Precipitation. Fractional

Fractional precipitation means the removal of part of the dissolved elements from solution by adding a chemical reagent to form a new less-soluble compound. In this respect, it is different from fractional crystallisation, where NO other compound is introduced in the solution. 

Hydroxides and double sulphate have extensively been used, as well as double chromate. The latter one especially has been used for the separation of yttrium from the other rare earths. The addition of sodium sulphate to the rare earth solution leads to the precipitation of double sulphates. The elements La, Ce, Pr, Nd and Sm form poorly-dissolvable double sulphates, whereas Ho, Er, Tm, Yb, Lu and Y form well-dissolvable double salts. The salts of Eu, Gd and Dy form salts of intermediate solubility. Generally, the use of this method cmdely separates the rare earth mixture 

4 Mineral Processing and Extractive Metallurgy of the Rare Earths 

Because less than 0.1% of the remains, we conclude that precipitation is essentially complete. 

Because [OH ] is maintained at a constant value, the calculation of is straightforward. 

PRACTICE EXAM PLEA A typical Ca concentration in seawater is 0.010 M. Will the precipitation of Ca(OH)2 be complete from a seawater sample in which [OH ] is maintained at 0.040 M  

The key factors in determining whether the target ion is essentially completely removed from solution in a precipitation are (1) the value of (2) the initial concentration of the target ion, and (3) the concentration of the common ion. In general, completeness of precipitation is favored by 

Fractional precipitation is a technique in which two or more ions in solution, each capable of being precipitated by the same reagent, are separated by the proper use of that reagent One ion is precipitated, while the other(s) remains in solution. The primary condition for a successful fractional precipitation is 

Even when both products are insoluble, we can still achieve some degree of separation by choosing the proper reagent to bring about precipitation. Consider a solution that contains Cl, Brand 1  

Sample FToblem 17.12 describes the separation of only two ions (CF and Br ), but the procedure can be applied to a solution containing more than two different types of ions. 

Silver nitrate is added slowly to a solution that is 0.020 M in CF ions and 0.020 M in Br ions. Calculate the concentration of Ag ions (in mol/L) required to initiate the precipitation of AgBr without precipitating AgCl. 

Setup The solubility equilibria, K p values, and equilibrium expressions for AgCl and AgBr are 

Solution Solving the AgBr equilibrium expression for Ag conc itration, we have and 

Thermogravimetry may be used to determine the composition of binary mixtures. If each component possesses a characteristic unique pyrolysis curve, then a resultant curve for the mixture will afford a basis for the determination of its composition. In such an automatic gravimetric determination the initial weight of the sample need not be known. A simple example is given by the automatic determination of a mixture of calcium and strontium as their carbonates. 

Both carbonates decompose to their oxides with the evolution of carbon dioxide. The decomposition temperature for calcium carbonate is in the temperature range 650-850 °C, whilst strontium carbonate decomposes between 950 and 1150°C. Hence the amount of calcium and strontium present in a mixture may be calculated from the weight losses due to the evolution of carbon dioxide at the lower and higher temperature ranges respectively. This method could be extended to the analysis of a three-component mixture, as barium carbonate is reported to decompose at an even higher temperature ( 1300 °C) than strontium carbonate. 

Thermogravimetric analysis has also been used in conjunction with other techniques, such as differential thermal analysis (DTA), gas chromatography, and mass spectrometry, for the study and characterisation of complex materials such as clays, soils and polymers.35 

QUANTITATIVE SEPARATIONS BASED UPON PRECIPITATION METHODS 

The simple theory of fractional precipitation has been given in Section 2.8. It 

If to a solution of a mixture of two salts, A and B, a third substance C, is added, in an amount insufficient to precipitate all A and B in the solution, more of one salt will be precipitated, as a rule, than the other. By redissolving the mixed precipitate and again partially precipitating the salts, we can, by many repetitions f the process, effect fairly good separations of substances otherwise intractable to any known process of separation. 

