THE EXAMINATION OE WATER 66i 



Pour iliis into the flask with its contents left from the above 

 experiment. 



Add a few pieces of pipe stem to prevent bumping. 



Distil over about four 50-c.c. Xessler glasses. 



Xesslerize and estimate as for saline ammonia. 



The amount in good water does not exceed o'oi part per 100, oco. 



(10) To Estimate the Aeration of Waters (O2 dissolved). 



Measure exactly the volume of a stoppered bottle about 250 c.c. 



Fill completely with water sample, avoiding air bubbles. 



By means of a long pipette, the rod inserted close to the bottom of 

 the bottle : — 



Add I c.c. of 40 per cent, manganese chloride. 



Add 3 c.c. of (50 per cent NaOH and 10 per cent. KI) solution. 



Insert stopper, avoiding air bubbles, shake. 



Subtract 4 c.c. from volume of bottle for reagents used. 



Allow to stand ten minutes. 



Run in 5 c.c. of concentrated HCl, holding pipette just above the 

 precipitate. 



Insert stopper and shake. 



Free iodine is liberated and a clear reddish brown fluid obtained. 



Transfer this fluid to a flask. 



Rinse out old flask with 10 c.c. of distilled water and add to new 

 flask. 



Run in this from a burette sodium thiosulphate solution (4 grm. 

 per 1,000 c.c.) until a faint yellow colour remains. 



Add 5 c.c. of fresh starch solution. 



Continue with thiosulphate until a blue colour is discharged. 



Now do a blank experiment : — 



In a flask place 50 c.c. of KMnO^ solution (i c.c. =o'oooi grm. O^). 

 20 c.c. of H2SO4 solution (i in 4). 

 10 c.c. of KI solution (10 per cent.). 



Titrate the liberated iodine as above. 



Suppose the blank required 25 c.c. of thiosulphate. 



Suppose the sample recjuired 15 c.c. of thiosulphate. 



50 c.c. KMnOj 50 X o'oooi = 0*005 grm. oxvgen. 



Hence 25 c.c. thiosulphate solution 0*005 grm. oxygen. 



Hence 15 c.c. thiosulphate solution oto5_x^j^ ^ p^^-, 



25 ^ "" 

 The volume of the bottle = (224 - 4 c.c.) 220. 



In 220 parts of the sample there are grm. 0*003 of O^;. 



T . r .1 1 .^1 o-ooi X 100, oco r 



. . In 100,000 parts 01 the sample there are — = i 30 



oxvgen absorbed. 



A good water should have about I'l parts per 100,000. 



