What is an Acid Base titration?

An acid-base titration is the method of quantitative analysis for determining the concentration of an acid and base by exactly neutralizing it with a standard solution of base or acid having known concentration.

Common Acids and Bases

You must be knowing the names of various acids like hydrochloric acid (HCl), sulphuric acid (H₂SO₄) or acetic acid (CH₃COOH), and bases like sodium hydroxide (NaOH), calcium hydroxide (Ca(OH)₂) or ammonia (NH₃). Are these chemicals found only in a chemistry laboratory?

No. We use these chemicals in our daily life. HCl is used for household cleaning, H₂SO₄ is used in car batteries, CH₃COOH is the main ingredient of vinegar, NaOH is used in detergents, ammonia in fertilizers, and so on. Antacids, come to our rescue on feeling indigestion or heartburn. Antacids contain bases that counteract the extra acid in our stomach, thus reducing the discomfort. Reaction of acid and base is called neutralization.

The neutralization reaction is the underlying basic principle of acid base titrations. This process is used to determine unknown concentration of acid or base. It is frequently used in food industry, pharmaceutical industry, cosmetic industry, waste water management, where determination of the concentration of acid/base is involved.

Titrimetric Analysis

Titration is quantitatively determining the unknown concentration of a solution with the help of a solution of known concentration (standard solution), in which the two solutions react in a definite and known proportion. The standard solution that is added to the solution of unknown concentration during titration is called titrant. The solution whose unknown concentration is determined is called titrand or analyte. In titration, we determine the volume of titrant required to react with the measured volume of the analyte. Titrant is usually taken in a burette and analyte in a conical flask (Fig. 1). The solution from the burette is gradually added to the conical flask till the reaction is complete.

Acid base titration is an experimental technique used to quantitatively analyze the unknown concentration of an acid or base when a standard solution of base or acid of known concentration exactly neutralizes it. Alkalimetry is the acid base titration carried out to determine the concentration of a base using a standard acidic solution. Acidimetry is when the same is done for determining the concentration of an acid using a standard basic solution.     

Let’s brush up the basic concepts of acids and bases before delving into the details.

The Arrhenius Acid and Base Theory

According to the Arrhenius theory, acid is a substance that produces hydrogen ions (H⁺) in water. H⁺ ions combine with H₂O molecules to form hydronium ions (H₃O⁺). Base is a substance that produces hydroxide ions (OH^-) in water. When an acid and a base react, salt and water are formed. This reaction is called neutralization reaction.   

For example, reaction between HCl (acid) and NaOH (base):

$HCl\left(aq\right) + NaOH\left(aq\right) \to NaCl\left(aq\right) + H_{2}O$

Brønsted–Lowry Theory of Acid and Base

According to Brønsted–Lowry theory, acid is a substance that has the ability to donate H⁺ ions (protons), and base is a substance that has the ability to accept protons. On removing H⁺ ion from the acid, conjugate base is formed. After accepting proton, base forms a conjugate acid.  According to this theory, acid base reaction (neutralization) is thus the removal of a proton from the acid that gets added to the base. In general, we can say:   

$HA + B \to B{H}^{+} + A^{-}$

where HA is the acid, B is the base, BH⁺ represents the conjugate acid of B, and A⁻ represents the conjugate base of HA.

Remember that, conjugate base of a strong acid and a weak acid is weak and strong, respectively. Conjugate acid of a strong base and a weak base is weak and strong, respectively.  

Acid Base Titration

Let’s take the example of titration of hydrochloric acid, HCl (acid) and sodium hydroxide, NaOH (base). Consider that NaOH is the titrant and HCl is the analyte. NaOH is taken in burette and measured volume of HCl in conical flask (Fig. 2a). Initial reading of NaOH from burette is noted. NaOH solution is then slowly added to the conical flask. As NaOH is added, it keeps neutralizing HCl. During the titration, a point is reached at which just exactly required NaOH is added to neutralize all of HCl. This point is called equivalence point. For an acid base titration, equivalence point is at which the exact number of moles of acid and base are reacted according to the stoichiometric balanced chemical equation. The aim during this titration is to find this volume of NaOH required exactly enough to neutralize the given volume of HCl.

But, how do we detect the equivalence point? When do we stop adding NaOH to the HCl solution? Reagents, known as indicators, are added to the analyte for this purpose. An indicator shows a visible change, like change of color in the solution on completion of the reaction i.e., at equivalence point. With the color change, we know that the equivalence point is reached. For acid base titration, pH indicators (or acid base indicators) that change color with change in pH are used to determine the equivalence point.

Phenolphthalein is a pH indicator which is colorless in acidic medium and imparts pink color in alkaline medium. If we add a few drops of phenolphthalein to the acid HCl in conical flask, the solution remains colorless. Immediately after completion of the reaction, when all of HCl has reacted away, with one extra drop of NaOH, solution turns pink due to alkalinity of the solution (Fig 2b). The point at which the indicator changes color in the solution is called the end point. Remember that the equivalence point is different from the end point. At the end point, the volume of NaOH used is recorded from the burette reading. Thus, we get the volume of titrant required to react with the given volume of analyte.

If concentration and volume of the titrant at equivalence point are known, concentration of the analyte can be determined. At equivalence point,

moles of acid = moles of base

Or $a1 \times M_{acid} \times V_{acid} = a2 \times M_{base} \times V_{base}$

where M is molarity (moles per litre of solution), V is volume, a₁ is basicity of acid, a₂ is acidity of base.

