Inorganic Synthesis

Copper(II) sulfate hydrates

Theory:

Copper(II) sulfate is naturally found as an anhydrous salt or as one of its hydrates. These are known as water-soluble minerals:

  • chalcocyanite – CuSO4 (orthorhombic)
  • bonattite – CuSO4·3H2O (monoclinic)
  • chalcantite – CuSO4·5H2O (triclinic)
  • boothite – CuSO4·7H2O (monoclinic)

The mineral chalcantite is the most stable and common form of copper(II) sulfate in nature. It can be synthetically prepared in a form of well-shaped triclinic crystals. Fully hydrated copper(II) sulfate known as boothite is quite rare. It dehydrates spontaneously to form chalcantite. Anhydrous form of copper(II) sulfate known as chalcocyanite and trihydrate known as bonattite are hygroscopic and not stable.

Bonattite

Bonattite [photo: Bob Downs]

Besides these minerals, monohydrate can be formed by heating copper(II) sulfate pentahydrate to 105 °C. This is however two-step process. At 90 °C the loss of two water molecules is observed leaving copper(II) sulfate trihydrate. Dehydratation goes to completition at 225 °C when anhydrous salt is left [1].

CuSO4·5H2O → CuSO4·3H2O → CuSO4·H2O → CuSO4

Copper(II) sulfate pentahydrate crystallizes from the solutions in temperature range 0-96 °C. Above 96 °C trihydrate is stable form of the salt [2]. Solubility curve for CuSO4 in grams per 100 g of water is shown below [3] along with solubility curve four pentahydrate. Among other divalent salts of 3d metals copper(II) sulfate has the lowest solubility.

solubility curve Cu sulfate

Trihydrate can be prepared by various methods. Lobry de Bruyn described the method of cooling 100 mL of saturated copper(II) sulfate pentahydrate solution in methyalcohol with 0.4 mL of distilled water in an ice-bath over the night. Sky-blue crystals are then filtered off and washed with little amount of methylalcohol [4]. Bristoti describes the procedure of adding 55 mL of concentrated sulfuric acid into a 45 mL of saturated copper(II) sulfate pentahydrate in water and heating the mixture at 60-70 °C for about 48 hours [1].

copper sulfate trihydrate

copper sulfate trihydrate II

Needle-like crystals of copper(II) sulfate trihydrate [photo: Michal Hegedus, Juraj Kmotorka]

cuso4

Copper(II) sulfate pentahydrate monocrystal

Copper(II) sulfate preparation:

CuO + H2SO4 → CuSO4 + H2O

A solution of 50% sulfuric acid (8,8 mL, 0,1mol) was made. Copper(II) oxide (7,95g, 100 mmol) was  added. The mixture was heated and after the reaction finished, the product was recrystallized in an ice bath. Yield: 93,45%.

CuSO4·3H2O crystals preparation:

Copper(II) sulfate pentahydrate (11 g, 44 mmol) was dissolved in distilled water (10mL). The solution was heated to 100°C. The solution was then let to cool a little and ethanol (99%) was slowly added to the solution with a pipette until the crystallization appeared. The product was filtered and dried at 60°C.  Molar ratio of the crystal water to central atom (3.27:1) was determined by the means of thermogravimetry.

Literature:

[1] Bristoti et al., J. Thermal Anal., Vol. 10, p. 279-281 (1976).
[2] Manomenova et al., Crystallography reports., Vol. 58, No. 3, p. 513-516 (2013).
[3] M. A. Hitchman, M. Lichon, and R. G. McDonald, J. Chem. Soc. Dalton Trans., 1817 (1987).
[4] Lobry de Rruyn, Rec. Trav. Chim., Vol. 11, p.113 (1892).