pH is a logarithmic scale used to measure the acidity or basicity of aqueous solutions. pH values below 7 indicate acidic solutions, while values above 7 indicate basic or alkaline solutions. A pH of 7 refers to solutions that have equal concentrations of hydrogen and hydroxide ions, like pure water at 25 degrees Celsius. The pH scale ranges from 0 to 14, and is determined by measuring the potential difference between a hydrogen electrode and a reference electrode in standard buffer solutions. The pH of a solution can be measured using a glass electrode and pH meter or indicators and is important in various applications.

1. Ph electrode
In chemistry, pH (/piːˈeɪtʃ/ pee-AYCH), also referred to as acidity or basicity, historically denotes
"potential of hydrogen" (or "power of hydrogen"). It is a logarithmic scale used to specify the
acidity or basicity of aqueous solutions. Acidic solutions (solutions with higher concentrations of
hydrogen (H + ) ions) are measured to have lower pH values than basic or alkaline solutions. The
pH scale is logarithmic and inversely indicates the activity of hydrogen ions in the solution where
[H + ] is the equilibrium molar concentration (mol/L) of H + in the solution. At 25 °C (77°F),
solutions with a pH less than 7 are acidic, and solutions with a pH greater than 7 are basic.
Solutions with a pH of 7 at 25 °C are neutral (i.e. have the same concentration of H + ions as OH −
ions, i.e. the same as pure water). The neutral value of the pH depends on the temperature and
is lower than 7 if the temperature increases above 25 °C. The pH range is commonly given as
zero to 14, but a pH value can be less than 0 for very concentrated strong acids or greater than
14 for very concentrated strong bases. [2] The pH scale is traceable to a set of standard solutions
whose pH is established by international agreement. [3] Primary pH standard values are
determined using a concentration cell with transference by measuring the potential difference
between a hydrogen electrode and a standard electrode such as the silver chloride electrode.
The pH of aqueous solutions can be measured with a glass electrode and a pH meter or a color-
changing indicator. Measurements of pH are important in chemistry, agronomy, medicine, water
treatment, and many other applications.

2. History
• In 1909, the Danish chemist Søren Peter Lauritz Sørensen introduced
the concept of pH at the Carlsberg Laboratory, [4] originally using the
notation "pH•", with H• as a subscript to the lowercase p. The concept
was later revised in 1924 to the modern pH to accommodate
definitions and measurements in terms of electrochemical cells. For
the sign p, I propose the name 'hydrogen ion exponent' and the
symbol pH• . Then, for the hydrogen ion exponent (pH• ) of a solution,
the negative value of the Briggsian logarithm of the related hydrogen
ion normality factor is to be understood.[4] Sørensen did not explain
why he used the letter p, and the exact meaning of the letter is still
disputed.

3. Definition
• The pH of a solution is defined as the decimal logarithm of the reciprocal of
the hydrogen ion activity, aH+. [14] Mathematically, pH is expressed as: For
example, for a solution with a hydrogen ion activity of 5×10−6 (i.e., the
concentration of hydrogen ions in moles per litre), the pH of the solution
can be calculated as follows: The concept of pH was developed because ion-
selective electrodes, which are used to measure pH, respond to activity. The
electrode potential, E, follows the Nernst equation for the hydrogen ion,
which can be expressed as: Definition pH where E is a measured potential, E
0 is the standard electrode potential, R is the gas constant, T is the
temperature in Kelvin, F is the Faraday constant. For H + , the number of
electrons transferred is one. The electrode potential is proportional to pH
when pH is defined in terms of activity.