Undissociated salt molecules

The chemistry relating to the use of dissolved salts as separating agents has not yet been fully understood. A principal reason for this is the complexity of effects that the salt can have, and how these can vary not only from system to system but, more significantly, within a given system as the concentrations of any or all of the system components are varied. For the apparently simple system defined earlier, consisting of two volatile components plus a dissolved salt, the salt could theoretically range from fully dissociated into two types of ions to totally associated. If its dissociation is anywhere between these two extremes as it probably is, it exists as three species two types of ions, plus undissociated salt molecules. All three species contribute to the salt effect on the ac-... 

The definition furthermore explains why the solid salt of phenolphthalein is red and that of p-nitrophenol is yellow. The salts have the constitution and, therefore, also the color of the ionogenic form, whether it be completely or only partially dissociated. This is in accord with the findings of Hantzsch (l.c.) and Hantzsch and Robertson (l.c.) mentioned early in this chapter, namely, that Beer s law holds for colored salts as long as the concentration is not too great. This is to be expected if the undissociated salt molecules are ionogenic and possess the same structure and color as do the ions. 

The situation becomes far more complicated for higher concentrations, say I > 0.03 molar, which still is fairly dilute, i.e., a few times 0.1% for many salts. The most important aspect may be that the association of ions into ion pairs, i.e., undissociated salt molecules (or ions, e.g., CaCl+), becomes significant. It is often assumed that salts completely dissociate into ions, unless the concentration is very high. This assumption is generally not true, and it would lead to considerable error in many foods. To be sure, most ion pairs are very short lived, but at any time a certain proportion of the ions is in the associated form. 

The undissociated NO molecules and the dissociation products can participate in secondary reactions in the mixed alkali-NO overlayers and result both in products which immediately desorb (e.g. N2) or further decompose (e.g. N20), and in alkali stabilized compound-like products (nitrite-like salts). As in the case of CO or C02 adsorption, the formation of such surface compounds is favoured at elevated temperatures and at alkali coverages higher than those corresponding to the work function minimum. 

Since E is known, and the e.m.f. of the cell (E) can be measured with various concentrations of acid, sodium salt and sodium chloride, i.e., for various values of mi, rrh and m3 in the cell depicted above, it is possible to evaluate the left-hand side of equation (14) or (15). In dilute solution, the sodium chloride may be assumed to be completely dissociated so that the molality of the chloride ion can be taken as equal to that of the sodium chloride, i.e., mcr is equal to m3. The acid HA will be partly in the undissociated form and partly dissociated into hydrogen and A ions the stoichiometric molality of HA is mi, and if nin is the molality of the hydrogen ions resulting from dissociation, the molality of undissociated HA molecules, i.e., maA in equation (15), is equal to mi — mn. Finally, it is required to knowm rriA- the A ions are produced by the dissociation of NaA, which may be assumed to be complete, and also by the small dissociation of the acid HA it follows, therefore, that mA is equal to m2 -f Since mu, the hydrogen ion concen-... 

This quantity in gram-molecules per litre is 0 0176 X 0 416 = 0 00732, and represents saturation with the undissociated salt, and remains the same on addition of the oxanilic acid, only the dissociated quantity being affected. Calling G the total concentration, we have in solution... 

The classical theory of catalysis supposed that the hydrogen and hydroxyl ions were the only effective catalysts in solutions of acids and bases. In a few instances early attempts were made to remedy some of the discrepancies encountered by attributing some catalytic power to undissociated acid molecules, but these attempts were mostly based on incorrect values for degrees of dissociation, and they did not take into account the possibility of primary or secondary salt effects. However, later work has shown definitely that species other than hydrogen and hydroxyl ions often can exert a catalytic effect, and the development of these ideas was closely linked with a closer understanding of the nature of the hydrogen ion in solution, and with the clarification of acid-base definitions (cf. Bell, 11). 

The exact mechanism of this addition is uncertain. It was postulated that the addition steps are through reactions of neutral lactam molecules with ammonium cations. Others felt, however, that the lactam molecules add to the undissociated salts. 

The extraordinarily low permeability can be explained by the fact that polyethylene as a non-polar medium can only be very weakly polarized and diffusion cannot lead to a separation of charge carrier. The ions are surrounded in the aqueous solution by a cloud of water molecules shielding the ion s charge. Cations and anions would therefore have to recombine from this hydrate shell to the molecule and become dissolved in the polyethylene or both become dissolved with their hydrate shell and diffuse. Such processes are thermodynamically rather unfavourable. The importance of dissociation of inorganic molecules for the migration becomes clear by permeation tests performed with concentrated hydrochloric acid. Undissociated HCl molecules are found to some extent in concentrated hydrochloric acid while the molecules are fully dissociated in aqueous NaCl or metallic salt solution. The available undissociated HCl molecules can become dissolved in the polyethylene and only then diffuse similarly to water molecules or undissociated acetic acid molecules. While no permeation of chlorine can be observed in permeation experiments with metal salts, diffused chlorine can be proven when using concentrated hydrochloric acid. 

Ionization of dicarboxylic acids has been shown to have a profound effect on the decarboxylation rate. The disodium salt of malonic acid (NaOOCCH2COONa) was found to be relatively stable with respect to decarboxylation (Fairclough 1938) up to temperatures of 125 °C, whereas the monosodium salt decomposed by a first-order reaction. Rates for the decarboxylation of both malonic and oxalic acids were slower in polar solvents in which a high degree of ionization was expected (Richardson and O Neal 1972). Similarly, the rate of decomposition of dibromomalonic acid was found to be proportional to the concentration of the undissociated acid molecule in solution (Muus 1935). 

