Iodine : Methods

Iodine

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Method of preparation:

1) By passing Cl₂ or Br₂ in aqueous KI solution :
2KI +Cl₂ ——-> 2KCl +I₂
2KI + Br₂ ——-> 2KBr +I₂

2) By the oxidation of KI(Potassium iodide) :

(a) H₂O₂ ——> H₂O + O
2KI +H₂O + O —–> 2KOH +I₂

Equation as a whole 
2KI +H₂O₂ ——> 2KOH + I₂

(b) O₃ —-> O₂ + O
2KI +H₂O+O —–> 2KOH +I₂

Equation as a whole 
2KI +H₂O + O₃ —–> 2KOH +I₂ +O₂

3) By the reaction of KI ( Potassium iodide) and copper sulphate :
CuSO₄ + 2KI ——->K₂SO₄ +CuI₂ (cupric iodide)
2CuI₂ ——>2CuI (cuprous iodide, white precipitate) +I₂

4) By heating a mixture of KI , MnO₂ and H₂SO₄ :

2KI + MnO₂ + 3H₂SO₄ ——> MnSO₄ +   2 KHSO₄ + 2H₂O + I₂

5) By the reaction of sodium Iodate and sodium bi-sulphite :
2NaIO₃ + 5 NaHSO₃ ——–>2 Na₂SO₄  + 3 NaHSO₄ + I₂+ H₂O

6) By the reaction of KI and KIO₃ with concentrated H₂SO₄:
5KI +KIO₃ + 6 H₂SO₄ ——> 6 KHSO₄ + 3 H₂O + 3I₂

Manufacture of Iodine from Sea Weeds :

Iodine is present in sea weeds such as Laminaria digitata and Laminaria stenophylla in the form of KI and NaI. Sea weeds are collected, dried and then burnt in shallow pits to obtained their ash which  is called kelp.It contains 0.5 to 1.3 % iodine in the form of KI and NaI.
Kelp is shaken with hot water then KI , NaI and impurities of NaCl, KCl, Na₂SO₄, Na₂CO₃ etc. come in solution. On cooling the solution impurities separate out while NaI and KI remain in solution. Concentrated H₂SO₄ and MnO₂ is added in mother liquor. Mother liquor is solution of  NaI and KI . It is  heated in an iron retort.Vapours of iodine are formed which are condensed  in series of aludels.

2NaI + MnO₂ +  3H₂SO₄ ——>  2 NaHSO₄ + MnSO₄ +  I₂ +  2H₂O

2KI + MnO₂ +  3H₂SO₄ ——> 2 KHSO₄ + MnSO₄ +  I₂ +  2H₂O

Manufacture of Iodine from Caliche :

Chile saltpeter (NaNO3) contains  5-20 % NaIO₃. Chile saltpeter is also known as caliche. It is an important source of iodine. Aqueous solution of caliche is prepared which on concentration gives crystal of NaNO₃. Crystals are filtered out. The filtrate contains NaIO₃. Required amount of NaHSO₃ is added in the filtrate. Iodine separates out.

2NaIO₃ + 5 NaHSO₃ ———-> 3 NaHSO₄ +  2Na₂SO₄ + H₂O + I₂

Purification of Iodine :

Iodine obtained by above method contains the impurity of ICl, IBr, ICN. These impurities are volatile and therefore these impurities  can not  be removed by sublimation method.Because iodine also have sublimation property so  KI is mixed with impure I2 and  heated . Then  vapours of pure I₂ are  formed which are condensed.

ICl + KI ——>  KCl +I₂

IBr +KI ——->KBr +I₂

ICN +KI ——->KCN +I₂

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Physical properties:

1. It is dark violet crystalline solid.
2. Iodine  has metallic lustre.
3. Iodine  sublimes on heating.
4. Vapours of iodine  are poisonous.
5. It is sparingly soluble  in water.