Buffering reactions

The sensitivity of the equilibrium constant to temperature, therefore, depends upon the enthalpy change AH . This is usually not a serious limitation, because most reaction enthalpies are sufficiently large and because we commonly require that the perturbation be a small one so that the linearization condition is valid. If AH is so small that the T-jump is ineffective, it may be possible to make use of an auxiliary reaction in the following way Suppose the reaction under study is an acid-base reaction with a small AH . We can add a buffer system having a large AH and apply the T-jump to the combined system. The T-jump will alter the Ka of the buffer reaction, resulting in a pH jump. The pH jump then acts as the forcing function on the reaction of interest. 

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. 

NHS-fluorescein is insoluble directly in aqueous solution and should be dissolved in organic solvent prior to addition of a small aliquot to a buffered reaction medium. Concentrated stock solutions may be prepared in DMSO or DMF. Such solutions are relatively stable if protected from light. Reaction conditions should be maintained at the optimal reactivity for NHS esters—pH 7-9. 

SAMSA-fluorescein is an orange solid compound. Dissolved in buffer at pH 9.0, its maximal wavelength of absorption or excitation is at 495 nm, while its emission wavelength maximum is 520 nm. The reagent and all solutions and derivatives made from it are light sensitive and should be stored in the dark. SAMSA-fluorescein is soluble in aqueous solutions above pH 6.0, but it can be dissolved in DMF to prepare a concentrated stock solution prior to adding a small amount to a buffered reaction mixture. 

Cascade Blue acetyl azide is soluble in aqueous solution, but the reactive azide group will hydrolyze and should be used immediately in a conjugation reaction. A concentrated stock solution may be prepared in water, dissolved quickly, and an aliquot quickly added to a buffered reaction medium. For aqueous reactions, a pH range of 7-9 is optimal. Avoid amine-containing buffers. 

The NHS ester end of NHS-LC-biotin reacts with amine groups in proteins and other molecules to form stable amide bond derivatives (Figure 11.4). Optimal reaction conditions are at a pH of 7-9, but the higher the pH the greater will be the hydrolysis rate of the ester. Avoid amine-containing buffers which will compete in the acylation reaction. NHS-LC-biotin is insoluble in aqueous reaction conditions and must be solubilized in organic solvent prior to the addition of a small quantity to a buffered reaction. Preparation of concentrated stock solutions may be done in DMF or DMSO. Nonaqueous reactions also may be done with this reagent for the modification of molecules insoluble in water. The molar ratio of NHS-LC-biotin to a... 

NHS-iminobiotin is insoluble in aqueous solution. It can be dissolved in organic solvent (DMF) prior to addition of a small aliquot to a buffered reaction medium. Don t exceed 10 percent DMF in the reaction to avoid protein precipitation problems. Optimal conditions for protein derivatization include non-amine-containing buffers at a pH of 7-9. The following... 

Biotin-BMCC is insoluble in water and must be dissolved in an organic solvent prior to addition to an aqueous reaction mixture. Preparing a concentrated stock solution in DMF or DMSO allows transfer of a small aliquot to a buffer reaction. The upper limit of biotin-BMCC solubility in DMSO is approximately 33 mM or 17 mg/ml. In DMF, it is only soluble to a level of about 7 mM (4 mg/ml). Upon addition of an organic solution of the reagent to an aqueous environment (do not exceed 10 percent organic solvent in the aqueous medium to prevent protein precipitation), biotin-BMCC may form a micro-emulsion. This is normal and during the course of the reaction, the remainder of the compound will be driven into solution as it couples or hydrolyzes. 

Fig. 15.1. Calculated effects on pH of reacting hydrochloric acid into a 0.2 molal NaCl solution and a 0.1 molal Na2CC>3 solution, as functions of the amount of HC1 added. The two plateaus on the second curve represent the buffering reactions between COJ- and HCOJ, and between HCO3 and C02(aq).

The simplified form is not as exact but is less cluttered than the full form and shows more clearly the nature of the buffering reaction. In this book, we will often make simplifications of this sort. 

Fig. 15.5. Concentrations of species in buffer reactions that contribute to the titration alkalinity of Mono Lake water, plotted against pH.

As shown in Table I, the AH values of the buffer reactions corresponding roughly to Ka], Ka2, and Ka3 (16.7, 50.0, and 83.3% neutralization, respectively) all have absolute values less than... 

Clay minerals are important to the crustal-ocean-atmosphere fectory, not just for their abundance, but because they participate in several biogeochemical processes. For example, the chemical weathering reactions responsible for their formation are accompanied by the uptake and release of cations and, thus, have a large impact on the chemical composition of river and seawater. This includes acid/base buffering reactions, making clay minerals responsible for the long-term control of the pH of seawater and, hence, of importance in regulating atmospheric CO2 levels. 

The buffer reactions of equations 5.286 to 5.290 have been accurately calibrated by various authors, and the fugacity (actually activity ) of O2 is usually expressed as a semilogarithmic function of absolute T. The buffer functions above are taken from Simons (1986) (eq. 5.286, 5.288, and 5.289), Huebner and Sato (1970) (eq. 5.287), and Eugster and Wones (1962) (eq. 5.290). 

One pecuhar oxygen buffer reaction is the well-known thermobarometric equation of Buddington and Lindsley (1964), which furnishes information on both T and the activity of molecular oxygen. The reaction uses the equilibrium among titanomagnetite (or ulvospinel ), magnetite, hematite, and ilmenite components in the hemo-ilmenite and spinel phases ... 

Hemingway, B.S. (1990) Thermodynamic properties for bunsenite, NiO, magnetite, Fe304, and hematite, Fe203, with comments on selected oxygen buffer reactions. Am. Min. 75 781-790... 

BSOCOES is a hydrophobic cross-linker and therefore must be dissolved in organic solvent prior to its addition to an aqueous reaction medium. Preparing a stock solution in DMF or DMSO and then adding an aliquot to the cross-linking reaction is recommended. Do not exceed a concentration of more than 10% organic solvent in the buffered reaction. 

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