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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 = -logcH+. 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, Kω = CH+ × COH- = K × cH2o = 1.8 × 10-16 × 55.56 = 1 x 10-14 (at 22° C). The constant Kωw is called the ion product of water. Because CH+ = COH- = in pure water (and in any neutral medium), pH = -log CH+ = 7. When acid is added to water, the CH+ increases and the pH decreases accordingly. So, for 0.01 molar (M) solution of HCl, the concentration CH+ = 10-2 and pH = -log CH+ = 2. When alkali is added to water, the COH- increases; then, CH+ = 10-14/cOH- decreases and the pH increases accordingly. For example, in a 0.01 M solution NaOH COH- = 10-2. This means that cH+ = 10-14/cOH- = 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, Kω 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 CH+ = COH- in a neutral medium.
REFERENCEKireev, V. A. Kratkii kurs fizicheskoi khimii, 4th ed. Moscow, 1969.
V. L. VASILEVSKII