The short answer
Copper II sulfate pentahydrate (also written copper sulphate pentahydrate, and known by the old name blue vitriol) is CuSO4·5H2O, molar mass 249.69 g/mol, CAS 7758-99-8. It is the ordinary, bright blue crystalline form of copper sulfate and the one almost every buyer means when they order “copper sulphate”.
It carries about 25.4 % copper by mass, and that copper is what the buyer is really paying for — as a fungicide, a feed supplement, a plating electrolyte or a reagent. That is why good specifications quote a minimum copper content, not just a purity.
Copper II sulfate pentahydrate formula and identifiers
| Item | Value |
|---|---|
| Molecular formula | CuSO4·5H2O |
| Molar mass | 249.69 g/mol |
| Synonyms | Copper sulphate pentahydrate, cupric sulfate pentahydrate, blue vitriol, blue stone |
| CAS number (pentahydrate) | 7758-99-8 |
| CAS number (anhydrous CuSO4) | 7758-98-7 |
| Copper content (theoretical) | 25.4 % |
| Water of crystallisation | 36.1 % |
| HS code | 2833.25 (sulphates of copper) |
Where the 249.69 comes from: copper at 63.55, sulphur at 32.06 and four oxygen at 16.00 give 159.61 for anhydrous CuSO4; five molecules of water add 5 × 18.02 = 90.08. Dividing gives the two numbers buyers use most: 63.55 ÷ 249.69 = 25.4 % copper, and 90.08 ÷ 249.69 = 36.1 % water. A lot that is 98 % pure pentahydrate therefore carries about 24.9 % Cu.
The “II” is the oxidation state of copper. Copper II (cupric) sulfate is the stable, commercial compound; copper I (cuprous) sulfate, Cu2SO4, is an unstable laboratory curiosity and is not what anyone ships.
Why is it blue, and is it ionic or covalent?
Copper sulfate is an ionic compound: Cu2+ cations and SO42− anions, with covalent bonds only inside the sulfate ion. In the pentahydrate, four water molecules sit directly around each copper ion and the fifth is held by hydrogen bonds. Those coordinated waters are what make the crystal blue: the copper ion surrounded by water absorbs red-orange light and we see the complementary blue.
Take the water away and the colour goes with it. That is the basis of the simplest test in the chemistry lab.
What happens when copper sulfate pentahydrate is heated?
Heated gently, the blue crystals lose their water of crystallisation in stages and turn into a white to greyish-white powder, anhydrous copper sulfate. The first waters leave readily; the last one is only driven off above about 200 °C.
CuSO4·5H2O → CuSO4 + 5 H2O
Add water to the white powder and it turns blue again, giving off heat. It is a reversible dehydration rather than a new substance being formed, and it is why anhydrous copper sulfate is used to detect water. At far higher temperatures the anhydrous salt breaks down to copper oxide and sulphur oxides, which matters for anyone calcining or incinerating copper-bearing waste.
Physical properties
| Property | Pentahydrate | Anhydrous |
|---|---|---|
| Appearance | Bright blue crystals, granules or powder | White to greyish-white powder |
| Crystal system | Triclinic | — |
| Density | About 2.29 g/cm3 | About 3.6 g/cm3 |
| Solubility in water | Readily soluble; far more soluble hot than cold | Soluble, with heat as it rehydrates |
| Solubility in ethanol | Practically insoluble | Practically insoluble |
| Behaviour in air | Slowly effloresces in very dry air | Hygroscopic; picks up water and turns blue |
Grades and what to specify
- Agricultural grade. For Bordeaux mixture, copper fungicide formulation and micronutrient fertilisers. Specified mainly on copper content, free acid, water-insoluble matter and iron. See our copper sulphate pentahydrate for agriculture and the agricultural grade page.
- Technical or industrial grade. For mining flotation, textiles, wood treatment and general chemical use. See copper sulphate pentahydrate technical.
- Feed grade. A copper source for premixes and mineral mixtures, with limits on arsenic, lead, cadmium and mercury and usually a fine, free-flowing powder. See copper sulphate pentahydrate feed grade.
- Electroplating grade. Tight limits on iron, nickel, chloride and insolubles, because impurities show up as rough or dull deposits. See copper sulphate pentahydrate plating grade.
- Reagent grades. Laboratory and analytical grades with a high assay and certified trace-metal limits. See copper sulfate pentahydrate AR.
Whatever the grade, write these into the order: minimum copper content (or assay as CuSO4·5H2O), iron, free acid (as H2SO4), water-insoluble matter, heavy metals (lead, arsenic, cadmium) where feed or food chains are involved, and the physical form — large crystals, small crystals or copper sulfate powder. Crystal size matters: fine powder dissolves fast for spray tanks and premixes, while larger crystals cake less in storage.
What is copper sulfate used for?
- Fungicides. The classic copper sulfate fungicide is Bordeaux mixture, copper sulfate neutralised with hydrated lime, used against downy mildew, blights and leaf spots on grapes, potatoes, fruit trees and many other crops. Copper sulfate is also a starting material for other copper-based fungicide formulations.
