Causes of Purple and Green Copper Patinas in Chlorinated Water

Understanding Why Copper Turns Purple as Well as Green in Chlorinated Water

Copper turning purple alongside its normal green patina in the presence of chlorine occurs due to distinct chemical reactions. The green is from typical copper corrosion producing copper carbonate and hydroxide. The purple results from copper ions reacting with chlorine compounds like dichloroisocyanuric acid, forming copper cyanurate or related complexes.

1. Typical Green Patina Formation on Copper

Copper develops a green patina when exposed to air and moisture. This is a common corrosion process resulting in the formation of stable copper compounds on its surface.

• Chemical process: Copper reacts with water, carbon dioxide, and oxygen.
• Reaction formula: 2Cu + H2O + CO2 + O2 → Cu(OH)2 + CuCO3
• Products: Copper hydroxide and copper carbonate form a green layer.

This green corrosion layer acts as a protective barrier slowing further oxidation. It is the “normal” verdigris appearance seen on statues, roofs, and other copper items exposed to air.

2. Purple Patina from Chlorine-Related Reactions

Chlorine in water alters the corrosion chemistry of copper surfaces significantly. Pools and other water systems often use chlorine-releasing chemicals, which interact with copper ions to create unique colored deposits.

2.1 Copper(II) Cyanurate and Copper Dichloroisocyanurate

When chlorine is introduced via compounds like sodium dichloroisocyanurate (NaDCC) or dichloroisocyanuric acid (DCCA), copper can form complex salts:

• Copper(II) cyanurate: A purple compound formed from copper reacting with cyanuric acid derivatives used in pool chlorination. This explains the purple or lilac colors seen.
• Copper dichloroisocyanurate complex: Sodium dichloroisocyanurate releases chlorine and interacts with copper ions, forming a distinctive lilac precipitate.

This purple coloration emerges when copper contacts water with elevated chlorine levels, especially with pool chlorine stabilizers present.

2.2 Effect of Chlorine Concentration

The intensity and likelihood of purple patina increase with chlorine concentration. Higher chlorine parts per million (ppm) create more reactive chlorine species.

• Greater chlorine ppm → higher chance of purple copper complexes forming.
• Maintaining elevated chlorine for sanitation purposes can inadvertently produce visible color changes on copper surfaces.

3. Role of Chlorine Compounds in Patina Formation

Chlorinated water often contains different chlorine sources that influence copper corrosion:

Chlorine Compound	Role in Copper Patina Formation
Sodium dichloroisocyanurate	Releases chlorine and forms copper dichloroisocyanurate complexes, resulting in purple hues.
Dichloroisocyanuric acid	Provides chlorine and reacts with copper to create copper(II) cyanurate, causing purple coloration.
Sodium hypochlorite	A common chlorine disinfectant that can contribute to copper oxidation but less directly linked to purple patina.

Chlorine released from these compounds interacts with copper ions producing secondary products distinct from the usual green corrosion layer.

4. Other Possible Factors for Purple Tints

While the interaction with chlorine compounds is the main cause, some other elements may subtly influence copper surface colors:

• Manganese oxidation: May cause pinkish or purple tones. Traces of manganese in water can oxidize and color copper surface layers.
• Minor impurities: Small amounts of iodide, organic matter, or contaminants like bird droppings could potentially affect coloration but are less documented.

These factors are secondary and usually less prominent compared to chlorine-related reactions.

Summary: Why Does Copper Turn Purple and Green in Chlorinated Water?

The typical green patina on copper forms through common corrosion reactions with air and moisture. When chlorine compounds are present, the water chemistry changes:

• Copper normally forms green copper carbonate and hydroxide layers on exposure to air and water.
• Chlorine released from sodium dichloroisocyanurate or dichloroisocyanuric acid reacts with copper ions.
• These reactions produce copper(II) cyanurate and related dichloroisocyanurate complexes with purple or lilac colors.
• Higher chlorine levels increase the chance and intensity of purple patina formation.
• Secondary factors like manganese presence can also influence coloration but are less significant.

Key Takeaways

• Copper’s green patina results from air oxidation forming copper carbonate and hydroxide.
• Purple patina emerges when copper reacts with chlorine-containing compounds used in water treatment.
• Sodium dichloroisocyanurate and dichloroisocyanuric acid, common chlorine sources, form purple copper complexes.
• Higher chlorine concentrations favor purple coloration development.
• Other elements such as manganese may contribute to additional tints but are minor causes.

Any ideas what’s causing this copper to turn purple as well as the normal green? (Chlorine is used in this water)

Have you ever glanced at a copper fountain or pipe and puzzled over why it’s sporting not just the typical verdant green but an unexpected purple hue? You’re not alone. This colorful curiosity often happens when chlorine is involved in the water. But why exactly does copper sometimes don a purple cloak instead of the usual green patina? The short and solid answer: the purple tint appears due to a chemical reaction between copper and chlorine-containing compounds, often used for water sanitation, creating unique copper-chlorine complex salts.

Now, let’s unravel this vibrant mystery piece by piece.

