Titration with universal indicator

Titration with a universal indicator that shows a range of pH values can produce a beautiful transition of colors, often resembling a rainbow. Universal indicators are mixtures of different pH-sensitive dyes that change color across a broad range of pH levels, typically from acidic (low pH) to basic (high pH). Here's how the process works in titration and what causes the "rainbow" of colors.

1. Setup of the Titration

• Titration is a technique where one solution (the titrant) is gradually added to another solution (the analyte) until the reaction is complete.
• In this case, you're using a universal indicator as a pH indicator that changes color depending on the pH of the solution.
• Common acids like hydrochloric acid (HCl) or sulfuric acid (H₂SO₄) can be titrated with a strong base such as sodium hydroxide (NaOH), or vice versa.

2. Color Transition During Titration

The universal indicator will change color depending on the pH of the solution, and these colors can range from red in very acidic conditions to violet in very basic conditions. As the titrant is added, the pH of the solution changes, and this results in a colorful display:

• At the beginning of the titration (Strongly Acidic pH ~ 1–3):

  • The solution is acidic, and the universal indicator will turn red or orange. This indicates a low pH.

• As titrant is added (Slightly Acidic pH ~ 3–6):

  • As the acid is neutralized, the pH slowly increases, and the color shifts through yellow. This is a visual cue of a weakly acidic solution.

• At the equivalence point (Neutral pH = 7):

  • When the acid and base are nearly balanced, the pH approaches neutral. At this point, the universal indicator will turn green, showing a pH of around 7.

• After the equivalence point (Slightly Basic pH ~ 8–10):

  • As the base continues to be added and the solution becomes more basic, the color shifts to blue, indicating a higher pH.

• In strongly basic conditions (Very Basic pH ~ 11–14):

  • As more base is added and the solution becomes strongly basic, the color further shifts towards violet or purple. This indicates a very high pH.

3. The Rainbow Effect

As the titration progresses and the pH changes, the solution will transition through the following colors, similar to a rainbow:

• Red → Orange → Yellow → Green → Blue → Violet/Purple.

This gradual color change is the result of the universal indicator responding to the changing pH as the titrant is added. If done carefully and slowly, the color transitions can be quite striking and visually resemble a rainbow.

4. Explanation of pH and Universal Indicator Colors

The different colors correspond to specific pH ranges because the universal indicator contains multiple dyes, each sensitive to different pH levels:

• Red: Strong acid (pH 1-3)
• Orange/Yellow: Weak acid (pH 4-6)
• Green: Neutral (pH 7)
• Blue: Weak base (pH 8-10)
• Violet: Strong base (pH 11-14)

By adjusting the rate at which the titrant is added, you can see the color transitions more clearly and appreciate the pH-dependent rainbow of colors.

5. Example of a Titration

If you start with hydrochloric acid (HCl) in a beaker and slowly add sodium hydroxide (NaOH), using a universal indicator, the solution would initially be red (acidic) and would gradually shift to yellow, green, blue, and finally purple as the acid is neutralized and the base starts to dominate.

Key Takeaway

Titration with a universal indicator provides not only an accurate way to find the equivalence point of a reaction but also a vivid display of colors that change as the pH progresses from acidic to neutral to basic. This gradual transition across the pH scale is what gives rise to the "rainbow" effect.