SATURATED SOLUTIONS AND SOLUBILITY

When an ionic solid, MX, is added to water, equilibrium 6.43 is established (if the ions formed are singly charged). When equilibrium is reached, the solution is saturated. 

Tabulated values of solubilities of ionic salts refer to the maximum amount of solid that will dissolve in a given mass of water to give a saturated solution. 

For very dilute solutions at 298 K, the numerical value of a concentration in molkg is equal to that in moldm, and the solubilities of sparingly soluble salts (see below) are generally expressed in mol dm.  

The solubility of a solid at a specified temperature is the amount of solid solute) that dissolves in a specified amount of solvent when equilibrium is reached in the presence of excess solid. The solubility may be expressed in several ways, for example  

It is crucial to state the temperature, since solubility may depend significantly on temperature as illustrated in Fig. 7.9 for KI and NaNOg. In contrast. Fig. 7.9 shows that between 273 and 373 K, the solubility of NaCl is essentially constant. 

9 Solubilities of ionic salts Solubility and saturated solutions 

Values of Alsp(298K) for selected sparingly soluble 

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The term solubility thus denotes the extent to which different substances, in whatever state of aggregation, are miscible in each other. The constituent of the resulting solution present in large excess is known as the solvent, the other constituent being the solute. The power of a solvent is usually expressed as the mass of solute that can be dissolved in a given mass of pure solvent at one specified temperature. The solution s temperature coefficient of solubility is another important factor and determines the crystal yield if the coefficient is positive then an increase in temperature will increase solute solubility and so solution saturation. An ideal solution is one in which interactions between solute and solvent molecules are identical with that between the solute molecules and the solvent molecules themselves. A truly ideal solution, however, is unlikely to exist so the concept is only used as a reference condition. 

A solution is saturated if it contains the maximum amount of solute that can possibly be dissolved in a solvent at the existing temperature and pressure conditions. If a solution is saturated, it need not be concentrated. Concentrated refers to the ratio of solute to solvent. A solution is concentrated if it contains a high ratio of solute to solvent. A solution is dilute if it contains a low ratio of solute to solvent. A solution of ammonia gas dissolved in water can be highly concentrated but not saturated. Ammonia gas dissolves easily in water. Barium hydroxide is slightly soluble. A saturated solution of barium hydroxide is very dilute. 

Silver iodide is only slightly soluble in ammonia, but dissolves in sodium thiosulphate, concentrated hydriodic acid, and saturated solutions of potassium iodide.7 It forms a series of double salts with silver bromide,8 with mercuric iodide, and with the iodides of the alkali-metals.10 Double compounds of silver iodide and ammonia of the formulae AgI,8NH3 (6-92) AgI,l NH3 (7-25) AgI,NHs (8-56) AgI,2NHs (7-05) and AgI, NH3 (11-59) have also been prepared,11 the figures in parentheses indicating the calculated heats of formation in large calories. 

From solute solubility data and saturated solution density data calculate the equilibrium concentration of solute (C ). 

Figure 7.2. Solubility and saturation. A schematic solubility diagram showing concentration ranges versus pH for supersaturated, metastable, saturated, and undersaturated solutions. A supersaturated solution in the labile concentration range forms a precipitate spontaneously a metastable solution may form no precipitate over a relatively long period. Often an active form of the precipitate, usually a very fine crystalline solid phase with a disordered lattice, is formed from oversaturated solutions. Such an active precipitate may persist in metastable equilibrium with the solution it is more soluble than the stable solid phase and may slowly convert into the stable phase.

In the case of formation of moderately soluble or well-soluble oxides the potentiometric curves are characterized by a wide non-saturated solution section and, as a rule, by a blurred pO drop at the equivalence point (sometimes this drop is absent) as is shown in Fig. 3.6.2 (curve 4). Precipitation of a soluble oxide is initiated in the solutions containing relatively high initial concentrations of oxide ions. In this case, the interaction can be detected from the dependence of the ligand number h on the initial concentration of the titrant it deviates considerably from zero. To divide the sections of non-saturated and saturated solutions using only the titration curve is a very difficult problem, especially when precipitation takes place from the concentrated titrant solution. 

We shall now consider the characteristic features of E-pf) dependences, which can be obtained for unsaturated and saturated solutions of the metal oxide possessing appreciable solubility in the studied melt. 

The introduction of seed crystals to a solution that is saturated or within the lower portion of the metastable zone prevents spontaneous nucleation (Karpinski et al. 1980). In most industrial cases, seeding is a manual operation. The indication of when to seed is derived from an indication of the process temperature, typically provided by the control system, and knowledge of the product solubility and current solution concentration, whether measured on-line, off-line, or calculated from charge amounts. Obviously, accurately calibrated temperature sensors in the laboratory, where the solubility relationship was established, as well as in the crystallizer, where the solubility relationship will be utilized, are necessary. 

The other members of the series are prepared in a similar manner. Increasing the number of carbon atoms in the cation increases the solubility of the corresponding compound in ethanol. If the solid product does not separate when the alkylammonium chloride is added to the blue solution of chromium(ll), the excess solvent should be removed from the flask by distillation. At the point where the crystals begin to appear, the oil bath is lowered and the solution (deep-blue color) is allowed to cool slowly. Beautiful shiny platelet crystals separate. An alternative procedure is to start with a smaller volume of solvent and saturated solutions of alkylammonium chlorides (e.g., in the case of (C2HsNH3)2[CrCl4], 40 mL of EtOH). 

