Determination of Thorium in Drinking Water
Method 910
Richard J. Velten
and
Betty 0. Jacobs
Environmental Monitoring and Support Laboratory
U.S. Environmental Protection Agency
...Cincinnati, Ohio 45268
May 1982
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DETERMINATION OF THORIUM
IN DRINKING WATER
1. Scope and Application
1.1 This method covers the measurements of total alpha and/or beta
emitting Isotopes of thorium.
1.2 Because of Its low specific activity, natural thorium (232) 1s
determined by a colorimetrlc method using arsenazo III reagent.
2. Summary of Method
2 1 An acidified aliquot of the sample 1s heated to boiling. Other
actinide elements after adjustment of the valence state using
sodium nitrite and thorium are copredpitated and concentrated on
ferric hydroxide at a high pH. Thorium 1s purified by selective
solvent extraction techniques using tri-isooctyl amine (TIOA) and
trioctyl phosphine oxide (TOPO). Thorium 1s copredpitated on
aluminum hydroxide to remove sulfate interferences prior to the
color development of the thorium-arsenazo complex. After the
colorimetric assay, the thorium is copredpitated on lanthanum
fluoride for radiometric measurement.
3. Sample Handling and Preservation
3.1 Samples should be analyzed as soon as possible after collection.
3.2 For storage, samples should be acidified with 5 ml of concentrated
hydrochloric acid.
».
4. Interference
4 1 Lanthanum carrier contains Interfering alpha activity to varying
degrees. This activity should be determined and the carrier
rejected 1f the associated alpha activity exceeds 0.2 alpha cpm per
milligram of lanthanum.
. * . » •
4.2 Measurement for the Isotope 234 should be made as soon as Possible
after the colorlmetrlc analysis as beta activity associated in the
decay of thorium 228 will cause high results.
4.3 Sulfates and nitrate reduce the Intensity of the thorium-arsenazo
color and should be removed.
5. Apparatus
5 1 Gas flow proportional counting system, low background beta counting
system, or scintillation detection system.
5.2 Spectrophotometer operating at 665 mu and having a near Infrared
filter (NIR).
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5.3 Hot plates
5.4 Separatory funnels, 60 mL
5.5 Large milllpore glass filtering apparatus and 47 mm 0.45y polyvinyl
chloride (PVC) membrane filters.
5.6 Stainless steel planchets, 2 Inch diameter
5.7 Aluminum foil for absorber material.
5.8 Glassware and drying ovens.
5.9 Centrifuge
5.10 MicrolUer plpet, adjustable 100-1000 yL.
6. Reagents
6.1 Acetone -alcohol solution - Mix equal volume of acetone and 95X
ethanol.
6.2 Aluminum carrier, 5 mg/mL. Dissolve 2.5g aluminum metal in 380 ml
of 6N HC1 in a 500 ml volumetric flask. Cool and dilute to volume
with~water. Mix thoroughly and filter.
6.3 Arsenazo III reagent, 0.02X. Dissolve 20±1 mg arsenazo III reagent
1n 100 mL of water.
6.4 Cyclohexane, reagent grade.
6.5 Ferric chloride, 20 mg/mL. Dissolve 9.6 g of FeCla • 6^0
Into 75 mL of 0.1N HC1 in a 100 mL volumetric flask. Dilute to
volume with 0.™ Tfcl.
6.6 Hydrochloric add, 8N Add 670 mL concentrated HC1 to 300 mL
water, cool and dilute to 1000 mL witn
water.
7H Dilute 350 mL of 8^ HC1 to 400 mL with
~~ water.
6N Dilute 150 mL of 8N HC1 to 200 mL with
water.
AH Dilute 200 mL of 8N HC1 to 400 mL with
%
0.5N. Dilute 25 mL* of 4N. HC1 to 200 mL with
0.1N Dilute 5 mL of 4N HC1 to 200 mL with water.
6.7 Hydrofluoric add, 48X.
6.8 Lanthanum carrier - 5 mg/mL. Dissolve 1.272 LaCl3 • 6H20 in
100 mL of 0.5N HC1.
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6.9 Sodium hydroxide, 6N. Dissolve 120 grains of NaOH 1n 300 ml of
water. Cool and dilute to 500 ml with water.
6.10 Sodium nitrite, IN. Dissolve 6.9g of NaN02 1n 75 ml of water 1n
a 100 ml volumetric flask. Dilute to volume.
6.11 Sulfurlc acid. 0.3M. Add 17 ml of concentrated sulfurlc to 700 mL
water 1n a 1-L volumetric flask and dilute to volume. Mix
thoroughly.
6 12 Standard thorium solution, 10 mg/mL. Dry approximately 5-10 grams
ThCNOO? • 4H?0 1n a drying oven at 107°C for 4 hours.
Remove and cool in a dedicator. Transfer 3-500 mg aliquot* , to
three previously fired, cooled and tared platinum or porcelain
crucibles. Place 1n a muff le furnace and slowly ncrease heat for
1 hour and then to 650°C for 1 hour. Remove, cool 1n a
desiccator and reweigh. Determine the X thorium In the thorium
nitrate. Weigh out sufficient dried Th(N03) • 4H20 equ valent
to 1000 mg thorium and dissolve In 50 ml 0.5N HC1 in a 100 ml
volumetric flask. Dilute to volume and mix thoroughly.
