9/16/98

UO2(C2H3O2)2.2H2O is 7.694 g/100 mL in 15 deg C water.

Leonard Corwin

Fort Dodge Animal Health

Princeton, NJ 08543-0400

corwinl@pt.cyanamid.com

In thinking about my own experiences with solubility of UA, I've noticed that

even at only 2, 3, 4 % w/v, there always seems to be a bit that doesn't go into

solution, even with heating. I consulted my vendor of UA and was told that there

is an insoluble component present in commercial UA, and thats what I was seeing.

This insoluble contaminant would then confuse efforts to determine saturation

point by visual obvservation - are those insoluble contaminants or are they

leftover UA crystals, having reached saturation? The yellow color of the

solution foils any attempt to detect differences in color of the two kinds of

crystals.

I did isolate these unknown crystals (forgot what color they were), did EDS

analysis on 10 clumps of crystals and found that in addition to moderate to

high amounts of U (quick, on-the-fly subjective semi-quant intuition), there

were also high amounts of titanium (one very high), moderate to high amounts of

silicon (one very high), low amount of aluminum (not always present), low amount

of iron (always present), varying carbon and oxygen, low phosphorus. One cubic

crystal was quite high in Si and Ti, only.

As U has one of the highest backscatter coefficients, perhaps the Ti, Fe, Al is

coming from stray x-rays generated inside the SEM chamber by BSE's. If not, then

my results indicate the basic composition of the contaminant crystals. Any idea

what they might be?

I should now take UA crystals out of the bottle, measure their spectrum, to

compare with the "contaminants" spectra measured above to sort this out.

But thanks to the above solubility data, we can at least make saturated solution

of UA, and ignore or filter out the contaminant crystals.

Gib Ahlstrand

Electron Optical Facility, University of Minnesota, Dept. Plant Pathology

495 Borlaug Hall, St. Paul, MN. USA. 55108 (612)625-8249

612-625-9728 FAX, giba@puccini.crl.umn.edu

We make up a saturated stock bottle which we draw from, and replenish

with UA and water (8-10g UA/100 ml) from time to time. The insoluble

material is described in the Merck Index as being due to insoluble basic

salts. It describes Uranyl Acetate as being "freely soluble in water

acidulated with acetic acid" For years, we have followed a modification of

a procedure from Millonig's 1976 book Laboratory Manual of Biological

Electron Microscopy (pg 53) and added a few drops of acetic acid per 100mls

of stock saturated UA (stored in a brown bottle). This seems to push the

ppt reaction the other way and give a clear solution. There seems to be

little difference in staining as long as only a few drops of acetic acid

are used. Changing the pH of the stain by much, is risky though as there

are numerous papers and procedures which modify the effects of UA stain by

doing so. We have raised the pH to the 4.5-5.5 (any higher and the UA will

ppt) and gotten improved staining but with unacceptable amounts of ppt on

the sections.

When compared with the other chemicals in the EM lab, UA would seem to be

relatively safe when used carefully. Ingestion and inhalation (exposure to

dust) are our major concern due to heavy metal toxicity as well as the

radiation hazards. Making sure that surfaces are not contaminated, and

cleaning any spillage immediately from bottles and tables before it dries

are important steps. Wearing gloves, and hand washing after glove removal

are also important safeguards.

The radiation exposure hazard under most operating conditions seems

minimal. The least exposure possible is desirable (ALARA), when you don't

need to handle it, don't be near it. Using Bill's number's, you would

still be well under the limits for occupational exposure if you were in

constant contact with .6 millirem/hr for a 2000 hr work year, (correct me,

but my references place the limits at 1.25 rem/quarter, 5 rem/year whole

body and 18.75 rem/quarter, 75 rem/year for extremities (Rayburn)) The

other factor to keep in mind is that we are not talking about a whole body

exposure, but just exposure to the hands. All in all, the amount of

exposure while making up and staining grids seems miniscule.

As an aside, the pretty flowered dinnerware from the 50's, the vivid

oranges and yellows are from uranium. If you have any, run a Geiger

counter over them, you'll be surprised the number of counts. Also the

mantles from gas and propane lanterns contain radioactive thorium. In the

past health physicist have suggested using them(sealed in their bags) for

check sources for counters.

Regardless, because of the toxicity, radiation hazard, as well as expenses

to purchase(well over $1.00/gm) and dispose of UA, minimizing the amount

needed to be discarded and wasted seems desirable. To the extents

possible, use of minimal amounts, and if considerable staining is done,

making stock saturated solutions which can be diluted to the desired

concentration as needed, are good ways to conserve UA, minimize radiation

exposure, and inhalation and ingestion hazards.

Now, if we are starting a poll for the chemicals in the EM Lab that make

us the most anxious, my vote is for cacodylate.

David Bentley

dlb@u.Arizona.EDU

can be exposed to a greater risk. The limit for the general population is

0.5 rem/year whole body, and I think this limit also applies to women who

are or may be pregnant. I do not know the status of graduate students;

I'd be inclined to err on the side of caution--especially since it is fairly

easy to keep exposure to UA low.

Yours,

Bill Tivol

tivol@wadsworth.org