Solubility of weak acids

The solubilities of weak acids and bases are dependent upon the pKa value(s) of the compound, the pH of the solution and the concentration of any counter ions to the... 

The total solubility of weak acid in a water/cosolvent system [HA] ot can be determined by the solubility of the un-ionized species in water, the solubilizing powers, the hydrogen ion concentration, and the dissociation constant of the weak acid. Equation (3.49d) illustrates that the total solubility of the weak acid will increase exponentially with respect to the volume fraction of the cosolvent. Even though the solubilizing power of the cosolvent for the un-ionized species is usually larger than that of the ionized species, the solubilization of the ionized species is very important in determining the total solubility when pH - pKa > oIIA - oa. ... 

Stuart M. and Box, K. (2005) Chasing equilibrium measuring the intrinsic solubility of weak acids andbases. Analytical Chemistry, 77, 983-990. 

Solubilities of Weak Acids, Weak Bases, and Pharmaceuticals as a Function of pH 851... 

Understanding the solubility of weak acids and bases as a function of pH is important in the formulation of pharmaceuticals for general use. From Illustration 15.3-1 we see that if we had to deliver a drug with low solubility in water that had a pK value similar... 

The solubility of weak acids and bases is a function of the pH of the medium. Therefore, differences in the dissolution... 

The key consideration in the analysis of any sustained release dosage form as previously discussed (see Sections II.A, II.B, II.C.l, and II.C.2rg) is to determine what solvent or solvent system will be most appropriate to assure the dissolution of the drug and its excipients to make it amenable to HPLC analysis. Aqueous solubility of weak acids and bases is governed by the pfCa of the compound and the pFI of the medium. In an acidic or low pFI medium, weak acids will be unionized and will be more soluble in organic solvents. The reverse is the case for basic compounds as previously discussed in Section II.B. Because the formulation of sustained release dosage forms tend to rely on the use of insoluble plastics (i.e., methyl acrylate-methyl methacrylate, polyvinyl chloride, and polyethylene), hydrophilic polymers (i.e., methylcellulose, hydroxypropyl-methylcellulose, sodium carboxymethyl cellulose, and carbopol 934), and fatty compounds (i.e., waxes such carnauba wax and glyceryl tristearate), similar hydro-organic solvents and sample preparation steps that have been discussed for tablets and capsules can also used for their analysis by HPLC (see Sections II.A, II.B, II.C.l, and II.C.2). 

The most common method of purification of inorganic species is by recrystallisation, usually from water. However, especially with salts of weak acids or of cations other than the alkaline and alkaline earth metals, care must be taken to minimise the effect of hydrolysis. This can be achieved, for example, by recrystallising acetates in the presence of dilute acetic acid. Nevertheless, there are many inorganic chemicals that are too insoluble or are hydrolysed by water so that no general purification method can be given. It is convenient that many inorganic substances have large temperature coefficients for their solubility in water, but in other cases recrystallisation is still possible by partial solvent evaporation. 

Compounds of Tl have many similarities to those of the alkali metals TIOH is very soluble and is a strong base TI2CO3 is also soluble and resembles the corresponding Na and K compounds Tl forms colourless, well-crystallized salts of many oxoacids, and these tend to be anhydrous like those of the similarly sized Rb and Cs Tl salts of weak acids have a basic reaction in aqueous solution as a result of hydrolysis Tl forms polysulfldes (e.g. TI2S3) and polyiodides, etc. In other respects Tl resembles the more highly polarizing ion Ag+, e.g. in the colour and insolubility of its chromate, sulfide, arsenate and halides (except F), though it does not form ammine complexes in aqueous solution and its azide is not explosive. 

The general approach illustrated by Example 18.7 is widely used to determine equilibrium constants for solution reactions. The pH meter in particular can be used to determine acid or base equilibrium constants by measuring the pH of solutions containing known concentrations of weak acids or bases. Specific ion electrodes are readily adapted to the determination of solubility product constants. For example, a chloride ion electrode can be used to find [Cl-] in equilibrium with AgCl(s) and a known [Ag+]. From that information, Ksp of AgCl can be calculated. 

