This article is about an essential class of titrimetry, i.e., acid-base titration. If you are following our titration series, you must have learnt about what is titration or titrimetry, followed by the basics of acid-base chemistry. In this article, we will discuss acid-base titration in detail, including its definition, procedure, examples and applications. So, let’s start reading!
Acid-base titration -Definition
The acid-base titration is a volumetric chemical analysis process. It involves a neutralization reaction in which an acid is reacted with an equivalent amount of base. An indicator whose endpoint coincides with the equivalence point at which [H+] = [OH–] in the titration mixture is used. The indicator detects this equivalence point by giving a color change. The titration volume is recorded, which is then used to determine the unknown quantity or concentration of the titrand.
To recall, there are two primary components of titration, i.e., a titrant or titrator and a titrand. The titrant is the chemical substance of known concentration, while the titrand is the chemical substance whose concentration is to be determined.
In an acid-base titration, either an acid or a base act as the titrant depending upon whether you have an unknown acid or a base concentration to be determined. The third important component in acid-base titration is the indicator.
An indicator is defined as a weak organic acid or a weak base that exhibits different colors in its protonated and deprotonated forms.
In a titration mixture containing acid as a titrant, there is a high concentration of H+ ions, so the weak acid (indicator) stays protonated, represented by HIn. As the alkali is titrated against the acid and an equivalence point is reached, the concentration of H+ becomes equal to OH–. The moment the OH– ions concentration exceeds H+ in the titration flask, the indicator gets deprotonated, represented by In–. It thus exhibits a color change at this point, marking the completion of the acid-base titration.
Methyl orange and phenolphthalein are two of the most commonly used indicators in acid-base titrations. Other examples of acid-base indicators are methyl red, litmus, bromothymol blue, alizarin yellow, etc.
Methyl orange gives a red color in an acidic solution, while it changes color to yellow under basic or alkaline conditions. It exhibits this color change between pH 3.2 to 4.4. Methyl orange is most commonly used in a strong acid-weak base titration. Phenolphthalein is colorless under acidic conditions and turns light pink under basic conditions. It changes its color between pH 9.4 to 10.6. Phenolphthalein is best used as an indicator in the titration of a strong or weak acid with a strong base.
Now let us see how an acid-base titration experiment is performed in the chemistry laboratory.
How to perform acid-base titration – Procedure
Step I: Preparation of standard solution
A standard solution of known concentration is prepared in a volumetric flask. For example, suppose the acid-base titration is being performed to determine the unknown concentration of NaOH (base) by using HCl (hydrochloric acid). In that case, a known concentration (say 0.1 M) of hydrochloric acid will be prepared in a specific volume using distilled water.
Step II: A specific volume of analyte solution is pipetted out in the titration flask.
A specific volume, such as 10 mL of analyte solution, is pipetted out in the titration flask. This acts as the titrand, so the titrand volume for this acid-base titration = 10 mL.
In any type of titration, the titrand goes into the titration flask while the titrant is taken in the burette.
Step III: A few drops of the acid-base indicator are added to the titration flask
Using a dropper, 2-3 fine drops of the acid-base indicator, such as phenolphthalein, are added into the titration flask containing the titrand.
The titration mixture is gently swirled to obtain a uniform mixing of the indicator.
Step IV: The titrant from the burette is added into the titration mixture dropwise.
The burette is filled with the standard solution of known concentration (titrant) which is then added into the titration mixture dropwise while continuously stirring the titration flask.
This dropwise addition of the titrant is stopped immediately as the indicator changes color.
The volume of titrant used is then noted. This is known as the titre volume.
The experiment is repeated three to four times so that an average titre volume can be obtained for greater accuracy.
Step V: The unknown concentration is determined as per stoichiometric principles
The balanced equation for the acid-base neutralization reaction between hydrochloric acid and sodium hydroxide and the supposed calculations given below can be used to determine the unknown concentration of NaOH in the above example, using the titration formula.
Titration formula:
Calculations
The example given above demonstrates a strong acid-strong base titration. There are other types of acid-base titrations that we have discussed in the upcoming section.
Depending upon the different types of titrations, particular titration curves can be obtained to determine the equivalence point of an acid-base titration.
Titration curves and equivalence point
A titration curve is a plot of the pH of the titration mixture versus titrant volume added during an acid-base titration.
The shape of the titration curve provides information about the acid-base reaction taking place in the titration flask.
Types of acid-base titration
Acid-base titrations can be classified on the basis of the strength of the acid and the base reacting with each other. As a general matter of fact, the titrant is always a strong acid or a strong base, while the titrand or analyte can either be a strong acid or base or a weak acid or base.
