Hydrogen Ions

HYDROGEN IONS are positively charged hydrogen atoms. A hydrogen atom consists of a positive nucleus and a single electron, upon the loss of which it turns into a hydrogen ion, denoted as H⁺; the hydrogen ion, thus, is identical to the atomic nucleus of hydrogen (a proton) and is the smallest material mass possessing a positive charge. The formation of free hydrogen ions from neutral atoms and molecules occurs in gases under the influence of an electric discharge, especially at low pressures (see Ionization of gases). Hydrogen ions easily combine with electrons, forming neutral atoms again. - Ions in solutions. Electrolytes dissolved in water or generally in a solvent with a high dielectric constant (see) undergo electrolytic dissociation, simultaneously yielding positively charged cations and negatively charged anions. The sum of the positive and the sum of the negative charges formed from neutral electrolyte molecules must naturally be equal to each other. - Acids and alkalis. Acids are substances that yield hydrogen ions upon dissociation in solution. Acids are distinguished by a number of characteristic chemical and physico-chemical properties. Thus, they act on metals with the evolution of hydrogen and the formation of corresponding salts, combine with alkalis (also yielding salts), are distinguished by a sour taste (which served as the reason for their name), etc. All these properties depend on the presence of the hydrogen ion common to all acids and are reactions of the H-ion. For example, upon action on metals, the reaction proceeds as follows: 2H⁺ + 2Me = 2Me⁺ + H₂. The metal enters the solution in the form of ions, and hydrogen ions are released in the form of molecular hydrogen. The more H⁺ contained in the solution, the more strongly the "acidic" properties of the solution are expressed, the greater its "acidity." In this respect, different acids differ strongly from one another. So-called "strong" acids are almost completely dissociated, i.e., almost all their molecules break down into hydrogen ions and corresponding anions. In weak acids, only a portion of the molecules break down, and the degree of dissociation is determined by the general laws of chemical equilibrium ("law of mass action") and is expressed by the following equation: In this equation, [H⁺], [A^-], and [AH] denote the corresponding concentrations of the hydrogen ion, the anion, and the undissociated acid molecules, and K represents the constant characterizing the strength of the acid and called the dissociation constant. The larger it is, the stronger the acid, i.e., the more hydrogen ions are in the solution at the same concentration of acid. For lactic acid, K equals 1.5·10^-4, for acetic acid 1.8·10^-5, for uric acid 1.5·10^-6. The opposite group of electrolytes are alkalis, the properties of which depend on the hydroxyl ion OH^- common to them all. Alkalinity, the alkaline reaction of a solution, is determined by the concentration of the OH-ion, just as is the case for acidity and H-ions. Due to the presence of hydroxyl ions, alkalis react with acids, whereby the OH^- of the former and the H⁺ of the latter combine into water molecules, and the cation of the alkali and the anion of the acid form a salt. This process is called neutralization; mutual binding leads to the disappearance of acidic and alkaline reactions and to the formation of a neutral solution. Dissociation of water. Some substances possess the ability to release both H⁺ and OH^- (see Ampholytes). Water belongs to them, the molecules of which are capable of breaking down into H⁺ and OH^-. Therefore, in an extremely small quantity, both these ions are found (in equal numbers) even in completely pure, neutral water containing neither acids nor alkalis. Their presence can be established quite accurately by measuring the electrical conductivity of pure water. The concentration of both water ions is determined by the same dissociation equation as in the case of acids: [H⁺]·[OH^-] / [H₂O]. Since in this case, in view of the very weak dissociation of water, the concentration of its undissociated molecules practically does not change, this equation can be simplified into the usual equation for the dissociation of water: [H⁺]·[OH^-] = K_w.

