Apparent stabilizers

Experiments showed that minute amounts of mercury vapour in the atmosphere are sufficient to discolour a blue iodide-starch paper, e.g. on heating a mixture of 0.2 g of barium nitrate with 0.8 mg of mercury to a temperature of 80°C there is immediate discoloration of the paper. 

Lenk von Wolfsburg, according to S. J. Romocki, Geschichte der Explosivstoffe, Bd. II, Oppenheim, Berlin, 1896. 

Schultze, Deutsche Industrie-Ztg. 10, III, 1865 Das tteue chemlsche Schiesspulver, Berlin, 1865. 

Hartig, Untersuchungen iiber den Bestand mid die Wirkungen der explosiven Baumwolle, Braunschweig, 1874. 

Volkmann, Austrian Pat. 21/208,21/257 (1871) also according to O. Guttmann, Zwanzig Jahre Fortschritte inExplosivstoffen, Berlin, 1909 Z. ges. Schiess- u. Sprengstoffw. 4, 16 (1909). 

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The factor at can be calculated from the known dissociation constants of EDTA, and since the proportions of the various ionic species derived from EDTA will be dependent upon the pH of the solution, a will also vary with pH a plot of log a against pH shows a variation of logoc = 18 at pH = 1 to loga = 0 at pH = 12 such a curve is very useful for dealing with calculations of apparent stability constants. Thus, for example, from Table 2.4, log K of the EDTA complex of the Pb2+ ion is 18.0 and from a graph of log a against pH, it is found that at a pH of 5.0, log a = 7. Hence from equation (30), at a pH of 5.0 the lead-EDTA complex has an apparent stability constant given by ... 

The extent of hydrolysis of (MY)(n 4)+ depends upon the characteristics of the metal ion, and is largely controlled by the solubility product of the metallic hydroxide and, of course, the stability constant of the complex. Thus iron(III) is precipitated as hydroxide (Ksal = 1 x 10 36) in basic solution, but nickel(II), for which the relevant solubility product is 6.5 x 10 l8, remains complexed. Clearly the use of excess EDTA will tend to reduce the effect of hydrolysis in basic solutions. It follows that for each metal ion there exists an optimum pH which will give rise to a maximum value for the apparent stability constant. 

EDTA is a very unselective reagent because it complexes with numerous doubly, triply and quadruply charged cations. When a solution containing two cations which complex with EDTA is titrated without the addition of a complex-forming indicator, and if a titration error of 0.1 per cent is permissible, then the ratio of the stability constants of the EDTA complexes of the two metals M and N must be such that KM/KN 106 if N is not to interfere with the titration of M. Strictly, of course, the constants KM and KN considered in the above expression should be the apparent stability constants of the complexes. If complex-forming indicators are used, then for a similar titration error KM/KN z 108. 

Apparent indicator constant 264, 267 Apparent stability constant 59 Aqua regia 111 Arc alternating current, 764 direct current, 763, 771 sensitivities of elements, (T), 766 Aromatic hydrocarbons analysis of binary mixtures, 715 Arsenates, D. of (ti) 357 Arsenic, D. of as silver arsenate, (ti) 357 as trisulphide, (g) 448 by iodine, (am) 634, (ti) 397 by molybdenum blue method, (s) 681 by potassium bromate, (ti) 406 by potassium iodate, (ti) 401 in presence of antimony, (s) 724 Arsenic(III) oxide as primary standard, 261... 

The chelate HgY2 must be considerably more stable than MY( l 4)+ if necessary one can mask M"+ to a certain desirable extent by means of a selective com-plexing agent A, so that KM becomes an apparent stability constant,... 

Table 4 shows worldwide imports of coffee, listing the primary importing nations.17 22 Since 1980, there has been an increase in overall coffee trade. Coffee imports have risen from 3.8 million metric tons overall to 4.9 million in 1993. Trends in coffee exports are reflecting in the patterns and volume of coffee imports. After a dramatic increase during the 10 year period from 1980 to 1990, there has been an apparent stabilization from... 

In general, vapor pressures are not all that important in preformulation, but it should always be kept in mind that a substance may have sufficiently high vapor pressure to (a) become lost to a sufficient extent to cause apparent stability problems and content uniformity problems and (b) exhibit a potential for interaction with other compounds and adsorption onto or sorption into package components [27],... 

In view of the ready commercial availability and apparent stability of the hexahy-drate, it is probable that the earlier report of explosion on impact, and deflagration on rapid heating [1] referred to the material produced by partial dehydration at 100°C, rather than the hexahydrate [2], The caked crystalline hydrated salt, prepared from aqueous perchloric acid and excess cobalt carbonate with subsequent heated evaporation, exploded violently when placed in a mortar and tapped gently to break up the crystalline mass, when a nearby dish of the salt also exploded [3]. Subsequent investigation revealed the probable cause as heating the solid stable hexahydrate to a temperature ( 150°C) at which partial loss of water produced a lower and endothermic hydrate (possibly a trihydrate) capable of explosive decomposition. This hazard may also exist for other hydrated metal perchlorates, and general caution is urged [4,5],... 

H20, or colomanite Ca[B30(0H)3] H2O 1 are added under identical reaction conditions, the reaction mixture remains colourless even after two months, and ribose can be detected, apparently stabilized by boric acid. The corresponding ribose-diborate complex (with a molecular weight of 307) has been clearly identified (Ricardo et al., 2004). 

By contrast, six-membered cyclic nitronates A containing the trimethylsilyl group at the C-6 atom are apparently stabilized through cleavage of the N-0... 

