Saturated solutions, definition

A saturated aqueous solution in contact with an excess of a definite solid phase at a given temperature will maintain constant humidity in an enclosed space. Table 11.4 gives a number of salts suitable for this purpose. The aqueous tension (vapor pressure, in millimeters of Hg) of a solution at a given temperature is found by multiplying the decimal fraction of the humidity by the aqueous tension at 100 percent humidity for the specific temperature. For example, the aqueous tension of a saturated solution of NaCl at 20°C is 0.757 X 17.54 = 13.28 mmHg and at 80°C it is 0.764 X 355.1 = 271.3 mmHg. 

Other oxides of phosphorus are less well characterized though the suboxide PO and the peroxide P2O6 seem to be definite compounds. PO was obtained as a brown cathodic deposit when a saturated solution of Et3NHCl in anhydrous POCI3 was electrolysed between Pt electrodes at 0°. Alternatively it can be made by the slow reaction of POBrs with Mg in Et20 under reflux ... 

In electrochemistry it is customary to multiply each of those quantities by Avogadro s constant and, when a few additional ions enter the already saturated solution, to speak of the entropy of solution per mole. Let the entropy of one mole of the crystalline solid be denoted by Scr and let Si and S2 denote, respectively, the entropy of the solution before, and after, the entry of the additional solute, both expressed in calories per mole. The total initial entropy is obviously (S + Si) and the final entropy is St. The difference between the final and the initial entropy is by definition AS,at. 

We shall now assume that it is possible to have a system in equilibrium composed of the various phases at a specified temperature and total pressure. This will be characterised by certain definite relations between the compositions of the phases (for example, a solid salt, saturated solution, vapour of the solvent). Let 77, T = total pressure, and temperature, of the system. n = number of components (cf. 84). r = phases ... 

Salts such as silver chloride or lead sulfate which are ordinarily called insoluble do have a definite value of solubility in water. This value can be determined from conductance measurements of their saturated solutions. Since a very small amount of solute is present it must be completely dissociated into ions even in a saturated solution so that the equivalent conductivity, KV, is equal to the equivalent conductivity at infinite dilution which according to Kohlrausch s law is the sum of ionic conductances or ionic mobilities (ionic conductances are often referred to as ionic mobilities on account of the dependence of ionic conductances on the velocities at which ions migrate under the influence of an applied emf) ... 

As was discussed above, it is essential to determine the effect, if any, that the emersion process has on the double layer. To do this, Wilhelm and colleagues have performed the definitive type of blank experiment. CO was adsorbed onto the Pt working electrode from sulphuric acid electrolyte. After adsorption, the CO-saturated solution was replaced with pure electrolyte. The potential of the electrode was then ramped in order to oxidise off the adsorbate, as C02, and the voltammogram so obtained is shown in Figure 2.118(a). The experiment was then repeated CO was adsorbed as before, but the electrode was emersed and transferred into the UHV chamber, before being re-immersed and the potential ramp applied. The voltammogram so... 

Electrolyte Activities. The activity of a solid electrolyte in equilibrium with its pure saturated solution, by definition is ... 

The crystal surface may be regarded as covered with a layer of saturated solution of a definite thickness through which the products have to diffuse. If the actual solvation of the solid proceeds rapidly in comparison to the process of diffusion the rate of solution wfil he essentially that of diffusion, and can accordingly be expressed by the Fick diffusion equation, the rate of solution per unit area of interface being given by... 

Note that this definition requires a saturated solution to be at equilibrium with undissolved solid. Substances that are more soluble at high temperature than at low temperature can sometimes form what are called supersaturated solutions, which contain a greater-than-equilibrium amount of solute. For example, when a saturated solution of sodium acetate is prepared at high temperature and then cooled slowly, a supersaturated solution results, as shown in Figure 11.5. Such a solution is unstable, however, and precipitation occurs when a tiny seed crystal of sodium acetate is added to initiate crystallization. 

This reduction potential relates, by definition, to 02-saturated solutions. For comparison with other values that are based on molarity, a value of-0.179 V should be taken (Wardman 1991). 

SOLUBILITY OF PRECIPITATES A large number of reactions employed in qualitative inorganic analysis involve the formation of precipitates. A precipitate is a substance which separates as a solid phase out of the solution. The precipitate may be crystalline or colloidal, and can be removed from the solution by filtration or by centrifuging. A precipitate is formed if the solution becomes oversaturated with the particular substance. The solubility (5) of a precipitate is by definition equal to the molar concentration of the saturated solution. Solubility depends on various circumstances, like temperature, pressure, concentration of other materials in the solution, and on the composition of the solvent. 

