7500-Cs RADIOACTIVE CESIUM*
7500-Cs A. Introduction
Radioactive cesium has been considered one of the more
hazardous radioactive nuclides produced in nuclear fission. Upon
ingestion, like potassium, cesium distributes itself throughout the
soft tissue and has a relatively short residence time in the body.
Half-lives of
134
Cs and
137
Cs are 2 and 30 years, respectively,
both being beta- and gamma-emitters.
7500-Cs B. Precipitation Method
1.
General Discussion
a. Principle: If the activity of cesium is high, radioactive
cesium can be determined directly by gamma-counting a large
liquid sample (4 L) or the sample can be evaporated to dryness
and counted. For lower-level environmental samples, add cesium
carrier to an acidified sample and collect the cesium as phospho-
molybdate. This is purified and precipitated as Cs
2
PtCl
6
for
counting. If total radiocesium determined by beta-counting ex-
ceeds 30 pCi/L, determine
134
Cs and
137
Cs by gamma spectrometry.
The quality control practices considered to be an integral part
of each method can be found in Section 7020.
2.
Apparatus
a. Magnetic stirrer with TFE-coated magnet bar.
b. Centrifuge, bench-size clinical, and centrifuge tubes.
c. Filter papers* and glass fiber filter, 2.4 cm diam.
d. pH paper, wide range, 1 to 11 pH.
e. Filtering apparatus: See Section 7500-Sr.B.2c.
f. Counting instruments: Use either a low-background beta
counter (see Section 7030B.1) or a gamma spectrometer (see
Section 7030B.5).
3.
Reagents
a. Ammonium phosphomolybdate reagent, H
12
Mo
12
N
3
O
40
P:
Dissolve 100 g molybdic acid (85% MoO
3
) in a mixture of
240 mL distilled water and 140 mL conc ammonium hydroxide
(NH
4
OH). When solution is complete, filter and add 60 mL conc
nitric acid (HNO
3
). Separately mix 400 mL conc HNO
3
and
960 mL distilled water. After both solutions cool to room tem-
perature, add, with constant stirring, the (NH
4
)
6
Mo
7
O
24
solution
to the HNO
3
solution. Let stand for 24 h. Filter† and discard
insoluble material.
Collect filtrate in a 3-L beaker and heat to 50 to 55°C (never
above 55°C). Remove from heating unit. Add 25 g sodium
dihydrogen phosphate (NaH
2
PO
4
) dissolved in 100 mL distilled
water, stir occasionally for 15 min, and let settle (approximately
30 min). Filter and wash precipitate with 1% potassium nitrate
(KNO
3
) and finally with distilled water. Dry precipitate and
paper at 100°C for 3 to 4 h. Transfer solid (NH
4
)
3
PMo
12
O
40
to
a weighing bottle and store in a desiccator.
b. Chloroplatinic acid, 0.1M: Dissolve 51.8 g H
2
PtCl
6
䡠6H
2
O
in distilled water and dilute to 1000 mL.
c. Cesium carrier: Dissolve 1.267 g cesium chloride (CsCl) in
distilled water and dilute to 100 mL; 1 mL ⫽10 mg Cs.
d. Calcium chloride, 3M: Dissolve 330 g CaCl
2
in distilled
water and dilute to 1000 mL.
e. Ethanol, 95%.
f. Hydrochloric acid, HCl, conc, 6N,1N.
g. Sodium hydroxide, NaOH, 6N.
4.
Procedure
a. To a 1-L sample, add 1.0 mL cesium carrier and enough
conc HCl to make the solution about 0.1NHCl (about 8.6 mL).
Slowly add1g(NH
4
)
3
PMo
12
O
40
and stir for 30 min using a
magnetic stirrer at 800 rpm. Let precipitate settle for at least 4 h
and discard supernatant by decanting or using suction (provided
by an inverted glass funnel connected to a vacuum source).
Using a stream of 1NHCl, quantitatively transfer precipitate to
a centrifuge tube. Centrifuge and discard supernatant. Wash
precipitate with 20 mL 1NHCl and discard wash solution.
b. Dissolve precipitate by dropwise addition of 3 to 5 mL 6N
NaOH. Heat over a flame for several minutes to remove ammo-
nium ions. (Moist pH paper turns green as long as NH
3
vapors
are evolved.) Dilute to 20 mL with distilled water. Add 10 mL
3MCaCl
2
and adjust to pH 7 with 6NHCl to precipitate
CaMoO
4
. Stir, centrifuge, and filter‡ supernatant into a 50-mL
centrifuge tube. Wash precipitate remaining in the original cen-
trifuge tube with 10 mL distilled water, filter through the same
filter paper, and combine the wash with filtrate. Discard precip-
itate and filter paper.
c. Add 2 mL 0.1MH
2
PtCl
6
and 5 mL ethanol. Cool and stir in
ice bath for 10 min. Using distilled water transfer to a tared
* Approved by Standard Methods Committee, 2000. Editorial revisions, 2011.
* Whatman No. 41, 9 cm diam; Whatman No. 42, 2.4 cm diam; or equivalent.
† Whatman No. 42 filter paper or equivalent. ‡ Whatman No. 41 filter paper or equivalent.
1
glass-fiber filter. Wash with successive portions of distilled wa-
ter, 1NHCl, and ethanol.
d. Dry at 110°C for 30 min, cool, weigh, mount on a nylon
disk and ring with polyester plastic§ cover, and beta-count or
gamma-scan for
134
Cs and
137
Cs.
5.
Calculation
Calculate the concentration of radiocesium as follows:
Cs, pCi/L ⫽C
2.22 ⫻EVR
where:
C⫽net count rate, cpm,
E⫽counter efficiency,
V⫽volume of sample, L, and
R⫽fractional chemical yield
⫽recovered Cs
2
PtCl
6
,mg⫻0.3945
added Cs carrier, mg
6. Bibliography
FINSTON, H.L. & M.T. KINSLEY. 1961. The Radiochemistry of Cesium.
Rep. NAS-NS-3035, U.S. Atomic Energy Comm.
KRIEGER, H.L. 1976. Interim Radiochemical Methodology for Drinking
Water. EPA-600/4-75-008 (revised), U.S. Environmental Protec-
tion Agency, Environmental Monitoring and Support Lab., Cincin-
nati, Ohio.
§ Mylar or equivalent.
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