Buffer mixture, acid-base

A mixture of acetic acid and sodium acetate is one example of an acid/base buffer. The equilibrium position of the buffer is governed by the reaction... 

A similar catalytic dimerization system has been investigated [40] in a continuous flow loop reactor in order to study the stability of the ionic liquid solution. The catalyst used is the organometallic nickel(II) complex (Hcod)Ni(hfacac) (Hcod = cyclooct-4-ene-l-yl and hfacac = l,l,l,5,5,5-hexafluoro-2,4-pentanedionato-0,0 ), and the ionic liquid is an acidic chloroaluminate based on the acidic mixture of 1-butyl-4-methylpyridinium chloride and aluminium chloride. No alkylaluminium is added, but an organic Lewis base is added to buffer the acidity of the medium. The ionic catalyst solution is introduced into the reactor loop at the beginning of the reaction and the loop is filled with the reactants (total volume 160 mL). The feed enters continuously into the loop and the products are continuously separated in a settler. The overall activity is 18,000 (TON). The selectivity to dimers is in the 98 % range and the selectivity to linear octenes is 52 %. 

Buffer mixtures are not confined to mixtures of monoprotic acids or monoacid bases and their salts. We may employ a mixture of salts of a polyprotic acid, e.g. NaH2P04 and Na2HP04. The salt NaH2P04 is completely dissociated ... 

A buffer is a mixture of a weak conjugate acid-base pair that stabilizes the pH... 

A 10 mM ionic strength universal buffer mixture, consisting of Good zwitterio-nic buffers, [174] and other components (but free of phosphate and boric acid), is used in the pION apparatus [116,556], The 5-pKa mixture produces a linear response to the addition of base titrant in the pH 3-10 interval, as indicated in Fig. 7.53. The robotic system uses the universal buffer solution for all applications, automatically adjusting the pH with the addition of a standardized KOH solution. The robotic system uses a built-in titrator to standardize the pH mapping operation. 

In the process of a weak acid or weak base neutralization titration, a mixture of a conjugate acid-base pair exists in the reaction flask in the time period of the experiment leading up to the inflection point. For example, during the titration of acetic acid with sodium hydroxide, a mixture of acetic acid and acetate ion exists in the reaction flask prior to the inflection point. In that portion of the titration curve, the pH of the solution does not change appreciably, even upon the addition of more sodium hydroxide. Thus this solution is a buffer solution, as we defined it at the beginning of this section. 

The reactions are accompanied by a considerable volume change, and a dilatometric method was employed by Bell and Higginson (1949), who added acetaldehyde-water mixtures (containing about equal quantities of MeCHO and MeCH(OH)2) to an excess of acetone, and thus measured kj, in presence of a large number of acid catalysts. The direct hydration of acetaldehyde in aqueous buffer solutions is inconveniently fast at room temperatures, but ( (j + A ) was measured dilatometrically at 0°C by Bell and Darwent (1950), who established the existence of general acid-base catalysis. 

All these electrolytes are neutral in Bronsted acid-base properties. Although rather exceptional, an acid, a base, or a pH buffer may be added to the supporting electrolyte of neutral salts. The acid-base system to be selected depends on the purpose of the measurement. We often use trifluoromethanesulfonic acid (CF3S03F1) as a strong acid acetic acid, benzoic acid, or phenol as a weak acid an amine or pyridine as a weak base and tetraalkylammonium hydroxide (ILtNOH) as a strong base. Examples of buffer systems are the mixtures of picric acid and its R4N-salt and amines and their PlCl04-salts. Here, we should note that the acid-base reactions in aprotic solvents considerably differ from those in water, as discussed in Chapter 3. 

Cells and organisms maintain a specific and constant cytosolic pH, keeping biomolecules in their optimal ionic state, usually near pH 7. In multicellular organisms, the pH of extracellular fluids is also tightly regulated. Constancy of pH is achieved primarily by biological buffers mixtures of weak acids and their conjugate bases. 

D. Sykora, E. Tesarova, and D. W. Armstrong, Practical Considerations of the Influence of Organic Modifiers on the Ionization of Analytes and Buffers in Reversed-Phase LC, LCGC 2002,20, 974 G. W. Tindall, Mobile-Phase Buffers. I. The Interpretation of pH in Partially Aqueous Mobile Phases, LCGC 2002,20, 102 S. Espinosa, E. Bosch, and M. Roses, Acid-Base Constants of Neutral Bases in Acetonitrile-Water Mixtures, Anal. Chim. Acta 2002,454, 157. 

