Buffer ratio

The solid lines in Fig. 15, which refer to the data for reaction of the 4,6-bis(phenylazo)resorcinol monoanion in 2-methylphenol buffers at two buffer ratios are plots of (31) using best-fit values of and and the... 

Fig. 16 Free energy/reaction coordinate diagram for proton transfer from the 4,6-bis(phenylazo)resorcinol monoanion to give the dianion in the presence of 2-methylphenol buffers at a 1 1 buffer ratio and at buffer concentrations of (a) 0.001 and (b) 0.10mol" dm. ...

Combination of Equations 8 and 10 leads to Equation 14, which can be used in conjunction with the buffer ratio defined by Equation 15 in obtaining values of Ka as shown previously (16, 24, 25). 

Total ionic strengths of solutions in the cells were varied from about 0.005M to ca. 0.02Af. The concentrations of solutions in cell C were made so that the buffer ratio in Equation 15 always had a value between 0.4 and 0.6. The nonaqueous cosolvents used in this study were Reagent Grade or better, and they were tested to be sure that they were free from significant quantities of potentially interfering substances such as halide ions, acids, and bases. Densities of tetra-hydrofuran-water mixtures were determined pycnometrically at 15° C and at 35°C. 

Examination of Equation 7.30 shows that the rate of an (E1cB)b reaction should be independent of the base concentration if the buffer ratio, B/BH+ is kept constant—that is, the reaction should exhibit specific base catalysis (see Section 7.1, p. 340 and Chapter 8, p. 405). An example of such a reaction is elimination of methanol from 33. Not only is specific base catalysis observed, but... 

The addition of buffers is required to maintain constant pH during the reaction when experiments are to be carried out in the range 3 < PH <11. However, keto-enol tautomerization reactions usually exhibit so-called general acid and base catalysis. 26 The observed rate acceleration with increasing buffer concentration implies that the components of the buffer participate in some rate-determining step of the reaction. In most cases, the rate of reaction increases linearly with increasing buffer concentration at constant buffer ratio, chb/cb3 = const (Fig. 4a). 

Fig. 4 (a) Buffer dilution plot (b) buffer slopes as a function of buffer ratio. 

With the buffer ratio held constant ([B-]/[BH] = 0.5), and the ionic strength maintained at 1 mol dm"3 with potassium chloride, initial rates show saturation kinetics over the range 0.01 < [B ] < 0.25 mol dm 3. This behaviour is compatible with the intermediacy of the carbanion 13, and the rate-determining step changing from its rate-limiting formation at low buffer concentrations to rate-limiting expulsion of quinuclidine from the carbanion, formed rapidly and reversibly, at high buffer concentrations [21]. 

In order to detect a general acid or general base catalysis, it is necessary to do a series of kinetic experiments in which [H+] or [OH-] and the ionic strength are kept constant while [HA] or [B] are varied in such a way that the buffer ratio remains unchanged within the same series. Details of studies of this type have been adequately described in the literature [1, 3, 4]. 

At low concentrations of BH+ such that k2 > k j [BH+], the observed first-order rate coefficient for conditions where buffer is present in excess over reactant is kx [B]. The rate of reaction is determined by a slow proton removal by base from carbon. At high concentrations of BH+, the observed first-order rate coefficient is (k1fe2/k-i)[B]/[BH+]. In this case, if the reaction is carried out in aqueous solution, the rate of reaction depends upon the hydroxide ion concentration and is independent of the buffer concentration at a fixed buffer ratio (specific base catalysis). The mechanism under these conditions consists of rapid pre-equilibrium formation of a carbanion followed by a slow step. Over the whole range of buffer concentration the first-order rate coefficient (M,hs) measured at fixed buffer ratio first increases (/ bs = kl [B]) with buffer concentration but reaches a limiting value (kohs = (ki k2 /k-i) [B] /[BH+]). This change in mechanism has been observed for a limited number of reactions [58]. Reactions (38) [58(a)] and (39) [58(b)] occurring in ethanol and reaction (40) [58(c)] in aqueous... 

