Solvation And Precipitation

Type of physical science: Chemistry

Field of study: Chemical processes

Solvation is the process that involves the association of solute with the solvent. The reverse process is called precipitation and provides the separation of the solid solute from the liquid solvent. The various forms of the precipitates and their use in quantitative determinations are examined by gravimetric analysis.

Overview

A solution is generally composed of two components: the solute and the solvent. The rule of thumb in solubility is that "like dissolves like." Thus, a polar solvent dissolves a polar or ionic solute, and a nonpolar solvent dissolves a nonpolar solute.

Solvation involves three steps. First, the solute has to be broken down into individual components; second, the intermolecular forces in the solvent will have to be broken in order to make room for the solute; finally, the solute and the solvent will interact to form the solution. The first two steps are normally endothermic (that is, they require energy), while the last step is exothermic (it releases energy). The energy change for the overall process is called enthalpy (or heat) of solution and might have a positive sign (indicating that energy has been absorbed) or negative sign (energy has been released). The association may involve chemical or physical forces or both, and as a result, a chemical compound may often form.

When a salt, such as calcium fluoride (CaF₂), is first added to water, ionization does not occur spontaneously, that is, no Ca⁺² or F^- ions are formed instantly. As the dissolution proceeds, however, the ions' concentration increases, and the possibility of them colliding to reform the solid becomes more distinct. Ultimately, dynamic equilibrium is reached, which is expressed in the form:

Ca⁺²(aqueous) +F^- (aqueous) = CaF₂ (solid)

At that point, the solution is said to be saturated, with the ions (Ca⁺² and F^-) in solution and the precipitate (CaF₂) out of it. The equilibrium expression relating solubilities is defined as: K_sp=[Ca⁺²]x[F^-]², where K_sp is the equilibrium constant and Ca⁺² and F^- are the solubilities of the ions expressed in moles per liter. When the product of the concentrations is higher than the value of the solubility product (a value characteristic for every compound), precipitation will start.

Solvation that occurs in aqueous solutions (with water as the solvent) is referred to as hydration. In this case, the highly polar water molecules surround the ions in the form of spheres of hydration. As a result of its interaction with water, the ion loses its mobility under an applied voltage. The extent of such hydration depends on several factors, such as the size and charge of the ion. Frequently, hydrated ions persist in the crystalline materials by the evaporation of aqueous solutions of salts. Solid hydrates in which the water molecules are coordinated to anions by hydrogen bonding are well known. Examples include calcium chloride dihydrate, CaCl₂·2H₂O, and cupric sulfate pentahydrate, CuSO₄·5H₂O.

Formation of a precipitate involves two phenomena, nucleation and crystal growth. Nucleation involves the formation of the smallest particles of a precipitate, the nuclei, within a supersaturated solution. It is followed by crystal growth, which is the deposition of the ions from the solution upon the surfaces of the already nucleated particles.

The particle size is governed by the number of nuclei formed in the nucleation step and is directly related to the extent of supersaturation of the solution. In general, the greater the extent of supersaturation, the smaller the size of individual nuclei. Thus, if large crystals are desired, the solutions that are to be mixed should be dilute, especially if the precipitate is extremely insoluble.

Nucleation can be spontaneous if a sufficiently large cluster of ions comes together. In most cases, however, induced nucleation occurs, which is promoted by the presence of many nucleation sites. Such sites include the surfaces of the container (the number of sites is influenced by type of container, degree of scratching of walls, and way of cleaning prior to use) as well as insoluble impurities that may be present.

Crystal growth follows nucleation and involves two steps: diffusion of the ions on the growing crystal and deposition of the ions on the surface. The rate of the latter depends on the nature and concentration of the ions, stirring, and temperature of the medium. The deposition rate depends on the shape of the crystal as well as the presence of impurities.

When first formed, the precipitate is in the form of colloidal particles that are suspended (dispersed) in solution. These particles have a high ratio of surface area (10^-5 to 10^-7 centimeters in diameter) to mass. As a result, they tend to allow ions from the mother liquid to adsorb at the negative or positive charge centers.

When a soluble substance precipitates along with an insoluble one, coprecipitation occurs. There are generally four different mechanisms leading to this phenomenon. First, surface adsorption, as already described, occurs when ions from the mother liquid adsorb onto the surfaces of precipitated particles. This adsorption involves a primary adsorbed ion layer tightly held and a secondary one that is, generally, loosely held.

