Solubility curves product

The shape of the equilibrium line, or solubility curve, is important in determining the mode of crystallization to be employed in order to crystallize a particular substance. If the curve is steep, i.e. the substance exhibits a strong temperature dependence of solubility (e.g. many salts and organic substances), then a cooling crystallization might be suitable. But if the metastable zone is wide (e.g. sucrose solutions), addition of seed crystal might be necessary. This can be desirable, particularly if a uniformly sized product is required. If on the other hand, the equilibrium line is relatively flat (e.g. for aqueous common salt... 

Figure 5. shows the solubility curves for a monotropic system of two polymorphs and will be used to discuss methods for controlling the polymorphic form of the product. In this instance the thermodynamically stable and thus least soluble polymorph is Form I. 

Whenever the solubility curve is crossed for the less stable Form II there is a risk that it will nucleate and contaminate the product. This situation is very probable when the solubility curves of the two polymorphs lie close together, as shown in Figure 21 of the Cimetidine case study. The addition of seed crystals of Form I, close to its solubility curve, and minimization of the supersaturation during the growth process is a good method of control in this instance. Solvent selection to extend the width of the Form II metastable zone would also be desired, as discussed in section 2.4.4. 

Where competing polymorphs may occur it is better to have systems where there is a large difference in the relative solubility of the two forms at the point of nucleation. This enables seeding of the crystallizer with the desired form at a temperature between the two solubility curves. A typical seed loading is 1 to 2 % by weight of the product. 

Hagenmaier (10) demonstrated that pH had little effect on water absorption of oilseed protein products, but solubility was pH dependent. He suggested that the differing degree of dependence on pH indicates that water absorption and protein solubility are not correlated. Contrastingly, Wolf and Cowan (28) reported the pH-water retention curve of soy proteins to follow the pH-solubility curve. Both solubility and water retention were minimal at the isoelectric point (4.5) and increased as the pH diverged from this point. Hutton and Campbell (20) reported that the effects of pH and temperature on water absorption of soy products paralleled those of solubility for the most part. 

Case III, An Excess of One Component Is Necessary.— It sometimes happens that an excess of one component is requisite for the formation of a double salt. Conversely, if the solid double salt is dissolved and the solution is evaporated, one of the components separates until the required excess of the other component has accumulated in the solution. This case is well illustrated by the mineral carnallite, KCl-MgCl2-6H20, one of the products of the Stassfurt mines. Let the point A (Fig. 20), represent a saturated solution of potassium chloride and the points, a saturated solution of magnesium chloride. An unsaturated solution of equivalent quantities of the two salts is then represented on the line OE, say at a. If the solution is evaporated at constant temperature (20°), the point a approaches the solubility curve of potassium chloride, viz., the line AC. At E, potassium chloride begins to separate and continues to do so until the representative point has moved to C, at which point carnallite makes its appearance. Since the separation of carnallite withdraws the two salts in equimolecular ratio from the solution, and since the solution now contains much more magnesium chloride than potassium chloride, the deposits of crystals of carnallite leave the solution unsaturated with respect to potassium chloride, and the latter salt steadily passes into solution again, while the deposit of carnallite increases. If, when the point C is reached, the crystals of potassium chloride are... 

The curve shown in Fig. 3 cannot proceed indefinitely in either direction. In the cathodic direction, the deposition of copper ions proceeds from solution until the rate at which the ions are supplied to the electrode becomes limited by mass-transfer processes. In the anodic direction, copper atoms are oxidized to form soluble copper ions. While the supply of copper atoms from the surface is essentially unlimited, the solubility of product salts is finite. Local mass-transport conditions control the supply rate so a current is reached at which the solution supersaturates, and an insulating salt-film barrier is created. At that point the current drops to a low level further increase in the potential does not significantly increase the current density. A plot of the current density as a function of the potential is shown in Fig. 5 for the zinc electrode in alkaline electrolyte. The sharp drop in potential is clearly observed at -0.9 V versus the standard hydrogen electrode (SHE). At more positive potentials the current density remains at a low level, and the electrode is said to be passivated. 

