Metal Ions and Plant Pigments

Use natural pigments to detect iron, mordant-dye fabric, and map how metal ions shift the whole pH–colour response

Intermediate🕐60 min▲Moderate hazardcomplex-ionsanalytical

Summary

The same plant pigments that read pH¹ also react with dissolved metals. Anthocyanins wrap around metal ions to form coloured complexes², which is why a splash of iron turns cabbage juice inky black and why alum “fixes” a plant dye to cloth. Here you’ll use pigments as a metal-ion test, dye fabric with different mordants³, and see how a metal shifts the entire acid–base colour response. This experiment builds directly on Natural pH Indicators — make an extract there first.

History

Using metal salts to bind plant colour to cloth — mordant dyeing — is one of humanity’s oldest pieces of chemistry. Alum was a major commodity of the ancient and medieval world precisely because it “fixed” dyes like madder and weld that would otherwise wash straight back out, and whole trades grew up around the alum mines. The word mordant comes from the Latin mordere, “to bite,” for the way the metal makes a dye grip the fibre.

The iron–tannin reaction you’ll use here to detect iron is the same one behind iron gall ink, the standard writing and drawing ink of Europe for well over a thousand years — and the deep blues of cornflowers and some hydrangeas come from anthocyanins wrapped around aluminium, the very metal–pigment complexes at the heart of this experiment.

Hazards & preparation

Warning

PPE: safety glasses and gloves.

• The metal salts here — iron(II)/iron(III), copper(II), zinc, alum — are low-to-moderate hazard but should not be ingested; keep them out of food areas and wash hands after handling.
• Avoid lead and other heavy-metal salts for the colour tests; they add nothing you can’t see with iron and copper, and they are genuinely toxic.
• Metal-salt solutions and precipitates should not go down the drain — settle, filter, and dispose of solids as directed on the Safety page.

Part A — Metal-Salt Colour Shifts

Materials:

• Red-cabbage or hibiscus extract (from Natural pH Indicators)
• Small cups
• A pinch each of the metal salts you have on hand: Epsom salt (Mg²⁺), calcium chloride (Ca²⁺), iron(II) sulfate, iron(III) chloride, copper sulfate (Cu²⁺), potassium alum (Al³⁺), zinc sulfate (Zn²⁺)

Steps:

1. Put an equal splash of extract in each cup.
2. Dissolve a different metal salt in each and stir.
3. Compare against a plain-extract control.

Metal salt	Ion	Colour change (anthocyanin)
Epsom salt	Mg²⁺	Slight blue shift
Calcium chloride	Ca²⁺	Minimal change
Iron(II) sulfate	Fe²⁺	Dark green-black
Iron(III) chloride	Fe³⁺	Intense dark green-black
Copper sulfate	Cu²⁺	Blue-green intensification
Alum	Al³⁺	Blue-purple, more stable
Zinc sulfate	Zn²⁺	Slight blue shift

Iron is the show-stopper — a tiny amount blackens the extract.

Part B — Iron Detection in Water

Because iron gives such a dramatic response, natural pigment makes a rough field test for iron in water.

Steps:

1. Add a few drops of cabbage or hibiscus extract to a water sample.
2. A shift toward dark green / black signals dissolved iron.
3. Compare the depth of colour against samples you’ve spiked with known amounts of iron sulfate to estimate concentration.

This is the same principle a well-water test kit uses, made visible with kitchen chemistry.

Part C — Mordant Dyeing

Metal ions “fix” a plant dye to fibre — the ancient craft of mordanting. The metal bridges the pigment to the fabric, and different metals give different final shades from one dye bath.

Materials:

• White cotton squares
• Cabbage or beet extract as the dye bath
• Separate solutions of alum, iron sulfate, and copper sulfate
• Vinegar

Steps:

1. Soak each cotton square in a different metal-salt solution for about an hour.
2. Rinse lightly.
3. Dye all the squares together in the same pigment bath.
4. Rinse and dry, then compare.

Mordant	Result
Alum	Truest to the original pigment colour
Iron	Darkens / “saddens” toward grey-green
Copper	Shifts toward blue-green
No mordant	Colour washes straight back out

Part D — The pH × Metal Grid

Metals don’t just add a colour — they move the whole pH response.