On the other hand, the incorporation of a small portion of the mother liquid into the growing crystal and subsequent coalescing into the precipitate leads to mechanical entrapment. Ordinary washing is not effective; neither is heating the precipitate, which can lead to eruption of the crystal but not to removal of the entrapped impurities.

Postprecipitation is a phenomenon very similar to surface adsorption. The crystals isolated (often called mixed crystals) occur with compounds that have the same anion and cations of similar size, shape, and charge. The difference between surface adsorption and postprecipitation is that in the latter case a precipitate (and not a simple ion adherence) results. This can be avoided if the first, already-formed precipitate (normally not a colloid) is promptly separated from its mother liquid.

Finally, occlusion occurs when an ion from the mother liquid is substituted for another one within a crystal. This leads to a permanent incorporation of an impurity, which can be avoided only by eliminating the replacement ion prior to precipitation of the desired compound. This contamination is particularly common with gelatinous precipitates.

Every precipitate has to be washed before being converted to the weighing form for the final determination. Washing generally removes the mother liquid but does not exclude the entrapped impurities. This can be accomplished with reprecipitation (double precipitation), which involves redissolving the crystals in pure solvent (for example, water) and reprecipitating them. One should be able to obtain far less contaminated crystals if the general precautions stated above are taken. Problems generally arise when the solid is redissolved with difficulty, or the formation of mixed crystals is likely. It is most desirable, therefore, to try to avoid any contamination by devising a procedure that will lead to a pure product from the beginning.

When a precipitate is allowed to be in contact with the mother liquid for some time, changes often occur. The crystals are of larger size and can, therefore, be filtered more easily. This process, called aging, or digestion, is believed to be a sequence of steps that involve dissolving of the smaller crystals and deposition of the nuclei on the larger ones. Aging is especially helpful with colloids and is most effective at higher temperatures. Dissolving and reprecipitating the small crystals also has the tendency of eliminating impurities that can be incorporated in the crystal lattice.

Generally, gravimetric analysis requires that the filtered precipitate be heated to remove all traces of solvent and volatile electrolytes until its weight is constant. Many times, however, the precipitate has to be ignited to decompose into a product of known composition. There is no definite temperature below or above which a solid is said to be dried or ignited; however, drying involves temperatures below 250 degrees Celsius, while ignition involves temperatures above 250 degrees Celsius (and up to 1,200 degrees Celsius). Precipitates that are to be dried should be collected on filter paper, in a Gooch crucible, sintered glass, or porcelain filtering crucibles. Precipitates that are to be ignited are normally collected on filter paper or porcelain filtering crucibles. Heating should be executed using the appropriate burners, or, alternatively, the crucibles can be placed in an electrically heated furnace.

Applications

In analytical chemistry, the separation of a solid phase from an aqueous solution is often accomplished by gravimetric analysis. The overall process includes precipitation, isolation by filtration or centrifugation, and weight determination.

Crystal growth is often aided by seeding. In the amination experiment of 3carbomethoxy-1,4-thiapyrone, Vaclav Horak and N. Kucharczyk observed, in the late 1970's, that the yields were either almost quantitative or zero. Thus, in successful experiments, the product would crystallize out in a very short period of time upon mixing of the reactants. On the other hand, using a supersaturated solution of the reactants did not lead to product formation and resulted in the decomposition of the reactant because of the high pH of the medium. Adding a few crystals of the product into the reactants' supersaturated solution led to quantitative precipitation of the desired product. In the process, seeding starts the nucleation step, which leads to crystal growth.

The difference of solubility product values has enabled the separation of two ions that are present in a mixture. Thus, the chloride ion can be isolated as silver chloride (AgCl) upon treatment with silver nitrate (AgNO₃) from a solution containing both chloride and iodide anions. This occurs despite the possibility of contamination with substances from the mother liquid. Because of its lower solubility product value, silver iodide will precipitate first, and by the time silver chloride will start precipitating, more than 99 percent of the iodide ions will have already been out of solution.

