Predictions buffer solutions

Danckwerts et al. (D6, R4, R5) recently used the absorption of COz in carbonate-bicarbonate buffer solutions containing arsenate as a catalyst in the study of absorption in packed column. The C02 undergoes a pseudo first-order reaction and the reaction rate constant is well defined. Consequently this reaction could prove to be a useful method for determining mass-transfer rates and evaluating the reliability of analytical approaches proposed for the prediction of mass transfer with simultaneous chemical reaction in gas-liquid dispersions. 

Before releasing a process column for chromatography, it is advisable to perform some test to measure efficiency, such as calculating height equivalent theoretical plates (HETP), both to forestall any problems in the column bed and to provide a benchmark by which to measure column reproducibility and predict degradation of the bed or material. Examples of compounds that are relatively innocuous for use in pharmaceutical applications are 1% NaCl (for gel filtration), concentrated buffer solutions (for ion exchange), and benzyl alcohol and parabens for reverse phase LC.10... 

Over the last several years, the number of studies on application of artificial neural network for solving modeling problems in analytical chemistry and especially in optical fibre chemical sensor technology, has increase substantially69. The constructed sensors (e.g. the optical fibre pH sensor based on bromophenol blue immobilized in silica sol-gel film) are evaluated with respect to prediction of error of the artificial neural network, reproducibility, repeatability, photostability, response time and effect of ionic strength of the buffer solution on the sensor response. 

Fig. 5. Relation between the average molecular weight between cross-links, Mc, and the volume fraction of gelatinized collagen (gelatin), v2. The swelling agent is 0.19 mol/1 citric acid - phosphate buffer solution, pH 7.4 at 80 °C. Experimental data obtained after cross-linking with various aldehydes solid circles, formaldehyde open circles, glutaraldehyde squares, glyoxal. The curve is predicted by Eq. (2) with X = 0.52 0.04 [38]...

If H+ ions from a strong acid are added to this buffer solution, Le Chatelier s principle predicts that the reaction will shift to the left and much of this H+ will be consumed to create more HC2H3O2 from 02 02 If a strong base that consumes H+ is added to this buffer solution, Le Chatelier s principle predicts that the reaction will shift to the right and much of the consumed H+ will be replaced by the dissociation of HC2H3O2. The net effect is that buffer solutions prevent large changes in pH that occur when an acid or base is added to pure water or to an unbuffered solution. 

Figure S.21 Resulting chromatogram at the optimum conditions predicted by figure S.20. pH = S.8 concentration of n-octylamine = 3.2 mM. ODS column mobile phase methanol-water (20/80) with 0.010 M acetate buffer. Solutes E = phenylethylamine, P = phenylalanine, V = vanillic acid, C = trans caffeic acid, M = trans p-coumaric acid, F = trans ferulic acid, A = phenylacetic acid, H = hydrocinnamic acid and N = trans cinnamic acid. Figure taken from ref. [559]. Reprinted with permission. 

During the drug discovery process the solubility in a mixture of water with another cosolvent like DM SO is commonly measured. The compounds are usually dissolved first in DMSO and then diluted in water and/or buffer solution. Therefore, a classification model has been successfully derived to predict the solubility in this solvent mixture using the VolSurf type of descriptors (Fig. 10.9(b)) for a set of 150 compounds with two latent variables (Table 10.2). 

The pH-solubility profiles are very similar to the pH-log D profiles however, it is more difficult to estimate the solubility of the charged form of molecules. The reason for this is that the counter-ions in the aqueous solutions influence the solubility of the charged species so while we can estimate the partition for the charged species into octanol, we cannot make definitive predictions or estimations of the solubility of the charged species in the presence of various counter-ions in the aqueous buffer solution. 

Describe how you would prepare one or more buffer solutions, including which compounds to use. Predict the pH of each solution. If your teacher provides the needed materials, measure the pH to test your prediction. 

Predict the results of two experiments A small amount of base is added to an unbuffered solution with a pH of 7 and the same amount of base is added to a buffered solution with a pH of 7. 

A buffer solution is a solution that resists changes in pH. If acid is added then, within reason, the pH does not fall if base is added, the pH does not rise. Buffers are usually composed of a mixture of weak acids or weak bases and their salts and function best at a pH equal to the pKa of the acid or base involved in the buffer. The equation that predicts the behaviour of buffers is known as the Henderson-Hasselbalch equation (named after chemists Lawrence Joseph Henderson and Karl Albert Hasselbalch), and is another vitally important equation worth committing to memory. It is derived as follows, by considering a weak acid that ionises in solution ... 

Substantial efforts have been devoted to the development of molecular sensors for dopamine. Raymo et al.70 reported a two-step procedure to coat silica particles with fluorescent 2,7-diazapyrenium dications sensing toward dopamine. The analysis of the fluorescence decay with multiple-equilibria binding model revealed that the electron deficient dications and the electron-rich analytes form 1 1 and 1 2 complexes at the particle/water interface. The interfacial dissociation constants of the 1 1 complexes were 5.6mM and 3.6mM for dopamine and catechol, respectively. Dopamine was dominated by the interaction of its electron-rich dioxyarene fragment with the electron-deficient fluorophore in neutral aqueous environments. Ahn et al.71 reported tripodal oxazoline-based artificial receptors, capable of providing a preorganized hydrophobic environment by rational design, which mimics a hydrophobic pocket predicted for a human D2 receptor. A moderate binding affinity, a dissociation constant of 8.2 mM was obtained by NMR titrations of tripodal oxazoline-based artificial receptor with dopamine in a phosphate buffer solution (pH 7.0). Structurally related ammonium ions, norepinephrine, 2-phenylethylamine,... 

According to HHS-FDA guidances, the determination of the equilibrium solubility should be carried out with the shake-flask method (other methods such as acid or base titration are permitted when their ability to predict the equilibrium solubility is justified). The experiments should be carried out at a temperature of 37 1°C. Further, a sufficient number of pH conditions should be chosen to cover the pH range of 1-7.5 and each determination should be carried out at least in triplicate. The buffer solutions given in the United States Pharmacopeia (USP) are appropriate for the tests, but other buffers are also allowed for these experiments. The pH value of each buffer solution should be checked before and after each experiment. Degradation of the API due to pH or buffer composition should be reported together with other stability data. 

Longhi, P, Mussini, T., and Rondinini, S., Predicting standard pH values for reference buffer solutions in solvent mixtures with water, Anal. Chem., 58, 2290, 1986. 

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