Since Mosander thus fractioned the gadolinite earths in 1839, the method has been extensively employed by W. Crookes (Ghem. News, 54, 131, 155, 1886), in some fine work on the yttria and other earths. The recent separations of polonium, radium and other curiosities have attracted some attention to the process. The mathematics of the reactions follows directly from the law of mass action. Let only sufficient C be added to partially precipitate A and B and let the solution originally contain a of the salt A, b of the salt B. Let x and y denote the amounts of A and B precipitated at the end of a certain time t, then a - x and b - x will represent the amounts of A and B respectively remaining in the solution. The rates of precipitation are, therefore, 

Let AB (Fig. 100) be any curve whose equation is known. It q B is required to find the area of the portion bounded by the curve the two coordinates PM, QN and that portion of the a axis, MN, included between the ordinates at the extremities of that portion of the curve under investigation. The area can be approximately determined by supposing PQMN to be cut up into small strips—called surface elements— perpendicular to the ic-axis finding the area of each separate strip on the assumption that the curve bounding one end of the strip is a straight Univ Calif - Digitized by Microsoft  

Area PQMN — Area PBLM + Area BQNL. 

In order to have some concrete image before the mind, let us find the area of PQNM in Fig. 101. Take any small strip PBSM let PM = y, BS = y + By OM = x and OS = x + 8x. Let SA represent the area of the small strip under consideration. If the short distance, PB, were straight and not curved, the area, SA, of the trapezium PBSM would be, (11), page 604, 

In this method a solution or dispersion of a plastic is stirred into at least 10 times its volume of a solvent, which acts as a precipitant for the dissolved plastic but which dissolves completely in the first solvent. The precipitated plastic is pulverised and repeatedly extracted with the precipitant/solvent or re-dissolved and precipitated. 

Or a non-solvent is added to a solution of the plastic until the first faint turbidities appear due to the high polymer plastic components. The solvent is then reduced on a water bath or by vacuum distillation. The solvent and the non-solvent must be completely miscible and the solvent must have a lower boiling point than the non-solvent. 

Or the plastic is dissolved in a solvent, which dissolves the plastic at higher temperatures - the plastic separates out on cooling while the additives remain in solution. 

Or plastic dispersions can often be separated by stopping the action of the emulsifier and/or dispersing agent, i.e., breaking the emulsion. The following methods can be used according to the type of emulsifier  

Precipitation of the emulsifier by the addition of acids or a interfacially active counterion. 

For AgBr to precipitate from solution, the silver ion concentration must exceed 3.9 X lO AT. Solving the AgCl equilibrium expression for the Ag concentration, we have 

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For preparative purposes batch fractionation is often employed. Although fractional crystallization may be included in a list of batch fractionation methods, we shall consider only those methods based on the phase separation of polymer solutions fractional precipitation and coacervate extraction. The general principles for these methods were presented in the last section. In this section we shall develop these ideas more fully with the objective of obtaining a more narrow distribution of molecular weights from a polydisperse system. Note that the final product of fractionation still contains a distribution of chain lengths however, the ratio M /M is smaller than for the unfractionated sample. 

Figure 8.3b shows that phase separation in polymer mixtures results in two solution phases which are both dilute with respect to solute. Even the relatively more concentrated phase is only 10-20% by volume in polymer, while the more dilute phase is nearly pure solvent. The important thing to remember from both the theoretical and experimental curves of Fig. 8.3 is that both of the phases which separate contain some polymer. If it is the polymer-rich or precipitated phase that is subjected to further work-up, the method is called fractional precipitation. If the polymer-poor phase is the focus of attention, the method... 

Separation Processes. The product of ore digestion contains the rare earths in the same ratio as that in which they were originally present in the ore, with few exceptions, because of the similarity in chemical properties. The various processes for separating individual rare earth from naturally occurring rare-earth mixtures essentially utilize small differences in acidity resulting from the decrease in ionic radius from lanthanum to lutetium. The acidity differences influence the solubiUties of salts, the hydrolysis of cations, and the formation of complex species so as to allow separation by fractional crystallization, fractional precipitation, ion exchange, and solvent extraction. In addition, the existence of tetravalent and divalent species for cerium and europium, respectively, is useful because the chemical behavior of these ions is markedly different from that of the trivalent species. 

