Which of the following is a secondary standard substance?

1.	Which of the following is a secondary standard substance?
Answer» `Na_(2)CO_(3). 10H_(2)O` `FeSO_(4).(NH_(4))_SO_(4). 6H_(2)O` `KMnO_(4)` `Na_(2)S_(2)O_(3). 5H_(2)O` Solution :This is because permanganate solutions are intrinsically unstable is acidic solution, and decompose slowly. Decomosition is catalysed by sunlight, so `KMnO_(4)` solutions should be stored in dark bottles and they must be standaredized frequently. `4MnO_(4)^(-)+4H^(+) rarr 4MnO_(2)+3O_(2)+2H_(2)O` Sodium carbonate, `Na_(2)CO_(3)10 H_(2)O` is a primary standard for solution of acids. Ferrous compounds are easily oxidized and so are difficult to obtain pure. However, the double salt `FeSO_(4)(NH_(4))_(2)SO_(4)6H_(2)O`, called Mohr's salt is used as a standard COMPOUND in volumetic ANALYSIS for titrations with oxidizing AGENTS such as dichromate, permanganate and citric solutions. hydrated sodium thiosu,phate `Na_(2)S_(2)O_(3)5H_(2)O` called 'hypo' is readily solube in water and solutions are used for iodinetitrations in volumetric analysis. Iodine very rapidly oxidizes thiosulphate ions `S_(2)O_(3)^(2-)` to tetrathionate ions `S_(4)O_(6)^(2-)`, and the `I_(2)` is reduced to `I_(-)` ions.

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