7.4.
URANYL STAINS
235
dry weight). Marinozzi and Gautier (1962) also have emphasized the
specific binding of uranyl acetate with nucleoproteins, the exact character
of the reaction depending in part upon the fixation employed (Chapter
7.9).
The solubility of uranyl acetate is quite temperature dependent. At
15°C a 7.7% solution is saturated. It decomposes relatively quickly in hot
water. In dilute alcoholic solution its solubility increases, but so does its
tendency to decompose. The fragility of this compound in solution has
caused most investigators to use only freshly prepared solutions as stains
even though it dissolves slowly, and this is a nuisance. Perhaps the most
convenient way of preparing a fresh solution is to add an appropriate
quantity of the salt to the solvent in a small covered container, and put
this on a shaker for the 20-30 minutes it probably will take to dissolve
the crystals. Then grids are floated on this fluid surface, section side
down. Depending upon the embedment, staining may be complete in
30 minutes, but often will take several hours.
We have had success in storing 5% stock solutions for months when
these have been kept undisturbed at room temperature in tall bottles.
A precipitate slowly develops, but sinks to the bottom where it accumulates, leaving the main volume clear. Solution to be used is carefully
drawn from its center with a pipette without disturbing the sediment.
Ordinarily we avoid contamination, and the solution remains an effective
stain. If staining is not to be prolonged beyond about 30 minutes, we
commonly float grids upon individual drops of stain, upon a sheet of
dental wax, in a Petri dish used as a moist chamber (Fig. 26, Chapter 7.3).
Heat can be used to expedite staining, but is apt to promote decomposition so that it must be applied judiciously. When a solution starts to
become cloudy it should be suspect.
On occasion we have had trouble with "dirt" and crystalline material
deposited on the sections. Sometimes undoubtedly this has come from a
reaction between the staining solution and contaminating substances on
the forceps tips which handled the grid. Thus, cleanliness is important
in transfer operations. Stored stock solutions of uranyl acetate sometimes
have been contaminated by minute crystals even though they seemed
clear. In such cases, filtration sometimes salvaged the solutions. However,
if filter paper is used with small quantities of solution, adsorption may
dilute the solution unduly.
Watson (1958) points out that acidification of uranyl acetate weakens
URANYL STAINS
235
dry weight). Marinozzi and Gautier (1962) also have emphasized the
specific binding of uranyl acetate with nucleoproteins, the exact character
of the reaction depending in part upon the fixation employed (Chapter
7.9).
The solubility of uranyl acetate is quite temperature dependent. At
15°C a 7.7% solution is saturated. It decomposes relatively quickly in hot
water. In dilute alcoholic solution its solubility increases, but so does its
tendency to decompose. The fragility of this compound in solution has
caused most investigators to use only freshly prepared solutions as stains
even though it dissolves slowly, and this is a nuisance. Perhaps the most
convenient way of preparing a fresh solution is to add an appropriate
quantity of the salt to the solvent in a small covered container, and put
this on a shaker for the 20-30 minutes it probably will take to dissolve
the crystals. Then grids are floated on this fluid surface, section side
down. Depending upon the embedment, staining may be complete in
30 minutes, but often will take several hours.
We have had success in storing 5% stock solutions for months when
these have been kept undisturbed at room temperature in tall bottles.
A precipitate slowly develops, but sinks to the bottom where it accumulates, leaving the main volume clear. Solution to be used is carefully
drawn from its center with a pipette without disturbing the sediment.
Ordinarily we avoid contamination, and the solution remains an effective
stain. If staining is not to be prolonged beyond about 30 minutes, we
commonly float grids upon individual drops of stain, upon a sheet of
dental wax, in a Petri dish used as a moist chamber (Fig. 26, Chapter 7.3).
Heat can be used to expedite staining, but is apt to promote decomposition so that it must be applied judiciously. When a solution starts to
become cloudy it should be suspect.
On occasion we have had trouble with "dirt" and crystalline material
deposited on the sections. Sometimes undoubtedly this has come from a
reaction between the staining solution and contaminating substances on
the forceps tips which handled the grid. Thus, cleanliness is important
in transfer operations. Stored stock solutions of uranyl acetate sometimes
have been contaminated by minute crystals even though they seemed
clear. In such cases, filtration sometimes salvaged the solutions. However,
if filter paper is used with small quantities of solution, adsorption may
dilute the solution unduly.
Watson (1958) points out that acidification of uranyl acetate weakens