A SELF-ADJUSTING BURET APPARATUS
In any laboratory where a large number of routine titrations are run, it is obviously of great advantage to have a buret arrangement which
will automatically fill itself exactly to the zero mark with standard solution and thus eliminate fully half of the buret readings that would otherwise be necessary. Various devices have been invented with this object but many of these require troublesome glass work for their manufacture. The apparatus shown in the accompanying diagram was worked out with the object of maintaining the utmost simplicity of design.
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JOURNAL OF CAEMICAL EDUCATION
S B ~ E M B E1928 R,
The tube C is simply a siphon connected through the soft rubber tube K to the buret. This soft rubber tube allows the buret to he adjusted up and down a little in position. B is an open glass tube with an opening in the lower end exactly on the level of the zero mark in the capillary tube D of the buret. A is a rubber tube connected with the air-space I and closed by apinch-cock or screw-clamp L. To start the apparatus working, the cock L is opened and suction applied a t A . The air space is evacuated and a stream of bubbles escapes a t G. Then the cock is closed tightly. It is obvious that all connections in and about the stopper J must be air-tight. Now we know that the diminished pressure of air in I plus that due to the column of liquid his just equal to the atmospheric pressure on the outside. In other words, the pressure effective a t G is that of the atmosphere so that the effective surface of the liquid is that of the level of the dotted line and the effective head is only the difference in level between G and the end of the siphon tube. The latter must of m s e be a little below the former. If the stopcock F is opened, the solution will flow into the buret and rise until i t reaches exactly the level of G in the capillary while a stream of bubbles will meanwhile flow from G, keeping the pressure constant. There will be a very slight difference in the levels attained in D depending upon whether a bubble has just left a t G or is just on the point of leaving, but the capillary is small enough to make this difference absolutely negligible for even very accurate work. If a little cap is used to close the capillary from the air when the apparatus is not in use, i t does not require cleaning for a long time and when it does, the buret may very easily be detached from the rubber tube I
