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Acid-Base Indicators

Middle schoolChemistryBuffered pH with approximate indicator acid-base fractions and illustrative colors, using water Kw = 10⁻¹⁴ at 25 °C

Choose a buffered sample and an indicator, then add a drop. Drag the orange dropper control to prepare a new sample pH.

3D

Drag the apparatus to adjust it. Drag the background to rotate. Scroll to zoom.

Measurement notebook

Change the setup, test a prediction and save the result. Up to 12 snapshots are kept.

About this experiment

Prepare a buffered sample, choose an indicator and add it with a three-dimensional dropper. Compare litmus, phenolphthalein, bromothymol blue and universal indicator across the pH range. The sample pH remains fixed so that the experiment isolates indicator response rather than changes caused by the reagent.

Learning goals

  • Connect pH to hydrogen and hydroxide concentrations.
  • Locate a color transition and distinguish it from a precise pH measurement.
  • Compare the information supplied by different indicators.

How to use it

  1. 1Prepare an acidic sample and add one litmus drop. Compare it with an alkaline sample.
  2. 2Choose phenolphthalein, vary pH around nine and observe its transition after adding a fresh drop.
  3. 3Repeat with bromothymol blue near neutral pH.
  4. 4Use universal indicator across the scale, then record sample pH, concentrations and drop count.

The model

Ion concentrations at the selected pH
Approximate indicator base-form fraction

Assumptions

  • The sample is an ideal buffer whose pH is unchanged by up to five indicator drops. Water Kw is fixed at 10⁻¹⁴.
  • Effective transition pKa values are 6.5 for the litmus surrogate, 9.3 for phenolphthalein and 7.1 for bromothymol blue. Litmus is actually a mixture, so its single-pKa treatment is illustrative.
  • Rendered colors and opacity are qualitative mixtures, not calibrated spectra. Universal indicator uses a piecewise color palette and has no single acid-base fraction. Selecting a new pH or indicator prepares a fresh undyed sample.

Try this in class

  1. Q1Why can the same color correspond to more than one pH?
  2. Q2Why does a one-unit pH change mean a tenfold concentration change?

Frequently asked questions

Does adding more indicator change the sample pH?

Not in this model. Buffering is assumed to isolate the color response.

Can the displayed color be used to calibrate a real camera or indicator solution?

No. The colors depend on an illustrative palette and browser rendering rather than measured absorption spectra.