Just Ask Antoine!
The molecular basis of indicator color changes
Turning water into wine is a classic magician's trick. The magician taps the edge of a glass of water with a wand and quickly pours it into an empty wine glass, and voila! The water is instantly changed into red wine. Pouring the wine into a third container changes it back into water.
Professional magicians sometimes use a pitcher or carafe with a hidden compartment to create the illusion. But anyone who has done an acid-base titration in freshman chemistry knows a simpler way to do the trick that is just as convincing- so long as no one insists on tasting the "wine" or the "water".
Since color change often accompanies chemical change, you might suspect that a chemical reaction is responsible for indicator action. This indeed the case.
HIn H+ + In-
The two forms of the indicator molecule have noticeably different colors. For example, bromocresol green has a yellow HIn form and a blue In form. When there are equal amounts of HIn and In, the solution looks bright green. Adding a drop of acid adds H+ ions which react with the In- ions to form HIn, and the solution becomes more yellow. Adding a drop of base converts HIn to In, and the solution becomes more blue.
But what exactly does attaching a hydrogen to the molecule do to cause a color change? To understand the answer, we'll have to know a little about how color and molecular structure are related.
Light delivers energy in little packets called photons. Different colors of light pack different amounts of energy in their photons. For example, photons of violet light have almost double the energy of those of red light.
All materials absorb photons of some energy. But only substances that absorb photons of visible light will have color. Molecules are very selective about what photon energies they will and will not absorb. In fact, the photon energies a molecule will absorb are so characteristic that they can be used as a 'fingerprint' to identify that molecule in a mixture. This preferential absorption can be explained by assuming that molecules have quantized energies; that is, they exist only in certain allowed energy states.
|Quantum theory shows how quantized energies arise naturally from the wavelike behavior of confined electrons.||
The photon will be absorbed only if its energy is exactly what is needed to take the molecule from one allowed state to another.
Since different molecules have different colors, it follows that molecular structure has something to do with the size of the energy transitions associated with absorption of visible light. The relationship is complex, but a simple model can be used to show many essential features. An electron bound in a molecule (or part of a molecule) is treated as though it is trapped in a uniform box with walls it cannot penetrate. This "particle in a box" model shows that confining electrons in a smaller space tends to make energy level spacings larger. The model shows that electrons restricted to a box the size of a covalent bond absorb in the ultraviolet, and so are colorless. Electrons that can spread over many atoms within a molecule absorb photons of lower energy, and if the box length is just so (a bit over 0.6 nm, and a bit less than 0.8 nm, according to the model) they'll absorb visible light. This explains why many organic materials that have color have structures with electrons that aren't pinned down in single covalent bonds.
|See Why things have color from Carnegie-Mellon for a Java applet that illustrates how electron confinement affects the color of a material.||
When a hydrogen ion combines with the base form of an indicator molecule, it will confine two formerly mobile electrons to a single covalent bond with the hydrogen, shifting the light that is absorbed towards the blue end of the spectrum. Indicator structures often undergo additional changes that amplify the change in electron confinement.
The color of a transparent object is due to the colors of light that can pass through the material. For example, white light passing through a glass of red wine looks red because the wine has absorbed the other colors, and lets only the red light pass through. To see this, try looking through a piece of red cellophane at objects of different colors. All colors but red vanish. Non-red objects become dark, with blue-green objects becoming areas become almost black- the cellophane absorbs light with these colors. The red areas still look red- red light is not absorbed.
Now we're ready to explain the molecular basis for the water-to-wine trick. Here is the colorless form of phenolphthalein:
The atom marked by the red arrow doesn't have a p-orbital available for pi-bonding, and it confines the pi electrons to the rings. The molecule absorbs in the ultraviolet, and this form of phenolphthalein is colorless.
In basic solution, the molecule loses two hydrogen ions. Almost instantly, the five-sided ring in the center opens and the electronic structure around the marked carbon changes. The pi electrons are no longer bottled up:
Many other indicators function in essentially the same way. For example, azo indicators (like methyl orange) are structurally quite different from the phthaleins, but again, a shift in electron structure around a key atom results in a change in electron confinement.
|Willstätter later received the 1915 Nobel Prize in Chemistry for his work on plant pigments.||
found that nearly any fruit or flower that is bright red, blue, or purple contains pigment molecules that are
based on cyanidin:
How can one molecular structure be responsible for all of these colors?
Like phenolphthalein, cyanidin's structure changes with pH. The form at left is found in acidic solution, and is a bright red. Notice the high formal charge on the oxygen in the structure. (You might expect that a molecule with a positive formal charge on oxygen would be rather unstable, but if you push the electrons around for a while, you see that many resonance structures can be drawn. The pi electrons are highly delocalized.)
Some flowers and berries have cyanidins with varying numbers of -OH groups on the rightmost ring, but the molecular blueprint is the same [Purdue]. In natural forms of the molecule, the hydrogens on at least one of the -OH groups are replaced with more sugar molecules. A cyanidin with attached sugars is called an anthocyan or anthocyanin. For example, the anthocyan that makes roses red and cornflowers blue has this structure:
|The sugar groups vary in structure and point of attachment for different species of plants.||
It's remarkable that two different species, with such different flower colors, have exactly the same molecules for pigmentation. The difference between the flowers is the pH of the fluid in pigment-bearing tissues: alkaline for cornflowers, and acidic for roses. Tampering with genes that regulate the pH in flowers has shifted petunia colors from red to blue [Griesbach], so perhaps one day, roses can be blue and violets, red.
References and Resources
Electron confinement and color
General Chemistry Online! Water to Wine
Copyright © 1997-2010 by Fred Senese