WATER ANALYSIS 33 



to carbonate. But as each c.c. of oxalic equals i mgr. 

 of CO 2 , then the number of them gives the total number 

 of mgr. of CO 2 in the water sample. The free CO 2 is 

 determined as before by N/2O sodium carbonate solution, 

 and the difference gives the amount present as bicarbonate. 



Dissolved Oxygen. Winkler, Dibdin, Thresh, and Mohr 

 have all devised methods for determining the amount of 

 oxygen dissolved in water samples. Any such method must 

 be simple, speedy, and accurate, and the water must not 

 be operated on in an inert atmosphere, or there might be 

 a rapid loss by diffusion. Winkler's is perhaps the most 

 simple and readily applied, and needs no special apparatus. 

 The following solutions are required : (a) Manganous 

 chloride solution free from iron (80 grm. MnCl 2 + 4H 2 O 

 in 100 c.c. of distilled water ; (b) KI and NaOH solution 

 (10 grm. KI in 100 c.c. of 33 per cent NaOH). This 

 solution when diluted, and sulphuric and starch solution 

 added, should not give any blue colour ; (c) N/ioo iodine 

 (1-27 grm. I and 2 grm. KI dissolved in I litre). This is 

 used to standardize thiosulphate ; (d) N/ioo thiosulphate 

 of soda solution (2-48 grm. Na 2 S 2 O 3 + 5H 2 O per litre) ; 

 (e) Starch solution. 



PROCESS. Take a glass bottle provided with a well- 

 fitting glass stopper and of about 300 c.c. capacity. 

 Determine accurately the capacity when stoppered. Wash 

 it out with some of the water to be examined, and then 

 fill it to overflowing with sample water (avoid splashing). 

 Introduce, by different pipettes, i c.c. of each of solutions 

 (a) and (b), doing this carefully so that they are delivered 

 close to the bottom of the bottle. Put in the stopper 

 tightly, enclosing no air bubbles. Mix the contents by 

 lightly swinging the bottle. A precipitate forms which is 

 allowed to settle. This takes a variable time ; usually 

 fifteen minutes is sufficient. When it has settled and the 

 upper part of the fluid is clear, introduce by pipette, so as 

 to fall on to the precipitate, 5 c.c. of strong HC1, replace 

 stopper and swing until precipitate dissolves, when the 

 fluid becomes yellow-coloured from liberated iodine. The 

 contents of the bottle are now poured into a clean beaker, 

 the bottle washed out with distilled water, and the wash- 

 ings added. It is then titrated with the N /ioo thiosulphate 