From a plot of the internalisation flux against the metal concentration in the bulk solution, it is possible to obtain a value of the Michaelis-Menten constant, Am and a maximum value of the internalisation flux, /max (equation (35)). Under the assumption that kd kml for a nonlimiting diffusive flux, the apparent stability constant for the adsorption at sensitive sites, As, can be calculated from the inverse of the Michaelis-Menten constant (i.e. A 1 = As = kf /kd). The use of thermodynamic constants from flux measurements can be problematic due to both practical and theoretical (see Chapter 4) limitations, including a bias in the values due to nonequilibrium conditions, difficulties in separating bound from free solute or the use of incorrect model assumptions [187,188],... 

Similarly, there are present in citrus juices active enzyme systems that destroy the important pectic constituents. These enzymes have to be rapidly inactivated to protect the desirable cloud (turbidity) which is apparently stabilized by pectic substances. ... 

However, the equilibrium constant must still be considered as pure and dimensionless numbers (according to the classical relation —AG° = RT In Ks). All molar concentrations in the expression of Ks should thus be interpreted as molar concentrations relative to a standard state of 1 mol dm-3 i.e. they are the numerical values of the molar concentrations5 . If the solution is not dilute enough, the equilibrium constants can still be written with concentrations but they must be considered as apparent stability constants. 

Apparent stability constants for interactions between l-chloro-2,4-dinitrobenzene and benzene or mesitylene were found to be 0.76 and 0.96 (moD1 dm3) respectively, and between 4-chloro-3-nitrotrifluoromethylbenzene and benzene or mesitylene the values were 0.96... 

TABLE 1. Apparent stability constant (Kc) of complexes between aromatic amines and l-fluoro-2,4-dinitrobenzene (unless otherwise indicated) at 40 °C... 

Recently262, the apparent stability constants of the complexes between aromatic fluoro derivatives and amines (shown in equilibrium 31, Kc in moL1 dm3) in toluene- were evaluated by 19F chemical shift measurements. 

TABLE 13. Apparent stability constants of molecular complexes between 2-hydroxypyridine and some aromatic nitro derivatives in hen/enevA at 25 °C125. Reproduced by permission of societa chimica Italiana from Reference 125... 

The study of molecular complexation was then extended to other aromatic nitro derivatives125. Although, as was described before, one of the more frequent methods of studying the formation of molecular complexes is by UV-visible spectrophotometry, the author did not observe detectable differences in the UV-visible absorbance spectra between the 2-hydroxypyridine-l-fluoro-2,4-dinitrobenzene (FDNB) mixtures and the sum of their separate components. The author observed that the signals of the 1II NMR spectra of FDNB in apolar solvents were shifted downward by the addition of 2-hydroxypyridine from solutions where [2-hydroxypyridine] [FDNB] he calculated the apparent stability constants, which are shown in Table 13. 

Even if the extraction of FeMoco in nmf slightly modifies the outer coordination of the MoFe7S9 cluster, in agreement with the experimental observation that the FeMo-protein can exist in the oxidized, semi-reduced and reduced forms, it exhibits two successive one-electron reductions (E st = -0.32 V and is d = -1.00 V,vj. NHE). In the absence of thio-phenol, the first reduction appears to be complicated by self-oxidation processes, whereas the presence of thiophenol apparently stabilizes the different oxidation states. 

The compounds Ln(C5H5)2Cl also have been made only with the lanthanides above samarium (772). These compounds are stable in the absence of air and moisture, sublime near 200 °C, are insoluble in non-polar solvents, and exhibit room temperature magnetic moments near the free ion values (772, 113). The chloride ion may be replaced by a variety of anions including methoxide, phenoxide, amide and carboxylate. Some of these derivatives are considerably more air-stable than the chloride — the phenoxide is reported to be stable for days in dry air. Despite their apparent stability, little is known about the physical properties of these materials. The methyl-substituted cyclopentadiene complexes are much more soluble in non-polar solvents than the unsubstituted species. Ebulliometric measurements on the bis(methylcyclopentadienyl)lanthanide(III) chlorides indicated the complexes are dimeric in non-coordinating solvents (772). A structmre analysis of the ytterbium member of this series has been completed (714). The crystal and molecular parameters of this and related complexes are compared in Table 5. 

NMR data of the per-O-acetyl derivatives. However, the proposed 2,3-cis-3,4-trans configuration of the 0-acetyl derivatives of 110 and 113 did not appear to be supported by coupling constants ( /2,3 = 2.5, 73,4= 10.5 Hz and 72,3 = 3.5, V3 4 = 9.5 Hz, respectively) and suggests that a reinterpretation of data is now required. A further feature that casts doubt on the validity of the structural claims is the apparent stability of leucopelargonidin (113) in contrast to the well-established reactivity of flavan-3,4-diols with 5,7-dihydroxy A-rings. °... 

An unique result of the electronic structure of the nitrosamine function is that it can apparently stabilize a positive or negative charge at the carbon adjacent to the amino nitrogen in a manner similar to the benzylic system. Hie stereoelectronic control of electroj ilic substitutions at the Or-position, to be discussed later, requires an orbital interaction of the nitrosamine function with the anionic electrons. Hie reactiv-... 

For complex formation between aldehydes and S(IV) to be important in the troposphere, the aldehydes not only must have high solubility but also be present in air at significant concentrations and form stable adducts with S(IV) at a sufficiently fast rate that it can occur during the lifetime of a typical cloud or fog event. Table 8.4 gives the rate constants /c,4 and kt5 for formation of the S(IV) complexes as well as the stability constants Ku and apparent stability constant K p, defined as... 

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