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Acid-Base Titration: Theory, Techniques, and Applications in Analytical Chemistry, Summaries of Analytical Chemistry

Mindanao State University (MSU)Analytical Chemistry

This comprehensive guide explores acid-base titration, covering neutralization theory, acid and base dissociation, autoionization of water, and the relative strengths of conjugate acid-base pairs. it delves into buffer solutions, titration curves, acid-base indicators, and the practical aspects of neutralization titrations, including the selection and use of reagents. Richly illustrated with figures and charts, enhancing understanding of complex concepts.

Typology: Summaries

2024/2025

Available from 04/27/2025

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radz-karl-daruca 🇵🇭

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CHAPTER 6: ACID-BASE TITRATION
THEORY OF NEUTRALIZATION
- It is a reaction where an acid and a base react to
form water and a salt.
BRONSTED-LOWRY THEORY OF ACIDS AND BASES
- Any compound that can transfer a proton to any
other compound is an acid, and the compound
that accepts the proton is a base.
Acid: Donates Protons
Base: Accepts Protons
Figure 1: Illustration of Proton Transfer Reaction
RELEVANT TERMS IN THEORY OF NEUTRALIZATION
1. STRONG ACIDS
2. WEAK ACIDS/BASE
3. AMPHIPROTIC SUBSTANCES
4. SOLVENT EFFECT
STRONG ACIDS
- Strong acids dissociate completely into ions.
i.e. ,
𝐻𝐶𝑙 𝐻𝑁𝑂3
WEAK ACIDS/BASE
- Weak acids and weak bases react to form salt
and water.
Example:
𝑁𝐻3+𝐻2𝑂 ↔ 𝑁𝐻4
++𝑂𝐻−
This reaction is reversible because as soon as (a weak base)
𝑁𝐻3
accepted a proton from , it becomes (a weak acid) and the
𝐻2𝑂 𝑁𝐻4
+
transfer of protons maintains equilibrium, allowing for their reversibility.
AMPHIPROTIC SUBSTANCES
- These are substances that can act either as an
acid or as a base.
Example: or water
𝐻2𝑂
As an acid:
𝑁𝐻3+𝐻2𝑂 → 𝑁𝐻4
++ 𝑂𝐻−
As a base:
𝐻𝑂𝐴𝑐+𝐻2𝑂↔𝑂𝐴𝑐−+𝐻3𝑂+
SOLVENT EFFECT
- The influence of a solvent on the properties and
behavior of molecules and chemical reactions.
Since water can act as a weak base, strong acids tend
to have the same strength in water.
Example:
𝐻𝐶𝑙𝑂4+𝐻2𝑂 →𝐻3𝑂++ 𝐶𝑙𝑂4
−
𝐻𝐶𝑙 + 𝐻2𝑂 → 𝐻3𝑂++ 𝐶𝑙−
NOTE: and have the same acid strength.
𝐻𝐶𝑙𝑂4𝐻𝐶𝑙
SOLUTIONS AND INDICATORS FOR ACID/BASE
TITRATION
- The standard solutions employed in the
neutralization titrations are strong acids (SA) or
strong bases (SB) because these substances
react more completely with an analyte than do
their weaker counterparts and thus yield sharper
end points.
- Sharp endpoints make pinpointing where the
reaction is complete in the titration easier.
ACID DISSOCIATION ( 𝐾𝑎)
𝐾𝑎=[𝐻+][𝑋−]
𝐻𝑋
BASE DISSOCIATION ( )
𝐾𝑏
𝐾𝑏=[𝐻𝑋][𝑂𝐻−]
[𝑋−]
AUTOIONIZATION OF WATER
- The process where water molecules react with
each other, forming hydronium ( ) and
𝐻3𝑂+
hydroxide ( ) ions.
𝑂𝐻−
𝐾𝑊=[𝐻+][𝑂𝐻−]
P-FUNCTIONS
pH= -log[ ] ; pOH= -log[ ]
𝐻+𝑂𝐻−
;
𝑝𝐻 + 𝑝𝑂𝐻 = 𝑝𝐾𝑤𝐾𝑎× 𝐾𝑏= 𝐾𝑤
Figure 2: Illustration of pH Chart
RELATIVE STRENGTHS OF CONJUGATE ACID-BASE
PAIRS
- The strength of a conjugate acid-base pair is
inversely related: the stronger the acid, the
weaker its conjugate base, and vice versa.
Prepared By: Radz Karl Daruca
CHM024- Analytical Chemistry
TOPICS:
1. THEORY OF NEUTRALIZATION
2. ACID DISSOCIATION
3. BASE DISSOCIATION
4. AUTO-IONIZATION OF WATER
5. P-FUNCTIONS
6. RELATIVE STRENGTHS OF CONJUGATE
ACID-BASE PAIRS
7. ACID-BASE PROPERTIES OF SALT
SOLUTIONS
8. TYPES OF SALTS
9. BUFFER SOLUTIONS
10. TITRATION CURVES
11. ACID-BASE INDICATORS
12. REAGENTS FOR NEUTRALIZATION
TITRATIONS
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THEORY OF NEUTRALIZATION