When an acid is added to this buffer solution, the added ions from the acid react with the CHjCOO ions from the salt, forming more undissociated CH COOH acid molecules. The overall result is that the concentration of ions stays the same, so the pH remains unchanged. 

The studies of Pauli he. cit.) and his co-workers, however, have revealed the fact that isohydric solutions of different acids do not effect equal combination with the isoelectric protein relatively more acetic acid for example being combined than hydrochloric acid in isohydric solutions. Again, both the actual position of these maxima as well as the magnitudes of the viscosities observed vary much with the nature of the acid employed. Thus the relatively weak oxalic acid appears to be a much stronger acid than sulphuric acid, whilst trichloracetic acid does not differ appreciably from acetic acid in its effect on the viscosity of albumin. It is probable that the degree of solvation of the protein molecules and of the protein salts must not be regarded as constant but that they vary both with the nature of the salt and in the presence of neutral salts which exert like alcohol a desolvating action more or less complete on the solvated isoelectric protein as well as on the undissociated protein salts. 

In alcoholic solution, carbohydrates possibly complex with undissociated molecules (ion pairs) of salt, as well as with free cations. Data from optical rotation experiments1 suggest that, even in aqueous solution, undissociated molecules of salt may be associated with the carbohydrate (see Section II, 7, p. 230). On the other hand,/ree anions do not appear to complex with carbohydrates in solution (see Section II, 8, p. 234). 

Binary electrolytes, such as KC1, although completely ionized, even in the solid state, lower the freezing point less than 2 x 1.86D, even when as dilute as I0-3M. This was at first attributed to incomplete ioni/aiion but is now explained by the long range of electrostatic forces. Note that Mg++ and SO4 are less independent than K+ and Cl- AgNOa, unlike KC1, etc., is a weak salt, and undissociated molecules increase rapidly with concentration. The ions nearer to an ion of one sign arc those of opposite sign, therefore electric conductivity is less than the sum of ionic conductivities extrapolated to zero concentration. 

Experiments have shown that any ionized silver salt, e.g., Ag2S04, AgC2H302, AgC103, may be substituted for AgN03, and any ionized chloride may be substituted for the NaCl, and the same results will be obtained. The union of Ag+ and Cl ions to form insoluble AgCl is in no wise affected by the other ions with which these ions are at the outset in electrical balance. These other ions will simply remain in the solution unless they, too, are the ions of some insoluble salt, for example, BaS04 or an undissociated molecule like H20. 

The most unique feature of EMFs, as distinguished from nonmetallic endofullerenes (such as N C6o) and empty fullerenes, is the strong interaction between the encaged metallic species and the fullerene cage, as represented by the electron transfer from the inner metallic species to the outer fullerene cage intramolecular charge transfer. Consequently, the EMF molecules are a type of superatom, or a type of salt, but remain undissociated in any solvent (see Figure 7.6). 

Many properties of electrolytic solutions are additive functions of the properties of the respective ions this is at once evident from the fact that the chemical properties of a salt solution are those of its constituent ions. For example, potassium chloride in solution has no chemical reactions which are characteristic of the compound itself, but only those of potassium and chloride ions. These properties are possessed equally by almost all potassium salts and all chlorides, respectively. Similarly, the characteristic chemical properties of acids and alkalis, in aqueous solution, are those of hydrogen and hydroxyl ions, respectively. Certain physical properties of electrolytes are also additive in nature the most outstanding example is the electrical conductance at infinite dilution. It will be seen in Chap. II that conductance values can be ascribed to all ions, and the appropriate conductance of any electrolyte is equal to the sum of the values for the individual ions. The densities of electrolytic solutions have also been found to be additive functions of the properties of the constituent ions. The catalytic effects of various acids and bases, and of mixtures with their salts, can be accounted for by associating a definite catalytic coefl5.cient with each type of ion since undissociated molecules often have appreciable catalytic properties due allowance must be made for their contribution. 

Solubility Equilibria The Solubility Product Principle.—It was seen on page 133 that the chemical potential of a solid is constant at a definite temperature and pressure consequently, when a solution is saturated with a given salt Mv A, the chemical potential of the latter in the solution must also be constant, since the chemical potential of any substance present in two phases at equilibrium must be the same in each phase. It is immaterial whether this conclusion is applied to the undissociated molecules of the salt or to the ions, for the chemical potential is given by... 

The solution of the precipitate cannot be attributed to increase in hydroxide-ion concentration, because sodium hydroxide does not cause it, nor to ammonium ion, because ammonium salts do not cause it. There remains undissociated NH OH or NHg, which might combine with the cupric ion. It has in fact been found that the new deep blue ion species formed by addition of an excess of ammonium hydroxide is the cupric ammonia complex Cu(NH3)4+, similar to the hydrated cupric ion except thar the four water molecules have been replaced by ammonia molecules. This complex is sometimes called the cupric tetrammlne complex the word ammine meaning an attached ammonia molecule. 

In addition, undissociated molecules, which may be oxides arising from the dissociation of ternary salts (MgO, ZnO, etc.), but also radicals and molecular species... 

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