GENERAL OPERATIONS " 45-
capacities between stated markings is calculated and a correction
is applied, if necessary.
Cleaning Solution.—Prepare a cleaning solution by dissolving 5 gin
of powdered commercial sodium dichromate in 500 cc of commercial
sulphuric acid. The solution may be kept in a bottle having a wide mouth,
such as those in which dry chemicals are purchased. Burettes may be
inverted and left standing in the bottle, the solution then being drawn up by
suction and held in the burette by closing the cock. For cleaning flasks
the solution may be allowed to remain in the flask for some time or a small
amount may be warmed and the flask rinsed with it. The chromic acid
produced by the interaction of sulphuric acid and sodium dichromate
oxidizes all organic matter and leaves the glass thoroughly free from it:
Na2Cr2O7 + H2S04 + H20 -» Na2S04 4~ 2H2Cr04,
4H2CrO4 + 3C + 6H2S04 -» 2Cr2(SO4)3 + 3C02 -f 10H2O.
Disappearance of the red chromate ion and the appearance of a green color,
due to the positive chromium ion of chromic sulphate, indicate exhaustion
of the solution.
Have the flask clean and quite dry. Place on a balance of capacity
sufficiently great to carry the filled flask. Counterpoise, then add
weights to the right pan at the rate of 997.18 gin for each liter.
Remove the flask from the balance and fill with recently boiled distilled
water at 20°, nearly to the point where it is thought that the mark will be
placed. Remove drops from the inside of the neck, above the level of the
water, using a roll of filter paper. Replace the flask upon the balance pan,
then carefully drop in water from a pipette until the balance is in equilibrium.
To mark the flask cut a strip of gummed label, long enough to reach
around the neck and about 5 mm wide. Carefully paste this with the
original straight edge at the level of the meniscus, where the mark is to
be made. Melt a small quantity of paraffin and brush a thin layer over
the label and over a space of about 3 cm on either side of it. Using the
point of a knife or of a sharpened piece of wood trace the straight edge
of the label around the neck of the flask, making a mark sufficiently wide
to be easily visible. The label here merely serves as a guide, making a
regular line possible. Using a small feather as a brush apply a few drops
of hydrofluoric acid and allow this to remain on the flask for two or three
minutes, after which the acid may be washed off and the paraffin removed
by warming.
In case the flask already has a graduation and the calibration shows
this mark to be incorrectly placed it is desirable to indicate the new mark
by making a small, well-defined arrow with the point resting exactly upon
the new mark. The operator's initials may be placed beside the arrow and
if this is done carefully, no interference will result.
If the flask contains no inscription etch the side in a manner similar to
that shown in Fig. 17, page 41.