Hydrogen Ion Concentrations

ASTM D1583-01 Standard test method for hydrogen ion concentration. 

The enzymic hydrolysis of carbohydrates and derivatives is influenced markedly by the hydrogen-ion concentration. There is an optimal region of pH, and at higher and lower values the activity decreases. Fig. 4 gives the pH activity curves for the hydrolysis of sucrose, raflSnose, and inulin by purified yeast invertase (12), 

An explanation (13) for the influence of the hydrogen-ion concentration is that these enzymes are amphoteric and that only the undissociated molecule is catalytically active on this basis, equations have been developed 

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Ph The phenyl group, CfiHs-. pH See hydrogen-ion concentration, phaeophorbide See chlorophyll, phaeophytin See chlorophyll, phalloidins See amanitins. 

By far the most widely known classes of pH-sensitive materials are the classes of chemical compounds that iaclude the acids, bases, and iadicators. The most interesting of these are the iadicators. These materials change colors as a function of pH and usually are totally reversible (see Hydrogen-ION concentration). 

Only acids are sour. Sourness is not identical to chemical acidity or pH, which is a function of the hydrogen ion concentration, but also appears to be a function of the entire acid molecule. A combination of pH and acid concentration determines the actual degree of the sour taste. At the same pH, any organic acid, eg, citric acid, exhibits a far greater sourness than a mineral acid, eg, hydrochloric acid (27,28). 

A particular concentration measure of acidity of aqueous solutions is pH which usually is regarded as the common logarithm of the reciprocal of the hydrogen-ion concentration (see Hydrogen-ION activity). More precisely, the potential difference of the hydrogen electrode in normal acid and in normal alkah solution (—0.828 V at 25°C) is divided into 14 equal parts or pH units each pH unit is 0.0591 V. Operationally, pH is defined by pH = pH(soln) + E/K, where E is the emf of the cell ... 

P-A4 = 7.7 (47). The rates of hydrolysis of the peroxodiphosphate ion are dependent on both pH and temperature, eg, as the hydrogen ion concentration iacreases, the rate iacreases (48). Peroxodiphosphoric acid hydroly2es to peroxomonophosphoric acid according to the foUowiag equation ... 

When equal amounts of solutions of poly(ethylene oxide) and poly(acryhc acid) ate mixed, a precipitate, which appears to be an association product of the two polymers, forms immediately. This association reaction is influenced by hydrogen-ion concentration. Below ca pH 4, the complex precipitates from solution. Above ca pH 12, precipitation also occurs, but probably only poly(ethylene oxide) precipitates. If solution viscosity is used as an indication of the degree of association, it appears that association becomes mote pronounced as the pH is reduced toward a lower limit of about four. The highest yield of insoluble complex usually occurs at an equimolar ratio of ether and carboxyl groups. Studies of the poly(ethylene oxide)—poly(methacryhc acid) complexes indicate a stoichiometric ratio of three monomeric units of ethylene oxide for each methacrylic acid unit. 

Diethyl ether is the principal by-product of the reaction of ethyl alcohol with diethyl sulfate. Various methods have been proposed to diminish its formation (70—72), including separation of diethyl sulfate from the reaction product. Diethyl sulfate not only causes an increase in ether formation but is also more difficult to hydroly2e to alcohol than is ethyl hydrogen sulfate. The equiUbrium constant for the hydrolysis of ethyl hydrogen sulfate is independent of temperature, and the reaction rate is proportional to the hydrogen ion concentration (73—75). 

Fig. 10-11. The pH scale is a measure of hydrogen ion concentration. The pH of common substances is shown with various values along the scale. The Adirondack Lakes are located in the state of New York and are considered to be receptors of acidic deposition. Source U.S. Environmental Protection Agency, Acid Rain—Research Summary," EPA-600/8-79-028, Cincinnati, 1979.