Example: Neutralization of 50.0 mL of HCl solution requires 25.0 mL of 1 M NaOH, calculate the concentration of HCl.

Sol. For HCl and NaOH titration, a₁ = a₂ = 1

So,  $M_{acid} = (M_{base} \times V_{base}) / V_{acid}$

                 =(1M × 25.0mL) / 50.0mL 

                 = 0.5M 

Types of Acid Base Titrations and Titration Curves

Titration of a Strong Acid and Strong Base

Example: HCl (strong acid) and NaOH (strong base)

Neutralization reaction:

$HCl + NaOH \to NaCl + H_{2}O$

At equivalence point, there are no excess hydrogen ions left in the solution on neutralization. The solution is neutral (pH ~ 7).

Let’s see how the titration curve for this titration looks like. A titration curve shows pH change of the analyte solution with volume of titrant added. It exhibits the strengths of acid and base, and shows the equivalence point with corresponding pH value. These titration curves are characteristic of each type of titration. Graphically, the point where the curve is most steep (or vertical) is the equivalence point. Fig 3 illustrates titration curve for strong acid (analyte) and strong base (titrant) titration. Equivalence point is obtained around pH 7.

Titration of a Weak Acid and Strong Base

Example: CH₃COOH (weak acid) and NaOH (strong base)

Neutralization reaction:

$C{H}_{3}COOH + NaOH \rightleftharpoons C{H}_{3}COONa + H_{2}O$

Consider that for the titration, analyte is CH₃COOH and titrant is NaOH. Fig 4 shows the titration curve. The solution in this case is not neutral at equivalence point, even though CH₃COOH is completely neutralized. It is basic (pH ~ 9). At the equivalence point CH₃COONa is present in the solution, which dissociates into acetate (CH₃COO^-) and sodium (Na⁺) ions. CH₃COO^- is a strong base being conjugate base of CH₃COOH which is a weak acid ($C{H}_{3}COOH + H_{2}O \rightleftharpoons C{H}_{3}CO{O}^{-} + H_3{O}^{+}$), and thus reacts with water (acting as acid) to form OH^- making the solution basic at equivalence point (Fig 5).

Titration of Strong Acid and Weak Base

Example: HCl (strong acid) and NH₄OH (weak base)

Neutralization reaction:

$N{H}_{4}OH + HCl \rightleftharpoons N{H}_{4}Cl + H_{2}O$

Consider that for the titration, analyte is HCl and titrant is NH₄OH.

Fig 6 shows titration curve. The solution in this case is acidic at equivalence point (pH ~ 6). The solution contains NH₄Cl at equivalence point which dissociates into NH₄⁺ and Cl^-. NH₄⁺ being a conjugate acid of a weak base, is a relatively strong acid. Thus, it combines with H₂O as follows:

$N{H_4}^{+} + H_{2}O \to N{H}_3 + H_3{O}^{+}$

Solution becomes acidic due to H₃O⁺ ions.

Weak Acid and Weak Base

Titration of weak acid and weak base is not often performed. Color change due to indicator at equivalence point in this case is very brief, therefore very difficult to observe.

The titration curve (Fig 7) shows a weak acid and a weak base titration where weak base has been taken as analyte (thus pH > 7 initially) and weak acid as titrant. There is no steep change in pH, and accurate equivalence point cannot be obtained.

Choice of Indicator

Acid base indicators change color with change in pH. It is very important to choose a suitable indicator to detect the end point of acid base titration accurately. The color change of the indicator should occur at the pH near the equivalence point of the reaction so that end point is detected close to the equivalence point. Remember that the end point is not same as the equivalence point. Three common indicators used for acid base titrations along with their color changes and pH range are given in table below:

From the pH range of these indicators, we can say that:  

• Phenolphthalein, methyl red and methyl orange can be chosen as indicators for strong acid and strong base titration for which pH at equivalence point is around 7.
• Phenolphthalein is suitable for weak acid and strong base titration as equivalence point for this titration is obtained around pH 8-10.
• Methyl orange and methyl red are suitable for strong acid and weak base titration as equivalence point for this titration is obtained around pH 6-4.

Context and Applications

An acid-base titration is used to determine the concentration of an unknown acid or base by neutralizing it with a known concentration of acid or base. This method is used to ensure the consistency of products sold in the pharmaceutical and wine industries. Crystallography is the study of crystal lattices and unit cells. Understanding unit cells aids in accurately assessing the structure of a solid. These will be covered in your advanced Chemistry studies. This subject is important in professional exams for both undergraduate and graduate courses, particularly for

Bachelors of Science in Chemistry

Masters of Science in Chemistry

Practice Problems

Q1. During titrimetric analysis which of the following is taken in the burette?

1. Titrant
2. Titrand
3. Analyte
4. Standard basic solution

Answer: Titrant

Q2. What is the conjugate base of H₂SO₄?

1. H₃O⁺
2. HSO₄⁺
3. HSO₄^2-
4. HSO₄^-

Answer: HSO₄^-

Q3. What is the concentration of the NaOH solution if It takes 80 mL of a 2 M HCl solution to neutralize 50 mL of NaOH solution?

1. 4 M
2. 3 M
3. 2 M
4. 6 M

Answer: 2M

Q4. The following titration curve is obtained in the titration of a ______ acid with a ______ base.

1. strong, strong
2. weak, weak
3. strong, weak
4. weak, strong

Answer: weak, weak

Q5. Which of the following indicator is suitable for the titration of CH₃COOH and NaOH?

1. Methyl orange
2. Methyl red
3. Phenolphthalein
4. Fluorescein

Answer: Phenolphthalein