HPLC) is a separation method to identify and quantify exact concentrations of nonvolatile components. Saturated filtered solutions are analyzed and compared with standard solutions with known concentrations [48]. Spectrophotometrical measurements are also a commonly used method to determine the absolute solubility. First, saturated solutions are filtered or centrifuged to obtain true solutions. Next, these solutions are further diluted and characterized by optical absorption measurements. By comparing the optical density (OD) of the investigated solutions with the OD of calibrated master solutions, the solubility of the component in the investigated media can be determined. Examples for determination of organic semiconductor solubility measurements with this method have been reported by Walker et al. and Machui et al. [46, 47]. 

A saturated solution may be either dilute or concentrated, depending on the solubility of the solute. A saturated solution can be conveniently prepared by dissolving a little more than the saturated amount of solute at a temperature somewhat higher than room temperature. Then the amount of solute in solution will be in excess of its solubility at room temperature, and, when the solution cools, the excess solute will crystallize, leaving the solution saturated. (In this case, the solute must be more soluble at higher temperatures and must not form a supersaturated solution.) Examples expressing the solubility of saturated solutions at two different temperatures are given in Table 14.3. 

Define (a) solvent, (b) solute, (c) solubility, (d) saturated solution, (e) unsaturated solution, and (f) insoluble solute. 

Solubility Equilibria Between Crystals and Saturated Solutions... 

The solubility of sugar is greater than 260 g/100 g H2O at all temperatures above 50°C. Therefore, the solution is unsaturated at 70°C and 60°C. At 50°C, the solubility is equal to the amount dissolved, so the solution is saturated. At 40°C, the solution contains more dissolved sugar than it should on the basis of solubility, and the solution is supersaturated. At 30°C, the excess sugar crystallizes from solution, and the resulting solution becomes saturated. From that point, excess sugar continues to crystallize from the solution, and the solution r ains saturated to 20°C. 

Boron trioxide is not particularly soluble in water but it slowly dissolves to form both dioxo(HB02)(meta) and trioxo(H3B03) (ortho) boric acids. It is a dimorphous oxide and exists as either a glassy or a crystalline solid. Boron trioxide is an acidic oxide and combines with metal oxides and hydroxides to form borates, some of which have characteristic colours—a fact utilised in analysis as the "borax bead test , cf alumina p. 150. Boric acid. H3BO3. properly called trioxoboric acid, may be prepared by adding excess hydrochloric or sulphuric acid to a hot saturated solution of borax, sodium heptaoxotetraborate, Na2B407, when the only moderately soluble boric acid separates as white flaky crystals on cooling. Boric acid is a very weak monobasic acid it is, in fact, a Lewis acid since its acidity is due to an initial acceptance of a lone pair of electrons from water rather than direct proton donation as in the case of Lowry-Bronsted acids, i.e. 

Suppose that a given volume of the solvent when cold can dissolve 15 g. of A and 5 g. of B. If too g. of the crude product are dissolved in this volume of the hot solvent, and the solution allowed to cool, then (ignoring the small mutual effect on the solubility of each compound caused by the presence of the other) it is clear that 82 g. of A will crystallise, whilst the whole of B will remain in solution, since the latter is not saturated with respect to B. 

Deliquescence and efflorescence. A substance is said to deliquesce (Latin to become liquid) when it forms a solution or liquid phase upon standing in the air. The essential condition is that the vapour pressure of the saturated solution of the highest hydrate at the ordinary temperature should be less than the partial pressure of the aqueous vapour in the atmosphere. Water will be absorbed by the substance, which gradually liquefies to a saturated solution water vapour will continue to be absorbed by the latter until an unsaturated solution, having the same vapour pressure as the partial pressure of water vapour in the air, is formed. In order that the vapour pressure of the saturated solution may be sufficiently low, the substance must be extremely soluble in water, and it is only such substances (e.g., calcium chloride, zinc chloride and potassium hydroxide) that deliquesce. 

Divide the saturated solution of n-butyl alcohol in water into three approximately equal parts. Treat these respectively with about 2-5 g. of sodium chloride, potassium carbonate and sodium hydroxide, and shake each until the soli have dissolved. Observe the effect of these compounds upon the solubility of n-butanol in water. These results illustrate the phenomenon of salting out of organic compounds, t.e., the decrease of solubility of organic compounds in water when the solution is saturated with an inorganic compound. The alcohol layer which separates is actually a saturated solution of water in n-butyl alcohol. 

An alternative method for isolating the n-butyl ether utilises the fact that n-butyl alcohol is soluble in saturated calcium chloride solution whilst n-butyl ether is slightly soluble. Cool the reaction mixture in ice and transfer to a separatory fimnel. Wash cautiously with 100 ml. of 2-5-3N sodium hydroxide solution the washings should be alkaline to litmus. Then wash with 30 ml. of water, followed by 30 ml. of saturated calcium chloride solution. Dry with 2-3 g. of anhydrous calcium chloride, filter and distil. Collect the di-n-butyl ether at 139-142°. The yield is 20 g. 

If the amine is soluble in water, mix it with a slight excess (about 25 per cent.) of a saturated solution of picric acid in water (the solubility in cold water is about 1 per cent.). If the amine is insoluble in water, dissolve it by the addition of 2-3 drops of dilute hydrochloric acid (1 1) for each 2-3 ml. of water, then add a sUght excess of the reagent. If a heavy precipitate does not form immediately after the addition of the picric acid solution, allow the mixture to stand for some time and then shake vigorously. Filter off the precipitated picrate and recrystaUise it from boiling water, alcohol or dilute alcohol, boiUng 10 per cent, acetic acid, chloroform or, best, benzene. 

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