Salt precipitation methods

For Volhard methods identified by an asterisk ( ) the precipitated silver salt must be removed before carrying out the back titration. 

Soluble sulfides such as sodium sulfide, potassium sulfide, and calcium polysulfides have been used to precipitate mercury salts from alkaline solutions. When this procedure is used, exercise of caution is requked to maintain the pH within a given alkaline range so as to prevent evolution of H2S. Because the solubiUty of mercuric sulfide in water is 12.5 flg/L at 18°C or 10.7 ppb of mercury, use of this method for removal of mercury is adequate for most purposes. However, the presence of excess alkah, such as sodium hydroxide or sodium sulfide, increases the solubiUty of mercuric sulfide as shown ... 

The precipitation method of separation involves the addition of salts such as ammonium sulfate or solvents such as polyethylene glycol to the reagent mixture to cause precipitation of the large molecular weight bound species. These methods of precipitation lack specificity and work well only when there is a large difference between the molecular weight of the material being measured and that of the bound complex of it. 

Tyree and Bynum [132] described an ion chromatographic method for the determination of nitrate and phosphate in seawater. The pre-treatment comprised vigorous mixing of the sample with a silver-based cation-exchange resin, followed by filtration to remove the precipitated silver salt. 

A better method has been described by Schwarz (S6). Washed red blood cells are lysed, precipitated with trichloroacetic acid below 0°C and the supernatant quickly neutralized. Speed and low temperatures are necessary to prevent hydrolysis of galactose-l-phosphate which is very sensitive to acid. Barium acetate and ethanol are added, and the precipitated barium salt of galactose-l-phosphate washed with 80% ethanol. The barium salt is then hydrolyzed by heating with dilute hydrochloric acid, acid and salts removed with mixed ion-exchange resins, and the galactose estimated by paper chromatography as described above. It is probably better to avoid the Amberlite MB-1 resin used by Schwarz and, instead, to use a weak base resin mixture, such as Amberlite MB-4. Recovery of added galactose-l-phosphate should be determined simultaneously. 

Method B Cl2 is bubbled through a stirred solution of BTMA-IC12 (34.8 g, 0.1 mol) in CH2C12 (300 ml) over a period of ca. 30 min and the precipitated tetrachloroiodate salt is collected by filtration. 

Elemental analysis A1 15.77% O 56.12% S 28.11%. A1 may he determined hy colorimetric method or hy atomic absorption or emission spectrophotometry sulfate may he determined by BaCb precipitation method in the aqueous solution of the salt. 

There are several processes for commercial thorium production from monazite sand. They are mostly modifications of the acid or caustic digestion process. Such processes involve converting monazite to salts of different anions by combination of various chemical treatments, recovery of the thorium salt by solvent extraction, fractional crystallization, or precipitation methods. Finally, metalhc thorium is prepared by chemical reduction or electrolysis. Two such industrial processes are outlined briefly below. 

For practical (real) catalyst systems, precipitation, ion exchange, impregnation and sol-gel processing procedures are used. In precipitation methods, a hydroxide or a carbonate of a metal may be precipitated from a solution of a metal salt onto the support material held in the solution. Thus, a copper-silica catalyst may be prepared using a Cu-nitrate solution in which silica is suspended. Additives of any alkali cause the precipitation of copper hydroxide onto the silica support. This is then dried and normally reduced in hydrogen at moderate temperatures ( 400-500 °C) to form the catalyst. In co-precipitation techniques , the support is precipitated simultaneously with the active catalyst. In the ion-exchange method, for example, highly dispersed Pt on... 

The precipitation of cuprous acetylide was introduced into analytical chemistry for the quantitative determination of copper. Since cupric acetylide was dangerous to handle, Makowka [108] worked out a method in which cupric salts are previously reduced, e.g. with hydroxylamine, to cuprous salts, when the acetylide is precipitated. Cuprous salts in a solution of hydroxylamine are employed as reagents for acetylene (e. g. Pietsch and Kotowski [114]). 

Table IV gives the Rf values determined under these conditions. Figure 3 shows the chromatograms of the reaction products of the re-esterification of a number of plasticizers. It is obvious that the method is suitable for identifying the alcohol components of all frequently used ester types. Difficulties have been met with phenolic components although they are present primarily in phosphoric acid esters. To identify these, evaporate to dryness a few drops of the reaction mixture of an alkaline saponification and mix this with a bit of the precipitated alkali salt. Add this residue to 10 to 20 drops of chloroform and gradually evaporate it repeat...

