Understanding Acid-Base Titration: Indicators and Mechanisms

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Introduction

An acid base titration is an experimental procedure depends on determining the concentration of an acid or base by neutralizing it using an acid or base of exactly known concentration. The process is carried out by adding gradually the standard solution known concentration and volume using a burette which is long, graduated measuring tube with a stopcock and a deliver tube at its lower end until we reach the end point of the reaction. The question is how we are going to know the end point;

The end point means the point at which the reaction is completed and that could by easier to be determined by using a suitable indicators which change its colour by changing the PH of the medium therefore the end point could be determined easily.

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Choosing a suitable indicator depends on the type of acid base titrations, each type has its own titration curve which tells us which indicator is going be used because each one has a characteristic transition range.

Acid base indicators are also known by PH indicators, they are weak acid or base which when dissolved in water dissociate into ions.

Indicators was first discovered by “Robert Boyle”, he recognized a certain plant extract which could change its colour after treating with an acid or base. In 1664, Boyle was able to describe the indicator through experiments while writing his book “experimental history of colour”. After that several indicators was discovered which helped a lot in analytical chemistry field. Acid base indicators classified into three types; colour, turbidity and fluorescence indicators.

In this research, we are going to discuss each type of them explaining the structure of common indicators used. [1], [2],[8]

Colour indicator: It’s a weak acids or bases that change its colour with changing the PH. Their main use is to detect end point in neutralization reaction and determine unknown PH of a solution. Colour indicator is classified into three types; the phthalaleins & sulphophthaleins indicators [eg; phenalophthalins], Azo indicators [e; methyl orange] and triphenylmethane indicators [eg; Malachite green]. There are two theories which discuss the mechanism of colour indicators; Ostwald-Arrhenius theory and chromophore theory. [2], [10]

Ostwald-Arrhenius Theory

According to this theory, indicators are either weak organic acids or bases which dissociates into ions. The colour of molecules differ from that of the ions and which makes the change in colour. On adding an acid or a base, the equilibrium will shift causing change in the colour. There’re objections that has been raised against this theory;

Adding small quantity of alkali to Phenolphthalein changes its colour from colourless into red and according to the theory adding more should increase the intensity of the colour but it was found that solution become colourless again
Indicators changes its colour in non-aqueous medium and that shows the colour change does not depend on dissociation of molecule as explained before since the existence of ions is rare in non-aqueous medium
In some reactions, the colour changes slowly while ionic reaction occur instantaneously.

Chromophore theory

It’s also known by “Quinonoid” theory, it said that the colour change of indicator depends on the organic structure of their molecules.
The acid base indicators exist in two tautomeric forms in equilibrium which are benzoid and quinonoid forms.
The two forms differ in colour, one of them exist in acidic medium and the other in alkaline medium.
Each structure have a functional group which is responsible for absorption a quantity of light as C=C, C=O, N=O.

This kind of functional group is called chromophore. There’s a part of the molecule responsible for increasing the intensity of colour in any compound which is called Auxochrome. This are functional group with lone pair of electrons. Therefore, the colour change of indicator is not due to the ionization only but it also happens as a result of an isomeric effect ; an intermolecular rearrangement which change the structure of indicator causing appearance or disappearance of the groups which influence the colour (chromophore & Auxochrome) causing colour change. This is considered as a completion of Ostwald theory.

Common indicators in this type: Phenolphthalein [ex of phenalophthalins and sulphophthaleins indicator] Phenolphthalein was discovered by a German scientist “Adolf von Baeyer” in 1871. Its weak acid with large organic molecule with the chemical formula of C20H14O4. It was prepared by mixing phenol and phthalic anhydride in the presence of zinc chloride or sulfuric acid. It’s generally used as an indicator in acid base titration where it’s added with the solution with unknown concentration and titrated gradually by another solution of known concentration. When the solution changes from colourless to first pink, it means that the end point has been reached.

PH range: it’s naturally colourless and remain in acidic medium and begins to change into red from PH (8.2) and it reached the maximum colour intensity of PH (10) and above then it becomes colourless above PH (12) due to formation of benzoid form.

Phenolphthalein has benzoid form in acidic medium that’s why it’s colourless while in has quinonoid form in alkaline medium which has pink colour other uses: It was used as a laxative but it was banned in the United States in 1999 because some researches shoes that it may be carcinogenic. [2], [3], [10] [b] Methyl orange [ex of azo dyes] It’s a pale orange powder that was discovered by a several workers; P. Griess in 1875; O.N. Witt in 1876; and Z. Roussin in 1876. Methyl benzene is formed the reaction of sulfanilic acid, sodium nitrite, and dimethylaniline. Molecular formula is [C14H14N3NaO3S] Its IUPAC name is sodium; 4-[[4-(dimethylamino) phenyl] diazenyl] benzenesulfonate.

It’s used in acid base titration as it has two colours; orange and red. [5], [6], [10] PH range: its range [3.1–4.4] as it has a red colour below PH (3.1) and become orange above PH (4.4) and then yellow in basic medium. Methyl orange has quinonoid form in acidic solution and benzenoid form in alkaline solution.

The color of benzenoid form is yellow while that of quinonoid form is red. Sometimes the colour indicators cannot be visible in some reaction so we can add additive substances to it or by mixing several kind of indicators.

EX: Screened indicator: it’s used when it’s not easy to detect the colour change in the light. In order to make the colour more sharper and more clear we could add to the indicator a dye like in methyl orange, aindigocarmine is added and change in colour become sharper from yellowish green to violet.

Mixed indicator: it’s used also in case the colour changed is difficult to be detected. We make the colour sharper through mixing different type pf indicators together but they should have the same PH. Ex; thymol blue and cresol red.

Universal or multi range indicators: they are useful in rough determination PH but it cannot be used in quantitative titration. It’s done by mixing certain indicators together producing a colour that may be made to extend over a considerable PH range.

Ex; a mixture of phenolphthalein, methyl red and bromothymol blue. [10] [2] Turbidity indicators It’s a kind of acid base titration that is used only if the colour indicator is not practical and this happens when titrating a dark coloured solution. The process consists of a cationic agent with anionic agent or the reverse that causes accumulation of colloidal precipitate near the end point. The end point of the titration can be determined in this case by maximum turbidity of the solution. [9], [10] [3]

Fluorescence indicators

It’s used in case of coloured and turbidity indicator cannot be used. Fluorescence indicators are a certain compounds that emit visible radiation or stop at certain PH when exposed to ultraviolet rays. Ex; Umbelliferon Umbelliferon: its molecular formula C9H6O3, it’s a hydroxycoumarin that is Coumarin substituted by hydroxyl group at the position number

Conclusion

Acid base indicators are very important, they help us determine the end point of the reaction and knowing the PH of some solutions. It’s classified into three types; coloured, turbidity and fluorescence indicators, each type deals with a specific kind of reaction. The most type of acid base indicators used is coloured indicators. They are generally weak acid or bases which are sensitive to the change of PH or more especially to the change of the concentration of H ions.

The change of the colour of the indicator is due to change in the structure of indicator molecule and its dissociation according to what Arrhenius and chromophore theory said. The common indicators used are phenolphthalein which is used in range 8.3–10.0 and methyl orange in range 3.1–4.4. Each changes its colour according to the medium. A wide range of indicators had been discovered which help in quantitate analysis in analytical chemistry field so it’s important to study the mechanisms of indicators as it’s also enters in many things like it can help us choose the suitable fertilizer by knowing the nature of the soil using them. Not only that indicators enters in many application in our life so we have to do more researches about it and develop it.