4TH GROUP OF CATIONS

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4TH GROUP OF CATIONS

Ba⁺², Ca⁺², Sr⁺²

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• The common characteristics of 4th group cations making them belonged to the same group together is their precipitation as their own carbonates with the addition of ammonunium carbonate ((NH₄)₂CO₃) at the alcaline media buffered with ammonium hydroxide (NH₄OH) and ammonium chloride (NH₄Cl).

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Ba

⁺²

• With K2CrO4 (Potassium chromate): A yellow-coloured (Barium chromate) precipitate occurred.

• Ba⁺² + K₂CrO_{4 } BaCrO₄+ 2K⁺

• Different from other 4th group cations, this precipitate does not dissolve in water and acetic acid; however, it dissolves in mineral acids.

• When an acid is added to barium chromate solution, the yellow colour transforms into reddish orange because of the emergence of Cr2O7-2

(Bichromate) anion.

• Therefore it can be inferred that for the precipitation of BaCrO₄, there should be no hydrogen ions with high concentration at the medium. If high concetration hydrogen ions exists the solution should be neutralized with the addition of sodium acetate (CH3COONa).

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Ba

⁺²

• Flame Experiment: When barium salts are burnt on a platinum wire at bunsen flame, the flame is coloured yellowish green.

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Sr

⁺²

• Flame Experiment: When strontium salts are burnt on platinum wire at bunsen flame, the flame is coloured with carmen red.

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Ca

⁺²

• Flame Experiment: When calcium salts are burnt on platinum wire at bunsen flame, the flame is coloured with tile red.

• With SO₄^-2: All the 4th group cations precipitate as their white- coloured sulphates when they are added with a salt containing H2SO4 or SO4-2.

• Only CaSO₄, different from other 4th group cations, dissolves by forming ammonium calcium sulphate salt ((NH4)2[Ca(SO4)]) when boiled with ammonium sulphate solution.

• BaSO4 and SrSO₄ do not dissolve under these conditions.

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SEPARATION OF GROUP IV AND GROUP V CATIONS

• Before separating group IV and group V cations, NH4+ should be searched.

• 1 mL (~20 drops) of sample that include group IV and group V cations is put in a test tube.

• 1 pinch of spatula or 10 drops (if it is liquid) 1M ammonium chloride (NH4Cl) is added into the tube and mixed.

• NH₄OH is added until it is basic.

• The basicity of medium is checked with litmus paper and (NH4)2CO3 is added until the precipitate complited.

• Then tube is heated in a water bath and centrifuged.

• There is group V cations in the supernatant and group IV cations in the precipitate.

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Points to note

• Centrifuge will be done everywhere the precipitate and supernatant are written, It will not be centrifuged only in the section where it is filtered while it is hot.

• Whether it is basic or not, must be checked with litmus paper.

• After centrifugation, several milliliters of purified water is added to the precipitate and the precipitate is washed by centrifugation again.

• The order of reagent addition is important for the separation of group IV cations from the group V.

• In order to form the media with the buffer of NH4OH - NH4Cl, first NH4Cl, and then NH4OH should be added. If NH4OH is added first, Mg²⁺ will precipitate as Mg(OH)2 which exists in the group V and it will remain at the precipitate with the 4th group cations.

• When NH4Cl, NH4OH and (NH4)2CO3 are added subsequently, all group IV cations will precipitate as their carbonates and Mg²⁺ will remain at the filtrate with the other group V cations.

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Experimental Procedure

• 20 drops of sample is put in a test tube, 8 drops of 5 M CH3COOH and 12 drops of 0.25 M K2CrO4 are added on and centrifuged. Sediment and filtrate are separated. (Weak acid is used because BaCrO4

does not precipitate in strong acidic medium)

• After the precipitate is washed, it is dissolved in concentrated HCl and if it is burned on Pt wire, a green flame is observed.

• NH₄OH is added to the filtrate until it become basic. Then 12 drops of 1.5 M NH4CO3 is added and centrifuged and sediment is removed.

• After the precipitate is washed, 25 drops of 5 M CH3COOH is added. It is heated until all CO2 is removed. After heating, 20 drops of 1 M (NH4)2SO4 and 1-2 crystal Na2S2O3 are added, boiled and filtered when it is hot.

• After the precipitate is washed, it is dissolved in concentrated HCl, and if it is burned on Pt wire, the reddish colour is observed.

• 20 drops of 5 M NH₄OH and a spatula of (NH4)2C2O4 are added on the filtrate, then centrifuged and sediment is removed.

• After the precipitate is washed, it is dissolved in concentrated HCl, and if it is burnt on Pt wire, tile red colour is observed