Hydrolysis has traditionally been used for the production of fatty acids and glycerols, which find widespread apphcation in soaps and detergents, cosmetics, pharmaceuticals, and food products (174). Hydrolysis of soybean (181), canola (147, 208, 209), sunflower (149, 181, 210), tuna (150), and blackcurrant oils (145), tri-palmitin (146), triolein (211), and ethyl stearate (202) in SCCO2 has been reported. These investigations employed a variety of lipases, including immobilized lipase from porcine pancreas (211), Novozyme 435 (146, 181), Lipozyme (147, 150, 208, 209), non-immobilized Candida rugosa (150, 181), Lipase OL (150), and Lipolase lOOT (149, 181, 210). The effects of water content, enzyme load, operating conditions (temperature and pressure), pH, enzyme/substrate ratio, oil/buffer ratio, and CO2 flow rate (for continuous reactions) on the hydrolysis reaction were reported. 

The ratio acid / base is the buffer ratio. Because the buffer ratio is not altered by dilution, neither is the pH, except in strong solutions where the attraction between ions becomes sufficiently great to interfere with the simple equilibria. Small additions of acid and alkali alter the buffer ratio and pH very little. Buffering is obviously most efficient when acid (base), i,e, when pH vK-... 

Our value (8.224) for the pMn of the 0.02m tris buffer in seawater II (34.2%o salinity) at 25°C is considerably higher than that (8.075) for the 0.005m buffer as given by Hansson (8). The buffer ratio is unity in both cases, in the absence of specific interactions as yet unrevealed, and the activity coeflBcients y are close to unity. Hence, pMn should be nearly the same as pK for tris in seawater, a quantity easily derived from E° (Table IV) and E given in Tables II and III. Our results yield pK = 8.185 in seawater I and 8.205 in seawater II, both values based on the molality scale. On the other hand, Hansson s value of pHs appears to be based on —log (mn)t, where (mn)t includes both free hydrogen ion and that combined with sulfate in the form of HS04". Consequently, pHs is expected to be lower than our pMn by about 0.12 unit (22) as found. 

Rat brain cortex membrane was isolated, and the membrane homogenate was used as a preparation containing somatostatin receptors. Four rats were sacrificed by cervical dislocation. The cerebral cortex tissue was washed three times with buffer (HEPES 20niM, pH7.3, containing lOmM MgCl2) and homogenized in ice-cold buffer, using a cortex to buffer ratio of 1 10, at... 

The cortices are immediately homogenized (e.g. using a Polytron or other similar homogenizer) three times in the same buffer with a tissue to buffer ratio of 1 10 at maximum speed for 10 s. 

The supernatant is discarded, and the pellet is resuspended in HEPES buffer with a pellet to buffer ratio of 1 10. 

With these provisos, buffer catalysis is generally measured from gradients of plots of observed first-order rate constants against buffer concentration the separation of the second-order buffer catalysis constant into catalytic constants for acid and basic components of the buffer is achieved by secondary plots of these second-order constants against buffer ratio. This is illustrated in Figure 1.17. 

For the further reduction of the reaction volume we tested even higher substrate concentrations and higher concentrated titrating solutions. As to the former, it turned out that the enzyme worked smoothly even at 20 to 30% (w/v) substrate concentration and, in addition, at substrate to buffer ratio of 1 1 (w/v) the enzyme still showed a useful activity. In all cases the product was obtained with excellent chemical and enantiomeric purity (both >99%). 

General acid-base catalysis is defined experimentally by the appearance in the rate law of acids and/or bases other than lyonium or lyate ions. For example, the hydrolysis of enol ethers 1.2 (Scheme 2.2) is general acid-catalyzed. In strong acid the rate expression will be the same as in Scheme 2.1, but near neutral pH the rate is found to depend also on the concentration of the buffer (HA + A ) used to maintain the pH. Measurements at different buffer ratios show that the catalytic species is the acid HA. (If more than one acid is present there will be an additional term kHAi[HA ][1.2] for each.)... 

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