exerts an important control over the sorption of metal ions for two reasons. First, the electrical charge on the sorbing surface tends to decrease as pH increases, lessening the electrical repulsion between surface and ions. More importantly, because hydrogen ions are involved in the sorption reactions, pH affects ion sorption by mass action. The sorption of bivalent cations such as Cu++,... 

Berridge MJ 1997 Elementary and global aspects of calcium signalling. J Physiol 499 291—306 Broad LM, Cannon TR, Taylor CW 1999 A non-capacitative pathway activated by arachidonic acid is the major Ca2+ entry mechanism in rat A7r5 smooth muscle cells stimulated with low concentrations of vasopressin. J Physiol 517 121-134 Byron KL, Taylor CW 1995 Vasopressin stimulation of Ca2+ mobilization, two bivalent cation entry pathways and Ca2+ efflux in A7r5 rat smooth muscle cells. J Physiol 485 455—468... 

Taylor However, there is evidence from us and other groups that the bivalent cation recognition properties of the sites that mediate stimulation and inhibition may be different. 

Taylor There is some evidence that speaks to this issue from Barbara Ehrlich, who has worked in bilayers on native cerebellar type 1 InsP3 receptor. She varied what is in effect the luminal bivalent cation from Sr2+, Ba2+ or Ca2+. Their order of being able to inhibit the channel is Ca2+> Sr2+ > Ba2+. The mean open times of those channels varied in the same way. This suggests something is happening quite immediately. 

It is reasonable to assume that the surface of metal sulfides show amphoteric behavior and it has been shown that uptake of bivalent cations is pH-dependent. Metal sulfide precipitates are efficient scavengers for heavy metal ions. 

Combining [Ca2+] and [Mg2+] in this expression, derived from ion exchange equilibrium considerations, is not strictly valid but cause litte deviation from more exact formulations and is justified because these two bivalent cations behave similarity during cation exchange. 

Some caution is required when comparing the association constants obtained from extraction experiments with those measured under anhydrous, homogeneous conditions. Iwachido et al. (1976, 1977) have shown that the extracted cation retains part of its aqueous solvation shell on complexation. In particular, the small univalent cations (Li+, Na+) and bivalent cations give high hydration numbers for their crown-ether complexes. Water molecules completing the co-ordination sphere of the cation have frequently been encountered in the solid state of crown-ether complexes (Bush and Truter, 1970, 1971). The effect of small amounts of water on the equilibria (1) has not been studied yet for crown ethers. However, it has been found that the presence... 

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