Passivation potential — Figure 2. Evaluation of XPS data on the chemical structure of the passive layer on Fe formed for 300 s in 1M NaOH as a function of potential with a two-layer model Fe(II)/Fe(III). Insert shows the polarization curve with oxidation of Fe(II) to Fe(III) at the Flade potential EP2, indication of soluble corrosion products Fe2+ and Fe3+, and passivation potential EPi in alkaline solution [i, iii]... 

Even though it may seem like a good idea to use any salt to melt ice or change the boiling point of water, not every salt can be dissolved completely in water. One of the salts that will not dissociate 100% into its ions is AgCl. Just how much AgCl can dissolve in water will be examined later when we examine solubility products in Chapter 8. You should also know that the temperature and amount of solvent used to dissolve a salt also alter how much of the salt can be dissolved. Because different amounts of solvent can be used, a standard of 100 grams of water has been set as the norm on solubility curves. The... 

Solubility. The results from the solubility experiments are given in Figures 4 and 5. The broken curve is the solubility curve for the original soy protein isolate (Purina 500 E) the nitrogen solubility of this product at neutral pH is below 40%, which indicates that the isolate has been partly denatured during its processing. The sample denoted as DH = 1.0% is the control and it is clearly demonstrated that the acid treatment has caused further aggregation and denaturation. The definitely positive DH-value... 

Degrees of supersaturation relative to specific hydrates may also be calculated. The solubility curve of CAH,o is more markedly dependent on temperature than are those of CjAHg or CjAHg. At 5°C, a solution obtained from CA rapidly becomes highly supersaturated in CAHjo, but much less so in C2AHg or AHj, but with increase in temperature this situation changes thus CAHiq is the major product at 5°C, but is undetectable above 30°C (C48). At 50°C, supersaturation is high only in CjAH, . which is rapidly formed. 

The addition of an antisoivent can be earned out in different ways, as indicated in Fig. 9-1, where the concentration of product is shown on the ordinate and the amount of antisoivent added is shown on the abscissa. A typical equilibrium solubility curve is indicated as A-B-C. (This curve could be concave or linear but is shown as convex for clarity.) The metastable region is indicated as the area between B-C and E-D. From point A to point B, addition of antisoivent will proceed without crystallization because the solution concentration is below the equilibrium solubility. At point B, equilibrium solubility is reached. As the addition of antisoivent continues, supersaturation will develop. The amount of supersaturation that can be developed without nucleation is system specific and will depend on the addition rate, mixing, primaiy and/or secondary nucleation rate, and growth rate, as well as the amount and type of impurities present in solution. 

The structure of halloysite is equivalent to that of kaolinite, but has a layer of water molecules between each pair of silica and alumina layers (see Chap. 9 Deer et al. 1992). Writing halloysite dissolution in the same form as in Eq. (7.35) for kaolinite, its solubility product is = 10 - (Hem et al. 1973 Stecfel and van Cappellen 1990), versus = 10" for kaolinite. In other words, halloysite is about 80 times more soluble than kaolinite. If plotted in Fig. 7.9, also assuming Si02(aq) = 17 ppm, the solubility curve for halloysite is parallel to but 1.9 log units above the curve for kaolinite at any pH. 

Limited by and hydrate formation. Usually design with AT = 2 to 3°C from the solubility curve. Washing crystals is critical too little, and product is contaminated too much, and crystals redissolve... 

Solubility and Transition Point.— The transition point, we have seen, is the point of intersection of four solubility curves (absence of vapour and constant pressure being assumed), and it has been shown by van t Hoff that at this point the product of the solubilities of the salts of the two salt-pairs is equal. At any other temperature, the salt-pair with the lower solubility product will be the stable salt-pair-Thus, from the solubility values of the single salts at a given tempera, ture, it is possible to state which of the salt-pairs is stable at that temperature. 

FIGURE 7.8-5 Liquid-liquid extraction and temperature swing recovery of solvent and product. Regeneration is accomplished by exploiting the temperature dependence of the mutual solubility curve. 

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