The Classic Green: Copper’s Natural Costume

First, the green. That familiar green patina coating on copper is something many have come to expect. It’s the natural result of copper’s interaction with air and water over time.

When copper hangs out with oxygen, water, and carbon dioxide—elements found abundantly in the atmosphere—it undergoes a slow oxidation process. This leads to the development of copper carbonate and copper hydroxide layers. Chemically speaking:

2Cu + H2O + CO2 + O2

→

Cu(OH)2 + CuCO3

These corrosion products combine to form what’s often called the “patina,” giving copper its signature green color. It’s slow, steady, and usually quite expected. In the context of statues, fountains, or pipes, this green patina acts as a protective layer, shielding the metal underneath from further corrosion.

The Surprise Purple: When Chlorine Gets Involved

Enter chlorine—the much-used disinfectant in pools, fountains, and water systems. Chlorine usually keeps water clean, but it also brings some chemistry party tricks of its own.

Water treated with chlorine compounds like sodium dichloroisocyanurate or dichloroisocyanuric acid exposes copper surfaces to new chemical interactions. These chemicals release chlorine steadily for water purification, but when copper ions are present, they don’t just play nice. Instead, they react to form unique copper complex salts.

• One main suspect for purple patinas is Copper(II) Cyanurate, a purple compound formed when copper meets dichloroisocyanuric acid.
• Also in the ring is a copper dichloroisocyanurate complex salt, showing up as lilac or bluish-purple deposits.
• Sometimes, this purple can look reminiscent of azurite—a naturally occurring copper basic carbonate mineral—but it’s chemically distinct due to the chlorine’s presence.

To put it simply: when copper meets these chlorine-releasing compounds, it’s like a chemical romance that results in a dazzling purple finish rather than the classic green.

How Does Chlorine Concentration Influence the Purple Hue?

Here’s a practical nugget: the stronger the chlorine concentration, the more likely it is that copper will develop these purple tones. One fountain maintenance professional reported that upping chlorine levels encouraged the aesthetic purple finish, and the customer loved it! So, yes—the higher the chlorine parts per million (ppm), the higher the chance purple will appear.

But let’s not jump to conclusions. Nature has a few other players who might contribute to the coloration.

Could Manganese or Other Elements Be Coloring the Copper?

Besides chlorine-based copper complexes, some speculate that manganese oxidation could tint copper surfaces with a pink or purplish hue. It’s less frequent but worth considering when multiple colors mingle on your copper.

On a lighter note, in casual chatter, people joke about “unicorn vomit” or other whimsical causes. While humorous, the reality lies solidly within the realm of chemistry.

Practical Tips for Managing Copper in Chlorinated Water

1. Monitor chlorine levels: Higher chlorine results in more purple patinas, so adjust chlorine carefully if you want to control color.
2. Regular cleaning: If the purple or green is unwanted, routine cleaning helps reduce buildup of these corrosion products.
3. Material choice: Consider using alternative materials or coatings if patina colors disrupt the design aesthetic.
4. Accept the art: Sometimes, the purple or green is a unique, natural finish that adds character. Customers and owners often find this look charming.

Why Does This Matter?

Understanding the chemistry of copper’s color changes in chlorinated water is not just academic. In practical terms, it helps owners, maintenance teams, and designers predict and manage the appearance and longevity of copper fixtures exposed to treated water. Knowing these factors also helps prevent surprises when your copper suddenly shifts from classic green to regal purple.

So next time you spot that mysterious purple sheen on copper in a swimming pool area or decorative fountain, you’ll know to tip your hat to the ingenious chemistry of copper reactions with chlorine compounds, perhaps enhanced by a dash of manganese. Colorful science at work, right in your backyard!

“Copper + chlorine = stunning purple! Nature’s chemistry lab never quits.”

Have you noticed this color switch in your own copper items? Share your stories and thoughts. Could you see the purple as a new design trend? Or do you prefer the timeless green? Either way, copper’s vibrant palette thanks to chemistry definitely spices up the scene!

Why is copper turning purple in chlorinated water instead of just green?

The purple color forms when copper reacts with chlorine compounds like sodium dichloroisocyanurate. This creates a copper cyanurate complex, which shows a lilac to purple patina. The green is from normal copper corrosion.

What chemicals in chlorinated water cause this purple patina on copper?

Sodium dichloroisocyanurate and dichloroisocyanuric acid, common chlorine sources in pools, react with copper ions. This reaction forms distinctive purple copper dichloroisocyanurate salts.

Does chlorine concentration affect the copper’s color change?

Yes, higher chlorine levels increase the chance of copper developing the purple patina. The more chlorine present, the stronger the reaction that forms the colored copper compounds.

Is the green patina on copper the same as the purple one?

No. The green patina results from copper oxidizing in air, forming copper carbonate and hydroxide. This is normal corrosion. The purple comes from a chemical reaction with chlorine compounds, not just oxidation.

Could other elements besides chlorine cause the purple color on the copper?

It’s possible manganese oxidation could contribute to pink or purple hues. However, in chlorinated water, copper reactions with chlorine chemicals are the main cause of purple patina.