In chemistry, neutralization (US spelling) or neutralisation (UK spelling), is a chemical reaction in which an acid and a base react quantitatively with each other. In a reaction in water, neutralization results in there being no excess of hydrogen or hydroxide ions present in solution. The pH of the neutralized solution depends on the acid strength of the reactants. Neutralization is used in many applications.
Meaning of "neutralization"
- acid + base = salt + water
- HCl + NaOH → NaCl + H2O
The statement is still valid as long as it is understood that in aqueous solution the substances involved are subject to dissociation, which changes the substances ionization state. The arrow sign, →, is used because the reaction is complete, that is, the said neutralization is a quantitative reaction. A more general definition is based on Brønsted–Lowry acid–base theory.
- AH + Bz → A- +BHz+1
AH represents an acid and B represents a base. z is an electric charge; negative for an anion, zero, or positive for a cation. When the reaction takes place in water and the base is the hydroxide ion, OH-, the reaction can be written as
- AH + OH- → A- + H2O
After an acid 'neutralization', there are no molecules of AH (or hydrogen ions produced by dissociation of the molecule) left, since all the acid molecules present initially in a neutral form are then ionized, i.e., charged, A-; the term 'acid neutralization' therefore implies acid (effects) rather than charge neutralization. In electrical terms, 'acid neutralization' thus corresponds to acid ionization. In the reaction, the amount of base added must moreover equal the amount of acid present initially. This amount of base is said to be the equivalent amount. In a titration of an acid with a base, the point of neutralization can also be called the equivalence point. The quantitative nature of the neutralization reaction is most conveniently expressed in terms of the concentrations of acid and alkali. At the equivalence point:
- volume (acid) × concentration (H+ ions from dissociation) = volume (base) × concentration (OH- ions)
In general, for an acid AHn at concentration c1 reacting with a base B(OH)m at concentration c2 the volumes are related by:
- n v1 c1 = m v2 c2
- [Ca,Mg]CO3(s) + H2SO4(aq) → [Ca,Mg]SO4(s) + CO2(g) + H2O
Such reactions are important in soil chemistry.
Strong acids and strong bases
- HCl(aq) → H+(aq) + Cl-(aq)
- NaOH(aq) → Na+(aq) + OH-(aq)
Therefore when a strong acid reacts with a strong base the neutralization reaction can be written as
- H+ + OH- → H2O
For example, in the reaction between hydrochloric acid and sodium hydroxide the sodium and chloride ions, Na+ and Cl- take no part in the reaction. The reaction is consistent with the Brønsted–Lowry definition because in reality the hydrogen ion exists as the hydronium ion, so that the neutralization reaction may be written as
- H3O+ + OH- → H2O + H2O
When a strong acid is neutralized by a strong base there are no excess hydrogen ions left in the solution. The solution is said to be neutral as it is neither acidic nor alkaline. The pH of such a solution is close to a value of 7; the exact pH value is dependent on the temperature of the solution.
Weak acids and strong bases
- AH + H2O H3O+ + A-
Acetic acid is an example of a weak acid. The pH of the neutralized solution is not close to 7, as with a strong acid, but depends on the acid dissociation constant (pKa)of the acid. The pH at the end-point or equivalence point in a titration may be easily calculated. At the end-point the acid is completely neutralized so the analytical hydrogen ion concentration, TH, is zero and the concentration of the conjugate base, A-, is effectively equal to the analytical concentration of the acid; writing AH for the acid, [A-]= TA. Defining the acid dissociation constant, pKa, as
- [HA] = Ka[A-][H+]; pKa = -log10Ka
and the self-dissociation constant for water, Kw, as
- Kw = [H+][OH-]; pKw = -log10Kw
the equation for mass-balance in hydrogen ions is easy to write down.
- TH= [H+] +Ka[A-][H+] - Kw[H]-1
The term Kw[H]−1 is equal to the concentration of hydroxide ions. At neutralization, TH is zero.
- [H+] + Ka[A][H+] - Kw[H ]-1 = 0
- [H+]2 + KaTA[H+]2 - Kw = 0
- [H+]2 = Kw / (1 + KaTA)
- log [H+] = 1/2 log Kw - 1/2 log(1 + KaTA)
- pH= 1/2 pKw - 1/2 log(1 + TA/Ka)
In most circumstances the term 1+TA/Ka is much larger than 1 and is equal to TA/Ka to a good approximation.
- pH ≈ 1/2 (pKw + pKa - log TA )
This equation explains explains the following facts:
- The pH at the end-point depends mainly on the strength of the acid, pKa.
- The pH at the end-point also depends on the concentration of the acid, TA.
