Solubility

The solubility of a substance indicates the extent to which a pure substance can be dissolved in a solvent. It refers to the property of the substance, under a homogeneous distribution to be mixed (as atoms, molecules or ions) in the solvent, i.e. to dissolve. Usually, the solvent is a liquid. There are also solid solutions, such as with alloys, glasses, ceramic materials and doped semiconductors. In dissolving gases in liquids, the term solubility refers to a coefficient which indicates the dissolved in the liquid quantity of gas at a certain pressure of the gas when the gas between the gas chamber and liquid in the diffusion equilibrium, i.e., as much in such diffuses out.

A distinction

• Qualitative solubility: If the substance in a particular solvent at all soluble in a recognizable degree?
• Quantitative Solubility: It specifies the exact amount of substance, the maximum dissolves in unit volume of a particular solvent.

The solubility is expressed in grams ( g) pure substance per 100 g of solvent.

Qualitative solubility

At temperatures above absolute zero, there are thermodynamic reasons ( entropy) for each substance in any other substance has a certain solubility, as described in the article pure substance. The increasing accuracy of the analysis confirms this a distinction between " soluble" and "insoluble" is dependent on the chosen boundary conditions. So there are relative statements such as poorly soluble, sparingly soluble or infinitely soluble a substance in another is.

A common classification of solubilities is given about the maximum amount of solute. Less than 0.1 mol / l solute is called sparingly, between 0.1 and 1 mol / l as moderately soluble and solubility greater than 1 mol / l are considered readily soluble.

In which liquids a solid is readily soluble, depends on the molecular properties of the substance and the liquid. As salt-like substances (ionic compounds) almost only in polar solvents such as water or, for example, hydrogen fluoride ( HF) are soluble. Many lipophilic ( " fat- loving " ) substances, however, are only in organic solvents such as benzene ( a nonpolar ( " apolar " ) solvents) appreciably soluble. " Polar " means in this context that the molecules of the solvent having a dipole moment, and therefore contact with charged (ions) or the polar molecules in turn solute interacts without causing a reaction. The polarity of solvents is scalable. Different polarities and thus different solubilities are used in the chromatographic process.

Many substances, for example ethanol or acetone, are readily miscible with both water as well as with non-polar solvents.

Quantitative solubility

Solubility and solubility product

The solubility of a substance in a solvent need not be limited. Thus sulfuric acid is miscible with water in any proportions.

In the mixture of phenol with water, there are two areas, a solution of phenol in water and a solution of water in phenol. In between is a range of "forbidden " mixing ratios. If phenol and water in these conditions mixed together, this leads to the formation of two liquid phases. The range of possible mixing ratios is not referred to as miscibility gap. Above a certain temperature, the upper critical solution temperature, in the case of phenol and water at about 340 K ( 67 ° C) the mixture, however, is completely miscible in all proportions.

The quantitative solubility or solubility is limited in soluble systems to the maximum concentration of a substance in the other, at which the mixture under equilibrium conditions is still single phase. The solubility is dependent on temperature. If the solubility limit is exceeded, a second phase is eliminated. Lack the necessary activation energy for diffusion or the resignation of the second phase, the mixture is single phase and above the solubility limit in a metastable, supersaturated state.

With salts followed by the solubility of the solubility under the condition that the salt AmBn is divided into m and n A B ions.

If none of the ions involved in addition of a mixture of in front of, the solubility of the salt in question can be calculated. The solubility of the salt is AmBn:

In mixed solutions, as in natural water with a variety of ions, tune the concentrations of anions and cations not stoichiometric pairwise agreement (as is the solution of individual salts correspond ). An example of this is the dissolved lime as calcium bicarbonate goes by dissolved carbon dioxide in solution and forms the major contribution to water hardness. About the dissociation of carbonic acid shifts with the changes in carbon dioxide content of the water ( by respiration and photosynthesis of aquatic organisms), the concentrations of carbonate and bicarbonate anions, the concentration of calcium cations remains unaffected. In this case, the product decision from the non-equivalent concentrations of calcium and carbonate ions as to whether and to what extent it is ( as scale or marl ) for failure to observe the solubility in precipitation of calcium carbonate.

The quantitative solubility, as generally indicated the concentration of solutions in different units:

• G / l solution ( mass concentration)
• G / g of solution (mass fraction, " percent by mass " in relation to 100 g)
• L / l solution (volume concentration, vol - % with respect to 100 l)
• Mol / l solution ( molar concentration )
• Eq / l solution ( normality, obsolete)
• Mol / kg of solvent ( molality )

Determination of solubility by conductivity measurements

For very sparingly soluble salts (eg, BaSO4, PbS, HgS, AgCl), the solubility can be determined from conductivity measurements with sensitive conductometers. To calculate the limiting equivalent conductivity of the salt is also needed at infinite dilution.

