Amphiprotic solutes

Ruelle, P., and V. W. Kesselring, The hydrophobic propensity of water towards amphiprotic solutes Prediction of molecular origin of the aqueous solubility of aliphatic alcohols , J. Pharm. Sci., 86,179-186 (1997b). 

Briefly de.scribe or define and give an example of (a) an amphiprotic solute. 

This type of behavior has been observed also for proteins. The solubility of a protein built out of amino acids is minimum when pH = pi, the isoelectric point. The minimum is a point not a band. As the pH moves away from the isoelectric point, the protein solubility increases. The presence of a net charge on the protein molecule increases its solubilization due to its interaction with dipolar water molecules (H+ as well as OH ). Kirwan and Orella (1993) have illustrated broadly a similar solubility behavior for other biological molecules of smaller molecular weight which produce zwitterions. Observe the solubility VcUiation with pH of amphiprotic solutes such as the p-lactam antibiotics, ampidllin, amoxicillin, etc., as determined by Tsujiet aZ. (1978) (Figure 5.2.11(b)). If the species is susceptible to hydrolysis at high pH, then pH effects are observed only at lower pH in the range of pH < pKi < pi. 

Amino acid zwitterions are internal salts and therefore have many of the physical properties associated with salts. They have large dipole moments, are soluble in water but insoluble in hydrocarbons, and are crystalline substances with relatively high melting points. In addition, amino acids are amphiprotic they can react either as acids or as bases, depending on the circumstances. In aqueous acid solution, an amino acid zwitterion is a base that accepts a proton to yield a cation in aqueous base solution, the zwitterion is an add that loses a proton to form an anion. Note that it is the carboxylate, -C02-, that acts as the basic site and accepts a proton in acid solution, and it is the ammonium cation, -NH3+, that acts as the acidic site and donates a proton in base solution. 

The acidic and basic properties of aqueous solutions are dependent on an equilibrium that involves the solvent, water. The reaction involved can be regarded as a Bransted-Lowry acid-base reaction in which the H20 molecule shows its amphiprotic nature ... 

Solder analysis of by EDTA, (ti) 337 Solochrome black 317, 692 Solochrome (Eriochrome) cyanine R 678 Solochrome dark blue 318 Solubility product 24 calculations involving, 25 importance of, 26 principal limitations of, 24 Solution of sample 110 Solvents amphiprotic, 282 aprotic, 282 ionising, 18 non-protonic, 18 protogenic, 18, 282 protophilic, 282... 

Because HS is amphiprotic, it is not immediately apparent whether an aqueous solution of NaHS will be acidic or basic. However, we can use the pKa and pfCh values of the HS ion to conclude that ... 

The pH of the aqueous solution of an amphiprotic salt is equal to the average of the pKlts of the salt and its conjugate acid. The pH of a solution of a salt of the final conjugate base of a polyprotic acid is found from the reaction of the anion with water. 

The two strands of the nucleic acid DNA are held together by four organic bases. The structure of one of these bases, thymine, is shown below, (a) How many protons can this base accept (b) Draw the structure of each conjugate acid that can be formed, (c) Mark with an asterisk any structure that can show amphiprotic behavior in aqueous solution. 

For all the amphiprotic solvents in the pure state the cation (or acid) concentration is by definition equal to the anion (or base) concentration in other words, the pure solvent is neutral therefore, adding a solute such as HC1 makes the solution acidic, whereas a solute such as NaOH makes it basic. 

Starting from the foregoing theory of amphiprotic solvents, we shall next consider their interaction with some reactive (inorganic or ionophoric) solutes as illustrated below. 

Autoprotolysis constants exist for any amphiprotic solvent and can be determined from electric conductivity studies of the solvent and solutions. A few examples include ... 

Among amphiprotic solvents of high permittivities, there are water-like neutral solvents (e.g. methanol and ethanol), more acidic protogenic solvents (e.g. formic acid), and more basic protophilic solvents (e.g. 2-aminoethanol). There are also amphiprotic mixed solvents, such as mixtures of water and alcohols and water and 1,4-dioxane. The acid-base equilibria in amphiprotic solvents of high permittivity can be treated by methods similar to those in aqueous solutions. If the solvent is expressed by SH, the acid HA or BH+ will dissociate as follows ... 

The pH scale in water is widely used as a measure of acid-base properties in aqueous solutions. It is defined by pH=-log a(H+). In Section 3.1, we dealt with the poH value, defined by poH=-log a(H+), for solutions in amphiprotic and aprotic solvents of high permittivity. Recently, however, the symbol pH has also been used for the value of -log o(H+) in such non-aqueous solutions. Therefore, hereafter, the symbol pH is used instead of paH-... 

Selectivity. Several additional analytes were examined. Extraction efficiencies from 1 mM solutions for a 2-min extraction time were as follows glucose, 2 salicylic acid, 34 oxine, 75 phenol, 22 and p-cresol, 79. 8-Hydroxyquinoline and p-cresol are significantly more extractable than salicylic acid. 8-Hydroxyquinoline can be back extracted either by basic solution (pH >12) or acidic solution (pH <3) because of its amphiprotic behavior (pKi = 5.0 pK2 = 9.90). This experiment was performed by using either 0.1 M NaOH or 0.01 M HC1 with essentially the same results. The extraction step should be performed at pH 7-10. 

A molecule that can both donate and accept a proton is said to be amphiprotic. The acid dissociation reaction (10-8) has a larger equilibrium constant than the base hydrolysis reaction (10-9), so we expect that a solution of leucine will be acidic. 

The isoelectric pH of a polyprotic compound is that pH at which the average charge of all species is 0. For a diprotic amino acid whose amphiprotic form is neutral, the isoelectric pH is given by pH = (pK + pK2). The isoionic pH of a polyprotic species is the pH that would exist in a solution containing only the ions derived from the neutral polyprotic species and from H20. For a diprotic amino acid whose amphiprotic form is neutral, the isoionic pH is found from [H + ] = VC / F + K K )l(Ki + F), where F is the formal concentration of the amino acid. 

Bismuth(III) oxide, Bi O is the compound produced by heating the metal, or its carbonate, in air. It is definitely a basic oxide, dissolving readily 111 acid solutions, and unlike the arsenic or antimony compounds, not amphiprotic in solution, although it forms stoichiometric addition compounds on heating with oxides of a number of other metals. It exists in three modifications, white rhombohedral, yellow rhombohedral, and gray-black cubical, Bismuth(II) oxide. BiO, has been produced by heating die basic oxalate. 

The three oxides of chromium, CrO, CriOj, and CrO , are of interest in exhibiting the transition in properties from basicity to acidity and from electrovalence io covalence, wilh increasing oxygen conlent. and the consequent increase in charge upon the Cr atom. Thus CrjOj. the middle oxide, is amphiprotic. dissolving either in strong alkali hydroxide solutions or in acids. 

An amphiprotic (also called amphoteric) salt is a salt that may ionize in solution either as an acid or a base, and react cither with bases or acids, according to the conditions. 

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