against potassium iodate, dissolve 1-2 grams of reagent grade potassium 

 iodide in 200 ml. of water in a 500 ml . Er lenmeyer flask, add ten drops 

 of concentrated hydrochloric acid and test for free iodine with starch. 

 If no free iodine is present, add 25.00 ml. of the potassium iodate 

 solution and titrate the liberated iodine with the thiosulfate immedi- 

 ately. The thiosulfate is added until the solution is a pale yellow, 

 then 2-3 ml. of starch solution are added and the titration continued 

 until the solution is colorless. The average of several titrations 

 which agree closely should be taken as the normality of the thiosulfate 

 solution. 



The thiosulfate solution should be prepared a year in advance. 

 After this time its concentration changes very little. 

 Tenth Normal Potassium Permanganate: 



Dissolve 3.25 grams of potassium permanganate in one liter of 

 distilled water and boil the solution for 10-15 minutes, let stand 

 overnight and then filter through sintered glass or asbestos. Stand- 

 ardize with tenth normal thiosulfate by measuring out 25.00 ml. of the 

 permanganate solution into a 500 ml. Erlenmeyer flask containing 3 grams 

 of potassium iodide and 5 ml. of concentrated hydrochloric acid dissolved 

 in 50 ml. of water. Let stand in the dark for 5 minutes, dilute to about 

 200 ml. with water and titrate with the thiosulfate, using starch as the 

 indicator. 



The addition of 10 grams of potassium hydroxide per liter of the 

 permanganate solution increases its stability. 

 Tenth Normal Potassium Bichromate: 



Dissolve 4.90 grams of potassium bichromate in distilled water and 

 make up to one liter. Standardize with tenth normal thiosulfate by mea- 

 suring out 25.00 ml. of the bichromate solution into a 750 ml. Erlen- 

 meyer flask containing 10 ml. of concentrated hydrochloric acid and 3 

 grams of potassium iodide in 50 ml. of water. Let stand in the dark for 

 5 minutes, dilute to about 400 ml. with water and titrate with the thio- 

 sulfate, using starch as the indicator. 



Owing to the green color of the final solution, it is advisable to 

 carry out the titration under artificial light. 

 Normal Silver Nitrate : 



Weigh out 169.89 grams of silver nitrate, previously dried at 110 , 

 dissolve in distilled water, make up to one liter and protect the solution 

 from the light. This solution is usually adequate for the standardization 

 of chloride solutions but it may be standardized by gravimetric determi- 

 nation as silver chloride. When greater accuracy is needed, the dry crys- 



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