c. The bore of the
jet should be of such a size as will allow the level of the liquid to
fall at the rate of about one foot in two minutes. If it runs more
quickly than this, an appreciable error arises from the varying amount
of liquid remaining, and damping the sides of the bulb. The flow of
liquid from a pipette must not be hastened by blowing into it. The lower
tube or nose of the pipette should be long enough to reach into the
bottle or flask containing the liquid about to be measured. The pipette
is filled by sucking at the open end with the mouth; this method of
filling renders the use of the instrument dangerous for such liquids as
strong acids, ammonia, and such poisonous solutions as that of potassic
cyanide. One attempt with a fairly strong solution of ammonia will teach
the beginner a very useful lesson. As soon as the liquid rises above the
upper mark in the pipette, the mouth is withdrawn, and the pipette
quickly closed by pressing the upper aperture with the index finger of
the right hand; it is well to have the finger slightly moist, but not
damp. The neck of the pipette should be long enough to allow its being
firmly grasped by the fingers and thumb of the right hand without
inconvenience. The pipette is first held in a vertical position long
enough to allow any moisture outside the tube to run down, and then the
liquid is allowed to run out to the level of the upper mark; this is
easily effected by lessening the pressure.
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