4TH GROUP OF CATIONS Ba+2, Ca+2, Sr+2

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).

Ba+2 With K2CrO4 (Potassium chromate): A yellow-coloured (Barium chromate) precipitate occurred. Ba+2 + K2CrO4  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).

Ba+2 Flame Experiment: When barium salts are burnt on a platinum wire at bunsen flame, the flame is coloured yellowish green.

Sr+2 Flame Experiment: When strontium salts are burnt on platinum wire at bunsen flame, the flame is coloured with carmen red.

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.

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.

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.

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