Solutions which prevent the hydrolysis of salts of weak acids and bases. If the precipitate is a salt of weak acid and is slightly soluble it may exhibit a tendency to hydrolyse, and the soluble product of hydrolysis will be a base the wash liquid must therefore be basic. Thus Mg(NH4)P04 may hydrolyse appreciably to give the hydrogenphosphate ion HPO and hydroxide ion, and should accordingly be washed with dilute aqueous ammonia. If salts of weak bases, such as hydrated iron(III), chromium(III), or aluminium ion, are to be separated from a precipitate, e.g. silica, by washing with water, the salts may be hydrolysed and their insoluble basic salts or hydroxides may be produced together with an acid ... 

Because the concentrations of ions in a solution of a sparingly soluble salt are low, we assume, just as we did for solutions of weak acids and bases (Section 10.7), that we can approximate Ksp by... 

Before leaving the subject of polarity and in relation to uptake and distribution, mention should be made of weak acids and bases. The complicating factor here is that they exist in solution in different forms, the balance between which is dependent on pH. The different forms have different polarities, and thus different values. In other words, the values measured are pH-dependent. Take, for example, the plant growth regulator herbicide 2,4-D. This is often formulated as the sodium or potassium salt, which has high water solubility. When dissolved in water, however, the following equilibrium is established ... 

An aqueous solution of a soluble salt contains cations and anions. These ions often have acid-base properties. Anions that are conjugate bases of weak acids make a solution basic. For example, sodium fluoride dissolves in water to give Na, F, and H2 O as major species. The fluoride anion is the conjugate base of the weak acid HF. This anion establishes a proton transfer equilibrium with water ... 

The preparation of salts of organic compounds is one of the most important tools available to the for-mulator. Compounds for both IM and IV solutions may require high solubility in order for the drug to be incorporated into acceptable volumes for bolus administration (see Table 1). Sodium and potassium salts of weak acids and hydrochloride and sulfate salts of weak bases are widely used in parenterals requiring highly soluble compounds, based on their overall safety and history of clinical acceptance. 

Streng, W. H. Tan, H. G. H., General treatment of pH solubility profiles of weak acids and bases, n. Evaluation of thermodynamic parameters from the temperature dependence of solubility profiles apphed to a zwitterionic compound, Int. J. Pharm. 25, 135-145 (1985). 

Because the glomerular filtrate contains many important body constituents (e.g., glucose), there are specific active uptake processes for them. Also, lipid-soluble chemicals diffuse back from the tubule into the blood, especially as the urine becomes more concentrated because of water reabsorption. The pH of the urine is generally lower than that of the plasma, and therefore pH partitioning tends to increase the reabsorption of weak acids. The pH of the urine can be altered... 

The unequal basicities of the three nitranilines can be illustrated by the following experiment. It is a general property of the salts of weak bases—as well as of weak acids—that in aqueous solution they are stable only if an excess of acid (or alkali) is present. When such solutions are diluted with water hydrolysis occurs as a result of the operation of the law of mass action. In the present case this phenomenon shows itself in the appearance of the yellow colour characteristic of the bases and finally, since the nitranilines are sparingly soluble in water, in their precipitation in crystalline form. The weaker the base the smaller is the amount of water which must be added in order to make the hydrolysis perceptible. 

Reverse-phase chromatography is used mainly for the separation of nonionic substances because ionic, and hence strongly polar, compounds show very little affinity for the non-polar stationary phase. However, ionization of weak acids (or weak bases) may be suppressed in solvents with low (or high) pH values. The effect of such a reduction in the ionization is to make the compound more soluble in the non-polar stationary phase but the pH of the solvent must not exceed the permitted range for bonded phases, i.e. pH 2-8. 

Soluble salts of weak acids and strong bases... 

Lutetium reacts slowly with water and is soluble in weak acids. Its crystals exhibit strong magnetic properties, which are important to the study of magnetism. 

The answer is D. Weak acids like lactic acid never completely dissociate in solution and are thus defined by the property that at least some of the protonated (undissociated acid) form and the unprotonated (conjugate base) form of the acid are present at all concentrations and pH conditions. The indicated of 5.2 is consistent with the idea that the lactate anion retains a strong affinity for protons, a hallmark of a weak acid. The lactate anion is highly water-soluble. All weak acids obey the Henderson-Hasselbalch equation. 

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