Inclusions in Precipitates of Potassium Perchlorate

Mixing solutions of 1.5 M KNO3 and 1.5 M HClO4 produces a white precipitate of KClO4. If permanganate ions, MnO4, are present, an inclusion of KMnO4 is possible. Impure precipitates of KClO4 are purple if an inclusion of KMnO4 is present. Shown below are descriptions of two experiments in which KClO4 is precipitated in the presence of MnO4.

Experiment 1. Place 1 mL of 1.5 M KNO3 in a test tube, add 3 drops of 0.1 M KMnO4, and swirl to mix. Add 1 mL of 1.5 M HClO4 dropwise, agitating the solution between drops. Destroy the excess KMnO4 by adding 0.1 M NaHSO3 dropwise. The resulting precipitate of KClO4 has an intense purple color (photo on left) because the precipitate forms under conditions where [MnO4] >> [ClO4].

Experiment 2. Place 1 mL of 1.5 M HClO4 in a test tube, add 3 drops of 0.1 M KMnO4, and swirl to mix. Add 1 mL of 1.5 M KNO3 dropwise, agitating the solution between drops. Destroy the excess KMnO4 by adding 0.1 M NaHSO3 dropwise. The resulting precipitate of KClO4 has a pale purple in color (photo on right) because the precipitate forms under conditions where [MnO4] << [ClO4].

Figure8.15

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