Titration Curve Calculator
Generate and analyze acid-base titration curves.
Titration Curve Calculator
Weak Acid - Strong Base Titration
Analyte (Weak Acid)
Titrant (Strong Base)
Titration
Titration is a technique where a solution of a known concentration is used to determine the concentration of an unknown solution. This calculator models the pH changes during the titration of a weak acid with a strong base, highlighting the buffer region, the half-equivalence point (where pH = pKa), and the equivalence point.
Understanding Titration Curves
The Visual Story of an Acid-Base Reaction.
What is a Titration Curve?
A Titration Curve is a graph that plots the pH of a solution (on the y-axis) as a function of the volume of titrant (a solution of known concentration) added (on the x-axis).
It provides a detailed, visual representation of how the acidity or basicity of a solution changes throughout a neutralization reaction.
The shape of the curve reveals key information about the acid and base being analyzed, including the equivalence point of the reaction and the pKa of a weak acid or the pKb of a weak base.
Example: The curve typically has an 'S' shape, with a very steep, nearly vertical section around the equivalence point where the pH changes rapidly.
Strong Acid - Strong Base Titration Curve
This is the simplest type of titration curve.
Initial pH: Starts very low (for an acid being titrated with a base).
Equivalence Point: The point where the moles of added base exactly equal the initial moles of acid. For a strong acid-strong base titration, this occurs at pH = 7.0.
Shape: The pH changes very slowly at the beginning, then rises extremely rapidly near the equivalence point, and finally levels off at a high pH.
Example: Titrating hydrochloric acid (HCl) with sodium hydroxide (NaOH) produces a classic strong acid-strong base curve.
Weak Acid - Strong Base Titration Curve
This curve has several distinct features:
Initial pH: Starts at a higher pH than a strong acid because the weak acid only partially dissociates.
Buffer Region: After some base is added, a buffer is formed (containing the weak acid and its conjugate base). In this region, the pH rises very slowly as the buffer resists changes in pH.
Half-Equivalence Point: At the exact midpoint of the buffer region, half of the weak acid has been neutralized. At this special point, pH = pKa of the weak acid.
Equivalence Point: The point where moles of base equal the initial moles of acid. For this type of titration, the equivalence point occurs at a pH > 7 because the conjugate base of the weak acid is itself a weak base, making the solution slightly alkaline.
Example: Titrating acetic acid (CH₃COOH) with sodium hydroxide (NaOH) is a classic example. The equivalence point is basic, and the pKa can be found at the half-equivalence point.
Weak Base - Strong Acid Titration Curve
This curve is essentially the inverse of a weak acid titration.
Initial pH: Starts at a moderately high pH (e.g., 9-11) because the weak base does not fully react with water.
Buffer Region: As acid is added, a buffer consisting of the weak base and its conjugate acid is formed, causing the pH to decrease slowly.
Half-Equivalence Point: At this point, half of the base has been neutralized. Here, pOH = pKb, which also means pH = 14 - pKb.
Equivalence Point: The point where moles of acid equal the initial moles of base. The equivalence point occurs at a pH < 7 because the conjugate acid of the weak base makes the solution acidic.
Example: Titrating ammonia (NH₃) with hydrochloric acid (HCl) is a common example. The equivalence point is acidic.
Real-World Application: Chemical Analysis
Titration curves are a fundamental tool in analytical chemistry.
Determining Unknown Concentrations: The primary use of a titration is to find the concentration of an unknown solution. The volume of titrant used to reach the equivalence point allows for a precise stoichiometric calculation.
Finding the pKa of a Weak Acid: By creating a titration curve for an unknown weak acid, a chemist can find the pH at the half-equivalence point, which directly gives them the acid's pKa. This is crucial for identifying the acid.
Choosing a pH Indicator: The equivalence point of a titration happens very quickly. To see it visually, a chemical indicator is used. A suitable indicator is one whose color change range (its own pKa) brackets the pH of the equivalence point.
Example:In the food industry, titration curves are used to determine the concentration of acids like citric acid in fruit juices or lactic acid in dairy products.
Key Summary
- A Titration Curve plots pH vs. volume of titrant added.
- The Equivalence Point is the steepest part of the curve, where moles of acid equal moles of base.
- For weak acid titrations, the equivalence point is at pH > 7, and the half-equivalence point reveals the acid's pKa (pH = pKa).
- For weak base titrations, the equivalence point is at pH < 7.
- These curves are essential for determining unknown concentrations and acid strengths.
Practice Problems
You are titrating a weak acid with a strong base. At the half-equivalence point, the measured pH of the solution is 4.76. What can you conclude about the weak acid?
Recall the special relationship that exists at the half-equivalence point of a weak acid titration.
Solution: At the half-equivalence point, pH = pKa. Therefore, you can conclude that the pKa of the weak acid is 4.76. This value is characteristic of acetic acid.
Examine a titration curve that starts at a very low pH (~1) and has an equivalence point at exactly pH 7.0. What type of titration is this?
Consider the starting pH and the pH of the equivalence point.
Solution: A very low starting pH indicates a strong acid. An equivalence point at pH 7.0 is the hallmark of a strong acid-strong base titration.
A titration curve for a weak base starts at pH 11 and has an equivalence point at pH 4.5. Is this reasonable? Where would you estimate the pKb?
Consider the properties of a weak base-strong acid titration. The equivalence point should be acidic. The half-equivalence point reveals the pKb.
Solution: Yes, an equivalence point at pH 4.5 is reasonable as it is in the acidic range. The half-equivalence point would occur at a higher pH, in the buffer region. If the half-equivalence point was at pH 9.25, then pOH would be 14 - 9.25 = 4.75, and that would be the pKb of the weak base.
Frequently Asked Questions
What is the difference between the equivalence point and the endpoint?
The equivalence point is the theoretical point where the moles of acid and base are stoichiometrically equal. The endpoint is the point in a titration where a physical change, like a color change from an indicator, is observed. In a well-designed experiment, the endpoint should be as close as possible to the equivalence point.
Why is the equivalence point of a weak acid-strong base titration basic (pH > 7)?
At the equivalence point, all of the weak acid (HA) has been converted to its conjugate base (A⁻). This conjugate base then reacts with water (A⁻ + H₂O ⇌ HA + OH⁻), producing a small amount of hydroxide ions (OH⁻), which makes the solution slightly basic.
What does the titration curve for a polyprotic acid look like?
A polyprotic acid has multiple acidic protons that it donates one by one. Its titration curve will have multiple 'S' shapes, with a distinct equivalence point for each proton that is neutralized. For example, a diprotic acid like carbonic acid (H₂CO₃) will have two equivalence points.
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