  • It is a reaction where an acid and a base react to form water and a salt. BRONSTED-LOWRY THEORY OF ACIDS AND BASES
  • Any compound that can transfer a proton to any other compound is an acid, and the compound that accepts the proton is a base. Acid : Donates Protons Base : Accepts Protons Figure 1: Illustration of Proton Transfer Reaction RELEVANT TERMS IN THEORY OF NEUTRALIZATION
  1. STRONG ACIDS
  2. WEAK ACIDS/BASE
  3. AMPHIPROTIC SUBSTANCES
  4. SOLVENT EFFECT STRONG ACIDS
  • Strong acids dissociate completely into ions. i.e. 𝐻𝐶𝑙, 𝐻𝑁𝑂 3 WEAK ACIDS/BASE
  • Weak acids and weak bases react to form salt and water. Example: 𝑁𝐻 3 + 𝐻 2 𝑂 ↔ 𝑁𝐻 4
  • 𝑂𝐻 − This reaction is reversible because as soon as 𝑁𝐻 3 (a weak base) accepted a proton from 𝐻 2 𝑂, it becomes 𝑁𝐻 4 + (a weak acid) and the transfer of protons maintains equilibrium, allowing for their reversibility. AMPHIPROTIC SUBSTANCES
  • These are substances that can act either as an acid or as a base. Example: 𝐻 2 𝑂or water As an acid: 𝑁𝐻 3 + 𝐻 2 𝑂 → 𝑁𝐻 4
  • 𝑂𝐻 − As a base: 𝐻𝑂𝐴𝑐 + 𝐻 2 𝑂 ↔ 𝑂𝐴𝑐 −
  • 𝐻 3 𝑂

SOLVENT EFFECT

  • The influence of a solvent on the properties and behavior of molecules and chemical reactions. Since water can act as a weak base, strong acids tend to have the same strength in water. Example: 𝐻𝐶𝑙𝑂 4 + 𝐻 2 𝑂 → 𝐻 3 𝑂
  • 𝐶𝑙𝑂 4 − 𝐻𝐶𝑙 + 𝐻 2 𝑂 → 𝐻 3 𝑂
  • 𝐶𝑙 − NOTE: 𝐻𝐶𝑙𝑂 4 and 𝐻𝐶𝑙have the same acid strength. SOLUTIONS AND INDICATORS FOR ACID/BASE TITRATION
  • The standard solutions employed in the neutralization titrations are strong acids (SA) or strong bases (SB) because these substances react more completely with an analyte than do their weaker counterparts and thus yield sharper end points.
  • Sharp endpoints make pinpointing where the reaction is complete in the titration easier. ACID DISSOCIATION (𝐾𝑎)

[𝐻

][𝑋

]

BASE DISSOCIATION ( 𝐾𝑏)

[𝐻𝑋][𝑂𝐻

]

[𝑋

]

AUTOIONIZATION OF WATER

  • The process where water molecules react with each other, forming hydronium ( 𝐻 3 𝑂) and + hydroxide ( 𝑂𝐻 ) ions. −

= [𝐻

][𝑂𝐻

]

P-FUNCTIONS pH= -log[ 𝐻 ] ; pOH= -log[ ]

𝑂𝐻 − 𝑝𝐻 + 𝑝𝑂𝐻 = 𝑝𝐾𝑤 ;𝐾𝑎 × 𝐾𝑏 = 𝐾𝑤 Figure 2: Illustration of pH Chart RELATIVE STRENGTHS OF CONJUGATE ACID-BASE PAIRS

  • The strength of a conjugate acid-base pair is inversely related: the stronger the acid, the weaker its conjugate base, and vice versa. Prepared By: Radz Karl Daruca TOPICS:
  1. THEORY OF NEUTRALIZATION
  2. ACID DISSOCIATION
  3. BASE DISSOCIATION
  4. AUTO-IONIZATION OF WATER
  5. P-FUNCTIONS
  6. RELATIVE STRENGTHS OF CONJUGATE ACID-BASE PAIRS
  7. ACID-BASE PROPERTIES OF SALT SOLUTIONS
  8. TYPES OF SALTS
  9. BUFFER SOLUTIONS
  10. TITRATION CURVES
  11. ACID-BASE INDICATORS
  12. REAGENTS FOR NEUTRALIZATION TITRATIONS

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ACID-BASE PROPERTIES OF SALT

