hydrogen ion exponent (pH) Department“ORGANIC CHEMISRTY AND TECHNOLOGY” Chemistry for Engineering Logistics hydrogen ion exponent (pH) 1. Dissociation of water 2. Acids and bases 3. The hydrogen ion exponent (pH) Assoc. Prof. V. Grozev

1. Dissociation of water The concept of pH was first introduced by Danish Chemist Sorensen at the Carlsberg Laboratory in 1909 and revised to the modern pH in 1924. 1.1. Water equilibrium The dissociation of water is an ionization reaction in pure water or an aqueous solution, in which a water molecule, H2O, loses the nucleus of one of its hydrogen atoms (H+) to become a hydroxide ion, OH−. The dissociation is an equilibrium process (water equilibrium): H2O H+ + OH- .

1. Dissociation of water 1.2. The water equilibrium constant The water equilibrium constant (Kw) is defined as: Kw = [H+] [OH-], where [H+] and [OH-] are concentrations of H+ and OH- ions in the solution. Experimentally, it has been found that the concentrations: [H+] = [OH-] = 10-7 Therefore Kw = [10-7][ 10-7] = 10-14 (1) (To multiply exponential numbers - simply add the exponents). The equation (1) shows that the multiplication of the concentrations [H+] and [OH-] is a constant value and equals to 10-14 at 250C. The sum of exponents in equation (1) must always be equal to 10-14.

2. Acids and bases 2.1. Definition of acid and bases (alkali) The first modern definition of acids and bases was devised by Svante Arrhenius (Sweden): - acid is a substance that dissociates in water to form only hydrogen ions (H+) and the concentration of H+ ions in aqueous solution increases: HCl H+ + Cl- - base is a substance that dissociates in water to form hydroxide (OH−) ions and the concentration of OH− ions in the solution increases: NaOH Na+ + OH-

2.2. Effect of Acids and Bases on Water Equilibrium If an acid (H+) is added to the water, the equilibrium shifts to the left and the OH- ion concentration decreases: H2O H+ + OH- If base (OH-) is added to water, the equilibrium shifts to the left and the H+ concentration decreases. In accordance with water equilibrium principle the multiplication of H+ and OH- ion concentrations must remain equal to 10-14. Both H+ and OH- ions are always present in any solution. A solution is: - acidic, if the H+ are in excess - [H+] > 10-7, for exsample 10-6, 10-5, 10-4; - basic, if the OH- ions are in excess ([H+] < 10-7 and [OH-] > 10-7); - neutral ([H+] = [OH-] = 10-7).

3. The hydrogen ion exponent (pH) The acidity or alkalinity of solutions can be described by means of hydrogen ion concentrations – 10-12, 10-10, 10-4 and etc. But this way is unsuitable and pH values have been introduced in 1924. 3.1. Definition pH is defined as the negative decadic logarithm of the hydrogen ion concentration. The equation is: pH = - log [H+], where - [H+] is hydrogen ion concentration; - log – decadic (or Brigsian) logarithm with base 10. The purpose of the negative sign is to give a positive pH value.

3. The Hydrogen Ion Exponent (pH) 3.2. Logarithm The logarithm of a number is the exponent to which another fixed value, the base, must be raised to produce that number. For example, the logarithm of 1000 to base 10 is 3, because 1000 is 10 to the power 3: 1000 = 10 × 10 × 10 = 103. If x = by, then y is the logarithm of x to base b, and is written y = logb(x), or y = logb(by), so log10(1000) = log10(103) = 3. Number Exponent Notation Logarithm of the Number  1000  103  3  100  102  2  10  101  1  0  0.1  10-1  -1  0.01  10-2  -2  0.001  10-3  -3

3. The Hydrogen Ion Exponent (pH) 3.3. Example If an acid has an H+ concentration of 0.0001, find the pH. Solution: First convert the number to exponential notation, find the logarithm, then solve the pH equation. [H+] = 0.0001 = 10-4; log of 10-4: log (10-4)= - 4; pH = - log [ H+] = - log (10-4) = - (-4) = +4 = pH

3. The Hydrogen Ion Exponent (pH) 3.4. pH scale The hydrogen ion concentration [H+], pH numbers and hydroxide ion concentration are represented in the table: pH values: Acidic pH<7 Neutral pH= 7 Basic pH >7

3. The Hydrogen Ion Exponent (pH) 3.5. Measuring of pH pH - values can be measured with universal indicator paper.

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