PEG precipitation works well for IgM, but is less efficient for IgG, and salt precipitation methods are usually recommended for the latter. PEG precipitation may be preferred in multistep purifications that use ion-exchange columns, because the ionic strength is not altered and therefore does not require dialysis before ion-exchange chromatography. Furthermore, it is a very mild procedure that usually results in little denaturation of antibody. This procedure is applicable to both polyclonal antisera and most MAb-containmg fluids. 

A systematic study of salting-out precipitation is carried out to obtain the operational limits within which this precipitation method can be applied for the production of fines (mean particle size <10 xm) with acceptable quality and productivity. The model substances glycine and sodium chloride are salted-out from their aqueous solutions by using ethanol as antisolvent. The main operational parameter is the initial supersaturation of the solutions. It is shown that the smallest particles can be produced at the limits of the metastability domain determined by three optional process parameters the initial solution concentration, the equilibrium solubility and the operational time. The product quality (crystallinity, polymorphic states, aggregation) and productivity considerably change with the operational conditions. 

Trimethylsilyl cyanide (0.54 ml, 4 mmol) was added to a stirred suspension of iodosyl triflate (0.58 g, 2 mmol) in dichloromethane (15 ml) at — 78°C under nitrogen. The mixture was allowed to warm to — 20°C and stirred at this temperature for 15 min until the formation of a clear solution. The solution was cooled to — 78°C and transferred to a cold stirred solution of the appropriate tributyltin heterocycle (4 mmol) in dichloromethane (15 ml). The mixture was brought to room temperature and crystallized by the addition of dry hexane (20-30 ml). The precipitated iodonium salt was filtered under nitrogen, washed with dry ether (30 ml) and dried in vacuo. Mono or bis hetaryl iodonium salts prepared by these and related methods also involved groups coming from selenophene [23], pyrazoles [24], benzothiophene [21], etc. 

Permanent hardness can also be estimated by the alkalimetric method of Wartha and Pfeifer. A measured volume (200 e.e.) of the water is boiled with 50 c.e. of a mixture of decmormal solutions of sodium carbonate and hydroxide in equal amounts after restoring to the original volume and allowing the solution to settle, the residual alkali is determined by titration with standard acid. As the bicarbonates do not cause any consumption of alkali, there is a direct proportionality between the quantity of alkali which disappears and the total amount of sulphates and chlorides of calcium and magnesium. Sodium carbonate alone does not efficiently precipitate magnesium salts from solution, but precipitation as the hydroxide is complete if excess of sodium hydroxide is present it is for this reasoii that a mixture of sodium carbonate and hydroxide is applied 3 (see also p. 211). 

Purification can also be achieved in other ways, through precipitation with salts, crystallization, and through aqueous two-phase extraction. Some of these methods are associated with substantial capital cost, low throughput, low yields, or waste issues. Fractional precipitation, one of the oldest protein separation technologies, can be surprisingly effective to separate a compound of interest from a complex broth. For instance, the process of fractional solvent precipitation of blood plasma components has been used since World War II. 

The double inverse microemulsion method was also used to synthesize per-ovskite-type mixed metal oxides [ 155]. One microemulsion solution contained nitrate salts of either Ba(N03)2/Pb(N03)2, La(N03)3/Cu(N03)2 or La(N03)3/ Ni(N03)2, and the other microemulsion contained ammonium oxalate or oxalic acid as the precipitant. These metal oxalate particles of about 20 nm were readily calcined into single phase perovskite-type BaPb03, La2Cu04 and LaNi03. The calcinations required for the microemulsion-derived mixed oxalates were 100-250 °C below the temperatures used for the metal oxalates prepared by a conventional aqueous solution precipitation method. 

The solubility of proteins and nucleic acids in aqueous solution depends on the solvation of the macromolecule by water this can be influenced by pH, ionic strength and temperature, and also by the addition of salts, or water-soluble organic solvents. We discuss below the various precipitation methods that have been used with proteins and nucleic adds, particularly with regard to concentration and fractionation procedures. 

---