The constant K_w, representing the product of the concentration of H⁺ and OH^- ('ionic product' of water), has a very small value; at 18° it is equal to 0.7·10^-14. In neutral water [H⁺] = [OH^-]; the concentration of each of these ions is approximately 10^-7. The same equation (2) determines the concentration of H⁺ and OH^- in acidic and alkaline solutions. Upon the addition of an acid, i.e., new H-ions, the concentration of OH-ions of water decreases, but the product of both ions remains constant; this product is just as constant in an alkaline solution. Therefore, in any aqueous solution, the concentrations of H⁺ and OH^- are conjugate values, inversely proportional to each other. Knowing one of them, one can calculate the other using equation (2). - Hydrogen number and hydrogen exponent. The data provided allow for a completely precise definition and a convenient measure of the reaction of an aqueous solution. Specifically, in view of the indicated inverse relationship between the concentration of H- and OH-ions, it is sufficient to indicate the concentration of any one of these ions to characterize the reaction of a liquid. As indicated in more detail in the article Active reaction, in practice it is customary to indicate the concentration of H-ions specifically. In this case, to avoid inconvenient designations in the form of a fraction with a large denominator or a whole number with a negative exponent, the so-called hydrogen exponent is usually used, i.e., the logarithm of the concentration of H-ions, whereby this logarithm (which is usually negative) is taken with the opposite sign. The concentrations of H-ions are expressed in powers of normality (see Normal solutions), whereby the concentration of H-ions in a completely dissociated normal acid solution is taken as unity. Suppose there is, for example, a solution 20,000 times weaker. In it, the concentration of H-ions will be equal to 1/20,000 or 0.00005, or 5·10^-5. The logarithm of this fraction will be 5.69897, or -4.30103. The latter value, taken with the opposite sign, will be the hydrogen exponent: pH=4.30 (the hydrogen exponent is in practice indicated to the second decimal place). The value of the hydrogen exponent provides a single scale for all reaction values, both acidic and alkaline. The difference between the two is, perhaps, even too smoothed out in this process. It is necessary to remember the position of the neutral point at a given t° in order to have an accurate idea of the degree of relative predominance of hydroxyl or H-ions corresponding to a given pH value. Meanwhile, at the very basis of the concept of reaction lies not the absolute concentration of H- and OH-ions, but their ratio. Therefore, Giribaldo recently proposed, instead of the hydrogen exponent, another measure of reaction, expressing the relative content of H-ions and the degree of deviation of the reaction in one direction or the other from neutrality. For this, the logarithm of the ratio [H⁺] : [OH^-] can serve, abbreviated, at the suggestion of Kopaczewski, by the sign pR: pR = log [H⁺] / [OH^-]. With this method of designation, neutrality in all cases is expressed by zero, acidic reaction by positive, and alkaline by negative pR values. This very illustrative designation has not, however, received wide distribution to date. Methods of measurement. For a long time, the titration method was used to measure the reaction. The acidity of a solution was expressed by the amount of alkali that needed to be added to bind all the H-ions contained in it and obtain a neutral solution. In a similar way, to measure alkalinity, the solution was titrated with an acid. However, titration allows one to measure the total amount of acid (resp. alkali) contained in the solution, but does not give any idea about the true concentration of free, actively acting H- and OH-ions, about the active reaction (see) of the solution (see Buffer properties). Only those methods whose application does not disturb the equilibrium of H⁺ and OH^- are suitable for measuring the reaction. The basic method here is the electrometric one. It is based on the fact that hydrogen adsorbed on the surface of a noble metal (usually platinum black) behaves like a metallic hydrogen electrode and, being immersed in a solution, acquires an electric charge, the potential of which depends on the concentration of H-ions in the solution. By measuring the magnitude of this potential, one can accurately determine the concentration of H-ions and the active reaction of the solution (see Gas chain). The colorimetric method, based on the use of indicators, serves the same purpose. The latter are weak acids or alkalis that change their color upon dissociation. For example, phenolphthalein is a weak acid, the molecules of which are colorless, while the free anion has a red color. Its dissociation is determined by the same equation (see equation 1) as the dissociation of other acids. An insignificant amount of free H-ions is sufficient to suppress its dissociation and decolorize the solution; conversely, in an alkaline medium, dissociation is enhanced, and the solution takes on a bright red color. Thus, one can judge the reaction of a solution by its color, and by colorimetric comparison with a series of standard solutions (see Buffer properties) of a strictly defined reaction, bring this determination to the desired degree of accuracy. - Biological role of H-ions. The reaction of the medium, the concentration of H⁺ and OH^-, has a huge influence on vital processes and therefore acquires special interest for biology and medicine. Colloids are distinguished by high sensitivity to the action of ions in general, and hydroxyl and H-ions in particular. The latter, at the same time, exert the greatest influence on chemical substances that have an amphoteric character (see Ampholytes). The construction of living matter from amphoteric colloids (which are primarily protein bodies) therefore explains the universal dependence of all living organisms on the concentration of H-ions. This dependence is expressed primarily in the fact that life is possible only at certain concentrations of H-ions, which for the majority of organisms approach neutrality. With an excessive increase in the concentration