Sparingly soluble substances

In the case of a sparingly soluble substance, if each of the quantities in (64) is divided by Avogadro s constant, we confirm the statement made above— namely, that, if AS at per ion pair is added to the contribution made to the entropy of the crystal by each ion pair, in this way we evaluate the contribution made by one additional ion pair to the entropy of the saturated solution and it is important to grasp that this contribution depends only on the presence of the additional pair of ions in the solution and does not depend on where they have come from. They might have been introduced into the solution from a vacuum, instead of from the surface of a solid. In (64) the quantities on the right-hand side refer to the solution of a crystal, but the quantity (S2 — Si) does not it denotes merely a change in the entropy of a solution due to the presence of additional ions, which may have come from anywhere. When Si denotes the entropy of a sufficiently large amount of solution, (S2 — Si) is the partial molal entropy of the solute in this solution. 

Before we can decide whether there is a large or a small difference between the unitary terms of the Cl" ions and the 1 ion, the difference in the cratic term must be calculated. The question is whether the difference of 41.0 e.u. between 116.8 and 75.8 merely represents the difference between the cratic terms, or whether it includes as well a difference between the uuitary terms of the I" ion and the Cl" ion. For ions provided by any two such sparingly soluble substances this question can be settled at once by a simple calculation. 

It is usual in writing equations involving equilibria between completely dissociated and slightly dissociated or sparingly soluble substances to employ the ions of the former and the molecules of the latter. The reaction is therefore written ... 

Appreciable errors may also be introduced by post-precipitation. This is the precipitation which occurs on the surface of the first precipitate after its formation. It occurs with sparingly soluble substances which form supersaturated solutions they usually have an ion in common with the primary precipitate. Thus in the precipitation of calcium as oxalate in the presence of magnesium, magnesium oxalate separates out gradually upon the calcium oxalate the longer the precipitate is allowed to stand in contact with the solution, the greater is the error due to this cause. A similar effect is observed in the precipitation of copper or mercury(II) sulphide in 0.3M hydrochloric acid in the presence of zinc ions zinc sulphide is slowly post-precipitated. 

If the solution is dilute (which restricts the theory to sparingly soluble substances) we have ... 

Determine Ksp for each of the following sparingly soluble substances, given their molar solubilities (a) AgBr, 8.8 X... 

Use the data in Table 11.4 to calculate the molar solubility of each sparingly soluble substance in its respective solution ... 

Levy, R. H. Rowland, M., Dissociation constants of sparingly soluble substances nonlogarithmic linear titration curves, J. Pharm. Sci. 60, 1155-1159 (1971). 

For quite sparingly soluble substances, formic acid, pyridine, bromobenzene, nitrobenzene, and occasionally also phenol, ethyl benzoate, aniline, and dioxan are used. A distinct relation exists between the constitution of solute and solvent, and is expressed by the old rule similia similibus solvuntur. Thus, as is well known, substances containing hydroxyl (e.g. sugars, carboxylic acids) are soluble in water, whereas hydrocarbons are more soluble in benzene and petrol ether than, for example, in alcohols. 

The extraction apparatus is used chiefly for dissolving out sparingly soluble constituents from mixtures, and for isolating natural products from (dry) vegetable or animal material. Occasionally it is very useful for recrystallising sparingly soluble substances from the extraction thimble by means of an appropriate solvent (especially ether). As a rule, crystals of the dissolved material separate already from the solution in the flask during the process of extraction. This solution soon becomes supersaturated even while hot. 

Mitchell, S. A method for determining the solubility of sparingly soluble substances, / Chem. Soc. (London), 1333-1337,1926. Mittal, P.K., Adak, T., and Sharma, V.P. Comparative toxicity of certain mosquitocidal compounds to larvivorous fish, PoeciZ/a... 

When preparing the cesium- and barium-saturated clays, the 1.0 M solutions used were decanted (after centrifuging) and analyzed semiquantitatively by emission spectroscopy. From those analyses, it appears that the following species were desorbed sodium, potassium, calcium, magnesium, and strontium. It further appeared that desorption of potassium was almost unique to cesium sorption whereas, desorption of the other species appeared to be common to both cesium and barium sorption. Small amounts of other elements such as nickel and copper were also detected by the analyses. However, to what extent the observed concentrations may represent desorption and to what extent they may represent the dissolution of sparingly soluble substances (particularly hydroxide species) is as yet-uncertain. The apparent concentrations of the desorbed species per gram of clay are given in Table III. 

Use the data in Table 11.5 to calculate the molar solubility of each sparingly soluble substance in its respective solution (a) silver chloride in 0.20 M NaCl(aq) (b) mercury chloride in 0.10 M NaCl(aq) ... 

