Dependence of Solubility on pH

An important example of the effect of pH on solubility is tooth decay. Tooth enamel contains the mineral hydroxyapatite, which is insoluble near neutral pH, but dissolves in acid because both phosphate and hydroxide in the hydroxyapatite react with H+  

Bacteria on the surface of our teeth metabolize sugars to produce lactic acid, which lowers the pH enough to slowly dissolve tooth enamel. Fluoride inhibits tooth decay because it forms fluorapatite, Cal0(PO4)6F2, which is more acid resistant than hydroxyapatite. 

To find the mass balance, we need to realize that all of the calcium and fluoride species come from CaF2. Therefore, the total fluoride equals two times the total calcium  

There are seven independent equations and seven unknowns, so we have enough information. 

For simplicity, we omit activity coefficients, but you do know how to use them. You would solve the problem with all activity coefficients equal to 1, find the ionic strength, and then compute activity coefficients with the Davies equation. Then you would compute effective equilibrium constants incorporating activity coefficients and solve the problem again. After each iteration, you would find a new ionic strength and a new set of activity coefficients. Repeat the process until ionic strength is constant. Wow You are smart  

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The effect of pH changes on precipitate solubilities merits special consideration. The dependence of solubility on pH may derive from the common ion effect where OH- or H30+ ions are generated by the dissolution of the precipitate, e.g. 

Region I may be further subdivided into a range where the solubility is essentially independent of the pH (pH exponential dependence of solubility on pH. 

Notes-. The empirical solubility model used was log (M ) = a + h pH + c log M.j. + d log OM, where a is an empirical constant (intercept) and the other three coefficients reflect the dependence of solubility on pH, total soil metal content and organic matter (or carbon), respectively. 

Fig. 3. Distribution of metal-2 -deoxymugineic acid (DMA) in solution as a function of pH. Metal binding by the soil solid phase is simultaneously considered using empirically derived solubilities. For Cd, Zn, Cu, and Ni the empirical solubility model used was log (M"+) = a + b pH + c log Mj + d log OM, where a is an empirical constant (intercept) and the other three coefficients reflect the dependence of solubility on pH, total soil metal content and organic matter (or carbon), respectively. Fe(OH)j had an assumed solubility of log K = —2.1 (Lindsay, 1979). The total soil metal concentration was 10 mg kg , organic C was 10 g kg and DMA was 25 iM.

Due to the greater solubility of silica and the greater size-dependence of solubility above pH 7 (Fig. 6), growth of the primary particles continues by Ostwald ripening. Particles grow rapidly to a size that depends mainly on... 

There appears to be an inverse relationship between the stability of the silica polymorphs and their solubility in water thus, opal is more soluble than quartz (BeleVtsev et al. [I960] Millot [I960]), and since many of the reactions of silica and silicates in soils are surface controlled, particle size must be considered (Jackson et al. [1949]). Because of the dependence of solubility on particle size, and hence the difficulty of determining the absolute solubility of the silica polymorphs, comparison cannot be made without qualification. Values of 120 to 140 ppm at 25°C have been quoted for the solubility of noncrystalline silica in water by Alexander et al. [1954], who also observed that the solubility rises sharply above pH 9, varies linearly with temperature, and is influenced by particle size and the number of internal OH groups. The solubility of cristobalite (prepared by heating 100 to 140 mesh quartz grains... 

Figure 3 shows both profile equations in action in a titration regime. The observed profile is a minimal combination of the neutral base and salt curves. Please note the dramatic decrease of solubility at pH< 2. It is a direct consequence of the common ion effect, the reciprocal dependence on the counterion concentration in Equation (9). 

Dependence of PZC on ZjR for 36 oxides, oxohydroxides and hydroxides (3 PZC values calculated assuming that PZC corresponds to the pH of minunum solubility, 6 self measured values, the other values taken from literature) is nearly linear [96], but the results (presented in from of graph) show substantial scatter. 

Many organizations use colon adenocarcinoma (Caco-2) for detailed study of permeability however, this method can be resource intensive. Parallel artificial-membrane permeability (PAMPA) [19] has proven to be a reliable predictor of passive transcellular permeability for intestinal absorption prediction. It is also useful to interpret results of cell-based discovery assays, in which cell-membrane permeability is limiting. Finally, pTf provides insight into the pH dependence of solubility and permeability. It can be measured [20] or calculated to get an understanding of the regions of the intestine in which the compound will be best absorbed, as well as to anticipate the effect of pH on solubility and pemieability. Permeability at the blood-brain barrier (BBB) also can be rapidly profiled [21]. 

Figure 4.10. Predicted pH-dependence of complexation on catechol-type groups of organic matter, assuming that Al(OH)3 precipitation-dissolution controls AP" solubility. Below pH 3, complexation based on the assumption of a large excess of AI(0H)3 (solid line) and the assumption of a limited quantity of Al(OH)j (broken line) is depicted. The 100 mmoles/kg level is the maximum complexing capacity of this model humic material.

The in silico models derived for solubility are based on intrinsic solubility as their experimental input data. The intrinsic solubility is the solubility value determined for the neutral (i.e., uncharged) species of the compound and is generally determined at 2 pH units above the pFCa value for bases and 2 pH units below the pKa value for acids. Ampholytes are determined at their isoelectric point. The solubility values used for the model development therefore seldom reflect the apparent solubility seen in the intestinal fluids. Hence, the predicted values obtained from the models need to be transferred to an in vivo situation, for instance, by use of the Henderson-Hasselbalch equation, which takes into account the pH dependency of solubility [16],... 

Let us first examine the points for H202. Both the H202 solubility and the effective first-order rate constant, fci,H2o2, depend only weakly on pH, resulting in tightly clustered points. The reason for the pH independence of i.h2o2 is, as we saw in Chapter 7, the nearly opposite pH dependences of fcH2o2 and //, [V that cancel each other in (12.97). 

Figure 5 Dependence of solubility of some metal hydroxides on pH. Solubility is expressed as pM, where M is the total concentration of all soluble metal species in equilibrium with the insoluble hydroxide.

Calcium oxalate and phosphate solubilities, on the other hand, strongly depend on the concentration of ions that form complexes with calcium, phosphate or oxalate, particularly citrate or magnesium ions [43-45]. Due to the protonation of these anions, the concentration of these complexes depends on pH, which contributes to the pH dependence of solubility. Whereas the solubility of calcium oxalate is only slightly pH dependant in the urinary pH range [45], the solubility of phosphates decreases with pH [43,44]. We have developed a urine model [43-45] based on the JESS Expert Speciation... 

It is often attempted to destroy enzymes in solution by heating. This procedure may result in serious misinterpretations, as experiments with trypsin and other of the pancreatic enzymes have shown. Trypsin is rapidly converted to a denatured protein at temperatures higher than 50°C., as shown by altered solubility in salt solutions and loss of enzymatic activity. On standing at low temperatures, however, both the solubility and activity become characteristic of the original trypsin. This type of reversible denaturation depends very much on pH. Similar reversible inactivations have been reported for other enzymes, including myokinase and hyaluronidase. 

Skornik NA, Serebrennikov VV (1965) Dependence of solubility of citrates of some rare earth elements on the pH of the medium. Zhum Neorg Khim 10 407 09... 

Sodium aluminate [1302-42-7] is another source of soluble aluminum made by leaching bauxite with caustic soda. As with alum, the active species are really its hydrolysis products which depend on the chemistry of the system to which it is added. It tends to raise the pH. It is available both as a soHd and as a solution (see Aluminum compounds, aluminates). 

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