of H⁺ or OH^-, the organism dies. The pH limits within which life is possible are not the same for different organisms. For some they are wide, for others they are extremely narrow. A pH value that kills one organism may be completely harmless or even optimal for another. As a result, pH acquires the significance of one of the most important factors in the distribution of organisms. Thus, different soils differ significantly from each other in the concentration of H-ions; these differences in many cases determine the preference shown by plants for one soil or another. In a similar way, different natural bodies of water have the most varied reactions—from very acidic to strongly alkaline. Representatives of the most acidic bodies of water can be some peat bogs, which have a pH of up to 3.5-4.0. A strongly alkaline reaction (pH = 10.0) is observed during the strong development of algae (the so-called 'water bloom'), which, during photosynthesis processes, decompose carbonic acid and alkalinize the water. Sea water has a weakly alkaline reaction, most often approaching pH = 8.0. The influence of pH is especially clearly revealed in bacteria, since they themselves often produce large amounts of alkaline (ammonia) or acidic products (butyric, lactic, and other acids) that kill other bacterial forms or even lead to self-poisoning. This phenomenon was used by Metchnikoff, who proposed using the Bulgarian bacillus of lactic acid fermentation to combat putrefactive bacteria of the intestine, which live in a weakly alkaline reaction. Action on individual biological processes. Even insignificant changes in pH, not exceeding the limits that allow the life of a given organism, often have a profound effect on individual vital processes occurring within it. Thus, the activity of enzymes reaches a maximum at a certain pH and falls more or less rapidly on both sides of it. Correspondingly, a similar dependence is shown by the metabolism of individual organs: for example, in an isolated heart muscle, it decreases several times when the pH is reduced from 7.35 to 6.6. Another example can be blood vessels: their muscular wall noticeably relaxes when the blood pH decreases by 0.2-0.3, contracting with the opposite change. The respiratory center in the medulla oblongata is distinguished by the greatest sensitivity to H-ions known to us, noticeably changing pulmonary respiration already with a change in the pH of the blood bathing it by 0.01. - pH of tissues and body fluids. Fluids with very different pH values are found in the body. This applies first of all to digestive juices, for which the following average values have been established for humans: pH of saliva = 6.0-7.0; pH of gastric juice = 1.7-2.0; pH of intestinal juice = 7.0-8.0. These pH values correspond exactly to the optimum action of the enzymes contained in the corresponding juices. Any significant change in the pH of these juices would disrupt or make the action of these enzymes completely impossible. In pathological conditions, such disturbances are especially observed in gastric juice.

In contrast to the diversity of hydrogen ion concentrations observed in digestive juices, blood is distinguished by the amazing constancy of its reaction. Its pH equals 7.3–7.5 and, even in the majority of pathological states, does not go beyond these limits. This value remains unchanged, despite the fact that acidic metabolic products are continuously entering the blood from tissues and that the blood, in turn, serves as material for the production of secretions and excretions of the most varied pH. The constancy of the hydrogen ion concentration in the blood, which is of extremely important significance for the vital activity of the organism, is maintained with the help of a whole series of highly perfect regulatory mechanisms. The first of these is the chemical composition of the blood itself, its buffer properties, thanks to which it steadfastly maintains its initial reaction even outside the organism, in vitro. An even more precise regulator in the organism is the respiratory apparatus (see Respiration), thanks to which any disturbance in the reaction of the blood is quickly compensated for by a corresponding change in the carbon dioxide content. While the lungs remove CO₂ from the blood, non-volatile acids are excreted by the kidneys. Urine therefore has an acidic (and little-constant) reaction; in humans, its pH usually lies between 5 and 7. Tissue fluid in a resting organ that is well supplied with blood has a neutral or weakly alkaline reaction, approaching the reaction of the blood. During muscular work, the hydrogen exponent decreases, and during strong muscular fatigue, the reaction in the muscle turns out to be weakly acidic (pH = 6.6–6.7). Even greater local acidification, local acidosis (see), occurs during inflammation (see). In an acute inflammatory process, Schade observed an acidic reaction (down to pH = 6.0 in a furuncle). The dilation of vessels and hyperemia of the inflamed area, just like pain sensations, are the direct result of a pathological increase in the concentration of hydrogen ions. Intracellular reaction. Measuring the pH inside a living cell is associated with particular difficulties. Most methods require the preliminary killing or damaging of the cell, which leads to a significant change (increase) in the initial concentration of hydrogen ions. Only in the very recent past, by means of microinjection of indicators, as well as the introduction of microelectrodes into a living cell (see Microsurgery), has it been possible in some cases to measure the reaction prevailing in the latter. It is apparently close to neutral, while still revealing local differences, in particular between the protoplasm and the nucleus. It is necessary, however, to remember that the entire doctrine of the hydrogen exponent is based, as indicated above, on the constancy of the ion product of water (equation 2) and, consequently, is applicable only to aqueous solutions. Therefore, the data provided also relate only to the aqueous phases of the cell. Regarding the concentration of hydroxyl and hydrogen ions in non-aqueous media, which likely play a very essential role in the life of the cell, there is as yet only very meager data.