True solubility

Solvent metal Temperature (°C) For true solubility, deduct from axis readirtg ... 

It is well known that many compounds are able to change their physical form whilst suspended in solution. For example, a compound of interest may change from one polymorphic form to another, while different crystalline aggregations of the same compound can have different solubility profiles. Impurities can mask the true solubility, and aggregation in solution can also change the thermodynamic equilibrium. Finally, errors which have been published in the literature data may in fact magnify from publication to publication. 

He cautions that the correlation may underestimate the true solubility, since due to ozone decomposition (depending on ionic strength 31 as well as the type of ions (see below) and... 

When fine powders of vitreous silica, quartz, tridymite, cristobalite, coesite, and stishovite of known particle-size distribution and specific surface area are investigated for their solubility in aqueous suspensions, final concentrations at and below the level of the saturated concentration of molybdate-active silicic acid are established. Experimental evidence indicates that all final concentrations are influenced by surface adsorption of silicic acid. Thus, the true solubility, in the sense of a saturated concentration of silicic acid in dynamic equilibrium with the suspended silica modification, is obscured. Regarding this solubility, the experimental final concentration represents a more or less supersaturated state. Through adsorption, the normally slow dissolution rates of silica decrease further with increasing silicic acid concentrations. Great differences exist between the dissolution rates of the individual samples. 

Hancock, B. C. and M. Parks. 2000. What is the true solubility advantage for amorphous pharmaceuticalsPharm ReSl 7 397-404. 

In environmental situations, dissolved organic matter such as fulvic acids frequently increase the apparent solubility. This is the result of sorption of the chemical to organic matter which is sufficiently low in molecular mass to be retained permanently in solution. The "true" solubility or concentration in the pure aqueous phase probably is not increased. The apparent solubility is the sum of the "true" or dissolved concentration and the quantity which is sorbed. 

The particle-size counting procedure overestimated the true solubility since there is a limit below which particle size cannot be counted. The mean solubility estimate was 1.05 mg/liter by this method in 0.9% NaCl (0.154 M), in good agreement with the spectrophoto-metric method. 

Phase Composition and Simultaneous Polymerization. Theoretically the phase composition of the SIN s should not be determined by the true solubility of one polymer in the other. Even though the true solubility of one polymer in the other is low because the components of the SIN s are incompatible, simultaneous polymerization and gelation are expected to cause entrapment of one component in the other. The degree of entrapment presumably will be controlled by the relative rates of the two reactions and their degree of simultaneity. The phase composition is reflected in the glass transition behavior of the material. Thus a close look at the dynamic mechanical spectra of the SIN s is necessary to determine the effect of simultaneous polymerization on phase composition. 

The steam distillation residue contains heavy oils and asphaltenes. These were separated by solubility differentiation. Both of these materials were sufficiently soluble in CCl, to permit examination by both IR and NMR. Principal featuris of the IR and NMR spectra are shown in Table VI. The possibility of colloidal dispersion of the asphaltenes instead of true solubility may have caused some loss of fine structure for the aromatic absorption regions. 

A9.3.5.7.1 These substances, usually taken to be those with a solubility in water of <1 mg/1, are frequently difficult to dissolve in the test media, and the dissolved concentrations will often prove difficult to measure at the low concentrations anticipated. For many substances, the true solubility in the test media will be unknown, and will often be recorded as < detection limit in purified water. Nevertheless such substances can show toxicity, and where no toxicity is found, judgement must be applied to whether the result can be considered valid for classification. Judgement should err on the side of caution and should not underestimate the hazard. 

A9.3.5.10.2 Polymers represent a special kind of complex substance, requiring consideration of the polymer type and their dissolution/dispersal behaviour. Polymers may dissolve as such without change, (true solubility related to particle size), be dispersible, or portions consisting of low molecular weight fractions may go into solution. In the latter case, in effect, the testing of a polymer is a test of the ability of low molecular mass material to leach from the bulk polymer, and whether this leachate is toxic. It can thus be considered in the same way as a complex mixture in that a loading of polymer can best characterize the resultant leachate, and hence the toxicity can be related to this loading. 

Hancock BC and Parks M. What is the True Solubility Advantage for Amorphous Pharmaceuticals. Pharm Res 2000 17 397-404. 

Centrifugation or ultracentrifugation may be preferable for certain samples that are difficult to filter. Solubility samples in co-solvent systems with high viscosity are such examples. If the solute is less dense than the solubility medium, it will float on the surface, making it difficult to sample the solution. This may be particularly problematic for compounds with low solubility where a single particle carried over to the solution may cause significant overestimation of the true solubility. 