Solubility is expressed as the concentration of a substance in a saturated solution at a defined temperature. The US Pharmacopeia (USP) gives the solubility definitions shown in Table 12-2. 

At a given temperature take a saturated solution of a definite salt, in the presence of an excess of the salt, at the pressure n S is the concentration of the solution. Give to the pressure a value r, slightly greater than n, keeping the temperature constant the equilibrium will be disturbed and the composition of the solution will vary until its concentiations has taken the value S which corresponds to saturation at the pressure n. ... 

Heat of solution in saturated solutions.— When a very small mass of salt m passes, at the temperature T, from the precipitate state into a solution nearly saturated at this temperature T, the phenomenon is accompanied by a certain absorption of beat the quantity of heat absorbed which, other things being equal, is proportional to the small mass tn, depends on the temperature T at which the phenomenon is produced this quantity of heat absorbed may be represented by the product mL, L being a fixed coefficient for a definite temperature, but variable with the temperature L is what is called the heat of solution in saturated solution of the salt considered, at the temperature T. 

It is evident that if, at a definite temperature, there exist two distinct saturated solutions of concentrations there... 

The concentration and the vapour pressure of the solution have a definite value for each temperature. Plotting the concentrations of the saturated solutions against the temperatures, we obtain the solubility curves of the various solid phases which are in equilibrium with the saturated solution. 

A nutrient is classified as water soluble if it can be dissolved in water. However, this property is relative, not absolute, since all materials dissolve to some degree in all solvents. Therefore, one might arbitrarily choose a concentration limit of 1 miJJimolar (1,0 mM) to define water solubility, that is, any compound whose saturated solution in water contains more than 1.0 mmol per liter (1,0 mol per 1000 liters) is considered water soluble. Cholesterol, for example, is definitely not water soluble. A saturated solution is only 0.001 mAI. A nutrient is classified as fat soluble if it can be dissolved in fats or oils. Again, limits of solubility must be established arbitrarily. 

CO adsorption on Cu electrode surface is interfered with by specifically adsorbed anions. CO can be adsorbed below a certain definite potential, determined by the adsorption strength of CO and the anion. When CO molecules displace the specifically adsorbed anions on Cu electrode, a voltammetric peak is observed as exemplified for Cu(lOO) in CO saturated phosphate buffer solution in comparison with N2 saturated solution (Fig. 29). Subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS) spectra in Fig. 30 demonstrates that CO is adsorbed at -0.8 V vs. SHE but not at -0.4 V, and adsorbed phosphate anion vice versa. " This process is equivalent to charge displacement adsorption of CO on Pt electrode revealed by Clavilier et al The profile of the voltammogram depends greatly on the crystal... 

The Saturated Solution.—From what has been said above, it will be seen that the condition of saturation of a solution can be defined only with respect to a certain solid phase if no solid is present, the system is undefined, for it then consists of only two phases, and is therefore bivariant. Under such circumstances not only can there be at one given temperature solutions of different concentration, all containing less of one of the components than when that component is present in the solid form, but there can also exist solutions containing more of that component than corresponds to the equilibrium when the solid is present. In the former case the solutions are unsaturated, in the latter case they are supersaturated with respect to a certain solid phase in themselves, the solutions are stable, and are neither unsaturated nor supersaturated. Further, if the solid substance can exist in different allotropic modifications, the particular form of the substance which is in equilibrium with the solution must be known, in order that the statement of the solubility may be definite for each... 

Since in systems of two components the two phases, solution and vapour, constitute a bivariant system, the vapour pressure is undefined, and may have different values at the same temperature, depending on the concentration. In order that there may be for each temperature a definite corresponding pressure of the vapour, a third phase must be present. This condition is satisfied by the system solid—liquid (solution)— vapour that is, by the saturated solution (p. 165). In the case of a saturated solution, therefore, the pressure of the vapour at any given temperature is constant. 

The solubility relations of sodium sulphate illustrate very clearly the importance of the solid phase for the definition of saturation and supersaturation. Since the solubility curve of the anhydrous salt has been followed backwards to a temperature of about 18 , it is readily seen, from Fig. 73, that at a temperature of, say, 20 , three different saturated solutions of sodium sulphate are possible, according as the anhydrous salt, the heptahydrate or the decahydrate, is present as the solid phase. Two of these solutions, however, would be metastable and supersaturated with respect to the decahydrate. 

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