Cobalt(II) sulfate represents a group of d-block transition metal sulfates that are readily soluble in water. It is a crystalline compound of ruby red colour (hexa-,heptahydrate) or violet to reddish-pink (monohydate, anhydrous) that crystallizes from water solution as a heptahydrate at normal conditions (even thoug, hexa-, penta- and monohydrate are known). The hydrates of the compounds were used previously as a pigment and they still serve as precursors to other cobalt salts. Heptahydrate has been known at least for two centuries (cobalt vitriol) . Despite this, not many papers have been published dealing with profound analysis and characterization of these simple compounds. We notice the paper on cobalt(II) sulfate hexahydrate thermal decomposition in air atmosphere  and heptahydrate decomposition , too. Thermodynamic studies about the heat of hydration were also published with the water solubility data for both hexa- and heptahydrate (solubility curves can be seen in Fig. 1) .
Fig. 1 Solubility curves for both hexa- and heptahydrate according to [Brodale, 1965].
In nature, heptahydrate it is rarely found as a mineral Bieberite CoSO4.7H2O. It easily dehydrates in air to form hexahydrate which is stable up to 320 K according to [Strazsko, 2000]. The compound crystallizes in a monoclinic crystal system with P21/c symmetry. No direct coordination of sulfate anions to metal atoms is observed in the crystal structure (shown below). Structure can be described as a 3D net of hydrogen bonded hexaaquacobalt(II) complex cations [Co(H2O)6]2+ and sulfate anion SO42-.
The crystal structure of hexahydrate is isomorphic with other minerals of mellanterite group. Till now, no pictures of large heptahydrate single crystal has been published on the Internet. In this article we provide a simple procedure for the preparation of cobalt sulfate salt and crystal growth.
- Deionizied water
- Cobalt(II) chloride hexahydrate
- Sodium bicarbonate
- Sulfuric acid (96%)
- Filtration apparatus
- Evaporation dish
- Crystallization misk
CoCl2 + 2NaHCO3 → 2NaCl + CoCO3 + CO2 + H2O
CoCl2.6H2O (m=20 g) was dissolved in 50 cc of H2O. NaHCO3 (m=15 g) was then added in slight excess to precipitate the carbonate. Attention needs to be paid not to overspill the content of the beaker, due to evolution of CO2 gas during this reaction (using Na2CO3 avoids this issue). Precipitate was then filtered and rinsed with distilled water to get rid of any chloride anions present.H2SO4 (m=8.58 g, w=96%) was diluted to w=50%.
CoCO3 powder was added to the beaker with the H2SO4 causing an exothermic reaction to take place:
CoCO3 + H2SO4 → CoSO4 +CO2 + H2O
This solution was then evaporated to measure the yield and later used for crystallization.
Crystallization of the heptahydrate:
Cobalt(II) sulfate heptahydrate can be crystallized from water solution at room temperature using evaporation method. It has to be noted though, that it´s not easy to obtain a perfect monocrystal and purity of the material seems to be crucial. Crystals are monoclinic and deep red in colour. They dehydrate very fast, but there are two ways to preserve them long term. They can be covered in clear nail polish, or if you don´t want to contaminate your specimen with other chemicals for what ever reason, you can use the second way, which was also used in our case. The crystal was placed inside of transparent, air-tight container, with several layers of tissue, and the bottom layers were soaked in a saturated solution of cobalt sulfate (not enough reach the crystal itself, but enough to keep the internal enviroment moist, thus preventing the crystal from dehydration).Crystallization of the hexahydrate:
Cobalt(II) sulfate hexahydrate can be obtained either by crystallization in the temperature range from cca. 45-65C [Brodale, 1965], or at room temperature by evaporation from solution of dilute sulfuric acid. Because little to no information about this ternary system can be found in the literature, we´ve had to experiment. With the experience with a similiar solution used to crystallize nickel(II) sulfate hexahydrate, we´ve tried similiar proportions. To 20mL of saturated CoSO4 solution 6mL of conc. sulfuric acid (w=96%) was added. At the contact of the acid with the solution a pink precipitate of monohydrate was formed. It has redisolved with stirring. Adition of sulfuric acid to the solution of salt causes drop in the solubility of the salt. But, suprisingly, no sudden crystallization occured. It may be caused by supersaturation of the solution, but that has not been confirmed yet. After a few hours, thecrystals of hexahydrate formed at the bottom of the beaker. They are monoclinic and strikingly similiar to those of β-NSH. These crystals are stable and do not dehydrate on exposure to air.
[Photo by Juraj Kmotorka]
 Sage (1791) Le Journal de physique et le radium, Paris: 39: 53 (as Cobalt Vitriol).
 Straszko, J. et al. (2000) Journal of Thermal Analysis and Calorimetry, p. 935-942.
 Sinha, S. G. Deshpande, N. D. (1989) Thermochimica acta, p. 1-10.
 Brodale, G. E. Glauque, W. F. (1968) The Journal of Physical Chemistry, p. 1268-1272.