Natural Pigments and Color Chemistry
Natural Pigments as Chemical Indicators
Many fruits, vegetables, and flowers contain pigments that change color in response to pH and metal ions. These natural indicators provide a beautiful and accessible way to explore acid-base chemistry and metal coordination.
This guide covers extracting pigments, testing their pH responses, and exploring color changes with metal salts.
Safety Notes
- Wear gloves to avoid staining hands
- Some metal salts are toxic - wash hands after handling
- Work in a well-ventilated area
- Adult supervision for children
Beet Juice (Betalains)
Beets contain betalains, a class of pigments distinct from the anthocyanins found in most other red/purple plants. The main pigment is betanin, which gives beets their deep red-purple color.
Extraction
- Grate or finely chop fresh beets
- Cover with water and simmer for 15-20 minutes
- Strain liquid - this is your beet extract
- Alternatively, use canned beet juice directly
pH Behavior
Betalains are less sensitive to pH than anthocyanins, but do show some changes:
| pH Range | Color |
|---|---|
| Acidic (pH 2-4) | Bright red-pink |
| Neutral (pH 7) | Deep red-purple |
| Basic (pH 10+) | Yellow-brown (degrades) |
Note: Betalains are unstable in strongly alkaline conditions and will break down, losing their color permanently. This is different from anthocyanins which reversibly change color.
Beet Juice + Epsom Salt (Magnesium Sulfate)
Materials: - Beet juice extract - Epsom salt (MgSO\(_4\) · 7H\(_2\)O) - Small beakers or cups
Procedure: 1. Pour beet juice into two containers 2. Add 1-2 teaspoons Epsom salt to one, stir to dissolve 3. Compare colors
Observations: - The Epsom salt solution may appear slightly lighter or more pink - Mg\(^{2+}\) ions don’t form strong complexes with betalains - The main effect is from the slightly acidic nature of the magnesium sulfate solution
Extension: Try heating both solutions gently - the plain beet juice may fade faster than the one with Epsom salt, as magnesium can slightly stabilize betalains.
Beet Juice + Other Metal Salts
| Metal Salt | Ion | Color Change |
|---|---|---|
| Epsom salt | Mg\(^{2+}\) | Slight lightening, more pink |
| Calcium chloride | Ca\(^{2+}\) | Minimal change |
| Iron(II) sulfate | Fe\(^{2+}\) | Darkens, becomes brownish |
| Iron(III) chloride | Fe\(^{3+}\) | Dark brown-black |
| Copper sulfate | Cu\(^{2+}\) | Purple-brown shift |
| Alum | Al\(^{3+}\) | Brightens slightly |
Key insight: Iron ions have the most dramatic effect on beet juice, turning it dark brown or black due to complex formation.
Red Cabbage (Anthocyanins)
Red cabbage is the classic natural pH indicator, containing anthocyanins that show a full rainbow of colors across the pH scale.
Extraction
- Chop red cabbage finely
- Boil in water for 10-15 minutes
- Strain - the deep purple liquid is your indicator
- Can also blend raw cabbage with water and strain
pH Behavior
Red cabbage shows the widest color range of any common natural indicator:
| pH | Color | Example Solution |
|---|---|---|
| 1-2 | Red | Strong acid (HCl) |
| 3-4 | Pink/Magenta | Vinegar, lemon juice |
| 5-6 | Purple | Weak acid |
| 7 | Blue-purple | Pure water |
| 8-9 | Blue | Baking soda solution |
| 10-11 | Blue-green | Washing soda |
| 12-13 | Green | Ammonia |
| 14 | Yellow | Strong base (NaOH) |
Red Cabbage + Metal Salts
Anthocyanins form coordination complexes with metal ions, causing dramatic color shifts:
| Metal Salt | Ion | Color Change |
|---|---|---|
| Epsom salt | Mg\(^{2+}\) | Slight blue shift |
| Calcium chloride | Ca\(^{2+}\) | Minimal change |
| Iron(II) sulfate | Fe\(^{2+}\) | Dark green-black |
| Iron(III) chloride | Fe\(^{3+}\) | Intense dark green-black |
| Copper sulfate | Cu\(^{2+}\) | Blue-green intensification |
| Alum | Al\(^{3+}\) | Blue-purple, more stable |
| Zinc sulfate | Zn\(^{2+}\) | Slight blue shift |
| Lead acetate | Pb\(^{2+}\) | Blue-purple (toxic - avoid) |
Experiment: Metal Ion Rainbow
- Prepare 6 small cups with cabbage extract
- Add different metal salts to each
- Compare the color changes
- Add acid or base to each - observe how metals affect the pH response
Science: Metal ions coordinate with the oxygen atoms in anthocyanin molecules, changing the electron distribution and thus the absorbed wavelengths of light.
