46 QUANTITATIVE AGRICULTURAL AXALYXIH

Calibration of Burettes.—The marking of a burette its too complex to be
easily changed and the calibration will therefore consist of finding what, if
any, corrections must be applied to the existing graduations.

First inspect the burette to determine whether it conforms to specili-
cations, especially with respect to outflow time. If not, make what altera-
tions are possible. A burette who.se outflow time is too short, will give
erratic measurements. Clean the burette with cleaning solution, followed
by distilled water. Fill with distilled water at 20°. Weigh accurately a
25 cc weighing bottle to the third decimal then measure 5 ce of water into
it from the burette, and reweigh. Add another 5 cc and weigh, continuing
until the bottle is full. Empty the bottle, reweigh and continue* the process
until the water from the entire graduated portion of the, burette has been
weighed. Repeat the process in order to have a check upon the work,
Calculate the true capacity of each of the ten portions, using the weight
0.99718 gm for 1 cc of water. Record as follows, the capacities in the last
two columns being recorded only m far as the second decimal place.

Mark

Weight of water,
each interval

True capacity, ouch
interval

Trtui total pnjuinty, z«-ru let
of mtrrvui

Construct a curve showing the true reading at all points. In ease any
marked irregularity is observed at any part of the burette; HO thai correct ions
taken from the curve would be inaccurate, recalibrate thin portion, using
1 cc at a time.
Calibration of Transfer Pipettes.—Determine whether the time of
outflow conforms to the requirements as net forth on page 43. If not, niter
the tip of the pipette before calibrating. Provide a weighing bottle having
a capacity of 10 cc, also a larger one having a capacity equal to that of the
pipette. Cut a strip of paper about 2 mm wide and 5 cm long and carefully
rule this in divisions of centimeters, rriarking from 0 to 5, and HuhdiviMions
of millimeters, using fine linen. Strips cut from coordinate paper are
suitable for this purpose. Determine the approximate location of the
capacity mark on the pipette by a rough experiment, unless the pipette
is already marked. Paste the paper strip on the Btem of the pipette with the,
division 2.5 at the supposed place for the capacity mark ami with the JSITO
toward the point of the pipette. Having cleaned the pipette with chromic,
acid solution it is drawn full of distilled water which i« at a temperature of
20°, and the water is allowed to flow out until the JM»ro mark is exactly
reached. The pipette must be held in a vertical petition and the eye inuwt
be in the same horizontal plane as in the meniscus. The, pipette* tip IK now
touched against the side of the beaker to remove the Iwtt drop. The
finger is then removed from the top of the pipette and the water in allowed
to flow, at full speed, into the larger weighing bottle, which has already been
weighed. The tip is immediately touched to the side of the weighing bottle
to remove the hanging drop. The weighing bottle is then Htopperwi arid