Page 40 - Petrology of Sedimentary Rocks

For pipette analysis, all material coarser than 44 (62 microns) must be removed
from the sample, either by wet or dry sieving; the best amount of sample to work with
is approximately I5 grams, although analyses can be run on samples as small as 5-10
grams. With more sample the grains interfere wth each other too much in settling and
may flocculate; with less sample, the experimental error in weighing becomes large
with respect to the sample.

It is assumed that you start with a sample already dispersed (see page 22). Pour
the dispersed sample in the liter cylinder, add dispersant and water to bring it up
exactly to 1000 ml. Stir vigorously and let the cylinder stand for a day; if it shows no
signs of flocculation, it is ready for analysis; if it starts to flocculate (see page 24)
dispersion will have to be redone or the sample discarded.

Prepare a data sheet giving diameters, depths and times of withdrawal listed on
it. Obtain an accurate timer (watch with second hand will do), a stirring rod, pipette,
and an array of 50 ml beakers.
Stir the cylinder vigorously, starting at the bottom and working up until all the
material (even the coarsest) is distributed uniformly throughout the column. End up
with long, smooth strokes the full length of the column, from the very bottom until the
rod breaks surface. THIS IS EXCEEDINGLY IMPORTANT. As soon as the stirring rod
emerges for the last time, start the time. At the end of 20 seconds, insert the pipette to
a depth of 20 cm and withdraw exactly 20 ml. This is the most important single step, as
on it the subsequent analyses are based, so make it as close to 20.0 ml as you possibly
can.

Continue withdrawals at the specified time intervals. It is unnecessary to stir the
suspension at any time after the first stirring; however if you need to you can stir over
again and start timing from the new stirring; for example if you forget to take the 2-
hour sample, you can stir the cylinder again and wait another two hours. In general this
is not desirable and should be avoided if possible.

Years of experience have shown that this is the best procedure for making
withdrawals: fifteen seconds before the time of withdrawal, grasp the rubber tube of
the pipette with the left hand and convey it to the mouth. With the right hand, insert
the glass part of the pipette to the proper depth, and steady it by resting the right hand
on the rim of the cylinder. With your left hand grasp the pipette at the place where the
rubber tube is attached to the glass, so you will be able to get an exact volume in the
pipette by using your left index finger to clamp the rubber tube shut against the end of
the glass tube. When the time arrives, begin sucking very rapidly and draw the
suspension up about an inch above the volumetric mark, and clamp with the left finger.
Then let the suspension down very slowly by slowly unclamping the finger, until the
liquid surface hits the volumetric mark. If you are off by no more than l/8” either way
it is sufficiently accurate. Remove the pipette, expel the suspension into a weighed 50
ml. beaker, suck up 20 ml of distilled water to rinse out the pipette, and expel the rinse
water into the same beaker.

Place the beakers in an oven with cover glass on top, and evaporate them to
dryness (they should be dried at least 25 hours at IOO-130’ C). Remove them from the
oven and let them cool to room temperature. Let them remain open in the room where
weighing is to take place for several hours, until they come to equilibrium with the
moisture content of the atmosphere (the thin clay films in the beakers will begin to
absorb water very rapidly when they first come out of the oven, consequently gain
rapidly in weight at first). Weigh them to 0.001 gm. on a chemical balance; if possible
weigh all beakers at nearly the same time.

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