This will be smallest for true solubility, larger for swelling to, say, 25%, and still larger for swelling... 

Some remarks should be made about these tests. First, the meaning of the word solubility is fundamentally different from the definition used by physical chemists, given at the beginning of this section. Suppose that a solubility of 50% is observed. If this were a true solubility, doubling of the amount of solvent would lead to 100%. For the protein preparation, doubling the amount of solvent may well leave the result at 50% in other words, half of the material would be well soluble, and the other half not at all. In most cases, however, the situation will be somewhere in between. This... 

The process used for determining a distribution of aqueous solubility values involves assuming a "true" solubility value equal to 1.00 arbitrary units, assigning an error distribution for each recognized error source, and calculating the effect on true value of the several sources of error. After assigning... 

The column method also has several steps which may result in a negative bias on measured aqueous solubility. The lack of water-solute equilibrium in the generator column itself may produce an outlet concentration lower than the "true solubility according to Stolzenburg and Andren (8). This result also has been modeled with a beta distribution (93-percent confidence level of -3 percent). 

The saturated aqueous solution of surfactant at 25°C is in equilibrium with a liquid crystalline phase which contains about 25 wt% water. This phase dispersed in solution is the same as the phase formed by water vapor sorption into initially dry surfactant, according to nmr spectroscopy (which virtually eliminates the possibility, mentioned in the Introduction, of a complicating triple point in the two-component system). This hydrated liquid crystal is probably lamellar, to judge by the similarities in texture with lamellar liquid crystals of phospholipids and water (36). It is not uncommon for surfactants for form liquid crystalline phases by absorption of water, or hydrocarbon, or both (37). Moreover the true solubility of many other surfactants (particularly alkyl aryl sulfonates) in water, in salt water, and in hydrocarbon is small, sometimes as small as 0.003 wt% in water, below the Krafft point (38,39). Hence the present finding of liquid crystalline phase in equilibrium with isotropic aqueous solution at surfactant levels above 0.1 wt% may be representative of broad classes of surfactants, including some of interest in connection with... 

This distinction from the true solubility product, ptme, becomes more essential with the increase in the complexation ability of the halide anion and its equilibrium molality in the melt. From equation (3.7.40), it is seen that there exist two possible reasons for the change in the solubility product of metal-oxides in the studied melts ... 

Discrepancies between the data sets were reconciled as discussed in ( ). The increasing scatter in for gibbsite with falling temperature is attributed primarily to contamination of Bayer process gibbsite with bayerite or other surface reactive precipitates, and is discussed further below. Those measurements which are believed to approach most closely the true solubility of gibbsite are reported by by Kittrick (jj ) and Russell et al. 

Not all of the surfactants are capable of forming micelles. The appropriate ratio between the size of hydrophobic (hydrocarbon chains) and hydrophilic (polar group) parts of surfactant molecules, which determines their hydrophile-lipophile balance (HLB, see Chapter VIII, 3), is necessary for the formation of micelles to take place. Sodium and ammonium salts of C12 - C20 fatty acids, alkylsulfates, alkylbenzenesulfonates, and other synthetic ionic and nonionic surfactants are the examples of micelle-forming surface active substances. The true solubility, i.e. the concentration of dissolved substance in its molecular or ionic form, of such surfactants is rather low for ionic surfactants it is on the order of hundredths and thousandths of kmol m 3, while for nonionic ones it can be even lower by one or two orders of magnitude. 

The amount of substance present in the micellar state, cmjc = mnmic / NA may exceed the concentration of it in the molecular solution by several orders of magnitude. The micelles thus play a role of a reservoir (a depot) which allows one to keep the surfactant concentration (and chemical potential) in solution constant, in cases when surfactant is consumed, e.g. in the processes of sol, emulsion and suspension stabilization in detergent formulations, etc. (see Chapter VIII). A combination of high surface activity with the possibility for one to prepare micellar surfactant solutions with high substance content (despite the low true solubility of surfactants) allows for a the broad use of micelle-forming surfactants in various applications. 

The solvent polarity (non-polarity), which determines the interactions of its molecules with polar and non-polar regions of surfactant molecules, plays an important role in the formation of micelles in non-aqueous medium. For micelle formation to take place, the medium has to be a good solvent for hydrocarbon chains only. Micelles do not form in the medium of nature similar to both parts of diphilic surfactant molecules the surfactants reveal only true solubility in such medium. Low alcohols (less than C5) which are good solvents for both polar and non-polar regions of surfactant molecules are the typical examples of such media. 

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