The primary polymerization product ia these processes has a relatively wide molecular weight distribution, and a separate step is often used to narrow the polydispersity. Such a narrowkig step may consist of high vacuum stripping to remove volatile polymer chains, often followed by a solvent fractionation step (35,36), sometimes a solvent fractionation step alone (37,38), or a fractional precipitation from organic solvent (32). The molecular weight distribution can also be narrowed by depolymerization at elevated temperatures ia the presence of a depolymerization catalyst (217—220). 

The theory of the process is as follows. This is a case of fractional precipitation (Section 2.8), the two sparingly soluble salts being silver chloride (Xsol 1.2 x 10 10) and silver chromate (Kso] 1.7 x 10 12). It is best studied by considering an actual example encountered in practice, viz. the titration of, say, 0.1M sodium chloride with 0.1M silver nitrate in the presence of a few millilitres of dilute potassium chromate solution. Silver chloride is the less soluble salt and the initial chloride concentration is high hence silver chloride will be precipitated. At the first point where red silver chromate is just precipitated both salts will be in equilibrium with the solution. Hence ... 

The precipitated metallic hydroxides or hydrated oxides are gelatinous in character, and they tend to be contaminated with anions by adsorption and occlusion, and sometimes with basic salts. The values presented in Table 11.2 suggest that many separations should be possible by fractional precipitation of the hydroxides, but such separations are not always practical owing to high local concentrations of base when the solution is treated with alkali. Such unequal concentrations of base result in regions of high local pH and lead to the precipitation of more soluble hydroxides, which may be occluded in the desired precipitate. Slow, or preferably homogeneous, precipitation overcomes this difficulty, and much sharper separations may be achieved. 

Purification of photoprotein. The dialyzed photoprotein solution was centrifuged to remove precipitates, and then subjected to fractional precipitation by ammonium sulfate, taking a fraction precipitated between 30% and 50% saturation. The protein precipitate was dissolved in 50 ml of 10 mM sodium phosphate, pH 6.0, containing 0.1 mM oxine ( pH 6.0 buffer ), dialyzed against the same buffer, and the dialyzed solution was adsorbed on a column of DEAE-cellulose (2.5 x 13 cm) prepared with the pH 6.0 buffer. The elution was done by a stepwise increase of NaCl concentration. The photoprotein was eluted at 0.2-0.25 M NaCl and a cloudy substance (cofactor 1) was eluted at about 0.5 M NaCl. The photoprotein fraction was further purified on a column of Sephadex G-200 or Ultrogel AcA 34 (1.6 x 80 cm) using the pH 6.0 buffer that contained 0.5 M NaCl. 

Although it is claimed that the Strecker reaction of 2 results in the exclusive formation of one isomer of 3 and that selective elimination of the minor isomer during isolation of the intermediate compound 4 is not possible, it is apparent that during the workup of the hydrolysis product 4, fractional precipitation or crystallization or other separation of the diastereomers may... 

These operations often select for impurity removal as well as further product concentration. Approaches include fractional precipitation. Other alternatives such as chromatography and adsorption are also considered as methods of process purification. 

Liquid samples might appear to be easier to prepare for LC analysis than solids, particularly if the compounds of interest are present in high concentration. In some cases this may be true and the first example given below requires virtually no sample preparation whatever. The second example, however, requires more involved treatment and when analyzing protein mixtures, the procedure can become very complex indeed involving extraction, centrifugation and fractional precipitation on reversed phases. In general, however, liquid samples become more difficult to prepare when the substances are present at very low concentrations. 

Water soluble protein with a relative molecular mass of ca. 32600, which particularly contains copper and zinc bound like chelate (ca. 4 gram atoms) and has superoxide-dismutase-activity. It is isolated from bovine liver or from hemolyzed, plasma free erythrocytes obtained from bovine blood. Purification by manyfold fractionated precipitation and solvolyse methods and definitive separation of the residual foreign proteins by denaturizing heating of the orgotein concentrate in buffer solution to ca. 65-70 C and gel filtration and/or dialysis. 

By extraction of Cinchona bark with aromatic hydrocarbons, conversion of the crude alkaloids into the sulfates and fractional precipitation with NaOH as sulfate. 

The data on the molecular mass and on the intrinsic viscosity confirmed the expectation that the fraction, precipitated under freezing, had a higher molecular mass (Table 1). 

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