At the time of filtration, the precipitate should not be in colloid form, because the particles are so fine that they pass through ordinary filtering media. Thus, it is imperative for the colloidal particles to coagulate and settle first. This is accomplished by neutralizing the adsorbed ions (in this example, Ag⁺) with a secondary layer, the counterion layer (in this case, NO₃^-). In general, the greater the concentration of counter ions in solution, the larger the degree of neutralization of the adsorbed ions, and the greater the degree of coagulation.

Coagulation of colloids through counterion adsorption is reversible and is called peptization. In the above example, this is accomplished by removing the secondary layer (the nitrate ions) when washing the coagulated, filtered crystals with pure water.

A typical example of postprecipitation is observed in the separation of calcium from magnesium by precipitation with oxalate. Magnesium oxalate does not precipitate by itself under ordinary conditions. When calcium oxalate is precipitated from a solution containing magnesium ions, however, magnesium oxalate coprecipitates if the precipitate is allowed to remain in solution for an extensive amount of time. This phenomenon can be explained by the adherence of excess oxalate ions onto the already formed crystals of calcium oxalate and the subsequent adherence of the magnesium ions.

Supersaturation has to be held to a minimum in order to obtain large crystals. Ideally, this can be achieved by generating (instead of merely mixing) the precipitating agent at a rate comparable to that of the product's crystal growth. This technique is called precipitation from a homogeneous solution and is used in quantitative determinations of certain ions whose precipitates are pH sensitive. For example, aluminum can be determined quantitatively by converting it to aluminum oxide (Al₂O₃). Treatment of aluminum with a base, such as ammonia (NH₄OH), however, yields hydrous aluminum oxide, a colloidal, gelatinous precipitate that is difficult to filter and is susceptible to coprecipitation. The difficulties are eliminated when the aluminum ion solution is treated with an acidic urea solution, which produces ammonia upon heating. The slowly produced ammonia raises the pH, and the formed crystals of hydrous aluminum oxide are less colloidal and of better crystalline form. The desired anhydrous aluminum oxide can then be easily obtained upon igniting.

Group II cations in qualitative analysis are separated by their conversion to the corresponding sulfides. The use of gaseous hydrogen sulfide also yields colloidal suspensions that are not easily separated by filtration. Thioacetamide can serve in the homogeneous precipitation, since it is slowly hydrolyzed in the proper medium to produce hydrogen sulfide, which yields less contaminated precipitates that can be filtered easily.

The precipitation of barium sulfate is a characteristic example of the success of aging in the improvement of the crystal quality in the filtration process. Leaving the solid in the mother liquid leads to the formation of larger, less contaminated crystals that can be filtered easily.

Burning the filter paper can often create problems. For example, in the ignition step (the final step of the isolation of barium sulfate), incomplete combustion of the filter paper produces carbon, which reduces barium sulfate to barium sulfide and thus leads to incorrect results in the quantitative analysis.

Context

Precipitation, in a broad sense, is a process that is an integral part of life. The condensation of water vapor in the atmosphere precipitates rain or snow. Water freezing in the winter leads to ice formation. The electrodeposition of a metal at the electrode upon passing an electric current through a solution containing ions is also a form of precipitation. Gravimetric analysis is the branch that deals with the precipitation phenomenon as it is commonly understood.

Gravimetric analysis includes a variety of techniques in which the mass of a product is used to determine the quantity of the original analyte. Since mass can be measured accurately and precipitation techniques are almost quantitative, gravimetric methods are among the most accurate in analytical chemistry. Such analysis historically has been the common method in the determination of elements in substances such as alloys and ores. The development of various modern techniques, such as chromatography, electrochemical methods, atomic absorption, spectroscopy, and spectrophotometry, have, in many instances, replaced this kind of analysis with equally good results. Nevertheless, combustion analysis, a method used to determine the empirical formula of an unknown compound, still utilizes precipitation techniques. The percent content of nickel in steel (via the dimethyl glyoxime complex) and iron in an alloy (as ferric oxide) is also gravimetrically determined.

For the quantitative isolation of a precipitate, the methods for reducing contamination should be applied. The application of the above-mentioned techniques, digestion, precipitation from a homogeneous solution, and use of dilute solutions, should give adequate results. Basic analysis will still use many of those precipitating methods well into the future.

