Concentration of H Ion and pH

Acids such as HCl and sulfuric acid (H2SO4) produce H+ ion, whereas bases such as sodium hydroxide and calcium hydroxide [NaOH and Ca(OH)2 respectively] produce hydroxide ion, OH . Molar concentrations of water solutions of hydrogen ion, [H+], range over many orders of magnitude and are conveniently expressed by pH defined as follows  

In absolutely pure water at 25 C, the value of [H+] is exactly 1 x 10 mole/L, the pH is 7.00, and the solution is neutral (neither acidic nor basic). Acidic solutions have pH values of less than 7, and basic solutions have pH values of greater than 7. 

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Note the relative concentrations of H+ ion and other cations as percents of the soil CEC values in Table 10.5. Important ions adsorbed on soils, but rarely measured, may include NH, and on waterlogged soils may also include Fe + and Mn. The data in Table 10.5 show that below about pH 5, protons occupy a large fraction of clay surface sites. Below pH 3 to 4, protons occupy all the sites and tend to destroy clay structures. This, of course, makes it difficult to study the surface and sorptive properties of clays in acid solutions. The proton competes effectively with other cations for exchange sites, even when its concentration is 10 to 100 times less than that of the cations. 

Calculating ion concentrations from pH Sometimes, you have to calculate the concentration of H+ ions and OH ions from the pH of a solution. Example Problem 18.4 shows how to do this. 

Addition of adds, especially strong ones, in a solution results in increased concentrations of H ion and a decrease of pH. The introduction of bases, especially strong ones, in a solution removes H and increas-ers pH. In either case equality a = is distorted but ionic product of water remains constant. For this reason under standard conditions persists... 

You should notice that the solutions in question 1, parts c, d and e, are alkalis they all have pH values greater than 7. Even though they are alkalis they each still have a small concentration of H ions, and this concentration is used to calculate the pH. They each have a small concentration of H ions because H2O H + OH- is an equilibrium. Even when there is an excess of OH- ions there is still a small concentration of H ions. In the same way, the solutions in question 1, parts a and b, have a small concentration of OH- ions, even though the solutions are acids. 

Confining attention to the case in which the concentrations of the acid and its salt are equal, i.e. of a half-neutralised acid then pH = pKa. Thus the pH of a half-neutralised solution of a weak acid is equal to the negative logarithm of the dissociation constant of the acid. For acetic (ethanoic) acid, Ka = 1.75 x 10 5 mol L 1, pKfl = 4.76 a half-neutralised solution of, say 0.1M acetic acid will have a pH of 4.76. If we add a small concentration of H + ions to such a solution, the former will combine with acetate ions to form undissociated acetic acid ... 

Solutions of different acids having the same concentration might not have the same pH. For instance, the pH of 0.10 M CH3COOH(aq) is close to 3 but that of 0.10 M HCl(aq) is close to 1. We have to conclude that the concentration of H,() ions in 0.10 M CH3COOH(aq) is lower than that in 0.10 M HCl(aq). Similarly, we find that the concentration of OH ions is lower in 0.10 M NH,(aq) than it is in 0.10 M NaOH(aq). The explanation must be that in water CH.COOH is not fully deprotonated and NH3 is not fully protonated. That is, acetic acid and ammonia are, respectively, a weak acid and a weak base. The incomplete deprotonation of CH3COOH explains why solutions of HC1 and CH3COOH with the same molarity react with a metal at different rates (Fig. 10.14). 

To confirm the relation between the Pd(II) reduction and the change in the concentration of H+ ions, the sonolysis of 1-propanol-water solution without Pd(II) was carried out and the change in the pH value was investigated. The pH value without Pd(H) decreased from 5.7 to 5.2 in the 10 min irradiation, while that with Pd(II) decreased from 3.5 to 2.7 in the 10 min irradiation. This result suggests that the change... 

Chemoreceptor response to increased arterial hydrogen ion concentration. An increase in arterial hydrogen ion concentration, or a decrease in arterial pH, stimulates the peripheral chemoreceptors and enhances ventilation. This response is important in maintaining acid-base balance. For example, under conditions of metabolic acidosis, caused by the accumulation of acids in the blood, the enhanced ventilation eliminates carbon dioxide and thus reduces the concentration of H+ ions in the blood. Metabolic acidosis may occur in patients with uncontrolled diabetes mellitus or when tissues become hypoxic and produce lactic acid. An increase in arterial hydrogen ion concentration has no effect on the central chemoreceptors. Hydrogen ions are unable to cross the blood-brain barrier. 

