Marcha Analitica Cationes Grupos I - II - Parte 02/03

Separation of Cations in Analytical Chemistry

Initial Steps in Cation Separation

• The process begins by adding diluted hydrochloric acid to a mixture of group one and group two cations, leading to the precipitation of group one cations as white chlorides. This results in a solution containing bismuth, antimony, copper, cadmium, tin, and possibly traces of lead that did not react.

Precipitation and Centrifugation

• After adding 6M hydrochloric acid (15-20 drops), any remaining cations in solution will continue to react. The mixture is agitated and centrifuged to separate the precipitate from the solution. Group two cations remain in the solution while group one forms a white precipitate.

Solubility and Separation Techniques

• To further separate the group one cations based on their solubility product constants (Kps), deionized water and heat are added to dissolve lead chloride due to its higher Kps compared to mercurous chloride and silver chloride. The mixture is then centrifuged again for separation.

Confirming Presence of Lead Ions

• Once lead chloride is dissolved, potassium chromate is added while still warm; if a yellow precipitate forms, it confirms the presence of lead(II) ions through the formation of lead chromate. This step involves careful observation during centrifugation for clear phase separation.

Reaction with Ammonium Hydroxide

• For silver chloride and mercurous chloride precipitates, 6M ammonium hydroxide is introduced which leads to black precipitate formation due to dismutation reactions involving mercurous chloride. A complex salt with silver ions also forms during this reaction which has a higher Kps than silver chloride allowing for its solubility.

Final Confirmation of Silver Ions

• Adding nitric acid to the solution containing diamin silver complex will result in white precipitate formation if silver ions are present; this indicates successful liberation from complex salts back into ionic form as they rejoin chlorides forming solid silver chloride again upon agitation and centrifugation for final confirmation.