Substances which are readily soluble in water, such as lower molecular weight alcohols, will dissolve freely in water if accidentally spilled and will tend to remain in aqueous solution until degraded. On the contrary, sparingly soluble substances dissolve more slowly and, when in solution, have a stronger tendency to partition out of aqueous solution into other phases. They tend to have larger air-water partition coefficients or Henry s law... 

In this case, Ps is also P,s. Thus the product xy is 1.0 and x is 1/y. Sparingly soluble substances are such because the value of y is large. 

Triterpenoids occur in hardwood parenchyma resin, and closely related sterols are also present in softwoods (Fig. 5-6). Sterols typefied by the abundant /3-sitosterol, mostly have a hydroxyl group in the C-3 position. They also appear as the alcohol component in fatty acid esters (waxes). Triterpenoids and sterols are sparingly soluble substances contributing to pitch problems in pulping and paper making. Some trees contain polyterpenes and their derivatives known as polyprenols. Betulaprenols, present in birch wood, belong to this category of substances (Fig. 5-7). 

Some precipitates are deposited slowly and the solution is in the state of supersaturation for a considerable time. Thus, when calcium oxalate is precipitated in the presence of larger amounts of magnesium ions, the precipitate is practically pure at first, but if it is allowed to remain in contact with the solution, magnesium oxalate forms slowly (and the presence of calcium oxalate precipitate tends to accelerate this process). Thus, the calcium oxalate precipitate becomes contaminated owing to post-precipitation of magnesium oxalate. Post-precipitation often occurs with sparingly soluble substances which tend to form supersaturated solutions, they usually have an ion in common with the primary precipitate. Another typical example is the precipitation of copper or mercury(II) sulphide in dilute acid solution, which become contaminated, if zinc ions are present, by post-precipitation of zinc sulphide. Zinc ions alone may not be precipitated with sulphide ions under identical circumstances. 

By the differential heat of solution is meant the heat which is evolved when 1 mol of the solute is dissolved in a very large quantity of the saturated solution. For very sparingly soluble substances whose saturated solutions may be regarded as infinitely dilute, the three kinds of heat of solution become identical. 

The solubility therefore increases with the temperature when Li< 0, i.e. when heat is absorbed in the process of solution. This relationship was discovered nearly simultaneously by Le Chateher and van t Hoff. It is important to remember that the equation is only vahd when Raoult s law holds, i.e. for solutions of sparingly soluble substances which do not conduct the electric current If the solute splits up into ions in solution, equation (IT) must be modified accordingly. This law has been confirmed experimentally by various investigators, but almost exclusively for solutions of electrolytes. 

As a counterpeud to the previous section consider now solutions of sparingly soluble substances, consisting of uncharged, non-polymeric molecules. As long as the solutions remain dilute (x. x ) the Gibbs surface excess r " or r " now... 

The experimental data regarding the solubility of sparingly soluble substances can be verified with the suggested test if accurate data for the activity coefficients of the constituents of the solute-free mixed solvent are available. 

The test was applied to a number of systems representing the solubilities of sparingly soluble substances in mixed solvents. First, the test was scrutinized for four nonaqueous systems for which accurate solubility data were available. Second, the suggested test was applied to a number of systems representing experimental data regarding the solubility of sparingly soluble substances in aqueous mixed solvents. 

The solubility-product expression permits the ready calculation of the solubility of a sparingly soluble substance that ionizes completely in water. 

The isotopically pure solvents H2O and D2O have very similar physical properties. The chief differences occur in those properties which are most sensitively dependent on small changes of liquid structure, for example the temperature of maximum density (which increases from 3-98 to 11-2°C on going from H2O to D2O) or the melting point (which is 3-8°C for D2O) (Kirshenbaum, 1951). The solubility differences of various sparingly soluble substances in H2O and DgO, which commonly go up to as much as 20% for both electrolyte and non-electrolyte solutes (see, e.g.. Shearman and Menzies, 1937 Chang and Chu 1939 Eddy and Menzies, 1940 Ben-Naim, 1965 Krescheck et al. 1965, Broadbank et al., 1968), have similarly been discussed in terms of structural changes (e.g. Swain and Bader, 1960 Nemethy and Scheraga, 1962). 

One of the most important properties of aqueous micellar solutions is their ability to enhance the solubihty of otherwise sparingly soluble substances. This seems to have been acknowledged as far back as 1846 when, according to McBain and Hutchinson, Persoz observed increased solubihty in soap solutions. 

Corswant, C.V., Thorean, P. and Engstrom, S. (1998) Triglyceride-based microemulsion for intravenous administration of sparingly soluble substances. /. Pharm. Sci, 87, 200-208. 

For sparingly soluble substances, the volume in standard vessel methods may not be sufficient to dissolve the dose. In this case, the USP IV flow-through method is beneficial, since it provides a continuous renewal of the dissolution fluid. However, the maximum flow rate will limit the apparent solubility in this procedure. Sufficient solubility will not be obtained for a rapidly releasing formulation of a drug with very low solubility in relation to the dose. 

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