- The pH rises more steeply at the end-point as the acid concentration increases.
When a weak acid is titrated with a strong base the end-point occurs at pH greater than 7. Therefore, the most suitable indicator to use is one, like Phenolphthalein, that changes color at high pH.
Weak bases and strong acids
The situation is analogous to that of weak acid and strong base.
- H3O+ + B H2O + BH+
The pH of the neutralized solution depends on the acid dissociation constant of the base, pKa, or, equivalently, on the base association constant, pKb.
The most suitable indicator to use for this type of titration is one, like Methyl orange, that changes color at low pH.
Weak acids and weak bases
When a weak acid reacts with an equivalent amount of a weak base complete neutralization does not occur.
- AH + B A- + BH+
The concentrations of the species in equilibrium with each other will depend on the equilibrium constant, K, for the reaction, which can be defined as follows.
- [A-][BH+] = K[AH][B]
Given the association constants for the acid (Ka) and the base (Kb).
- A- + H+ AH; [AH] = Ka[A-][H+]
- B + H+ BH+; [BH+] = Kb[B][H+]
it follows that K = Ka / Kb
A weak acid cannot be neutralized by a weak base, and vice versa.
Chemical titration methods are used for analyzing acids or bases to determine the unknown concentration. Either a pH meter or a pH indicator which shows the point of neutralization by a distinct color change can be employed. Simple stoichiometric calculations with the known volume of the unknown and the known volume and molarity of the added chemical gives the molarity of the unknown.
In wastewater treatment, chemical neutralization methods are often applied to reduce the damage that an effluent may cause upon release to the environment. For pH control, popular chemicals include calcium carbonate, calcium oxide, magnesium hydroxide, and sodium bicarbonate. The selection of an appropriate neutralization chemical depends on the particular application.
There are many uses of neutralization reactions that are acid-alkali reactions. A very common use is antacid tablets. These are designed to neutralize excess gastric acid in the stomach (HCl) that may be causing discomfort in the stomach or lower esophagus. This can also be remedied by the ingestion of sodium bicarbonate (NaHCO3).
Also in the digestive tract, neutralization reactions are used when food is moved from the stomach to the intestines. In order for the nutrients to be absorbed through the intestinal wall, an alkaline environment is needed, so the pancreas produce an antacid bicarbonate to cause this transformation to occur.
Another common use, though perhaps not as widely known, is in fertilizers and control of soil pH. Slaked lime (calcium hydroxide) or limestone (calcium carbonate) may be worked into soil that is too acidic for plant growth. Fertilizers that improve plant growth are made by neutralizing sulfuric acid (H2SO4) or nitric acid (HNO3) with ammonia gas (NH3), making ammonium sulfate or ammonium nitrate. These are salts utilized in the fertilizer.
Industrially, a by-product of the burning of coal, sulfur dioxide gas, may combine with water vapor in the air to eventually produce sulfuric acid, which falls as acid rain. To prevent the sulfur dioxide from being released, a device known as a scrubber gleans the gas from smoke stacks. This device first blows calcium carbonate into the combustion chamber where it decomposes into calcium oxide (lime) and carbon dioxide. This lime then reacts with the sulfur dioxide produced forming calcium sulfite. A suspension of lime is then injected into the mixture to produce a slurry, which removes the calcium sulfite and any remaining unreacted sulfur dioxide.
Neutralization is covered in most general chemistry text-books. Detailed treatments may be found in text-books on analytical chemistry such as
- Skoog, D.A; West, D.M.; Holler, J.F.; Crouch, S.R. (2004). Fundamentals of Analytical Chemistry (8th ed.). Thomson Brooks/Cole. ISBN 0-03-035523-0. Chapters 14, 15 and 16
- Stumm, W.; Morgan, J.J. (1996). Water Chemistry. New York: Wiley. ISBN 0-471-05196-9.
- Snoeyink, V.L.; Jenkins, D. (1980). Aquatic Chemistry: Chemical Equilibria and Rates in Natural Waters. New York: Wiley. ISBN 0-471-51185-4.
- Millero, F.J. (2006). Chemical Oceanography (3rd ed.). London: Taylor and Francis. ISBN 0-8493-2280-4.
- Metcalf & Eddy. Wastewater Engineering, Treatment and Reuse. 4th ed. New York: McGraw-Hill, 2003. 526-532.
- Jarvis, Alan & Beavon, Rod. "Periodicity Quantitative Equilibria and Functional Group Chemistry", 16 January 2001.
- Steven S. Zumdahl (2009). Chemical Principles (6th ed.). New York: Houghton Mifflin Company. pp. 319–324.