Solubility product measurement

Fluorides of the lantlmnides, yttrium, and scsmdium normally precipitate from aqueous media as the hemihydrates, LnF3-MsH20. It is these hemihydrates which are in equilibrium with saturated solution, and the solubility-product measurements refer to these hemihydrates. 

In propagating errors it is generally advisable to keep one or two extra digits in intermediate results, and round off only when you get to the final result. Applying this rule to the solubility product measurement, the mass of dissolved silver chloride is 0.888 0.0142 mg, and the formula weight of silver chloride is 143.3279 0.00042 g- mol"1. 

Solubility products measured in pure systems may not represent soil conditions very well. The impurities in solids affect their aqueous solubility soil minerals are characteristically impure. Nonetheless, predictions of soil solution concentrations usually assume the solubility products of pure minerals apply. The water molecule is ignored in stability constant and solubility product equations. The concentration of water is assumed to be unity because water is present in great excess and does not change significantly during the reaction. This assumption is good in all but the most concentrated aqueous solutions and in dry soils. 

Smith °" have demonstrated asymmetric interactions in stereospecific substitution reactions. An estimate of the magnitude of the asymmetric interaction in the case of the d and / form of d.s-[CoCl2(en)2] with the solvent (—)-2,3-butanedioF °, from solubility product measurements, suggests that antiracemisationin systems where solvolytic interference is less important (perhaps with m-[Co(N02)2(en)2] ), will lead to interesting kinetics. In this solvent, as well as 1,2-propanediol, such studies, when compared with the racemisation rates of the resolved complexes in the racemic or meso forms of the solvents, must lead to a greater understanding of the role of the solvent in a class of reactions whose precise mechanism has proved difficult to define. 

The thermodynamic solubility product for Pbl2 is determined in this experiment by measuring its solubility at several ionic strengths. 

The general approach illustrated by Example 18.7 is widely used to determine equilibrium constants for solution reactions. The pH meter in particular can be used to determine acid or base equilibrium constants by measuring the pH of solutions containing known concentrations of weak acids or bases. Specific ion electrodes are readily adapted to the determination of solubility product constants. For example, a chloride ion electrode can be used to find [Cl-] in equilibrium with AgCl(s) and a known [Ag+]. From that information, Ksp of AgCl can be calculated. 

The solubility product is learned from measurements of the solubility. In turn, it can be used as a basis for calculations of solubility. Suppose we wish to know how much cuprous chloride, CuCl, will dissolve in one liter of water. We begin by writing the balanced equation for the reaction ... 

The value of Kip is the same as that listed in Table 11.4. Many of the solubility products listed in tables were determined from emf measurements and calculations like this one. 

The trivalent [P04] and [As04] ions react similarly. Examples of anions that give insoluble Hg(I) compounds in this way include halides, pseudohalides, halates, carboxylates and sulfate. A trace of HNO3 or HCIO4 is often added to the solution of the Hg(I) nitrate or perchlorate to prevent disproportionation induced by alkali. Table 1 lists common Hg(I) derivatives prepared in this way and includes values of the solubility products of the sparingly soluble Hg(I) compounds where these are measured. A similar reaction is used to prepare HgjCO, from a soluble bicarbonate ... 

In the tables we find i Ag+/Ag = 0.7996 V and 2 Agci/Ag = 0.2223 V. From the above it is clear that primarily the silver-silver chloride electrode functions as a pAg electrode, i.e., it measures oAg+ at an ionic strength above 0.01 (cf., extended Debye-Hiickel expressions) the calculation of [Ag+ ] becomes more difficult, and even more so for [Cl ], where the solubility product value is also involved. 

According to the foregoing theory, any halide electrode can measure Ag+, but not a halide whose solubility product is lower than that of the halide in the membrane so, although an Agl electrode measures I, Br and Cl, an AgCl electrode measures only Cl". 

The solubility product is measured by determining the ion concentrations by a suitable analytical method and the results are extrapolated to zero ionic strength, where P — P. 

Potentiometry is used in the determination of various physicochemical quantities and for quantitative analysis based on measurements of the EMF of galvanic cells. By means of the potentiometric method it is possible to determine activity coefficients, pH values, dissociation constants and solubility products, the standard affinities of chemical reactions, in simple cases transport numbers, etc. In analytical chemistry, potentiometry is used for titrations or for direct determination of ion activities. 

Ksp, known as the solubility product, is widely used as a measure of the solubility of sparingly soluble salts. It should be noted that the dimensions of this constant will change according to the stoichiometry of the reaction. 

The solubility product as a measure of solubility was introduced in Chapter 3. For the solubility equilibrium... 

Equilibrium with precipitation. The previous example calculated carbonate speciation admitting unrestricted solubility of all species. Actually, it is easily verified that the calculated calcium and carbonate concentrations exceed calcium carbonate solubility as measured by the solubility product... 

Once the composition of the aqueous solution phase has been determined, the activity of an electrolyte having the same chemical formula as the assumed precipitate can be calculated (11,12). This calculation may utilize either mean ionic activity coefficients and total concentrations of the ions in the electrolyte, or single-ion activity coefficients and free-species concentrations of the ions in the electrolyte (11). If the latter approach is used, the computed electrolyte activity is termed an ion-activity product (12). Regardless of which approach is adopted, the calculated electrolyte activity is compared to the solubility product constant of the assumed precipitate as a test for the existence of the solid phase. If the calculated ion-activity product is smaller than the candidate solubility product constant, the corresponding solid phase is concluded not to have formed in the time period of the solubility measurements. Ihis judgment must be tempered, of course, in light of the precision with which both electrolyte activities and solubility product constants can be determined (12). 

Some ionic salts dissolve only imperceptibly, e.g. 10 -10 mol dm . A simple measure of how much material actually enters solution is the solubility constant (also called the solubility product, and sometimes given the symbol Tsp)- An expression for may be formulated simply by multiplying the activity... 

In the context of the desirability of removing sulfur compounds from fuels, a bacterial strain has been identified that will metabolize thianthrene to water-soluble products under aerobic conditions (83MI5). A thermophilic organism, Sulfolobus acidocaldarius, removed 38% of the sulfur, as measured by sulfate release, in 4 weeks at 70°C (87MI2). 

This relationship enables AG for a reaction to be calculated from the experimentally determined equilibrium constant. Alternatively, if K is difficult to measure experimentally (as, for example, in the case of certain solubility products) the same expression may be used to obtain K from AG . 

It has long been recognized that ferric iron is a moderately strong acid. As early as 1896, Goodwin (5) concluded from conductometric measurements that simple dilution of ferric chloride solutions led to the formation of FeOH2+. The insolubility of ferric hydroxide has of course been appreciated even longer. The best current estimate of the solubility product constant for Fe OH)s at 25° (in 3 M NaC104) is (d). 

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