Oxidation-reduction titrations involve the titration between an oxidizing agent and a reducing agent. Iodometry and Iodimetry, respectively, are widely utilized titration methods that revolv…Full description
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Q.1. What is the difference between Iodimetric titrations and Iodometric titrations? Ans. IODIMETRIC TITRATIONS
When an analyte that is a reducing agent (like hypo) is titrated directly with a standard iodine solution, the method is called "iodimetry". In iodimetric titrations, free iodine is used. Since it is difficult to prepare the solution of iodine (iodine sublimates and is less soluble in water) it is dissolved in KI solution. KI+I 2 ---> KI3 In an Iodimetric titrations (reduction of iodine) the direct use of iodine as an oxidizing agent in neutral or slightly acidic medium using starch as an indicator is made. The various reducing agents used in these titrations are thiosulfates, sulfites, arsenites or antimonites. 2-
-
2-
I2+ S2O3 ----> 2I +S4O6 thiosulfate
tetra thionate
2-
2-
I2+SO3 + H2O ----> 2I- + SO 4 + 2H+ sulfite
sulfate 2-
2-
I2+ AsO3 + H2O - ----> 2I-+ AsO4 + 2H+ arsenite
Arsenate 3-
2-
I2 + SbO3 + H2O ----> 2I-+ SbO4 + 2H Antimonite
Antimonite -
+
4+
-
H2S+I2--> S + 2I +2H 2+
+
Sn + I2---> Sn + 2I
N2H4 + 2I2---> N2 + 4H+ + 4IIODOMETRIC TITRATIONS
When an analyte that is an oxidizing agent is added to excess iodide (like KI) to produce iodine, and the iodine produced is determined by titration with sodium thiosulfate, the method is called "iodometry".
In the first step oxidizing agents such as KMnO4, K 2Cr2O7 , CuSO4 , peroxides etc are treated with an excess of KI when iodine is liberated quickly and quantitatively. For eg.
-
+
-
2MnO4 +16 H +10 I ----> 2Mn 2-
+
-
2+
+ 5I2 + 8H2O
+
Cr2O7 +14 H +6I ---> 2Cr3 +3I2 +7H2O 2+
-
2Cu + 4I ----> Cu2I2+ I2
In the second stepThe liberated iodine is titrated against a standard solution of sodium thiosulfate using starch as an indicator. All such titrations in which iodine is liberated from potassium iodide with the help of an oxidizing reagent and is titrated against a standard solution of sodium thiosulfate are called iodometric titrations.
2Fe + 2I- ----> 2Fe + I2 Reaction of H2O2 H2O2 + 2I- +2H+ ---> 2H2O + I2 Reaction of As (V) H3AsO4 + 2I- +2H+ ---> H3AsO3 +I2 + H2O Reaction of Cu2+
2Cu
2+
+ 2I- ----> 2Cu I + I 2
Reaction of HNO2 2 HNO2 + 2I- ---> I2 + 2NO + H2O Reaction of SeO322-
SiO3 + 4I- ----> 2CuI + I 2 Reaction of O3 +
O3 + 2I- + 2H ---> O2 + H2 + H2O Reaction of Cl2 Cl2 + 2I- ---> 2 Cl- + I 2 Reaction of Br2 Br2+ 2I- ----> 2Br- +I2 Reaction of HClO HClO+2 I-+ 2H+----> Cl- + I 2 + H2O Practice Problem:
1. A mixture of K2Cr2O7 and KMnO4 was treated with excess of KI in an acidic medium. The iodine liberated required 100 cm3 of 0.15 N sodium thiosulfate solution for titration. Find the % amount of each compound in the mixture. Answer of this problem will be sent to u in next mail.