Solubility and precipitation will always be closely associated with everyday life. The concept of solubility finds an application in tooth decay. Food decomposition produces acids, which dissolve tooth enamel, which is saved by the incorporation of fluoride onto the enamel to form a new compound called fluorapatite, which is less soluble to acids than the original enamel. The extremely low solubility of barium sulfate makes it a safe and an excellent agent to improve the clarity of X rays of the gastrointestinal tract. The relatively low solubility of calcium salts (phosphate and oxalate) is responsible for the formation of kidney stones.

Principal terms:

COAGULATION: the cohesion of colloid particles to produce larger particles that can precipitate

COLLOIDS: particles with a high ratio of surface area to mass (diameter of about 10 to the power of -7 to 10 to the power of -5 centimeters) that are electrically charged, repel one another, and resist coagulation

COPRECIPITATION: the precipitation of a soluble substance along with an insoluble one

CRYSTAL GROWTH: the step following nucleation, resulting from deposition of the ions from the solution upon the surfaces of the already nucleated particles

DIGESTION: the process in which the precipitate stands in contact with the mother liquor to form larger crystals; the process is also known as aging

FRACTIONAL PRECIPITATION: the procedure used to separate ions from a mixture by forming precipitates that have a common counterion; this is possible if there is a considerable difference in the solubility product values of the precipitates

NUCLEATION: the formation of the smallest particles of a precipitate, the nuclei, once the value of the solubility product of the precipitate is surpassed; further growth of the nuclei leads to crystal formation

OCCLUSION: the phenomenon in which an ion from the solution is substituted for another ion within a crystal; it is often encountered in gelatinous precipitates

SEEDING: the process in which addition of crystals to a supersaturated solution induces nucleation and further crystal growth

SOLVATION: the process in which solvent molecules surround the solute particles

SUPERSATURATED SOLUTION: a solution whose solute concentration exceeds the equilibrium solubility

Bibliography

Brady, James E., and G. E. Humiston. GENERAL CHEMISTRY. 5th ed. New York: John Wiley & Sons, 1990. An excellent text for the general chemistry course. Chapter 11 covers the physical properties of solutions and colloids, including solvation.

Flaschka, H. A., A. J. Barnard, Jr., and P. E. Sturrock. QUANTITATIVE ANALYTICAL CHEMISTRY. 2d ed. Boston: Willard, 1980. Chapter 6 of this good quantitative analysis book discusses the principles of gravimetric analysis and, in particular, the contamination, purification, separating, and washing of precipitates, as well as digestion. Examples are included in the next chapter, "Applied Gravimetric Analysis."

Harris, Daniel C. QUANTITATIVE CHEMICAL ANALYSIS. San Francisco: W. H. Freeman, 1982. An excellent college text for quantitative analysis that devotes several chapters to the solubility and precipitation of organic compounds. Section 8-2 discusses the precipitation process; chapter 5 covers the solubility of ionic compounds; chapter 9 gives a series of precipitation titrations.

Horak, Vaclav, and N. Kucharczyk. JOURNAL OF CHEMICAL EDUCATION 55 (1978): 580. A short article on the amination of 3-carbomethoxy-1,4-thiapyrone that illustrates the success of seeding in precipitating a solid.

Ramette, Richard W. CHEMICAL EQUILIBRIUM AND ANALYSIS. Reading, Mass.: Addison-Wesley, 1981. The beginning of chapter 6 of this thorough book deals with the mathematical principles of solubility determinations based upon precipitation. Section 6.2 gives a closer look at the precipitate particles and discusses the different processes involved.

Skoog, Douglas A., and Donald M. West. FUNDAMENTALS OF ANALYTICAL CHEMISTRY. 3d ed. Philadelphia: Saunders, 1980. One of the oldest yet most complete and thorough texts of analytical chemistry. Chapter 6 involves calculations involving precipitates, while chapter 7 discusses the particle size and purity of precipitates.

Vogel, Arthur. A TEXT-BOOK OF QUANTITATIVE INORGANIC ANALYSIS, INCLUDING ELEMENTARY INSTRUMENTAL ANALYSIS. 3d ed. New York: Wiley, 1961. A classic and one of the first quantitative analysis books with an excellent section on the theory of gravimetric analysis. It includes, among other topics, precipitation methods, the colloidal state, supersaturation, the purity of the precipitate, and quantitative separations based on precipitation. An outstanding book overall.

Concentrations in Solutions

Solutes and Precipitates