Solvent basicity measurement methods

A third method of estimating solvent basicity is provided by the donor number concept 14 ). The donor number of a solvent is the enthalpy of reaction, measured in kcal per mole, between the solvent and a Lewis add such as antimony (V) chloride. (Other Lewis acids, such as iodine or trimethyltin chloride, may be used, but the scale most often reported is that for SbCl5.) Available values for the SbCls donor number have been included in Table 1. Plots of the Walden product versus solvent basicity (A//SbC1 ) for several solvents are shown for lithium, sodium, and potassium ions in Fig. 10 and for the tetraalkylammon-... 

The ability of a solvent to donate a pair of electrons of one of its donor atoms towards the formation of a coordinate bond with an acceptor atom of a solute is a measure of its Lewis basicity. Several methods have been proposed over the years to express this donor ability or donicity, but only few of them have proved to be viable and of any real usefulness. 

Equation (4-11) illustrates the role of solvent basicity in determining the strength of a solute acid. Equation (4-11) is, however, of little direct use in analytical acid-base measurements because we have no methods of evaluating the absolute constants acidity basicity suppose we wish to Compare the ionization constants of two acids HA and HA" in a solvent SH. From (4-11)... 

Several adaptations of the basic fluorometric method are now available for specific application to plasma (48), and to food extracts (49,50). A spectrophotofluorometric assay procedure also has been devised using DEA-Sephadex column to separate erythorbic (isoascorbic acid) and ascorbic acid (51). This is of particular importance since erythorbic acid, a very common food additive, has been suggested to be an antagonist of ascorbic acid (52,53,54). In general, the spectrofluorometric assay procedures involve measurements of fluorescence on solvent extracts of the acidified samples at 365-348 nm and 435-450 nm, as the wavelengths of maximum excitation and emission, respectively (48,50,51). 

A popular method of estimating solvent basicity is based on the donor number scale, DN, introduced by Gutmann [44, 45]. The donor number is defined as the value of the heat of reaction of the polar solvent with the strong Lewis acid SbCls when these reactants are dissolved in 1,2-dichloromethane (fig. 4.14). The scale suffers from the problem that DN cannot be measured directly for protic solvents because of the instability of SbCls in these systems. However, values of DN for these solvents have been estimated by a variety of other techniques [46]. 

Another method of estimating solvent basicity makes use of the solvatochro-mism of a Cu + complex, namely Cu(II) A,A,A, A -tetramethylethylenediamine acetoacetonate [47]. This parameter which is designated is especially convenient because it may be measured directly for all the solvents considered here, both protic and aprotic. The relationship between and DN is... 

On page 287, in paragraph 1 under Method for Measuring Solvent Basicity , on line 2, we used should read we had to use . 

Values of AG° may also be obtained by partition measurements of the solute between pairs of immiscible solvent, but this method of course is restricted in terms of solvents. In all investigations of AG° values, it is necessary to specify the concentration units employed, and to ensure that in the basic equations and the concentration units of the rate constant (for second-order constants) are the same as those used in the calculation of the AG° values. In the present paper, the concentration units in solution will be mol fractions, and all second-order rate constants will be corrected to units of (mol fraction) s before calculation of the 5AG values(34). 

Basically the methods of measuring this concentration are the same as those for the determination of the solubility of any compound in a given solvent with the additional difficulties due to the presence of a colloidal solution. As these solubilization properties are temperature sensitive, it is important that all these methods should be subject to adequate temperature control a factor often missing in much experimental work. 

A few scales of Lewis affinity and some spectroscopic scales of Lewis basicity (see below) have been constructed by carrying out the reaction A - - B AB on a dilute solution of the acid in pure, liquid base as solvent. This pure base method will be studied in Chapter 4. It gives solvent basicity scales which are not strictly equivalent to solute basicity scales measured on a dilute solution of the acid and the base in an inert solvent. 

In many applications in mass spectrometry (MS), the sample to be analyzed is present as a solution in a solvent, such as methanol or acetonitrile, or an aqueous one, as with body fluids. The solution may be an effluent from a liquid chromatography (LC) column. In any case, a solution flows into the front end of a mass spectrometer, but before it can provide a mass spectrum, the bulk of the solvent must be removed without losing the sample (solute). If the solvent is not removed, then its vaporization as it enters the ion source would produce a large increase in pressure and stop the spectrometer from working. At the same time that the solvent is removed, the dissolved sample must be retained so that its mass spectrum can be measured. There are several means of effecting this differentiation between carrier solvent and the solute of interest, and thermospray is just one of them. Plasmaspray is a variant of thermospray in which the basic method of solvent removal is the same, but the number of ions obtained is enhanced (see below). 

A method for determining the basicities of phosphoryl compounds has been decribed which is based on 31P n.m.r. chemical shift measurement and a two phase system consisting of an organic solvent and 12H sulphuric acid.245 The gas-phase protonation of aliphatic phosphine oxides have been determined by cyclotron resonance. There was a good correlation of pK4 with Kabachnik constants and HO... 

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