Processing kodak motion picture films, module 3 analytical procedures

g. Titration
After the initial reading is made and the final
drop removed, the standard solution is added to
the titration vessel with constant swirling. See
Figure 7 for the proper way to turn a stopcock.
As the end point is approached, the rate of
addition is decreased, until finally the titrant is
added dropwise or as split drops. At this point,
tilt the vessel and remove each drop by touching
the tip with the wall of the vessel at a level just
above the surface of the liquid. Tilt the vessel
slightly more to rinse in the drop. Generally the
end point is defined as a specified color change
that persists for at least 15 seconds. When the
end point has been reached, there should be no
final drop to remove. If an end point is not sharp,
or if it is unfamiliar, it may be difficult to decide
when the end point has been reached. Then after
a drop, record the buret reading, add another
drop, and note the change produced in the
indicator. Continue this procedure until the
specified color change has occurred.
Figure 7 How to Turn a Stopcock
Thumb and forefingers wrap
around handle of stopcock to
turn cock and apply inward
pressure to keep the plug
seated.
F002_0907AC
Seating pressure absorbed by
last two fingers pushing
against tip of buret.
h. Drainage Error
Unless the titration to the end point has been
slow and gradual, wait 30 seconds before taking
the final reading of the meniscus, so that the
effect of further drainage will be negligible.
Read meniscus at eye level.
i. Cleaning Buret
Clean the buret with cleaning solution. Prevent
the concentrated cleaning solution from coming
into contact with the stopcock lubricant. A
convenient way to clean burets without
removing any of the stopcock lubricant is to
invert them in a pipet jar containing enough
cleaning solution to fill the buret to the stopcock.
After a few minutes, rinse inside and out with
tap water, then rinse three times with small
quantities of reagent water. Store in a vertical
position.
j. Capping
Capping the buret with an inverted test tube will
aid in preventing evaporation of the solution and
contamination by dust. If the solution is not
alkaline and does not contain fluorides or
phosphates in acid solution, it is generally safe to
allow the solution to stand in the buret. A full
buret will stay clean longer than a dry or
partially filled buret.
k. Greasing Stopcock (glass stopcocks)
If the stopcock sticks or leaks, remove the old
lubricant by wiping with a cloth, using methanol
or acetone if desired. Replace with fresh
“Lubriseal”. Apply only a thin film since too
much lubricant may plug the hole. Unless the
parts of the stopcock are dry before lubricating
and sealing the plug, the seal may be defective.
Teflon stopcocks are not to be greased.
l. Offset Tip
A buret with an offset tip is useful when titrating
with a potentiometer or when the apparatus is
crowded into a small space.
m. Plugged Tip
Occasionally a buret tip becomes plugged with a
small amount of lubricant. The plug can be
expelled in the following manner: Open the
stopcock so that the pressure of the liquid
column is on the plugged tip. Insert the tip in a
beaker of warm water. If this treatment does not
dissolve the plug, it will be necessary to
disassemble the stopcock and thoroughly clean
the buret with cleaning solution, after which the
stopcock must be relubricated. In certain
instances the use of a thin wire probe (pipet
probe), or buret wire, is a satisfactory means of
unplugging buret tips.
6 Processing KODAK Motion Picture Films, Module 3, Analytical Procedures H24.03