  • What explains how salts can affect the pH of the aqueous solution is HYDROLYSIS. HYDROLYSIS
  • It is a chemical reaction where water breaks down a compound. In salts, it involves the interaction of salt's ions with water molecules, leading to the formation of either 𝐻 3 𝑂 or

𝑂𝐻 − ions. TYPES OF SALTS

  1. NEUTRAL SALTS
  2. BASIC SALTS
  3. ACIDIC SALTS
  4. SALTS OF WEAK ACIDS/WEAK BASES NEUTRAL SALTS
  • These salts are made up if the cation is from a strong base, and the anion is from a strong acid. In that case, neither ion will react with water and the solution will be neutral. pH = 7 BASIC SALTS
  • These salts are made up if the cation is from a strong base, and the anion is from a weak acid. In that case, neither ion will react with water and the solution will be basic. pH > 7 ACIDIC SALTS
  • These salts are made up if the cation is from a weak base, and the anion is from a strong acid. In that case, neither ion will react with water and the solution will be neutral. pH < 7 SALTS OF WEAK ACIDS/WEAK BASES
  • A salt formed between a weak acid and a weak base can be neutral, acidic, or basic depending on the relative strengths of the acid and base.
  1. If 𝐾𝑎 > 𝐾𝑏, then the solution of the salt is acidic.
  2. If 𝐾𝑎 < 𝐾𝑏, then the solution of the salt is basic.
  3. If 𝐾𝑎 = 𝐾𝑏, then the solution of the salt is neutral. BUFFER SOLUTIONS
  • Solution of conjugate acids or bases that resist pH changes.
  • When a weak acid is titrated with a strong base or a weak base with a strong acid, a buffer solution consisting of a conjugate acid/base pair is formed. Figure 3: Illustration of Buffer Solution HENDERSON-HASSELBACH EQUATION
  • This equation is used to calculate the pH of the buffer solution. 𝑝𝐻 = 𝑝𝐾𝑎 − 𝑙𝑜𝑔 [𝑎𝑐𝑖𝑑] [𝑐𝑜𝑛𝑗𝑢𝑔𝑎𝑡𝑒 𝑏𝑎𝑠𝑒] 𝑝𝑂𝐻 = 𝑝𝐾𝑏 − 𝑙𝑜𝑔 (^) [𝑐𝑜𝑛𝑗𝑢𝑔𝑎𝑡𝑒 𝑎𝑐𝑖𝑑][𝑏𝑎𝑠𝑒] PROPERTIES OF BUFFER SOLUTION
  1. EFFECT OF DILUTION
  2. EFFECT OF ADDED ACIDS AND BASES EFFECT OF DILUTION
  • Decreases concentration of the weak acid and conjugate base or vice versa.
  • pH level is ot significantly affected. EFFECT OF ADDED ACIDS AND BASES
  • The addition of acid and base results in minimal changes in pH level because the buffer system neutralizes them. BUFFER CAPACITY
  • This refers to the amount of acid or base that a buffer solution can neutralize before its pH changes significantly (usually by one unit).
  • Usually defined as the number of moles of a strong acid or base that causes 1.00L of the buffer to undergo 1.00-unit change in the pH.
  • Buffer capacity does not only depend on the total concentration of the component but also on their concentration ratio. MAXIMUM BUFFER CAPACITY:
  1. Acid to salt ratio is equal
  2. Base to salt ratio is equal EFFECTIVE BUFFER
  3. [ 𝐻] =

𝐾𝑎

  1. [ 𝑂𝐻] = − 𝐾𝑏
  2. 𝑝𝐻 = 𝑝𝐾𝑎
  3. 𝑝𝑂𝐻 = 𝑝𝐾𝑏
  4. [𝐵𝐻] =[𝐴 − ]
  5. [𝐵] = [𝐵𝐻

] PREPARATION OF BUFFER

  • A buffer solution of any desired pH can be prepared by combining a weak acid with its conjugate base or a weak base with its conjugate acid.
  • This solution resists changes in pH upon the addition of small amounts of strong acid or base. TITRATION CURVES
  • Consist of a plot of reagent volume as the horizontal axis and some function of the analyte or reagent concentration as the vertical axis.
  • Two most widely used endpoints are: a. color change due to the analyte/indicator b. change in potential of the electrode that responds the the reagent/analyte Prepared By: Radz Karl Daruca

