Becker & Abbott — Method of Estimating Dissolved Oxygen^ ^-c. 251 



The side tube E, for introducing solid potash into the apparatus, is of 2 em. bore, 

 and is carefully shaped so that a rubber cork will fit it perfectly. Above the 

 8-way tap one of the capillaries is connected to an absorption pipette containing 

 alkaline pyrogallol, while the other is bent sideways and downwards, and projects 

 in front of the tap. W, Y, and Z are rests for the mercury reservoir on the 

 wooden stand to which the apparatus is fastened. Y was at such a height that 

 when the reservoir was placed on it the mercury level fell to E, leaving a vacuum 

 in the upper part of the apparatus. 



Marks were etched on the glass at A, B, and G, and the volumes OA, OB, and 

 OG were determined by weighing their contents of mercury. 



Experimental Procedure. 



The mercury level is brought to rest at E, and secured in this position by a 

 screw clip at G. Then the potash sticks, broken up into pieces 1 to 1^ inches 

 long, are introduced. The potash used was Merck's (85 to 86 per cent. KOH). 

 The same amount can be introduced into the apparatus for each estimation by 

 using the same length of stick, but in nearly all our experiments a quantity 

 sufficient to ensure saturation was used. As a general rule it may be taken that 

 1 gram of potash should be used for each ciibic centimetre of water sample, 

 although possibly less than this may suffice. The cork is replaced and secured 

 by a metal clip M passing around the bulb. The mercury level is then raised 

 until the capillary S is full of mercury, whereupon the tap is closed. On placing 

 the mercury reservoir in the bottom rest Y the mercury falls to the vicinity of 

 E, leaving the potash sticks exposed and in a partial vacuum. When the reservoir 

 is raised, a bubble collects in OB, and is expelled from the apparatus via S. 

 This operation is repeated until no more air can be detected in the apparatus. 

 Three exhaustions usually suffice. 



The apparatus and capillary S being full of mercury, the sample to be tested 

 is drawn in through S until the mercury level reaches the mark G. By this 

 means a definite volume of water OG is measured. Having closed the tap, the 

 reservoir is placed iii Y, whereupon the water sample falls on the potash sticks, 

 and the dissolved gas is expelled with great vigour in one to two minutes. When 

 boiling ceases, the mercury level is made the same inside and outside the 

 apparatus, and the bubble is measured hj making its top correspond with 0, and 

 measuring the distance of its bottom from A. The height of the potash column 

 is also measured. 



The bubble and a little of the potash are then forced into P, and the rest of 

 the potash is removed through S. After gently rocking the stand for one to two 

 minutes, the bubble is drawn back to OB and re-measured. This operation is 

 repeated to ensure complete absorption of oxygen. Slightly over five minutes 

 are required for the total gas determination, while the oxygen determination 

 increases the time by about ten minutes. From the measurements it is easy to 

 calculate the volume of gas per 100 c.c. of sample. 



The alkaline pyrogallol reagent is best prepared by dissolving 5 grams of 

 pyrogallol in 30 c.c. of water, and saturating with potash in the apparatus, in 

 the same way as a water sample. The reagent can then be driven into the 

 pipette without contact with the air. 



Before starting a new determination it is essential to wash all traces of 

 pyrogallol from OG. It is also advisable to wash the tap with water and acid 

 after a series of experiments. 