Friedrich Kohlrausch and Arnold F. Holleman developed this method of determination.

Temperature dependence of the solubility

The solubility of a substance in another depends to a first approximation of the solution enthalpy: Is the solution reaction is endothermic ( positive solution ), the solubility increases upon heating. In an exothermic reaction solution, the solubility decreases upon heating. Is the enthalpy of solution approximately zero, such as in saline, then the solubility of hardly changes upon heating.

Example of the Löslichkeitsberechnung a salt

Calculate the solubility L for aluminum sulfate in water at a known KL

This means that from each mole of aluminum sulfate are formed in the solution 2 mol of aluminum and 3 moles of sulfate ions. Furthermore, the following relationships apply:

The factors in front of L, as the exponent in the above equation are the stoichiometric factors. Used in the equation for KL, we have:

Which:

The numerical values ​​for the solubility products are obtained from the standard free enthalpies.

Verbal, the solubility according to the European Pharmacopoeia

The European Pharmacopoeia defines the following terms, at 15 ° C to 25 ° C:

Solution of gases in liquids

At the interface between gas and liquid occurs due to diffusion of gas molecules to the exchange between the solution and the gas space. In this case, the entry of molecules in the solution is proportional to the partial pressure of the gas, and the outlet is proportional to the concentration of the gas in the solution ( see Henry's law ). In the so-called saturation concentration, there is a dynamic equilibrium between the two diffusion directions. The saturation concentration is proportional to the partial pressure in the gas space. The linking proportionality constant is referred to as solubility, more precise than the solubility coefficient, :

The index i refers to the gas in the possible mixed solutions, such as in the solution of the gas mixture "air" in water.

Usually, this solubility of gases in liquids decreases with increasing temperature. Likewise, dissolved solids affect in reducing water to the gas solubility from. Therefore, for example, in sea water is less soluble than oxygen in fresh water.

A deviation from proportionality between gas pressure and equilibrium concentration makes itself only at very high pressures (compared to atmospheric pressure) noticeable.

Solution in solids

The laws of solubility for solids are retained even in principle. Here, too, different phases can be observed. If a single-phase solid mixture, there is a solution. Crystalline substances thereby form solid solutions or intermetallic compounds. Separate phases are often present very finely distributed. This is particularly the case in relation to the diffusion speed in the formation of high cooling rate or at a eutectic composition. Therefore, with the naked eye to assess the solubility usually not possible, especially as single-phase systems are often grained crystalline and also intergranular grain boundary phases may arise. Aids are diagrams of the temperature profile of cooling, where phase transitions appear as breakpoints or creases, and light microscopy as well as further crystallographic studies with even greater effort.

Much more common than in liquids are in solids before metastable solutions. These occur especially when the already solidified mixture at temperature decrease the solubility decreases and sufficient diffusion rate is no longer provided.

The solubility of solids in one another and their multi-phase, and the change of the property of the temperature is crucial for the establishment of technical alloys. It is also often a multiphase desired. Copper / nickel, and silver / gold are examples of systems with complete solubility at each composition, the latter comes as electron naturally as well. Steels, however, are predominantly multi-phase (ferrite, austenite, martensite, cementite ) as single-phase alloys are alloyed ferritic chromium steels with 15% to 30 % chromium and less than 0.1 % of carbon of technical importance. The system of copper / zinc is at room temperature for five different phases, which are separated by gaps mixture. Depending on the composition is only one of them or the mixture gap between two phases, a mixture of the two. In the technically relevant range (brass. Copper content min 50 %) are two of these phases.

The system copper / tin, tin bronze belongs to the oldest technical as alloy is an example of metastable conditions: between 520 ° C and 586 ° C tin in copper with a share up to 15.8 % of the already solid at this temperature alloy soluble. Even at room temperature in equilibrium, the solubility of tin is close to zero, is material which has such a concentration of tin is not exceeded at the elevated temperature at which further cooling, single-phase, since now hardly occurs diffusion. On the other hand, segregates already in the previous solidification the melt and the last solidified material, the boundary of 15.8 % tin already exceed at significantly lower tin content of the alloy. Due to the large diffusion of inertia of the tin these concentration differences can be compensated only by prolonged annealing at about 750 ° C. Both effects together lead to cast tin bronze, depending on the cooling rate in tin contents up to 4% to 6 % is single-phase and single-phase can be even up to 15 % can be achieved by annealing, although at room temperature no appreciable solubility is given.