Strong acid vs strong base titration
In a strong acid vs strong base titration, both the titrant and the titrand get completely ionized.
Example:
The titration of hydrochloric acid (HCl) with 0.1 M sodium hydroxide (NaOH).
The complete ionization of HCl yields H+ and Cl– ions, while NaOH completely dissociates into Na+ and OH– ions. Na+ and Cl– ions combine to form NaCl (salt), while H+ and OH– ions combine to produce H2O (water).
The net result of this neutralization reaction is the conversion of an acidic solution into a neutral solution containing NaCl and H2O.
In accordance with this, the titration curve below shows that the titration mixture containing HCl solution initially had a low pH value.
With the addition of NaOH from the burette, the titration mixture’s pH increases to a neutral pH of 7.
The acid-base neutralization reaction reaches completion at this equivalence point. The pH of the titration mixture changes rapidly near the equivalence point. It stays fairly constant on either side of the equivalence point. The pH changes again after the neutralization reaction is complete and the concentration of OH– ions exceed H+ ions in the reaction mixture.
The acid-base indicator, such as phenolphthalein, changes color at the pH where its endpoint coincides with the equivalence point.
The titration mixture finally reaches an alkaline pH as the concentration of NaOH exceeds HCl and NaCl in the titration flask.
Weak acid vs strong base titration
In a weak acid vs strong base titration, the weak acid only partially dissociates in the titration mixture.
Example:
The titration of acetic acid with 0.1 M sodium hydroxide.
The titration curve below shows that the pH of the titration mixture initially was 3 as acetic acid only partially ionize to release a limited number of H+ ions in the titration mixture.
The pH increases as sodium hydroxide is added from the burette. The titration curve for a weak acid-strong base titration exhibits a gradual pH change at the equivalence point as opposed to the rapid pH change at the equivalence point in a strong acid-strong base titration.
Equivalence point occurs at an alkaline pH for a weak acid vs strong base titration. Bromothymol blue and phenolphthalein are the most suitable indicators that gives color changes within this pH range.
As the concentration of NaOH exceeds in the titration flask after the equivalence stage is crossed, a high pH level is maintained in the reaction mixture.
Weak base vs strong acid titration
The titration curve for a weak base versus strong acid titration is exactly opposite to that obtained for weak acid versus strong base titration.
Example:
The titration of ammonia or ammonium hydroxide with 0.1 M hydrochloric acid.
The titration starts with a pH level around 11 in the presence of a weakly basic solution of ammonium hydroxide in the titration flask.
With the addition of hydrochloric acid from the burette, the pH of the titration mixture drops rapidly. NH4OH partially dissociates into NH4+ and OH– ions. HCl undergoes complete ionization into H+ and Cl– ions. The equivalence point is approached gradually as the H+ ions of the titrand completely neutralize the OH– ions present in the titration flask.
A solution of NH4Cl is obtained at the equivalence point. NH4Cl is a weak Bronsted acid, which also partially ionizes to yield an acidic solution. H+ ions from the dissociation of NH4Cl and H+ ions from the complete dissociation of HCl result in an acidic solution at the equivalence point.
Consequently, an acid-base indicator, such as methyl orange, is selected that gives color change at this acidic pH.
The pH decreases further until a completely acidic solution is obtained in the titration flask.
More information on acid-base indicators here.
Acidimetry vs Alkalimetry
Acid-base titrations are also sometimes classified based on whether an acid or a base is the titrant.
If acid-base titration is used to determine the unknown concentration of a base using a standard acid of known concentration, then it is called Acidimetry. Contrarily, if the concentration of an acid is unknown, which is determined using a base of known concentration, then this type of acid-base titrimetry is known as Alkalimetry.
Why is acid-base titration important – Applications
Acid-base titrations are important for the following:
- Determining the unknown concentration of an acid or a base in the chemistry laboratory.
- Quantitative chemical analysis in research and development.
- Environmental analysis.
- Volumetric analysis in the pharmaceutical industry.
Read more about the uses and applications of titration here.
Also solve some interesting acid-base titration numerical problems.
Here is another acid-base titration worksheet for you.
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References
1. C.Harris, Daniel. 2010. Quantitative Chemical Analysis (W.H Freeman and Company), 8th Edition.
2. Christian, Gary D., Purnendu K. Dasgupta, and Kevin A.Schug. 2013. Analytical Chemistry, 7th Edition, Pg. 281-297.