High-pH separation

We are not limited to using only salt displacement to remove the sample cation. If it is a weak cation, its charge can be removed by running a pH gradient to high pH. Separation will be in order of pKb, with the lowest pKb coming off first. Since many of the naturally occurring cations are amines, it will be necessary to go to pH around 12 and a saturation column will be required. 

Regenerate the capillary surface. The capillary should be regenerated with a minimum two capillary volume rinse with regenerating solution before re-equilibration with separation buffer. For neutral to high pH separations, 0.1-1.0 M NaOH is effective while 0.1-0.5 M phosphoric acid (or the acid equivalent of your separation buffer) is adequate for low pH separations. This should be followed with a short (one capillary volume) rinse with distilled water. 

The relation between pH and the distribution ratio for a weak base (curve 3) is therefore the opposite of a weak acid so that it is possible to separate acids from bases in a mixture either by extracting the acids at low pH or the bases at high pH. Separating mixtures of acids or mixtures of bases is possible only if their dissociation or protonation constants differ by several pK units. 

Most plasmids are topologically closed circles of DNA. They can be separated from the bulk of the chromosomal DNA by virtue of their resistance to alkaline solution. The double-stranded stmcture of DNA is denatured at high pH, but because the two strands of the plasmid are topologically joined they are more readily renatured. This property is exploited in rapid procedures for the isolation of plasmid DNA from recombinant microorganisms (5,6). 

Tannins occur in many plants and are separated by extraction. At present, only quebracho extract is used as a mud thinner in significant quantity in the United States. Quebracho is an acidic material and performs best at high pH. It is an excellent thinner for lime-treated and cement-contaminated muds. However, it is not effective at high salt concentrations. Sulfomethylated tannin products are functional over a wide range of pH and salinity and have either been treated with chromium for good thermal stabiUty (58) or are chrome free. Concentrations of tannin additives are ca 1.5—18 kg/m (0.5—6 lb/bbl). 

Capillary Electrophoresis. Capillary electrophoresis (ce) is an analytical technique that can achieve rapid high resolution separation of water-soluble components present in small sample volumes. The separations are generally based on the principle of electrically driven ions in solution. Selectivity can be varied by the alteration of pH, ionic strength, electrolyte composition, or by incorporation of additives. Typical examples of additives include organic solvents, surfactants (qv), and complexation agents (see Chelating agents). 

The pH effect in chelation is utilized to Hberate metals from thein chelates that have participated in another stage of a process, so that the metal or chelant or both can be separately recovered. Hydrogen ion at low pH displaces copper, eg, which is recovered from the acid bath by electrolysis while the hydrogen form of the chelant is recycled (43). Precipitation of the displaced metal by anions such as oxalate as the pH is lowered (Fig. 4) is utilized in separations of rare earths. Metals can also be displaced as insoluble salts or hydroxides in high pH domains where the pM that can be maintained by the chelate is less than that allowed by the insoluble species (Fig. 3). 

Neutralization Acidic or basic wastewaters must be neutrahzed prior to discharge. If an industry produces both acidic and basic wastes, these wastes may be mixed together at the proper rates to obtain neutral pH levels. Equahzation basins can be used as neutralization basins. When separate chemical neutralization is required, sodium hydroxide is the easiest base material to handle in a hquid form and can be used at various concentrations for in-line neutralization with a minimum of equipment. Yet, lime remains the most widely used base for acid neutr zation. Limestone is used when reaction rates are slow and considerable time is available for reaction. Siilfuric acid is the primary acid used to neutralize high-pH wastewaters unless calcium smfate might be precipitated as a resmt of the neutralization reaction. Hydrochloric acid can be used for neutrahzation of basic wastes if sulfuric acid is not acceptable. For very weak basic waste-waters carbon dioxide can be adequate for neutralization. 

Each term in this equation represents an independent pathway. The low-pH arm in the figure is equivalent to reaction (6-57), or one similar to it, in which the proton attacks the substrate directly. The high-pH pathway represents the unimolecular reaction of the substrate or else its reaction with water. As this discussion illustrates, a reaction whose pH profile shows upward bends can be analyzed in terms of separate pathways. A complex profile can be separated into regions at each upward bend each region is a distinct pathway. 

Togami, D. W., Poulsen, B. J., Batalao, C. W., and Rolls, W. A., Separation of carbohydrates and carbohydrate derivatives by HPLC with cation-exchange columns at high pH, BioTechniques, 10, 650, 1991. 

Townsend, R. R., Atkinson, R H., and Trimble, R. B., Separation of high-mannose isomers from yeast and mammalian sources using high pH anion-exchange chromatography, Carb. Res., 215, 211. 1991. 

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