The role that acid and base catalysts play can be quantitatively studied by kinetic techniques. It is possible to recognize several distinct types of catalysis by acids and bases. The term specie acid catalysis is used when the reaction rate is dependent on the equilibrium for protonation of the reactant. This type of catalysis is independent of the concentration and specific structure of the various proton donors present in solution. Specific acid catalysis is governed by the hydrogen-ion concentration (pH) of the solution. For example, for a series of reactions in an aqueous buffer system, flie rate of flie reaction would be a fimetion of the pH, but not of the concentration or identity of the acidic and basic components of the buffer. The kinetic expression for any such reaction will include a term for hydrogen-ion concentration, [H+]. The term general acid catalysis is used when the nature and concentration of proton donors present in solution affect the reaction rate. The kinetic expression for such a reaction will include a term for each of the potential proton donors that acts as a catalyst. The terms specific base catalysis and general base catalysis apply in the same way to base-catalyzed reactions. 

Mathematically, pH is the logarithm (base 10) of the reciprocal of the hydrogen ion concentration. The pH may range from 0 to 14, where 0 is most acidic, 14 most basic, and 7 is neutral. Natural waters usually have a pH between 6.5 and 8.5. 

A decrease in reaction order with respect to hydrogen ion concentration witli increasing acidity has frequently been observed for reactions involving weak acids. 

The notation g- o/7 is the unit tox gram-ion, which represents Avogadro s number (6.02 X 10 ) of ions. The pH is a direct measine of the hydrogen ion concentration and is defined by... 

The pH scale is widely used in biological applications because hydrogen ion concentrations in biological fluids are very low, about 10 M or 0.0000001 M, a value more easily represented as pH 7. The pH of blood plasma, for example, is 7.4 or 0.00000004 M H. Certain disease conditions may lower the plasma pH level to 6.8 or less, a situation that may result in death. At pH 6.8, the H concentration is 0.00000016 M, four times greater than at pH 7.4. 

Hydrogen was recognized as the essential element in acids by H. Davy after his work on the hydrohalic acids, and theories of acids and bases have played an important role ever since. The electrolytic dissociation theory of S. A. Arrhenius and W. Ostwald in the 1880s, the introduction of the pH scale for hydrogen-ion concentrations by S. P. L. Sprensen in 1909, the theory of acid-base titrations and indicators, and J. N. Brdnsted s fruitful concept of acids and conjugate bases as proton donors and acceptors (1923) are other land marks (see p. 48). The di.scovery of ortho- and para-hydrogen in 1924, closely followed by the discovery of heavy hydrogen (deuterium) and... 

The now universally used measure of the hydrogen-ion concentration was introduced in 1909 by the Danish biochemist S. P. L. S0iensen during his work at the Carlsbetg Breweries Biochem. Z 21. 131. 1909) ... 

C 0.5012 mol 2 at 15°C). It is clearly unwise to associate a pH meter reading too closely with pH unless under very controlled conditions, and still less sensible to relate the reading to the actual hydrogen-ion concentration in solution. For further discussion of pH mea.surements, see Pure Appl. Chem. 57, 531-42 (1985) Definition of pH Scales, Standard Reference Values, Measurement of pH and Related Terminology. Also C E News, Oct. 20. 1997. p. 6. 

The dissociation constant for the first process is only 1.1 X 10 lmol at 25°C this corresponds to pKa 2.95 and indicates a rather small free hydrogen-ion concentration (cf. CICH2CO2H, p ffl 2.85) as a result of the strongly H-bonded, undissociated ion-pair [(H30)" F ]. By contrast, K2 = 2.6 X 10 lmol pK2 0.58), indicating that an appreciable number of the fluoride ions in the solution are coordinated by HF to give HF2 rather than by H2O despite the very much higher concentration of H2O molecules. 

The oxidizing power of the halate ions in aqueous solution, as measured by their standard reduction potentials (p. 854), decreases in the sequence bromate > chlorate > iodate but the rates of reaction follow the sequence iodate > bromate > chlorate. In addition, both the thermodynamic oxidizing power and the rate of reaction depend markedly on the hydrogen-ion concentration of the solution, being substantially greater in acid than in alkaline conditions (p, 855). 

It is the rapid increase in rates of hydration with increasing hydrogen ion concentration that prevents measurement with existing apparatus of the -pKa values of anhydrous bases such as pteridine. For example, at pH 1, hydration of the anhydrous cation is half-complete in 0.01 sec at 20°. Conversely, it is the comparative slowness of the reactions in near-neutral solutions that makes it possible, by adding acid solutions to near-neutral buffers, using the stopped-flow technique, to determine the p STa values of the hydrated species. 

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