Steps:

1. Lay out a grid: 4 metal conditions (none, alum, iron, copper) × 3 pH conditions (acidic vinegar, neutral, basic baking soda).
2. Add cabbage extract to all twelve wells.
3. Document the twelve distinct colours.

Reading across a row shows the familiar pH rainbow; reading down a column shows how a bound metal shifts that rainbow — direct evidence that the two effects combine.

What you should see

In Part A the iron cups turn near-black while magnesium and calcium barely move — a clear ranking of how strongly each ion binds. Part B turns that into a yes/no iron test. In Part C one dye bath yields three different fabric colours depending only on the pre-soak metal, and the un-mordanted square fades to almost nothing. The Part D grid shows twelve colours from one pigment: proof that pH and metal coordination are independent knobs on the same molecule.

Symptom	Likely cause	Fix
No colour shift with a metal	Ion binds weakly (Mg²⁺, Ca²⁺) or too dilute	Expected for those ions; use more salt or try iron
Dye washes out of fabric	No/weak mordant, or under-soaked	Pre-soak longer in alum or iron; add a splash of vinegar
Everything goes muddy brown	Too much iron, or pigment degrading in base	Use less iron; keep the dye bath near neutral

The Science

Metal coordination

Anthocyanins carry neighbouring hydroxyl (–OH) groups on their ring system. A metal ion can sit between two of them, donating into empty orbitals and forming a coordination complex:

\[\ce{Anthocyanin + M^{n+} -> [Anthocyanin-M]^{(n-1)+}}\]

Binding the metal redistributes electron density in the chromophore — the light-absorbing part of the molecule — so the absorbed wavelengths, and therefore the colour, shift. Iron is the most dramatic because Fe²⁺/Fe³⁺ form strong, deeply coloured complexes; the softly-binding Mg²⁺ and Ca²⁺ scarcely register.

Mordanting

The same bridging chemistry explains mordant dyeing. The metal ion anchors to the fibre on one side and chelates the pigment on the other, locking a dye that would otherwise rinse away and tinting it toward the metal’s characteristic complex colour. Alum gives clean, true hues; iron dulls and darkens; copper cools toward blue-green.

pH and metal are independent

Part D works because protonation (pH) and metal binding act on the anthocyanin through different mechanisms, so their effects stack. This is why a real indicator system has to control for dissolved metals — an iron-rich sample would read the “wrong” colour for its pH.

Questions to Explore

1. Why does iron blacken the extract while calcium does almost nothing? Both are common cations.

   Hint / answer

   Iron forms strong, deeply coloured coordination complexes with the pigment’s –OH groups, dominating what light is absorbed. Ca²⁺ binds only weakly and barely perturbs the chromophore, so the colour hardly moves.

2. How can a single dye bath give three different fabric colours? Nothing about the pigment changed.

   Hint / answer

   Each fibre carries a different pre-bound metal, and the metal both fixes the dye and shifts its complex colour. The pigment is identical; the mordant decides the final shade.

3. Why does an un-mordanted square lose its colour in the wash? What is the mordant physically doing?

   Hint / answer

   Without a mordant the pigment only sits loosely on the fibre and rinses away. The metal ion bridges pigment to fibre — chelating one, anchoring to the other — so the colour stays put.

4. Why would dissolved iron make a natural pH test unreliable? Connect it back to the previous experiment.

   Hint / answer

   Iron binds the anthocyanin and shifts its colour independently of pH, so an iron-rich sample shows a colour that no longer maps cleanly onto the pH chart. The metal response overlays the acid–base response.

Going further

• Rank your whole shelf. Test every metal salt you own and order them by how strongly they shift the extract — a home-made spectrochemical ranking.
• Try other pigments. Blueberry, grape, and red-onion anthocyanins all coordinate metals; beet’s betalain behaves differently again.
• In the Acid–Base track: having seen metals and protons act on one molecule, revisit The pH Landscape to see buffering hold pH — and colour — steady on purpose.

Footnotes

1. pH — A 0–14 scale measuring how acidic (low) or basic (high) a solution is; 7 is neutral.

2. Complex ion — A central metal ion surrounded by bound molecules or ions (ligands), often intensely coloured.

3. Chelation — The binding of a metal ion by a molecule that grips it at several points at once, like a claw.