- Fertilisers. A soluble copper micronutrient for copper-deficient soils, applied to soil, in blends or as a foliar spray.
- Animal feed. A widely used copper source in premixes for poultry, pigs and cattle. Sheep are unusually sensitive to copper, so feed formulators set species limits carefully.
- Algae and water. An algaecide for reservoirs, irrigation canals and ponds. Its effect on fish depends strongly on water hardness and alkalinity, so dosing must follow the registered product label and local rules.
- Electroplating and refining. The copper source in acid copper plating baths, together with sulphuric acid, used on printed circuit boards, decorative plating and electroforming, and the electrolyte in copper electrorefining.
- Mining. An activator in froth flotation, most notably for zinc sulphide (sphalerite) ores.
- Textiles, leather and wood. A mordant in dyeing, and a source of copper in some wood preservative formulations.
- Laboratories and education. A component of Fehling’s, Benedict’s and biuret reagents, a water indicator in its anhydrous form, and the standard salt for growing crystals.
Is copper sulfate toxic?
It is hazardous, but well understood. Under GHS copper sulfate is harmful if swallowed (H302), causes skin irritation (H315) and serious eye irritation (H319), and is very toxic to aquatic life with long-lasting effects (H400/H410). The environmental classification is the one that shapes shipping and storage: it is transported as UN 3077, environmentally hazardous substance, solid, n.o.s. (copper sulphate), Class 9, packing group III, and marked as a marine pollutant.
In practice: wear gloves and eye protection, avoid raising dust, keep it away from food and feed stores, and never let spills or rinse water reach drains or watercourses. Store it dry and away from strong bases and from reactive metals such as iron, zinc and aluminium, which displace copper from solution; mild steel tanks are a poor choice for its solutions.
Packing and export documents
- Packing. Usually 25 kg or 50 kg HDPE woven or multi-wall paper bags with a polyethylene liner, palletised; jumbo bags for large users; smaller packs for reagent grades.
- Labelling. GHS labels with the environment pictogram and the Class 9 and marine pollutant marks for sea freight.
- Documents. A certificate of analysis stating copper content or assay, iron, free acid, insolubles and heavy metals as specified; a 16-section GHS safety data sheet; the dangerous goods declaration for UN 3077; commercial invoice and packing list; and a certificate of origin.
For another sulphate salt that is bought the same way — by metal content and hydrate form — see our guide to zinc sulphate heptahydrate vs monohydrate, and for a sulphate used alongside it in water treatment, aluminium sulphate and alum.
Enquire with a specification
Scapex Exports supplies copper sulphate pentahydrate from India in agricultural, technical, feed, electroplating and reagent grades, to an agreed specification, with a certificate of analysis and a 16-section GHS safety data sheet on every batch and full export documentation.
Send the grade, the minimum copper content, the crystal size or powder, the pack size, the quantity and the destination port, and you will get a quotation written against that specification rather than a generic offer.
Frequently asked questions
What is copper II sulfate pentahydrate used for?
Copper II sulfate pentahydrate (CuSO4·5H2O) is used mainly in agriculture, as the copper source in Bordeaux mixture and other copper fungicides and as a copper micronutrient in fertilisers; as a copper supplement in animal feed; as an algaecide in water; in acid copper electroplating and copper electrorefining baths; as an activator in mineral flotation; as a textile mordant; and as a laboratory reagent in Fehling's, Benedict's and biuret solutions.
What is the molar mass of CuSO4·5H2O?
249.69 g/mol. Anhydrous copper sulfate (CuSO4) contributes 159.61 g/mol and the five molecules of water contribute 90.08 g/mol. That means the pentahydrate is about 36.1 percent water by mass and contains about 25.4 percent copper, which is why commercial grades are often specified as a minimum percentage of Cu.
Is copper sulfate toxic?
It is harmful if swallowed, irritating to the skin and eyes, and very toxic to aquatic life with long-lasting effects, so it is classified as hazardous under GHS and shipped as an environmentally hazardous substance (UN 3077, Class 9). Used as directed on registered agricultural or water-treatment products it is widely permitted, but it must be kept away from food, children and waterways, and handled with gloves and eye protection.
Is CuSO4 soluble in water?
Yes. Copper sulfate pentahydrate dissolves readily in water to give a blue solution, and it is considerably more soluble in hot water than in cold, which is how large crystals are grown. It is practically insoluble in ethanol.
Is CuSO4 ionic or covalent?
Copper sulfate is an ionic compound made of Cu2+ cations and SO4 2- anions. The bonds inside the sulfate ion are covalent, but the compound as a whole is held together by ionic bonding, which is why it dissolves in water and conducts electricity in solution. In the pentahydrate, four water molecules are coordinated to each copper ion and the fifth is held by hydrogen bonds.
What happens when copper sulfate pentahydrate is heated?
It loses its water of crystallisation in stages and turns from bright blue to a white or greyish-white powder, anhydrous copper sulfate. The last water molecule is removed only above about 200 °C. Adding water back reverses the change and the powder turns blue again with the release of heat, which is why anhydrous copper sulfate is used as a simple test for water. At much higher temperatures the anhydrous salt decomposes to copper oxide and sulphur oxides.