Buffers concentration

Buffer solutions are prone to bacterial growth and should be prepared regularly and an expiry date assigned. In order to flush out buffer salts from the system, a mobile phase should be prepared that contains water instead of the buffer element. The system, including the column, should be washed thoroughly with this mixture. The column should then be flushed with organic solvent for storage purposes. 

---

The experimental detection of general acid catafysis is done by rate measurements at constant pH but differing buffer concentration. Because under these circumstances [H+] is constant but the weak acid component(s) of the buffer (HA, HA, etc.) changes, the observation of a change in rate is evidence of general acid catalysis. If the rate remains constant, the reaction exhibits specific acid catalysis. Similarly, general base-catalyzed reactions show a dependence of the rate on the concentration and identity of the basic constituents of the buffer system. 

Notice that specific acid catalysis describes a situation in which the reactant is in equilibrium with regard to proton transfer, and proton transfer is not rate-determining. On the other hand, each case that leads to general acid catalysis involves proton transfer in the rate-determining step. Because of these differences, the study of rates as a function of pH and buffer concentrations can permit conclusions about the nature of proton-transfer processes and their relationship to the rate-determining step in a reaction. 

Figure 8.P28 gives the pH-rate profile for conversion of the acid A to the anhydride B in aqueous solution. The reaction shows no sensitivity to buffer concentration. Notice that the reaction rate increases with the size of the alkyl substituent, and, in fact, the derivative with R = = CHj is still more reactive. Propose a mechanism which is... 

An effective experimental design is to measure the pseudo-first-order rate constant k at constant pH and ionic strength as a function of total buffer concentration 6,. Very often the buffer substance is the catalyst. Let B represent the conjugate base form of the buffer. Because pH is constant, the ratio (B]/[BH ] is constant, and the concentrations of both species increase directly with 6 where B, = [B] -t-[BH"]. 

Figure 6-4. Plol of Eq. (6-46) for the hydrolysis of cinnamic anhydride at 25°C and ionic strength 0.1 M in carbonate buffers. fi, represents the total buffer concentration. From top to bottom, pH = 10.06, 9.76, and 9.14.

Throughout this section the hydronium ion and hydroxide ion concentrations appear in rate equations. For convenience these are written [H ] and [OH ]. Usually, of course, these quantities have been estimated from a measured pH, so they are conventional activities rather than concentrations. However, our present concern is with the formal analysis of rate equations, and we can conveniently assume that activity coefficients are unity or are at least constant. The basic experimental information is k, the pseudo-first-order rate constant, as a function of pH. Within a senes of such measurements the ionic strength should be held constant. If the pH is maintained constant with a buffer, k should be measured at more than one buffer concentration (but at constant pH) to see if the buffer affects the rate. If such a dependence is observed, the rate constant should be measured at several buffer concentrations and extrapolated to zero buffer to give the correct k for that pH. 

These are rate constants for the hydrolysis of cinnamic anhydride in bicarbonate-carbonate buffers. The pK of bicarbonate is 10.22. Find the rate constant for hydrolysis, at each pH, at zero buffer concentration. Analyze the data to determine if the acid or base component of the buffer, or both, are responsible for catalysis, and give the catalytic rate constant(s). 

The following data are for the hydrolysis of cinnamic anhydride in (2-amino-2-hydroxymethyl-1,3-propane diol buffers. Extrapolate them to zero buffer concentration, and, together with data from Problem 9, plot the pH-rate profile. Determine the order with respect to hydroxide, and calculate the rate constant for hydrolysis. 

These rate constants are for the hydrolysis of cinnamic anhydride in carbonate buffer, pH 8.45, total buffer concentration 0.024 M, in the presence of the catalysts pyridine, A -methylimidazole (NMIM), or 4-dimethylaminopyridine (DMAP). In the absence of added catalyst, but the presence of buffer, the rate constant was 0.005 24 s . You may assume that only the conjugate base form of each catalyst is catalytically effective. Calculate the catalytic rate constant for the three catalysts. What is the catalytic power of NMIM and of DMAP relative to pyridine ... 

FIGURE 16.11 Specific and general acid-base catalysis of simple reactions in solution may be distinguished by determining the dependence of observed reaction rate constants (/sobs) pH and buffer concentration, (a) In specific acid-base catalysis, or OH concentration affects the reaction rate, is pH-dependent, but buffers (which accept or donate H /OH ) have no effect, (b) In general acid-base catalysis, in which an ionizable buffer may donate or accept a proton in the transition state, is dependent on buffer concentration. 

In some cases, impurities in the ionic liquids resulted in dramatic pH shifts, causing enzyme inactivation. This could sometimes be overcome simply by titration or higher buffer concentrations. In other cases, purification of the ionic liquid or an improved synthesis might be necessary. 

Borate-gluconate eluant. Prepare a buffer concentrate by dissolving the following substances in water and making up to 1 L with distilled, de-ionised... 

The kinetics of decarboxylation of 4-aminosalicylic acid in some buffer solutions at 50 °C were studied. The first-order rate coefficients increased with increasing buffer concentration, though the pH and ionic strength were held constant (Table 217). This was not a salt effect since the rate change produced by substituting potassium chloride for the buffer salt was shown to be much smaller. It follows from the change in the first-order rate coefficients (kx) with... 

NON-DEPENDENCE OF RATE COEFFICIENTS k2 AND k3 UPON BUFFER CONCENTRATION FOR REACTION OF PhB(OH)2 WITH H202 AT 25 °C727... 

As in the case of bromodeboronation, an increase in acetate buffer concentration reduced the rate, the biggest effect arising from the acid component, and again the effect of increasing the concentration of acetate could not be explained. Chelates produced large rate accelerations as with bromodeboronation and sodium fluoride similarly produced a very large effect, which was considered to be related to the stability of the tetrafluoroborate anion. 

The kinetics have also been examined by Brown et al.132 using buffer solutions and with similar results they pointed out that the reduction in rate caused by increasing the buffer concentration might conceivably have arisen from a slight change in pH since the rate coefficients are markedly dependent upon this a change in pH of 0.03 units would have been sufficient to account for the results of Kuivila and Williams731. 

The buffer concentration also directly affects the size of droplets produced - the higher the buffer concentration, then the smaller they are, and this is desirable. The buffer concentration, however, has an effect on the ionization efficiency and at high buffer concentrations (>10 M) the relationship between detector response and analyte concentration is not linear. As indicated earlier in Figure 2.6, this situation must be avoided for precise quantitative measurements. 

Hydration of compounds 2, 3, 4, 5 was found to be first order both in substrate and in hydronium ion (4-10). Furthermore, a careful kinetic study of compounds 2c-g and the sulfur analog 4 revealed that the hydration rate at constant ionic strength was dependent on the buffer concentration and hence was general acid catalyzed. 

FIGURE 6 Effect of pH on the degradation of PC at 72°C (buffer concentration = 0). The lines were calculated with linear regression analysis. (From Grit et al., 1989.)... 

Figure 3. Typical chromatogram of UV-ahsorhing constituents of a 0.15-ml sample of urine. Conditions 150-cm anion exchange column (1215 fi, Aminex A ZI) bujfer, acetate at pH 4.4 gradient, buffer concentration from 0.015M to 6M flow rate, 10.5 ml/hr, coUimn temperature, 25°C for first 5 hr and 60°C thereafter (34).

If several buffer concentrations are used, extrapolation can be carried out to zero ionic strength, and the pKa can be determined. For initial studies, however, a pK in... 

---