WATER ANALYSIS 45 



substances reduce different proportions of permanganate, 

 and slight variations in temperature and acidity influence 

 the readiness of the permanganate to part with its oxygen 

 to an appreciable extent. Nevertheless, the process yields 

 results which, taken in conjunction with other analytical 

 facts, aid materially in forming an opinion of a water 

 sample. 



Reagents required : (a) Standard solution of potassium 

 permanganate : 



2K 2 Mn 2 O 8 +6H 2 SO 4 =2K 2 SO 4 +4MnSO 4 +6H 2 O+5O 2 

 2 x 316 5 x 32 ; 



that is, 632 parts of potassium permanganate liberate 160 

 parts of oxygen, or i part of O will be set free by 3-95 parts 

 of permanganate. Hence, if 3-95 grm. of the latter be 

 dissolved in i litre of aq. dest., then i c.c. = i mgr. O. 

 The solution is usually diluted ten times in use, so that 

 10 c.c. = i mgr. O. (b) KI solution, 10 per cent in aq. 

 dest. (c) Starch solution, i grm. per half litre, freshly 

 boiled and filtered, (d) Sodium thiosulphate solution, 

 i grm. to the litre of distilled water, (e) Sulphuric acid, 

 25 per cent in aq. dest. 



PROCESS. Take two stoppered flasks or bottles of at 

 least 300 c.c. capacity, and into one put 250 c.c. of sample 

 water, and into the other put 250 c.c. of distilled water. 

 To each add 10 c.c. of the 25 per cent sulphuric acid, and 

 place them both on a water-bath at 80 F. or 26 C. When 

 the required temperature is reached, 10 c.c. of the perman- 

 ganate solution are added to each lot. A pink colour will 

 result. Maintain the temperature, and observe carefully 

 whether the pink colour is discharged. If so, then another 

 10 c.c. of the permanganate solution is added to the sample 

 and the control, and more if necessary to keep them 

 markedly pink. Further addition of sulphuric acid is not 

 needed. At the end of a specified time, which may be 

 fifteen minutes, half an hour, one hour, two hours, three 

 hours, or four hours, or any combination of these (the 

 commonest being fifteen minutes and four hours), the 

 oxidizing process is stopped by the addition of i c.c. of the 

 KI solution, when the unused permanganate reacts thus, 

 through its loosely held oxygen ; 50 2 + 2oKI -f loH 2 O = 



