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ILLINOIS ACADEMY OF SCIENCE 



The Winkler method checks iwith the gasometric method 

 with uniformly good results 1 . This method gives high re- 

 sults if nitrites are present in the solution. Organic matter is 

 said to reduce the iodine to some extent. "Standard Methods" 

 reads, "If nitrites be present, correction must be made." This 

 correction is not proportional to the amount of nitrite present 

 since the reaction is catalytic. The first reaction is, 



HN0 2 +HI=H 2 0+NO+I. 



The nitric oxide then combines with oxygen from the air to 

 form nitrous acid again, and this liberates more iodine. Wink- 

 ler appreciated the fact that the second reaction does not take 

 place until the iodine solution is exposed to the air, and oxi- 

 dized the nitrous acid quantitatively in the bottle, by forming 

 manganic chloride (MnCl 3 ) before the addition of potassium 

 iodide. In a separate sample the amount of manganic chloride 

 required for the destruction of the nitrous acid (HN0 2 ) was 

 determined. The equivalent of this amount, in terms of dis- 

 solved oxygen, is deducted from the total dissolved oxygen. 



Two modifications have been proposed for counteracting 

 the effect of nitrites. The first was proposed by Rideal and 

 Stewart 3 . This modification is based upon the principle that 

 nitrites and organic matter may be oxidized by potassium 

 permanganate in acid solution. A few drops of sulfuric acid 

 are first added, then an excess of permanganate. After allow- 

 ing the oxidation to proceed for 20 minutes, a strong solution 

 of potassium oxalate (K 2 C 2 4 ) is added; this is followed by 

 the usual Winkler procedure. 



The second modification was proposed by Hale-Melia 

 This modification is evidently based upon the principle of the 

 suppression of both nitrite reactions by the suppression of the 

 hydrogen ion concentration. In this modification, after the 

 sample has been acidified in carrying on the usual Winkler 

 procedure, an excess of potassium acetate, KC 2 H 3 2 , is added. 

 This reacts With the sulfuric acid, as follows : 



H 2 S0 4 +2KC 2 H 3 2 =2HC 2 H 3 2 + K 2 S0 4 



Acetic acid has a low hydrogen ion (H + ) concentration; Hale 

 and Melia claim that the interaction of HN0 2 and KI does 

 not take place under these conditions. 



In considering Hale and Melia's modification, Elvove* 

 has shown that iodine is destroyed by potassium acetate when 



1. Winkler; Ber. 21, 2843-54, 1888. Kisch ; Zeit. Angen. Chem., 1891, 105-8. 

 Chlopin; Arch. f. Hyg., 27, 18-33, 1896. Spitta; Arch. f. Hyg., 38, 220, 1900. 

 Birge and Juday ; Wis. Sur. Bull., 22, 11-12. 



2. Analyst, 26, 141-8. 



3. Tour. Ind. and Eng. Chem., Dec. 1913, pp. 978. 



4. Bull. 96, U.S. Hyg. Lab. pp. 26. 



