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Qualitative inorganic analysis
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=== 3rd analytical group of cations === The ''3rd analytical group of cations'' includes ions which form hydroxides that are insoluble even at low concentrations. Cations in the 3rd group are, among others: [[iron|Fe<sup>2+</sup>]], Fe<sup>3+</sup>, [[aluminium|Al<sup>3+</sup>]], and [[Chromium|Cr<sup>3+</sup>]]. The group is determined by making a solution of the salt in water and adding [[ammonium chloride]] and ammonium hydroxide. Ammonium chloride is added to ensure low concentration of hydroxide ions. The formation of a reddish-brown precipitate indicates Fe<sup>3+</sup>; a gelatinous white precipitate indicates [[aluminium|Al<sup>3+</sup>]]; and a green precipitate indicates Cr<sup>3+</sup> or Fe<sup>2+</sup>. These last two are distinguished by adding sodium hydroxide in excess to the green precipitate. If the precipitate dissolves, Cr<sup>3+</sup> is indicated; otherwise, Fe<sup>2+</sup> is present.
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