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A topic from the subject of Titration in Chemistry.

  • 1 year ago
  • 6 min read
Phenolphthalein as an Indicator
Introduction

Phenolphthalein is a chemical compound commonly used as an acid-base indicator. It's colorless in acidic solutions and turns pink in basic solutions. This color change is due to a structural change phenolphthalein undergoes when protonated in acidic solutions. The protonated form is colorless, while the deprotonated form is pink.

Basic Concepts

A solution's pH measures its acidity or alkalinity. The pH scale ranges from 0 to 14, with 7 being neutral. Solutions below 7 are acidic, while those above 7 are basic. Phenolphthalein changes color at approximately pH 8.2, meaning it's colorless in acidic solutions and pink in basic solutions.

Equipment and Techniques

Using phenolphthalein as an indicator requires a burette, pipette, flask, and a sample of the solution to be tested. The solution is placed in the flask, and the burette is filled with a phenolphthalein solution. The phenolphthalein is added dropwise until the solution turns pink. The number of drops needed indicates the solution's pH.

Types of Experiments

Phenolphthalein is used in various experiments, including:

  • Titration: A technique to determine the concentration of an unknown solution. A known volume of a solution with known concentration is added to an unknown solution until the reaction is complete. Phenolphthalein indicates the reaction's endpoint.
  • Acid-base neutralization: A reaction between an acid and a base. Phenolphthalein indicates the reaction's completion point.
Data Analysis

Data from experiments using phenolphthalein determines the solution's pH. The number of phenolphthalein drops needed to turn the solution pink is a measure of the pH. More drops indicate a more acidic solution; fewer drops indicate a more basic solution.

Applications

Phenolphthalein has various applications:

  • Acid-base titrations: A common indicator to determine the endpoint where the acid and base have completely reacted.
  • Acid-base neutralization: Determines when an acid and base have completely neutralized each other, useful for determining the concentration of an unknown acid or base.
  • pH indicator: Used to determine a solution's pH by observing its color.
Conclusion

Phenolphthalein is a versatile chemical compound with various applications. It's commonly used as an acid-base indicator and can determine a solution's pH, making it a valuable tool for chemists and scientists.

Phenolphthalein as an Indicator

Phenolphthalein is a widely used pH indicator in chemistry due to its distinct color change over a narrow pH range. It's a weak organic acid that changes color depending on the pH of the solution it's in.

Key Points:
  • Colorless in acidic solutions (pH < 8.2)
  • Turns bright pink in basic solutions (pH > 10.0)
  • Transition interval: pH 8.2 - 10.0 (The color change occurs gradually within this range.)
  • Endpoint in titrations: approximately pH 9.1 (This is the pH at which the sharpest color change is observed during a titration.)
  • Useful for titrations involving weak acids and strong bases (Because the equivalence point of these titrations often falls within the phenolphthalein transition range.)
Main Concepts:
  • Weak acid equilibrium: Phenolphthalein acts as a weak acid (HIn) and undergoes a reversible color change depending on the concentration of hydroxide ions (OH-):

    HIn + OH- ⇌ In- + H2O

    Where HIn represents the colorless acid form and In- represents the pink conjugate base form.

  • Equilibrium constant and pH range: The equilibrium constant (Ka) for this reaction determines the pH range over which the color change occurs. A larger Ka would result in a sharper color change at a higher pH.
  • Endpoint and overtitration: The endpoint color change is not instantaneous. This means that a slight excess of titrant (the solution being added) might be needed to observe the full color change, potentially leading to overtitration if not carefully monitored.
  • Limitations: Phenolphthalein is not suitable for all titrations. For example, it is unsuitable for titrations involving weak acids and weak bases because the equivalence point often lies outside the phenolphthalein transition range.
Phenolphthalein as an Indicator

Objective: Demonstrate the use of phenolphthalein as an indicator for acid-base titrations.

Materials:

  • Phenolphthalein solution (1% w/v)
  • Sodium hydroxide solution (NaOH), approximately 0.1 M
  • Hydrochloric acid solution (HCl), approximately 0.1 M
  • Distilled water
  • Two beakers (100 mL)
  • Burette
  • Pipette
  • Pipette filler
  • White tile or background

Procedure:

  1. Using a pipette, add 25 mL of HCl solution to a beaker.
  2. Add 2-3 drops of phenolphthalein solution to the HCl solution.
  3. Fill the burette with the NaOH solution. Record the initial burette reading.
  4. Slowly add the NaOH solution from the burette to the HCl solution while constantly swirling the beaker.
  5. Observe the color change. The solution will remain colorless until the equivalence point is reached, at which point a persistent faint pink color will appear.
  6. Record the final burette reading and calculate the volume of NaOH used.
  7. Repeat steps 1-6 using a beaker containing distilled water instead of HCl solution. Note the difference in results.

Observations:

  • The phenolphthalein solution in HCl will remain colorless until the NaOH is added to neutralize it.
  • A faint pink color will appear in the HCl solution at the equivalence point (the point where the acid and base have completely reacted).
  • The phenolphthalein solution in distilled water, upon addition of NaOH, will immediately turn pink.
  • Record the volume of NaOH used to reach the equivalence point in each case.

Conclusion:

Phenolphthalein acts as an acid-base indicator, changing color from colorless in acidic solutions to pink in basic (alkaline) solutions. The experiment demonstrates its use in determining the equivalence point of an acid-base titration. The change in color indicates the pH has risen above its transition range (approximately pH 8.2-10.0). The difference in the behavior of the indicator in acidic and basic/neutral solution is clear. The distilled water serves as a control to show the behavior of the indicator in a neutral environment.

Significance:

Phenolphthalein's sharp color change near a neutral pH makes it a useful indicator in many titrations, especially those involving strong acids and strong bases. Its application allows for precise determination of the concentration of an unknown solution.

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