How to prepare & standardization 0.1 N Potassium dichromate

0.1 N Potassium Dichromate Solution

Preparation of 0.1 N Potassium dichromate (K2Cr2O7)  Solution             

• Transfer 6 g of potassium dichromate (K2Cr2O7) to a platinum dish and dry at 120°C for 4 h. Cool in a desiccator. 
• Place 4.9 g of the dried K2Cr2O7 in a 1-L volumetric flask, and
• Add 100 mL of water. Swirl to dissolve and when solution is complete, dilute to the mark with water and mix. 
• Store the solution in a glass-stoppered bottle.

NOTE —If desired, the solution also may be prepared on a determinate basis by accurately weighing dried primary standard potassium dichromate,9 and diluting the solution carefully to volume.

 Standardization of 0.1 N Potassium dichromate (K2Cr2O7) Solution      

• 
  Place 40 ml of water in a 250-mL glass-stoppered conical flask. and
• Add 40 mL, accurately measured, of the K2Cr2O7 solution. Stopper the flask, swirl to mix, remove the stopper, and 
• Add 3 g of potassium iodide (KI), 2 g of sodium bicarbonate (NaHCO3), and 5 mL of hydrochloric acid (HCl). 
• Stopper the flask quickly, swirl to ensure mixing, and let stand in the dark for 10 min. 
• Rinse the stopper and inner walls of the flask with water and titrate with freshly Standardized 0.1 N Sodium thiosulfate solution (Na2S2O3) until the solution is yellowish green. 
• Add 2 mL of starch solution (10 g/L), and continue the titration to the disappearance of the blue color.

 Calculation             

Calculate the normality of the K2Cr2O7 solution, as follows:
A = B * C / D

  where:
   A = normality of the K2Cr2O7 solution,
   B = milliliters of Na2S2O3 solution required for titration of the solution,
   C = normality of the Na2S2O3 solution, and
   D = milliliters of K2Cr2O7 solution used.

 Stability                 

Re-standardize monthly.