Figure 18.2—Measurement of pH. The concentration of H+ ions can be determined from the potential difference between the reference electrode and the glass electrode. Details of the membrane, which is permeable to the H1 ion, are shown. When an H+ ion forms a silanol bond, a sodium ion moves into the solution to preserve electroneutrality. A cross-section of the membrane showing this exchange reaction is presented (IUPAC conventions are not followed to improve clarity in the diagram). Prior to its use, the pH meter is calibrated with a buffer solution of known pH.

Combined measuring probe Has a thin-walled glass bulb at its tip which detects H+ ions in solution, and produces a small voltage (around 0.06 V per pH unit), and as such provides a signal related to the concentration of H+ ions which can be converted into a measurement of pH. For simplicity, the pH probe can be... 

For most oxides, as the pH is increased, the adsorption of potential determining ions, H" and OH, changes in correspondence with the concentration of these species in solution. For each surface, therefore, a point is reached at which the concentration of positive ions and negative ions just balance, the point of zero charge. The pH where the zeta potential, is 0, is called the isoelectric point. The isoelectric point for various ceramic materials is given in Table 9.11. 

Because the cell potential is sensitive to the concentrations of the reactants and products involved in the cell reaction, measured potentials can be used, to determine the concentration of an ion. A pH meter is a familiar example of an instrument that measures concentration from an observed potential. The pH meter has three main components a standard electrode of known potential, a special glass electrode that changes potential depending on the concentration of H+ ion in the solution into which it is dipped, and a potentiometer that measures the potential between the two electrodes. The potentiometer reading is automatically converted electronically to a direct reading of the pH of the solution being tested. 

Every HCI molecule produces one H+ ion. The bottle labeled 0. IM HCI contains 0.1 mole of H+ ions per liter and 0.1 mole of Cl ions per liter. Eor all strong monoprotic acids, the concentration of the acid is the concentration of H+ ion. Thus, you can use the concentration of the acid for calculating pH. 

Equilibrium constants for these reactions are indicated byK The total capacity of the surface for adsorption of H+ ions and the equilibrium constants are determined by potentiometric titration, which involves addition of a known amount of acid or base and then measuring the change in solution pH. The difference between the amount of titrant added and its concentration in the solution represents the amount of H+ that has adsorbed onto, or reacted with, the surface. 

Use of the pH notation allows all degrees of acidity and alkalinity normally encountered in chemistry to be expressed on a scale from 0 to 14, corresponding to the concentrations of H+ ions contained in the solution. Solutions with a pH < 7 are considered acidic, solutions with a pH > 7 are alkaline, while a solution with a pH = 7 is neutral. 

The pH scale is a logarithmic scale representing the concentration of H ions in a solution. Remember that as the H concentration increases the OH concentration decreases and vice versa. If we have a solution with one in every ten molecules being H, we refer to the concentration of HT ions as 1/10. Remember from algebra that we can write a fraction as a negative exponent, thus 1/10 becomes 10 Conversely 1/100 becomes 10, 1/1000 becomes 10, etc. Logaridims are exponents to which a number (usually 10) has been raised. For example log 10 (pronounced "the log of 10") = 1 (since 10 may be written as lO ). The log 1/10 (or 10 ) = -1. pH, a measure of the concentration of H ... 

Strictly speaking, hydrogen ion activity should be substituted for [H+], but effectively the latter is sufficiently accurate. Water is considered to be neutral, but it is partially ionized into Fl+ and OFF ions. The concentration of H+ ions in pure water is 10 7 mol dm-3, so a neutral solution has pH = 7. Below this value a solution becomes increasingly acidic as pH falls, while above it conditions become increasingly alkaline as pH rises. 

Differences in the temporal development of release rates for the individual elements are connected with their sorption/desorption behaviour, which is primarily due to pH-effects but may also be influenced by com-plexation, e.g. by elevated concentrations of chloride ions. With respect to the pH-effects, however, there are significant differences in the response of the various solid substrates to the addition of H+-ions, and it may be argued that the pH-values on the solid surfaces -which can be estimated from "pH-titration tests" - are decisive for the behavior of the particular element rather than the pH-values determined in solution. 

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