AQUEOUS SOLUTIONS AND PRECIPITATION

The mixture of diastereomers has been separated into its two principal components by Izatt, Haymore, Bradshaw and Christensen who had previously identified the two principal diastereomers as the cis-syn-cis and cis-anti-cis isomers. Their previous separation technique involved a protracted chromatography on alumina but the new method relied upon the difference in water solubility between the lead perchlorate and hydroniur perchlorate complexes. The lead perchlorate complex is essentially insoluble in aqueous solution and precipitates from it. Using this method, one may obtain 39% of the high-melting polymorph (mp 83—84°) and 44% of the low-melting compound (mp 62—63°). Note that the former also exists in a second crystalline form, mp 69—70°. 

Introduction and Orientation, Matter and Energy, Elements and Atoms, Compounds, The Nomenclature of Compounds, Moles and Molar Masses, Determination of Chemical Formulas, Mixtures and Solutions, Chemical Equations, Aqueous Solutions and Precipitation, Acids and Bases, Redox Reactions, Reaction Stoichiometry, Limiting Reactants... 

To isolate carboxylic acid rather than the salt, it is essential to add acid (e.g. dilute HC1) to the aqueous solution. The acid protonates the carboxylate salt to give the carboxylic acid that (in most cases) is no longer soluble in aqueous solution and precipitates out as a solid or as an oil. 

Not yet familiar with these results, we oxidized [BnHn]2- with FeCl3 in an aqueous solution and precipitated [NBzlEt3]2[B22H22] on addition of the corresponding ammonium chloride in a 62 % yield. 

The work often started by wetting the finely ground mineral powder with concentrated sulfuric acid. On heating to dull red heat, sulfates of the RE elements were formed and could be dissolved in ice water (the solubility increases with decreasing temperature). Oxalic acid was added to the aqueous solution and precipitated RE oxalates, which were transformed to oxides by calcination. This oxide mixture was the raw material in the separation work. 

Minerals dissolve into aqueous solution and precipitate from aqueous solution by the water-rock interaction process. The rate of dissolution of minerals for one component system is simply expressed as... 

Amines extract rare earths in reversed order compared to organophosphates. Aliphatic amines form salts with acids in aqueous solution and precipitate the hydroxides of metals. The solubility of amines in water decreases with increase in molecular weight. Primary amines extract from sulfate solutions while the tertiary amines extract from nitrate solutions. The lighter rare earths are preferentially extracted by amines and when used with aqueous soluble aminocarboxylic acid, chelating reagents preferentially extract the heavy rare earths. 

To hydrolyse an ester of a phenol (e.g., phenyl acetate), proceed as above but cool the alkaline reaction mixture and treat it with carbon dioxide until saturated (sohd carbon dioxide may also be used). Whether a solid phenol separates or not, remove it by extraction with ether. Acidify the aqueous bicarbonate solution with dilute sulphuric acid and isolate the acid as detailed for the ester of an alcohol. An alternative method, which is not so time-consuming, may be employed. Cool the alkaline reaction mixture in ice water, and add dilute sulphuric acid with stirring until the solution is acidic to Congo red paper and the acid, if aromatic or otherwise insoluble in the medium, commences to separate as a faint but permanent precipitate. Now add 5 per cent, sodium carbonate solution with vigorous stirring until the solution is alkaline to litmus paper and the precipitate redissolves completely. Remove the phenol by extraction with ether. Acidify the residual aqueous solution and investigate the organic acid as above. 

The actinide elements exhibit uniformity in ionic types. In acidic aqueous solution, there are four types of cations, and these and their colors are hsted in Table 5 (12—14,17). The open spaces indicate that the corresponding oxidation states do not exist in aqueous solution. The wide variety of colors exhibited by actinide ions is characteristic of transition series of elements. In general, protactinium(V) polymerizes and precipitates readily in aqueous solution and it seems unlikely that ionic forms ate present in such solutions. 

The chlorides, bromides, nitrates, bromates, and perchlorate salts ate soluble in water and, when the aqueous solutions evaporate, precipitate as hydrated crystalline salts. The acetates, iodates, and iodides ate somewhat less soluble. The sulfates ate sparingly soluble and ate unique in that they have a negative solubitity trend with increasing temperature. The oxides, sulfides, fluorides, carbonates, oxalates, and phosphates ate insoluble in water. The oxalate, which is important in the recovery of lanthanides from solutions, can be calcined directly to the oxide. This procedure is used both in analytical and industrial apptications. 

Acids that are solids can be purified in this way, except that distillation is replaced by repeated crystallisation (preferable from at least two different solvents such as water, alcohol or aqueous alcohol, toluene, toluene/petroleum ether or acetic acid.) Water-insoluble acids can be partially purified by dissolution in N sodium hydroxide solution and precipitation with dilute mineral acid. If the acid is required to be free from sodium ions, then it is better to dissolve the acid in hot N ammonia, heat to ca 80°, adding slightly more than an equal volume of N formic acid and allowing to cool slowly for crystallisation. Any ammonia, formic acid or ammonium formate that adhere to the acid are removed when the acid is dried in a vacuum — they are volatile. The separation and purification of naturally occurring fatty acids, based on distillation, salt solubility and low temperature crystallisation, are described by K.S.Markley (Ed.), Fatty Acids, 2nd Edn, part 3, Chap. 20, Interscience, New York, 1964. 

