pH Value

a negative decimal logarithm of the hydrogen ion concentration, or more accurately, hydrogen ion activity (in gram ion per liter), of a given solution: pH = -logc_H+. The pH value serves as a quantitative measure of a solution’s acidity, which has a significant influence on the direction and speed of many chemical and biochemical processes. At normal temperatures (more accurately, at 22° C) pH = 7 for neutral solutions, pH < 7 for acidic solutions, and pH > 7 for alkaline solutions. The accurate measurement and regulation of pH are necessary not only for chemical and biochemical laboratory research but also for numerous technological processes in industry, for the evaluation of soil properties, and for initiating procedures to increase the soil’s productivity.

Water dissociates into ions as shown by the equation H₂O ⇄ H⁺ + OH^-. According to the law of mass action,

where c is the molar concentration and K is a constant at a given temperature (1.8 × 10^-16 gram ion per liter at 22° C). One liter of water contains 1,000/18.016 = 55.56 gram moles of water (the quantity of dissociated molecules is so small that it can be ignored). Therefore, K_ω = C_H+ × C_OH- = K × c_H₂o = 1.8 × 10^-16 × 55.56 = 1 x 10^-14 (at 22° C). The constant K_ωw is called the ion product of water. Because C_H+ = C_OH- = in pure water (and in any neutral medium), pH = -log C_H+ = 7. When acid is added to water, the C_H+ increases and the pH decreases accordingly. So, for 0.01 molar (M) solution of HCl, the concentration C_H+ = 10^-2 and pH = -log C_H+ = 2. When alkali is added to water, the C_OH- increases; then, C_H+ = 10^-14/c_OH- decreases and the pH increases accordingly. For example, in a 0.01 M solution NaOH C_OH- = 10^-2. This means that c_H+ = 10^-14/c_OH- = 10^-12 and pH = 12.

Poteiitiometric methods are primarily used for accurate pH measurements. When the determination of pH does not require high accuracy, pH is measured using a group of indicators, with each indicator changing its color at a certain pH. The pH scale is usually used for solutions that have a hydrogen ion concentration of 1 gram ion per liter and less. A neutral water solution has a pH of 7 only at room temperature. At higher temperatures dissociation of water increases, K_ω increases, and at 100° C pure water has a pH of approximately 6. At temperatures below 22° C, pure water has a pH greater than 7. However, in all cases C_H+ = C_OH- in a neutral medium.

REFERENCE

Kireev, V. A. Kratkii kurs fizicheskoi khimii, 4th ed. Moscow, 1969.

V. L. VASILEVSKII

pH value

A number denoting the degree of acidity or of basicity (alkalinity); 7 is a neutral value; acidity increases with decreasing values below 7; basicity increases with increasing values above 7.

单词	ph value
释义	pH value Translations pH Value pH Value a negative decimal logarithm of the hydrogen ion concentration, or more accurately, hydrogen ion activity (in gram ion per liter), of a given solution: pH = -logc_H+. The pH value serves as a quantitative measure of a solution’s acidity, which has a significant influence on the direction and speed of many chemical and biochemical processes. At normal temperatures (more accurately, at 22° C) pH = 7 for neutral solutions, pH < 7 for acidic solutions, and pH > 7 for alkaline solutions. The accurate measurement and regulation of pH are necessary not only for chemical and biochemical laboratory research but also for numerous technological processes in industry, for the evaluation of soil properties, and for initiating procedures to increase the soil’s productivity. Water dissociates into ions as shown by the equation H₂O ⇄ H⁺ + OH^-. According to the law of mass action, where c is the molar concentration and K is a constant at a given temperature (1.8 × 10^-16 gram ion per liter at 22° C). One liter of water contains 1,000/18.016 = 55.56 gram moles of water (the quantity of dissociated molecules is so small that it can be ignored). Therefore, K_ω = C_H+ × C_OH- = K × c_H₂o = 1.8 × 10^-16 × 55.56 = 1 x 10^-14 (at 22° C). The constant K_ωw is called the ion product of water. Because C_H+ = C_OH- = in pure water (and in any neutral medium), pH = -log C_H+ = 7. When acid is added to water, the C_H+ increases and the pH decreases accordingly. So, for 0.01 molar (M) solution of HCl, the concentration C_H+ = 10^-2 and pH = -log C_H+ = 2. When alkali is added to water, the C_OH- increases; then, C_H+ = 10^-14/c_OH- decreases and the pH increases accordingly. For example, in a 0.01 M solution NaOH C_OH- = 10^-2. This means that c_H+ = 10^-14/c_OH- = 10^-12 and pH = 12. Poteiitiometric methods are primarily used for accurate pH measurements. When the determination of pH does not require high accuracy, pH is measured using a group of indicators, with each indicator changing its color at a certain pH. The pH scale is usually used for solutions that have a hydrogen ion concentration of 1 gram ion per liter and less. A neutral water solution has a pH of 7 only at room temperature. At higher temperatures dissociation of water increases, K_ω increases, and at 100° C pure water has a pH of approximately 6. At temperatures below 22° C, pure water has a pH greater than 7. However, in all cases C_H+ = C_OH- in a neutral medium. REFERENCE Kireev, V. A. Kratkii kurs fizicheskoi khimii, 4th ed. Moscow, 1969. V. L. VASILEVSKII pH value A number denoting the degree of acidity or of basicity (alkalinity); 7 is a neutral value; acidity increases with decreasing values below 7; basicity increases with increasing values above 7. MedicalSeepHFinancialSeePH
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