pH Value

Also found in: Dictionary, Thesaurus, Medical, Financial, Wikipedia.

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 = -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- = pH 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.


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


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.
References in periodicals archive ?
However, the veracity of their claims has not been investigated, as the majority of these soaps and shampoos do not disclose their pH values.
01 accuracy, in the range of 1-14, working temperature range 0-100[degrees]C was used to measure the pH value of MWCNT/water nanofluid.
At low pH values the adsorption of phenol decreases because of the H+ ions which suppress the ionization of phenol and thus its uptake on polar adsorbent is reduced.
This is then digitally converted and from the digital voltage value the output pH value is obtained from the simple line equation given by y = 0.
Sorption efficiency in 2 mg/ formaldehyde solution (dependence on time and pH value with standard deviations)
2 mmol of lactate recorded in the "interrupted trial", shows that the effort is performed within a lactate tolerance zone (S1) and, correlated to the lowest pH value, it shows that, during the exercises involving sprints, muscle glycogen (anaerobic glycolysis) generates a large amount of lactic acid and H+ ions, which results in a decrease of the pH value that may reach a level between 7.
Moreover, owing to increasing concerns regarding the significance of continuous pH value monitoring in industrial processes, the demand for online pH meter is projected to witness fastest growth rate over the next five years,” said Mr.
With the addition of the CHIC fair with the Spring Edition for the first time, as well as the return of PH Value and Yarn Expo Spring, buyers and exhibitors praised the increased business opportunities on offer.
The effects of pH value, ionic type, and ionic strength of swelling medium on swelling behaviors were investigated.
This is explained by the fact that reserves of glycogen become depleted during lairage and after a longer lairage there is a lower muscle glycogen content and consequently higher pH value of meat.