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TITRATION CURVE OF A WEAK BASE WITH A STRONG ACID

  • The derivation of a curve for the titration of weak base is similar to that of the weak acid. Figure 8: Illustration of Titration Curve of a Weak Base with a Strong Acid TITRATION OF A WEAK ACID WITH A WEAK BASE OR OF A WEAK BASE WITH A WEAK ACID
  • It is not feasible as it does not proceed to completion, hence the endpoint is not sharp enough to provide an indicator that can be used in a titration. EFFECT OF CONCENTRATION ON TITRATION CURVE
  • The greater the concentration of analyte and titrant, the steeper the slope at the equivalence point region, hence the sharp end point. EFFECT OF REACTION COMPLETENESS ON TITRATION CURVES
  • The strength of the acid or base depends on the 𝐾𝑎 or 𝐾𝑏 value. Lower values mean weaker acids or bases. When they're weaker, the titration curve becomes less steep near the equivalence point. EFFECT OF ACID STRENGTH ON TITRATION CURVE
  • Strong acids titrate more rapidly and sharply near the equivalence point compared to weak acids, resulting in a steeper slope on the titration curve. Figure 9: Illustration of Titration Curve affected by the Acid Strength EFFECT OF BASE STRENGTH ON TITRATION CURVE
  • Stronger bases exhibit steeper pH changes near the equivalence point compared to weaker bases, resulting in a more pronounced vertical portion on the curve. Figure 10: Illustration of Titration Curve affected by the Base Strength ACID-BASE INDICATORS
  • It is a weak organic acid or base whose undissociated form differs in color from its conjugate acid/base form. 𝐻𝐼𝑛 + 𝐻 2 𝑂 ⇔ 𝐼𝑛 −
  • 𝐻 3 𝑂
  • If HIn is red and In– is yellow, the observed color in a solution depends on the pH, which determines their relative concentrations. Then;
  1. At a very acidic pH, HIn dominates and the solution is red.
  2. At a very basic pH, 𝐼𝑛 dominates and − the solution is yellow. Case #1 exists if: [𝐻𝐼𝑛] [𝐼𝑛_]

Case #2 exists if: [𝐻𝐼𝑛] [𝐼𝑛_]

1 10 ACID DISSOCIATION CONSTANT 𝐾

=

[𝐻 3 𝑂
][𝐼𝑛
]
[𝐻𝐼𝑛]

In logarithmic form: 𝑝𝐻 = 𝑝𝐾

− 𝑙𝑜𝑔

[𝐻𝐼𝑛]
[𝐼𝑛

−

]
∆𝑝𝐻 (Related to Yellow and Red Indicator)

∆𝑝𝐻 = 𝑝𝐻𝑏𝑎𝑠𝑒 − 𝑝𝐻𝑎𝑐𝑖𝑑 = (𝑝𝐾𝑎 + 1) − (𝑝𝐾𝑎 − 1) = 2 Thus, the minimum change in pH, pH, required to cause a detectable color change from red to yellow is 2 pH units. Indicator range = pKa ± 1 COMMON TYPES OF INDICATORS

  1. PHTHALEIN INDICATORS
  2. SULFONEPHTHALEIN INDICATORS
  3. AZO INDICATORS PHTHALEIN INDICATORS Colorless in mildly acidic conditions, phenolphthalein displays various colors in alkaline media and gradually fades in strong alkalinity. It is the most recognized phthalein indicator. Prepared By: Radz Karl Daruca

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Figure 11: Illustration of Phenolphthalein Indicator SULFONEPHTHALEIN INDICATORS These indicators change color in two ranges: one in acidic settings and another in neutral to slightly basic ones. Their basic form stays stable even in strong alkali. Phenol red is a simple example of this type of indicator. Figure 12: Illustration of Phenol Red Indicator AZO INDICATORS Most shift from red to yellow as basicity increases, with transition ranges typically acidic; common examples include methyl orange and methyl red. Figure 13: Illustration of Methyl Orange Indicator TITRATION ERROR WITH ACID/BASE INDICATORS

  1. SYSTEMATIC ERROR
  2. RANDOM ERROR SYSTEMATIC ERROR
  • Occurs when the pH at which the indicator changes color differs from the pH at chemical equivalence. RANDOM ERROR
  • originates from the limited ability of the eye to distinguish reproducibly the intermediate color of the titration. VARIABLES THAT INFLUENCE INDICATORS’ BEHAVIOUR
  1. TEMPERATURE
  2. IONIC STRENGTH
  3. PRESENCE OF ORGANIC SOLVENTS AND COLLOIDAL PROPERTIES REAGENTS FOR NEUTRALIZATION TITRATIONS
  4. STANDARD ACID SOLUTIONS
  5. STANDARD BASE SOLUTIONS STANDARD ACID SOLUTIONS
  • prepared by diluting an approximate volume of concentrated reagent, commonly used is HCl.
  • standardized against weighed quantities of sodium carbonate, and other primary standards [ TRIS or THAM tris (hydroxymethyl) aminomethane]. STANDARD BASE SOLUTIONS
  • sodium hydroxide is the most commonly prepared standard base solution; KOH and Ba(OH)2 are also encountered.
  • standardized using weak organic acids, most commonly used is potassium hydrogen phthalate (KHP). Prepared By: Radz Karl Daruca

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