Purple Potato (Anthocyanins)
Purple potatoes (and purple sweet potatoes) contain anthocyanins similar to red cabbage, primarily petunidin and malvidin derivatives.
Extraction
- Peel and chop purple potatoes
- Boil in water for 15-20 minutes
- Strain the purple liquid
- Alternatively, mash cooked potatoes in water and strain
pH Behavior
| pH Range | Color |
|---|---|
| Acidic (pH 2-4) | Pink-red |
| Neutral (pH 7) | Purple |
| Basic (pH 10+) | Green-blue, then yellow |
The response is similar to red cabbage but often with less vivid color changes.
Purple Potato + Metal Salts
Similar to red cabbage, but often with more muted responses:
| Metal Salt | Color Change |
|---|---|
| Iron salts | Dark gray-green |
| Copper sulfate | Blue-green |
| Alum | Brightens purple |
Other Natural Indicator Sources
Blueberries/Blackberries
Pigment: Anthocyanins (primarily delphinidin, cyanidin)
Extraction: Crush berries, add water, strain
pH Response: - Acidic: Red-pink - Neutral: Purple - Basic: Blue-green
Metal response: Similar to red cabbage; iron turns dark, alum stabilizes color.
Hibiscus Flowers
Pigment: Anthocyanins (delphinidin-3-sambubioside)
Extraction: Steep dried hibiscus in hot water (like tea)
pH Response: - Acidic: Bright red (hibiscus tea color) - Neutral: Purple - Basic: Blue-green
Hibiscus is particularly vivid and makes an excellent indicator.
Turmeric
Pigment: Curcumin (not an anthocyanin - a curcuminoid)
Extraction: Dissolve turmeric powder in rubbing alcohol or make a paste with water
pH Response: - Acidic to Neutral: Yellow-orange - Basic (pH 8+): Red-brown
Turmeric only changes at high pH, making it useful for detecting bases specifically.
Metal response: Iron salts turn turmeric brown-black.
Red Onion
Pigment: Anthocyanins
Extraction: Soak chopped red onion skins in water
pH Response: Similar to red cabbage but less vivid
Butterfly Pea Flower
Pigment: Anthocyanins (ternatin)
Extraction: Steep dried flowers in hot water
pH Response: - Acidic: Purple-pink - Neutral: Deep blue - Basic: Green
Popular in cocktails for dramatic color changes when citrus is added.
Grape Juice (Concord)
Pigment: Anthocyanins
pH Response: - Acidic: Red - Neutral: Purple - Basic: Green
Use unsweetened pure grape juice for best results.
