IV] WATERS 39 



OXYGEN REQXTIRED. 



Method I.— Official. 



20 REAGENTS. 



(a) Standard potassium permanganate solution. — Dissolve 0.3952 gram of potas- 

 sium permanganate in 1 liter of water; each cc. has 0.1 mg. of oxygen available for 

 oxidation. 



(b) Standard oxalic acid solution. — Dissolve 0.7875 gram of crystallized oxalic acid 

 in 1 liter of water. 



Determine the value of the oxalic acid in terms of the permanganate by boiling 

 10 cc. of the oxalic acid and 200 cc. of redistilled water (prepared by treating distilled 

 water with alkaline permanganate and distilling) with 10 cc. of sulphuric acid (1 

 to 3) and titrating, while still boiling, with the standard permanganate to the appear- 

 ance of a pink color. 



21 DETERMINATION. 



Add 10 cc. of sulphuric acid (1 to 3) to 200 cc. of the water in a porcelain dish and 

 heat to boiling. Add from a burette the standard permanganate until the water 

 is distinctly red and boil for 10 minutes, adding more of the standard permanganate 

 from time to time to maintain the red color. Add 10 cc. of the standard oxalic acid 

 and titrate back with the standard permanganate to a pink color. From the total 

 number of cc. used of the permanganate subtract the number of cc. equivalent to 10 cc. 

 of the oxalic acid. The result gives the number of cc. of the permanganate required 

 for 200 cc. of the water. Correct for sulphids, nitrites and ferrous salts, if present, by 

 subtracting the number of cc. of the standard permanganate absorbed by another 

 200 cc. portion of the sample when treated as above, digesting at room temperature 

 for 3 minutes. 



Method II. {Schulze-Trommsdorf Method.^) — Tentative. 

 (To be used when the chlorin content of the sample is high.) 



22 REAGENTS. 



(a) 50% sodium hydroxid solution. 



Other reagents and standard solutions are described under 20. 



23 DETERMINATION. 



Introduce 100 cc. of the water to be examined in a 300 cc. flask, add 0.5 cc. of the 

 sodium hydroxid and 10 cc. of the permanganate, boil for 10 minutes, allow to cool to 

 50°-60°C. and add 5 cc. of the dilute sulphuric acid and 10 cc. of the standard oxalic 

 acid. As soon as the liquid has become perfectly colorless, and while constantly agita- 

 ting, cautiously add from a burette, drop by drop, the standard permanganate, until 

 the liquid acquires a faint permanent redness. The permanganate required to 

 effect this is the quantity required for the decomposition of the organic matter in 

 the 100 cc. of water. 



If 100 cc. of the water require more than 4 cc. of the permanganate for the oxidation 

 of organic matter, a second determination must be made using more of the per- 

 manganate and a correspondingly larger quantity of the sodium hydroxid, as unde- 

 composed permanganate remaining after boiling must be at least twice as great as 

 the quantity decomposed. 