Copper(II) also forms stable complexes with O-donor ligands. In addition to the hexaaquo ion, the square planar /3-diketonates such as [Cu(acac)2l (which can be precipitated from aqueous solution and recrystallized from non-aqueous solvents) are well known, and tartrate complexes are used in Fehling s test (p. 1181). 

A mixture of 37.6 g of N-acetyl-L-glutamine and 1.000 ml of water is heated to 40°C, and 900 ml of an isopropanol solution containing 40 g of aluminum isopropoxide isadded to the warm mixture with stirring. The stirring is continued for 10 minutes. The reaction mixture is filtered and the filtrate is concentrated under reduced pressure. Isopropanol is added to the aqueous solution and the salt precipitates in the solution. The precipitates are collected by filtration and upon drying, 48.5 g of the crystalline-like aluminum salt of N-acetyl-L-glutamine are obtained. 

Poloxamers are used primarily in aqueous solution and may be quantified in the aqueous phase by the use of compleximetric methods. However, a major limitation is that these techniques are essentially only capable of quantifying alkylene oxide groups and are by no means selective for poloxamers. The basis of these methods is the formation of a complex between a metal ion and the oxygen atoms that form the ether linkages. Reaction of this complex with an anion leads to the formation of a salt that, after precipitation or extraction, may be used for quantitation. A method reported to be rapid, simple, and consistently reproducible [18] involves a two-phase titration, which eliminates interferences from anionic surfactants. The poloxamer is complexed with potassium ions in an alkaline aqueous solution and extracted into dichloromethane as an ion pair with the titrant, tet-rakis (4-fluorophenyl) borate. The end point is defined by a color change resulting from the complexation of the indicator, Victoria Blue B, with excess titrant. The Wickbold [19] method, widely used to determine nonionic surfactants, has been applied to poloxamer type surfactants 120]. Essentially the method involves the formation in the presence of barium ions of a complex be-... 

Actually, Pbl2 is not completely insoluble. Tiny amounts of Pb and I ions remain in the aqueous solution after precipitation, but these amounts are so small that we consider Pbl2 to be an insoluble salt. Only 4.1 X 10 mol of Pbl2 dissolve in one liter of water at 25 °C, whereas 3.5 mol of KNO3 dissolve in one liter of water at 25 °C. Figure 4 highlights this difference in solubility. 

Ions not solvated are unstable in solutions between them and the polar solvent molecules, electrostatic ion-dipole forces, sometimes chemical forces of interaction also arise which produce solvation. That it occurs can be felt from a number of effects the evolution of heat upon dilution of concentrated solutions of certain electrolytes (e.g., sulfuric acid), the precipitation of crystal hydrates upon evaporation of solutions of many salts, the transfer of water during the electrolysis of aqueous solutions), and others. Solvation gives rise to larger effective radii of the ions and thus influences their mobilities. 

An example for chemical preparation that can be carried out within seconds in a beaker is this Dissolve pyrrole in dilute sulfuric acid. Add ferric chloride as an aqueous solution and watch the black polypyrrole precipitate. The oxidizing Fe ions are reduced to Fe, imparting one +-charge and donating their now excessive Cr ion as dopant ion to the polymer. 

The materials that accumulate to form sedimentary rocks are (i) products of disintegration (weathering) of rocks, (ii) volcanic ejecta, (iii) insoluble decomposition products, (iv) precipitated substances from aqueous solutions, and (iv) bio-organically derived substances. 

Contamination is not restricted to natural products. Organic pigment powders are often precipitated from aqueous solutions and washed with water prior to drying. These have been found to contain high numbers of microbes and have been implicated in contamination instances. 

Glasner A., Weiss D. The crystallization of calcite from aqueous solutions and the role of zinc and magnesium ions. I. Precipitation of calcite in the presence of Zn2+ ions. J Inorg Nucl Chem 1980 42 655-663. 

DPDPB is insoluble in aqueous solutions and should be initially dissolved in an organic solvent prior to addition of a small aliquot to a buffered reaction medium. Preparation of a stock solution in DMSO at a concentration of 25 mM DPDPB works well. The addition of an aliquot of this stock solution to the conjugation reaction should not result in more than about 10 percent organic solvent by volume in the buffered mixture or protein precipitation may occur. 

Although NHS-LC-biotin and sulfo-NHS-LC-biotin are very popular reagents for biotinylation, they both result in hydrophobic aliphatic biotin modifications on proteins and antibodies. Unfortunately, these groups have a tendency to aggregate in aqueous solution and may cause protein precipitation or loss of activity over time. For this reason, the use of more hydrophilic PEG-based biotin compounds of approximately the same spacer length may be a better alternative for maintaining water solubility of modified proteins (Chapter 18). 

---