thumb|The endpoint of an acid–base titration is reached as the indicator suddenly changes colour.|261x261px
An acid–base titration is a method of quantitative analysis for determining the concentration of Brønsted-Lowry acid or base (titrate) by neutralizing it using a solution of known concentration (titrant). A pH indicator is used to monitor the progress of the acid–base reaction and a titration curve can be constructed. Although these types of titrations are also used to determine unknown amounts of substances, these substances vary from ions to metals. This method's precision and simplicity makes it an important tool in quantitative chemical analysis, contributing significantly to the general understanding of solution chemistry.
History
left|thumb|255x255px|Svante Arrhenius
The history of acid-base titration dates back to the late 19th century when advancements in analytical chemistry fostered the development of systematic techniques for quantitative analysis. The origins of titration methods can be linked to the work of chemists such as Karl Friedrich Mohr in the mid-1800s. This theoretical foundation, along with ongoing experimental refinements, contributed to the evolution of acid-base titration as a precise and widely applicable analytical method.
thumb|444x444px|Titration of a standard solution using methyl orange indicator. Titrate is in Erlenmeyer flask, titrant is in burette.
:acid + base → salt + water
For example:
:HCl + NaOH → NaCl + H<sub>2</sub>O
Acidimetry is the specialized analytical use of acid-base titration to determine the concentration of a basic (alkaline) substance using standard acid. This can be used for weak bases and strong bases. An example of an acidimetric titration involving a strong base is as follows:
:Ba(OH)<sub>2</sub> + 2 H<sup>+</sup> → Ba<sup>2+</sup> + 2 H<sub>2</sub>O
In this case, the strong base (Ba(OH)<sub>2</sub>) is neutralized by the acid until all of the base has reacted. This allows the viewer to calculate the concentration of the base from the volume of the standard acid that is used.
Alkalimetry follows uses same concept of specialized analytic acid-base titration, but to determine the concentration of an acidic substance using standard base.
Indicator choice
A suitable pH indicator must be chosen in order to detect the end point of the titration. The colour change or other effect should occur close to the equivalence point of the reaction so that the experimenter can accurately determine when that point is reached. The pH of the equivalence point can be estimated using the following rules:
- A strong acid will react with a strong base to form a neutral (pH = 7) solution.
- A strong acid will react with a weak base to form an acidic (pH < 7) solution.
- A weak acid will react with a strong base to form a basic (pH > 7) solution.
These indicators are essential tools in chemistry and biology, aiding in the determination of a solution's acidity or alkalinity through the observation of colour transitions.
!Indicator name
!Indicator colour
!Transition interval (pH range)
!Color after high pH conditions
|-
|Methyl Orange
|Orange/red
|3.1 - 4.4
|Yellow
|-
|Methyl Red
|Red
|4.4 - 6.3
|Yellow
|-
|Congo Red
|Blue
|3.0 - 5.2
|Red
|-
|Phenolphthalein
|Colourless
|8.3 - 10.0
|Pink
|-
|Thymolphthalein
|Colourless
|9.3 - 10.5
|Blue
|-
|Bromophenol Blue
|Yellow
|3.0 - 4.6
|Blue
|-
|Bromocresol Green
|Yellow
|3.8 - 5.6
|Blue
|-
|Thymol Blue
|Red
|1.2 - 2.8; 8.0 - 9.6
|Blue
|-
|Cresol Red
|Yellow
|7.2 - 8.8
|Violet
|-
|Neutral Red
|Red
|6.8 - 8.0
|Yellow
|}
left|thumb|397x397px|Three different points in an acid-base titration using phenolphthalein as the indicator
Phenolphthalein is widely recognized as one of the most commonly used acid-base indicators in chemistry. Its popularity is because of its effectiveness in a broad pH range and its distinct colour transitions. However, weak acids are not often titrated against weak bases because the colour change shown with the indicator is often quick, and therefore very difficult for the observer to see the change of colour.
The point at which the indicator changes colour is called the endpoint. This excess titrant leads to an outcome where the solution becomes slightly more alkaline or over-acidified. and using more precise titration techniques, to mitigate the impact of overshooting and obtain reliable and precise measurements. Understanding the causes, consequences, and solutions related to overshot titrations is crucial in achieving accurate and reproducible results in the field of chemistry.
Mathematical analysis: titration of weak acid
thumb|258x258px|Titration of a weak acid with a strong base showing pH level, volume of titrant, and different points throughout the titration process
For calculating concentrations, an ICE table can be used.
- initial pH
- pH before the equivalence point
- pH at the equivalence point
- pH after the equivalence point
1. The initial pH is approximated for a weak acid solution in water using the equation: that describes the titration of a weak acid with a strong base from start to finish is given below:
: <math chem="">\phi = \frac{C_b V_b }{C_a V_a}</math>
where
" φ = fraction of completion of the titration (φ < 1 is before the equivalence point, φ = 1 is the equivalence point, and φ > 1 is after the equivalence point)
: thumb|223x223px|Monoprotic acid titration curve. Highlighted pink region depicts equivalence point.
<math>C_a, C_b</math> = the concentrations of the acid and base respectively
: <math>V_a, V_b</math> = the volumes of the acid and base respectively
Graphical methods
Identifying the pH associated with any stage in the titration process is relatively simple for monoprotic acids and bases. A monoprotic acid is an acid that donates one proton. A monoprotic base is a base that accepts one proton. A monoprotic acid or base only has one equivalence point on a titration curve.
Acid–base titration is particularly valuable in quantifying acidic or basic functional groups with pharmaceutical compounds. Additionally, the method is employed for the analysis of additives or ingredients, making it easier to adjust and control how a product is made. Quality control laboratories utilize acid-base titration to assess the purity of raw materials and to monitor various stages of drug manufacturing processes. The measurement of parameters such as pH, total alkalinity, and acidity is essential in evaluating the environmental impact of industrial discharges, agricultural runoff, and other sources of water contamination. Monitoring pH levels is important for preserving aquatic ecosystems and ensuring compliance with environmental regulations.