Acid-base buffer systems preparing

Particularly interesting is the mixture of an acid and a base, wherein we must then explicitly model the change in pfC of the base and mobile phase. Initially, we consider the aqueous systems that are shown in Fig. 4a and b. These are the pH/retention factors for an acid and a base with no acetonitrile. Based on this, the pH choices could be either less than 2.5, or more than 7.5. Most chromato-graphers would elect to choose a pH of 2.5 or so. However, if we are going to use an appreciable concentration of an organic modifier, the picture changes. If we assume that we are going to prepare an acid-based buffer, the elution profile for the acid remains the same however, for the base, the pH shift will be in opposite directions for buffer and analyte. The elution profile will shift. 

This very simple result makes it easy to make an initial choice of a buffer we just select an acid that has a pfC, close to the pH that we require and prepare an equimolar solution with its conjugate base. When we prepare a buffer for pH > 7 we have to remember that the acid is supplied by the salt, that the conjugate base is the base itself, and that the pKa is that of the conjugate acid of the base (and hence related to the pKh of the base by pR l + pKh = p/conjugate acid and base have unequal concentrations—such as those considered in Examples 11.1 and 11.2—are buffers, but they may be less effective than those in which the concentrations are nearly equal (see Section 11.3). Table 11.1 lists some typical buffer systems. 

With a given weak acid, a buffer soiution can be prepared at any pH within about one unit of its p vaiue. Suppose, for exampie, that a biochemist needs a buffer system to maintain the pH of a soiution ciose to 5.0. What reagents shouid be used According to the previous anaiysis, the weak acid can have a p Z a between 4.0 and 6.0. As the p deviates from the desired pH, however, the soiution has a reduced buffer capacity. Thus, a buffer has maximum capacity when its acid has its p as ciose as possibie to the target pH. Tabie 18-1 iists some acid-base pairs often used as buffer soiutions. For a pH - 5.0 buffer, acetic acid (p Za — 4.75) and its conjugate base, acetate, wouid be a good choice. 

A practical problem in solution preparation usually requires a different strategy than our standard seven-step procedure. The technician must first identify a suitable conjugate acid-base pair and decide what reagents to use. Then the concentrations must be calculated, using pH and total concentration. Finally, the technician must determine the amounts of starting materials. The technician needs a buffer at pH = 9.00. Of the buffer systems listed in Table 18-1. the combination of NH3 and NH4 has the proper pH range for the required buffer solution. 

This catalytic system was very flexible because by simple modification of the reaction conditions it was possible to prepare oxidized polymers with the desired level of carboxyl and carbonyl functions. No waste was formed because the process did not involve any acids, bases or buffer solutions. The incipient wetness process is very easy to scale up. Hydrophilic starch was prepared in batches of 150 L and incorporated successfully in paint formulations. Good results were also obtained with in vitro and in vivo tests for cosmetic formulation. Interestingly, this is a rather unique example of a heterogeneous catalytic process involving a soluble catalyst and a solid substrate. 

Because buffer solutions are widely used in the laboratory and in medicine, prepackaged buffers having a variety of precisely known pH values are commercially available (Figure 16.5). The manufacturer prepares these buffers by choosing a buffer system having an appropriate pKa value and then adjusting the amounts of the ingredients so that the [base]/[acid] ratio has the proper value. 

Preparation of Buffer Solutions.—The buffer capacity of a given acid-base system is a maximum, according to equation (77), when there are present equivalent amounts of acid and salt the hydrogen ion concentration is then equal to and the pH is equal to pfca. If the ratio of acid to salt is increased or decreased ten-fold, i.e., to 10 1 or 1 10, the hydrogen ion concentration is then lOfca or O.IAto, and the pH is pA a — 1 or pfca + 1, respectively. If these values for cn are inserted in equation (76), it is found that the buffer capacity is then... 

Control of pH is vital in synthetic and analytical chemistry, just as it is in living organisms. Procedures that work well at a pH of 5 may fail when the concentration of hydronium ion in the solution is raised tenfold to make the pH 4. Fortunately, it is possible to prepare buffer solutions that maintain the pH close to any desired value by the proper choice of a weak acid and the ratio of its concentration to that of its conjugate base. Let s see how to choose the best conjugate acid-base system and how to calculate the required acid-base ratio. 

Two buffer systems can be prepared from a weak dibasic acid and its salts. The first consists of free acid H2A and its conjugate base NaHA, and the second makes use of the acid NaHA and its conjugate base Na2A. The pH of the latter system is... 

Adsorption of PLL-g-PEG as well as all in situ experiments and protein studies was conducted in a 4-(2-hydroxyethyl)-piperazine-l-ethane-snUbnic acid- (HEPES-) based buffer (Flu-ka, Buchs, Switzerland) adjusted to pH 7.4 with NaOH 6 M (Fluka, Buchs, Switzerland). The choice of the buffer system is important for the type of studies performed in this work While phosphate-containing buffers result in strong binding of phosphate to transition metal cations and therefore change the surface properties in a time-dependent manner, HEPES is a buffer system with only weak interaction with metal oxide surfaces. The ionic strength of the buffer varied between 1 and 160 mM, either by dilution or by addition of NaCl (Fluka, Buchs, Switzerland). The following buffers were prepared 1... 

Thus, to prepare a buffer solution, we choose a weak acid whose pATg is close to the desired pH. This choice not only gives the correct pH valne of the buffer system, but also ensures that we have comparable amounts of the acid and its conjugate base present both are prerequisites for the buffer system to function effectively. 

Purely diffuse-layer models have become quite popular since the database for hydrous ferric oxide has been published by Dzombak and Morel [76]. The model calculations by these authors have shown that it is possible to describe a wide range of experimental sorption data within a relatively simple model framework. However, the description of acid-base properties of hydrous ferric oxide with this model is not convincing. Substantial failures with respect to true predictions can therefore be expected whenever dynamic systems involving the transport of protons are considered and variations of pH are possible (Lutzenkirchen et al., in preparation). Nevertheless, for conditions in which this is not the case (i.e., buffered systems), the database is very useful but should not be used in the context of mechanistic discussions. 

The HPLC was equipped with a UV-Vis detector (VWD = 210 nm), a water symmetry Cig column, 150 mm x 3.9 mm, 5 p,m. The mobile phase was 25% acetonitrile in 75% 20 mM phosphate buffer (pH 2.8). The flow rate was set at 1.5 mL/min. Equal volumes (25 p,L) of standard and sample solutions were injected into the chromatographic system. Quantification of polysorbate 80 is based on a comparison of the response of oleic acid in sample and that of oleic acid in standard solution [6]. When polysorbate 80 was quantified by GC, the released oleic acid can be detected without derivatization and prepared according to the HPLC method. 

For the determination of CCA in biological samples, methods not based on LC-MS/MS technology [39, 41-43] and methods that used LC-MS/MS [40, 52] have been reported. Most of the sample extraction methods used liquid-liquid extraction (LLE) technology, since this extraction method is simpler and able to minimize matrix effects. Consequently, LLE methods are considered to provide cleaner samples as compared to solid phase extraction (SPE) methods. Since LC-MS/MS methodology uses nonvolatile solvents or a combination of nonvolatile and volatile solvents, difficulties in the evaporation process and associated interferences when samples are injected onto the system can arise [51]. However, Bahrami as well as Souri [42,43] applied a combination of nonvolatile and volatile solvents in which the nonvolatile solvents were acidic buffers (pH 5 or less). Analytes eluted from SPE prepared samples did not undergo evaporation as applied commonly encountered in extraction procedures [37, 45]. 

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