Acids
There are three scientifically correct definitions that define an acid substance. Arrhenius defines an acid as a substance that made up of hydrogen that when dissolved in water breaks apart increasing the concentration of hydrogen ions (H+) in the water. Bronsted-Lowry defines an acid as a proton (H+ ion) donor. Lewis describes acids as substances that accept electron pairs. Acids have a sour taste and are highly reactive with metals (Brent, 2009). Nitric acid is used as fertilizers, and hydrochloric acid is used to bricks.
Bases
A basic substance can also be described using definitions from the three authors. Arrhenius defines a basic substance as one that contains the hydroxide group and breaks down to produce hydroxide ions (OH-) whenever it is dissolved in water. Bronsted-Lowry defines a base as a proton acceptor and Lewis defines it as an electron pair donor (Brent, 2009). Solutions that contain bases are known as alkaline and are bitter tasting with most of them feeling slippery. Sodium hydroxide is used to manufacture soap and ammonia hydroxide is used as a cleaning agent.
Measuring Acidity or Alkalinity of a Substance
The acidity or alkalinity of an aqueous substance is determined using the concentration of the hydroxide on a logarithmic scale known as the pH scale. The strength of an acid and a base is characterized by the amount of hydrogen ions and hydroxide ions in the substances respectively. The number of these ions in aqueous solutions can be very large up to hundreds of millions hence the use of a logarithmic scale (Petrucci, 2007). To understand how the pH scale works it is important to look at water that can act as both an acid or a base and can undergo auto ionization though rarely. The process helps find the equilibrium constant known as ionization constant for water (Kw). The constant is calculated at 25° C because it is affected by temperature.
H2O (l) + H2O (l) ↔ H3O (aq) + OH-(aq)
Kw = [H3O+] [OH-] = 1.0 × 10-14
[H3O+] = [OH-] = 1.0×10-14 = 1.0 × 10-7
The pH of water can thus be calculated as:
pH = -log [H3O+] = -log [1.0 × 10-7] = 7
When we add acid to water the H3O+ ions increases and the OH- ions reduces, adding a base to water increases the OH- ions and reduces the H3O+ ions. This is because the product of these ions should always remain constant:
[H3O+] [OH-] = 1.0 × 10-14
Increase in H3O+ ions reduces the pH and vice-versa hence any pH less than seven means a substance is acidic, and any pH over seven signifies a substance is basic.
Acid-base Reaction
Most acids and bases have the general formula HA and BOH respectively. When an acid and base solution combines, a neutralization process takes place. The hydrogen ion from the acid and the hydroxide ion from the base combine to form water.
H+ + OH- = H2O
The A- that is the acid anion and the B+ that is the base cation also combine to form a neutral salt. An example of such a reaction is when hydrochloric acid and sodium hydroxide react with each other to form salt and water.
HCl(aq) + NaOH(aq) → H2 O(l) + NaCl(aq)
Acid-base Titration
Acid-base titrations are used to determine the unknown concentrations of different solutions. A solution that has a known concentration known as titrant is added to a solution of known volume but unknown concentration known as titer until a neutralization reaction occurs (Gros, Bruttel, & Von Kloeden, 2005). The titrant is added slowly, and its volume recorded. One of the two solutions has to be a base and the other an acid. A color indicator is added to the titer to signal if neutralization has occurred. The amount of moles required to neutralize the other is equal, and the concentration of the known solution can be used to calculate the concentration of unknown solution.
References
Brent, L. (2009). Acids and Bases. New York: Crabtree.
Gros, L., Bruttel, P. A., & Von Kloeden, M. (2005). Practical titration training manual for Titrimetric volumetric analysis. Retrieved from http://www.rfpc.ir/uploads/Practical_Titration.pdf
Petrucci, R. H. (2007). General Chemistry: Principles and Modern ApplicationsMacmillian.