Prior to the chromatographic separation of amino acids on Dowex 50 columns, Carsten (C5) first desalts the urine sample on Amberlite IR 100 or Duolite C 3 and removes most of the nitrogenous bases on Amberlite IRA 400. This preliminary treatment allows for amino acid separations at ordinary temperatures using 2M and 4M HC1 on H+ columns for elution, instead of buffer mixtures a single column of 25 g of Dowex 50 is sufficient for all amino acids and 350-375 one-milliliter fractions are collected. The resolving power of this method does not seem to be as satisfactory as Moore and Stein s procedures, and it is not less time nor labor consuming. 

Polyprotic Systems In the same fashion as equation 91 has been derived, expressions for the buffer intensity of polyprotic acid-base systems can be developed. In Table 3.8, the buffer intensity of a diprotic acid-base system is derived. A polyprotic acid can be treated the same way as a mixture of indi-... 

The reaction in a mixture of a weak acid and its salt, or a weak base and its salt. Buffer mixtures or regulators. 

Solutions of any desired pH may be obtained simply by mixing a weak acid or base with one of its salts in various proportions. It is evident from equation (48) that even small amounts of strong acids and bases have only a slight effect on the pH of such mixtures. Certainly the small quantities of alkali from glass and carbon dioxide from the atmosphere can exert no perceptible influence. Such mixtures which are resistant to a change in reaction were called Buffer Mixtures by S. P. L. SOkensen. L. Michaelis coined the term Regulators. They may also be referred to as Ampholytes because of the amphoteric character of such mixtures. AU mixtures of weak adds and their salts,... 

Fels was the first to take advantage of such buffer mixtures. They were frequently employed by SSrensen. In the colori-metrie determination of hydrogen ion concentration they are almost indispensable. We see that the hydrogen or hydroxyl ion concentration always lies in the neighborhood of the dissociation constant of the acid or base used. When the acid and salt concentrations are equal,... 

Buffer solutions are liquids of such composition as to resist appreciable changes in hydrogen ion concentration. Addition of traces of acids or bases leaves their pH practically unaltered. For this reason they are indispensable as comparison media in colorimetric pH determinations. Buffer mixtures can be retained unchanged for two months if stored in closed flasks made of good glass and containing a disinfectant (a minute th3unol crystal). It is preferable, however, to prepare fresh solutions each month. 

The sensitivity of indicator papers depends upon a number of factors which will be discussed below. Suffice it to say that this sensitivity is always smaller than that of the indicator solution when determined with strong acids or bases. When buffer mixtures are used, however, the sensitivity of the paper towards hydrogen or hydroxyl ions is the same as that of the solution of the corresponding indicator. 

Furthermore, it was necessary to remodel completely the remaining contents. First the theoretical portion had to be extended considerably. In all acid-base equilibria, activities and not concentrations determine the equilibrium conditions. Therefore, from a practical viewpoint, a summarizing description of the modern theory of strong electrolytes and of the activity concept is indispensable, especially since otherwise phenomena such as influence of dilution on the pH of a buffer mixture, or the salt vcrror of indicators, etc. have no quantitative explanation. 

The product a0 oq has a maximum at pH = pKa, where a0 = ax = Vi, so that the product has the value A. For a single monoprotic buffer mixture (i.e., the mixture of a monoprotic acid and its conjugated base), the pH in the region of maximal buffer action, pH = pKa, can often be estimated from the Henderson approximation... 

A buffer is defined as a solution that resists change in pH when a small amount of an acid or base is added or when the solution is diluted. This is very useful for maintaining the pH for a reaction at an optimuiri vrdue. A buffer solution consists of a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid at predetermined concentrations or ratios. That is, we have a mixture of a weak acid and its salt or a weak base and its salt. Consider an acetic acid-acetate buffer. The acid equilibrium that governs this system is... 

You may have wondered why, in buffer mixtures, the salt does not react with water to hydrolyze as an acid or base. This is because the reaction is suppressed by the presence of the acid or base. In Equation 7.28, the presence of appreciable amounts of either HA or OH will suppress the ionization to a negligible amount. In Equation 7.33, the presence of either B or H3O+ will suppress the ionization. 

Most commonly, the components of a buffer are the conjugate acid-base pair of a weak acid. The buffer used in Figure 19.2, for example, is a mixture of acetic acid (CH3COOH) and acetate ion (CH3COO ). 

As illustrated in equations (22) and (23), the addition of either a strong base or a strong acid produces one of the components of the buffer mixture and so the pH does not change. Buffers are hmited in their buffer capacity, that is, the amount of a strong acid or strong base that can be added before the pH changes by 1 pH unit. 

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