The strontium chloride is an inorganic compound consisting of strontium, alkaline earth metal (Mr Becamgbara) and chlorine halogen. Because both elements have very different electronegativities, the compound is an ionic solid whose chemical formula is SrCl 2.
As it is an ionic solid, it is made up of ions. In the case of SrCl 2, they are one Sr 2+ cation for every two Cl - anions. Their properties and applications are similar to those of calcium and barium chlorides, with the difference that strontium compounds are relatively rare to obtain and, therefore, more expensive.
Like calcium chloride (CaCl 2), it is hygroscopic and its crystals absorb water to form the hexahydrate salt, in which six water molecules are present in the crystalline lattice (SrCl 2 · 6H 2 O, upper image). In fact, commercially availability is greater than the hydrate SrCl 2 anhydrous (without water).
One of its main applications is as a precursor to other strontium compounds; that is, it constitutes the source of strontium in certain chemical syntheses.
Chemical structure
The upper image represents the deformed rutile-like crystal structure of anhydrous SrCl 2. In this, the small green spheres correspond to Sr 2+ ions, while the bulky green spheres represent Cl - ions.
In this structure each Sr 2+ ion is "trapped" by eight Cl - ions, consequently having a coordination number equal to 8 and, possibly, a cubic geometry around it. That is, four green spheres make up the roof of the cube, while the other four make up the floor, with the Sr 2+ being located in the center of it.
What would the structure be in the gas phase? The Lewis structure for this salt is Cl-Sr-Cl, apparently linear and assuming a covalence of one hundred percent of its bonds. However, in the gas phase -SrCl 2 (g) - this "line" shows an angle of approximately 130º, being in reality a kind of V.
This anomaly could not be explained successfully, considering the fact that strontium does not have unshared electrons occupying electronic volume. Perhaps it can be caused by the participation of a d orbital in the bonds, or a nucleus-electron disturbance.
Applications
SrCl 2 · 6H 2 O has been used as an additive in organic polymers; for example, in polyvinyl alcohol, in order to modify its mechanical and electrical properties.
It is used as a strontium ferrite in the manufacture of ceramic magnets and glass used to make the colored front glass of television.
Reacts with sodium chromate (Na 2 CrO4) to produce strontium chromate (SrCrO 4), which is used as a corrosion resistant paint for aluminum.
When heated with fire, strontium compounds glow with a reddish flame, which is why they are used for making sparklers and fireworks.
Medicinal
Strontium chloride radioisotope 89 (the most abundant isotope is 85 Sr) is used in the medical field to reduce bone metastases, selectively injected intravenously into bone tissue.
The use of diluted solutions (3-5%) for more than two weeks in the treatment of allergic rhinitis (chronic inflammation of the nasal mucosa), shows improvements in the reduction of sneezing and nasal rubbing.
It was once used in toothpaste formulations to decrease tooth sensitivity by forming a barrier over dentinal microtubules.
Studies of this compound demonstrate a therapeutic efficacy compared to prednisolone (a metabolite of the drug prednisone) in the treatment of ulcerative colitis.
Their results are based on the model of the organism of rats; even so, it represents hope for those patients who also suffer from osteoporosis, since they can use the same medicine to combat both diseases.
It is used to synthesize strontium sulfate (SrSO 4), even more dense than SrCl 2. However, its minimal solubility in water does not make it light enough to be applied in radiology, unlike barium sulfate (BaSO 4).
Preparation
Strontium chloride can be prepared by the direct action of hydrochloric acid (HCl) on the pure metal, thus a redox-type reaction occurs:
Sr (s) + HCl (aq) => SrCl 2 (aq) + H 2 (g)
Here, the strontium metal is oxidized by donating two electrons to allow the formation of hydrogen gas.
Likewise, strontium hydroxide and carbonate (Sr (OH) 2 and SrCO 3) react with this acid when synthesizing it:
Sr (OH) 2 (s) + 2HCl (aq) => SrCl 2 (aq) + 2H 2 O (l)
SrCO 3 (s) + 2HCl (aq) => SrCl 2 (aq) + CO 2 (g) + H 2 O (l)
Applying crystallization techniques, SrCl 2 · 6H 2 O is obtained. It is then dehydrated by thermal action until finally producing anhydrous SrCl 2.
Properties
The physical and chemical properties of this compound depend on whether it is in its hydrated or anhydrous form. This is because the electrostatic interactions change as the water molecules add to the crystalline lattice of SrCl 2.
Anhydrous
Strontium chloride is a white crystalline solid, with a molecular weight of 158.53 g / mol, and a density of 3.05 g / mL.
Its melting points (874 ºC) and boiling (1250 ºC) are high, indicative of the strong electrostatic interactions between Sr 2+ and Cl - ions. Likewise, it reflects the great crystalline lattice energy that its anhydrous structure possesses.
The enthalpy of formation of solid SrCl 2 is 828.85 KJ / mol. This refers to the thermal energy released by each mole formed from its components in their standard states: gas for chlorine and solid for strontium.
Hexahydrate
In the hexahydrate form, it has a higher molecular weight than its anhydrous form (267 g / mol), and a lower density (1.96 g / mL). This decrease in its density is due to the fact that the water molecules "expand" the crystals, increasing the volume; therefore, the density of the structure decreases.
It is almost twice as dense as water at room temperature. Its solubility in water is very high, but in ethanol it is slightly soluble. This is due to its organic character despite its polarity. That is, the hexahydrate is a polar inorganic compound. Finally, at 150 ° C it is dehydrated to produce the anhydrous salt:
SrCl 2 · 6H 2 O (s) => SrCl 2 (s) + 6H 2 O (g)
References
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