Comparison Chart: All Natural Indicators
| Source | Pigment Type | Acid Color | Neutral | Base Color | Metal Sensitivity |
|---|---|---|---|---|---|
| Beet | Betalain | Pink-red | Red-purple | Brown (degrades) | Low |
| Red cabbage | Anthocyanin | Red | Purple | Green-yellow | High |
| Purple potato | Anthocyanin | Pink | Purple | Blue-green | Medium |
| Blueberry | Anthocyanin | Red-pink | Purple | Blue-green | High |
| Hibiscus | Anthocyanin | Red | Purple | Blue-green | High |
| Turmeric | Curcumin | Yellow | Yellow | Red-brown | Medium |
| Butterfly pea | Anthocyanin | Purple | Blue | Green | High |
Experiments with Metal Salts
Experiment 1: Iron Detection
Iron is the most dramatic metal for color changes with natural pigments.
Materials: - Cabbage or hibiscus extract - Iron(II) sulfate or iron(III) chloride - Unknown water samples
Procedure: 1. Add indicator to water sample 2. If it turns dark green/black, iron is present 3. Compare intensity to known iron concentrations
Application: This can detect iron contamination in well water.
Experiment 2: The Mordanting Effect
Metal ions “fix” natural dyes to fabric - this is called mordanting.
Materials: - White cotton fabric pieces - Cabbage or beet extract - Alum, iron sulfate, copper sulfate solutions - Vinegar
Procedure: 1. Soak fabric pieces in different metal salt solutions for 1 hour 2. Rinse lightly 3. Dye all pieces in the same pigment extract 4. Rinse and dry 5. Compare colors - each metal produces a different shade!
Results: - Alum: Truest to original color - Iron: Darkens/saddens colors (gray-green) - Copper: Shifts toward blue-green - No mordant: Color washes out easily
Experiment 3: pH + Metal Combinations
Materials: - Red cabbage extract - Metal salt solutions - Acids and bases
Procedure: 1. Make a grid: 4 metal conditions × 3 pH conditions 2. Metal conditions: none, alum, iron, copper 3. pH conditions: acidic (vinegar), neutral, basic (baking soda) 4. Add cabbage extract to each 5. Document the 12 different colors!
This demonstrates how metal ions shift the entire pH-color response curve.
The Chemistry Behind Color Changes
Anthocyanins and pH
Anthocyanins exist in different structural forms depending on pH:
- Flavylium cation (low pH): Red color, positively charged
- Quinoidal base (neutral pH): Purple-blue, neutral
- Carbinol pseudobase (higher pH): Colorless
- Chalcone (high pH): Yellow, ring-opened form
The equilibrium between these forms creates the rainbow of colors.
\[\ce{AH+ (red) <=>[$-$H+] A (purple) <=>[$-$H+] A^- (blue) -> chalcone (yellow)}\]
Metal Coordination
Metal ions coordinate with the hydroxyl (-OH) groups on anthocyanins:
\[\ce{Anthocyanin + M^{n+} -> [Anthocyanin-M]^{(n-1)+}}\]
This changes the electron density in the chromophore (color-producing part), shifting absorbed wavelengths. Iron is particularly effective because it forms strong, deeply colored complexes.
Betalains vs. Anthocyanins
Betalains (from beets) have a completely different chemical structure: - Contain nitrogen in the chromophore - Less pH-sensitive - Unstable at high pH (irreversible degradation) - Different metal binding behavior
This is why beet juice behaves differently from red cabbage despite both being red-purple.
Practical Applications
Natural pH Paper
- Soak coffee filters in cabbage extract
- Let dry completely
- Cut into strips
- Use to test pH of solutions - colors match the pH chart
Food Science
- Why do blueberry muffins sometimes have green spots? Baking soda (basic) around berries turns anthocyanins green.
- Why does beet soup (borscht) need acid? Vinegar or lemon keeps beets bright red.
- Why do pickled onions turn pink? Acid changes anthocyanins in red onions.
Art and Dyeing
Natural pigments can dye fabric, paper, and eggs. Different mordants create different shades from the same dye bath.
Resources
Videos:
Websites: - Compound Interest: Anthocyanins - RSC: Natural Indicators
Books: - “A Garden to Dye For” by Chris McLaughlin - “The Art and Science of Natural Dyes” by Joy Boutrup