Home
Common Compounds
Exam Guide
FAQ
Features
Glossary
Construction Kits
Companion Notes
Just Ask Antoine!
Simulations
Slide Index
Toolbox
Tutorial Index

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".

Fill a glass with water. Make the water slightly alkaline by adding a few drops of sodium hydroxide solution.
Hide a few drops of phenolphthalein solution in the bottom of a wine glass. Phenolphthalein is an organic compound that is colorless in acidic and neutral solution. It has an intense red color in alkaline solution.
Pour the alkaline water into the wine glass to convert the phenolphthalein to its red form.
Hide a few drops of a concentrated acid in the bottom of a third glass. When the "wine" is poured into this glass, the acid neutralizes the base, and the phenolphthalein is converted back into its colorless form.

But knowing how to do the trick doesn't make it any less mysterious. Just why the color change occurs when the pH

changes is the subject of this article.

Since color change often accompanies chemical change, you might suspect that a chemical reaction is responsible for indicator action. This indeed the case.
Indicators are weak acids or bases with differently colored acid and base forms.
The indicator reaction is pH dependent because it involves either the release or capture of hydrogen ions:

HIn H⁺ + In^-

where "HIn" and "In" stand for the indicator molecule with and without an attached hydrogen ion.

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.
Different molecules absorb different colors of light, depending on their electronic structure.

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.

Confining electrons to a smaller space makes the light they absorb bluer.

While many other factors can come into play, we have a general principle to guide our examination of indicator structures: Color changes can be caused by changes in electron confinement. More confinement makes the light absorbed bluer, and less makes it redder.

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.
The color of a solution comes from the light it doesn't absorb.
The color most strongly absorbed is the complement of the color that passes through the material. A solution that appears sea green absorbs red light; a purple solution absorbs green light.

Now we're ready to explain the molecular basis for the water-to-wine trick. Here is the colorless form of phenolphthalein:

Phenolphthalein in acidic solution.
Click the image for a 3D Chime structure.
How are electrons confined in this molecule? Every atom involved in a double bond has a p orbital which can overlap side-to-side with similar atoms next to it. The overlap creates a 'pi bond' which allows the electrons in the p orbital to be found on either bonded atom. These electrons can spread like a cloud over any region of the molecule that is flat and has alternating double and single bonds.

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:
Phenolphthalein in basic solution.
Click the image for a 3D Chime structure.

Removing H⁺ frees confined electrons.
If you have the Chime plugin, take a look at the three dimensional shape of this molecule by clicking on the structure. Notice that it is almost flat compared to the acidic form, which allows the pi cloud to extend over most of the molecule. The absorption shifts from the ultraviolet to the blue-green region of the spectrum, which makes this form of the molecule red (see the color complement chart).

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.

Why roses are red and violets are blue
Many plant pigments act as acid-base indicators, and we can now appreciate why some of these pigment molecules behave the way they do. The blue and red pigments of flowers were isolated and extensively studied by R. M. Willstätter , just before the outbreak of the first World War.

Willstätter later received the 1915 Nobel Prize in Chemistry for his work on plant pigments.

Willstätter found that nearly any fruit or flower that is bright red, blue, or purple contains pigment molecules that are based on cyanidin:

Cyanidin chloride in acidic solution.
Click the image for a 3D Chime structure.
Cyanidin-based compounds make apples, autumn leaves, roses, strawberries, and cranberry juice red. They make blueberries, cornflowers, and violets blue. They also make some grapes, blackberries, and red cabbage purple.

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.)

Cyanidin in basic solution.
Click the image for a 3D Chime structure.

In basic solution, removal of a hydrogen from the OH group on the rightmost ring results in the same sort of structure we saw in the basic form of phenolphthalein. This form of cyanidin is blue or violet. So the complementary color (red) is being absorbed. Red light carries less energy than blue, so (as with phenolphthalein) the electrons are less confined in the base than in the acid form. In very strongly basic solutions, the hydrogens on the remaining -OH groups start popping off, and as the electrons become even less confined, the blue color becomes bluer because the light absorbed becomes redder.

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.

Cyanidin diglucoside, the color in roses and cornflowers.
Click the image for a 3D Chime structure.

The sugar groups are usually glucose (blood sugar). Because they're connected to the rest of the molecule via an sp³ hybridized carbon, they don't change electron confinement much. The sugar rings bristle with -OH groups, which can hydrogen-bond to water. Their chief effect is to increase the pigment's solubility in water.

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

Why things have color (Carnegie-Mellon U.): A Java applet that illustrates the effect of electron confinement in a one-dimensional box on the color of the material.

Acid-Base Indicators

Acid Base Indicators and Titrations (Wyn Locke, Imperial College): Chime structures for many other indicators, part of the Virtual Chemistry Library project.

Plant Pigments

Nobel Prize Presentation, 1915 (The Nobel Foundation: Press release issued in 1915 describing Willstätter's pioneering work in plant pigment chemistry.
Basic Structures of the Anthocyanidins and flavonoids (Horticulture, Purdue U.): The names of many variant anthocyanidins obtained by moving -OH groups on the basic structure or attaching sugars or methyl groups to them.
Structure of anthocyanidins (Robert Lancashire, UWI, Mona Jamaica): Chime structures for several variant anthocyanidins not discussed here, including pelargonidin, delphinidin, peonidin, petunidin, malvidin, apigeninidin, luteolinidin, tricetinidin, aurantinidin, 6-hydroxy-cyanidin, 6-hydroxy-delphinidin, rosinidin, hirsutidin, 5-methyl-cyanidin, pulchellidin, europinidin, and capensinidin. The page also includes a UV/Vis spectrum of an anthocyanin. Another superb natural products resource from Robert Lancashire.
Plant Chemistry (Ohio State U.): An overview of secondary compounds produced by plants, with structures. The anthocyanidin pelargonidin (an orange-red flower pigment) is shown.
Plant Pigment Chemistry (Industrial Research Limited): Plant pigment chemistry is still an active area of research with important applications in genetic engineering, agriculture, food chemistry, and botany. A team of experts in New Zealand describe their ongoing investigations.
The Chemistry of Autumn Colors (B. Shakhashiri): A beautiful page that describes the succession of pigments that causes leaves to turn in the fall. Anthocyanins produce the reds in maples and oaks.
The Effect of The Ph6 Gene on the Color of Petunias (R. Griesbach): Tinkering with petunias in the U. S. National Arboretum.

General Chemistry Online! Water to Wine