Hutchinson — On the Estimation of Oxygen in Water. 139 



from wMch is found 



25 cub. cents, indigo = 218-75 cub. cents, of water. 



temperature of water = 12*6° C. 



barometric pressure = 744 m.m.s. 



"We therefore have 



744 

 V = 0-0262 X 0-031024 x 218-75 x — - = 1-392 ; 



9o 



therefore 



25 cub. cents, indigo = 1-392 cub. cents, oxygen. 



I made determinations on different days, at different conditions of 

 temperature and pressure. The following shows the quantity of 

 oxygen 25 cub. cents, of indigo was calculated to yield in each case : 



Temp. 12-6^ C. ) ^ ^.393 ^^^_ ^^^^^^ 

 Pressure, 744 m.m.s. ) 



Temp. 15-5^ C. ) j.^^g 

 Pressure, 771 m.m.s. ) >; m 



Temp. 14-75° C. ) ^ ^.g^^ 

 Pressure, 752 m.m.s. ) ?; >> 



Mean, = 1-398. 



The small amount of variation, under widely different conditions, 

 -shows this method of standardization to be a reliable one. 



I now proceed to give the method for the estimation of oxygen 

 contained in a water. Owing to the change which the hyposulphite 

 undergoes, it is necessary that a comparison between it and the indigo 

 should be made each day. After this has been done, and the appara- 

 tus freed from air by means of the hydrogen, 200 cub. cents, of warm 

 water (temp, about 50° C.) are then added ; 50 cub. cents, of indigo are 

 now run in. This I usually effected in portions of about 1 5 cub. cents, at 

 a time, decolourizing each portion by means of the hyposulphite, thus 

 utilizing this step for the comparison of the two re-agents ; effecting 

 thereby a saving of time and material. The liquid in the apparatus 

 being now brought to the yellow neutral tint, a measured volume of 

 the water under experiment is added — 75 cub. cents. I found a con- 

 venient quantity — taking care that no air is admitted at the same 

 time. The bleached indigo will now become re-oxidised, turning from 

 yellow to blue, in proportion to the amount of oxygen present in the 

 water. The hyposulphite is now cautiously added, until we again 

 arrive at the yellow tint, free from green ; a single drop of the re- 

 agent is sufficient to effect the colour change at the proper point. 

 From the quantity used, we find the amount of oxygen present in this 

 75 cub. cents, of water. The operation can be repeated over again on 