10 yg/mL. Transfer 1.0 ml of the 10 mg/mL standard thorium
solution to 900 mL 4N HC1 in a 1-11 ter volumetric flask, dilute to
volume with 4N HC1 and mix thoroughly.
6.13 Tri-isooctyl amine, 10X. Dilute 50 mL of tri-isooctyl amine with
xylene in a 500 mL volumetric flask.
6 14 THoctyl phosphine oxide, 0.1 M. Dissolve 19. 3g TOPO in 400 mL of
cycohexane in a 500 mL volumetFic flask. Dilute to volume with
cyclohexane.
6.15 Zylerie, reagent grade
7. Calibration
7.1 Thor1um-230. Determine the counter n
tina Th-230 by transferring a known quantity of a Th-230 stand
. to a beake? containing 50 ml of 0.5M HC1 and 1 mL of lanthanum
minutes. Count for sufficient time to collect 10000 counts
A R
Thorium-230 Counting Efficiency, E » - £ -
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where: A • Gross cpm of Th-230 standard
B - Alpha Reagent Blank of Lanthanlum Carrier, cpm
C • dpm of Th-230 standard.
7.2 Thorium-234. Transfer approximately 250 dprn of NBSUranlum-238
standard to a beaker containing 50 ml 0.5M HC1 and 1 mL of
lanthanum carrier and add approximately iWmg ascorbic acid. Warm
to dissolve. Cool and add 0.1 ml of 20X TICl* and mix. Add 2 ml
48X HF with stirring and allow to stand for 20 minutes, stlrr ng
occasionally. Filter through a 47 mm diameter 0.45 W^ricel
filter, wash one time each with 10 mL of water and ethanol. Trans-
fer thi filter to a 2-1nch stainless steel planchet previously
coated with rubber cement and containing 5 drops of 1:1 acetone:
alcohol mixture. Dry at 80<>C for 3-5 minutes. Cover the : fi Her
with a circle of alumlum foil ( 4.5 mg/cmZ) and beta count.
Count for sufficient time to accumulate 10000 counts.
A ft
Thor1um-234 Counting Efficiency, E « - •; -
L
where: A - Gross Th-234 + Pa 234«" cpm
B • Beta Reagent Blank of lanthanum carrier, cpm
C « dpm Uran1um-238 standard
7.3 Thorium-232 (Natural Thorium)
7 3 1 Transfer 0, 100, 400, 700, and 1000 yL of the standard 10
Jg/mL Ihorlum solution to each of five 10 ml volumetric
flasks.
732 Add 1.0 ml of the aluminum carrier solution and 5 ml of 4N
" " HC1, stopper, and shake to mix thoroughly.
7 3.3 To each standard and to each sample from step 8.31, add 1.0
ml of the arsenazo III reagent solution.
7.3.4 Dilute to mark with 4N HC1, stopper, and shake to mix
thoroughly.
7.3.5 Read each sample and standard at 665 mu using a near
Infrared filter and a 2 cm light path cell.
7 3 6 Using the standard sample readings, plot a curve using
absorbancy readings as the ordinate versus the ug of thorium
£ ?he abscissa. oV compute the equation for a straight line
by least squares analysis using the absorbancy as the
dependent variable.
8.0 Procedure
8.1 Acidify a 1-liter sample of tap water with 10 mL of 6N HC1. (If
sample was acidified for storage as in 3.2., omit 8.1.)
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8.2 Add 1 mL of the FeCl3 carrier solution and 10 ml of IN NaNOg
and heat to boiling.
8.3 With continuous stirring, add sufficient 6N NaOH to make alkaline
to pH 10 and continue heating for 5 minutes.
8.4 Remove from heat, cool and settle 1n a cold water bath.
8.5 Filter through a 47 mm 0.45 v membran^e filter, discarding the
filtrate.
8.6 Place a 40 mL cone bottom centrifuge tube In a Fisher filtrator and
attach the filtering apparatus to the filtrator.
8.7 Add 10 ml of 8N HC1 to the filter to dissolve the predpltatate.
8.8 With vacuum, filter the solution Into the centrifuge tube.
8.9 Repeat steps 8.7 and 8.8 one more time.
8.10 Rinse the filtering apparatus twice using 5 ml of 8N HC1,
filtering between each addition.
8.11 Transfer the solution to a 60 ml separatory funnel, rinsing the
tube twice with 5 ml portions of 8Ji HC1.
8.12 Add 20 ml of a 10X tri-1sooctyl amine/xylene solution and shake
for two minutes.
8.13 Allow the two phases to separate for 5 minutes, d™1n};|9 *h* f
bottom layer into a second 60 ml separatory funnel. (Note time of
separation if Th-234 is being determined.)
8.14 Wash the organic phase once with 5 ml of 8N HC1 by shaking for 1
minute.
8.15 Allow the wash to separate and combine it with the acid phase in
the second funnel.
8 16 Add 6 ml of water to the second separatory funnel to reduce the
acid concXtlration to 7N and add 10 mL of 0.1H trioctyl phosphine
oxide/cyclohexane (TOPOT solution.
-i
8 17 Extract for two minutes, allow to separate, and drain the acid
phase into a third 60 mL separatory funnel.
8.18 Add 10 mL of the TOPO solution to the acid phase and extract a
second time for two minutes.
8.19 Allow the phases to separate completely, drain and discard the
acid phase.
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8.20 Combine the second TOPO phase with the first extraction, and wash
the combined phases with 10 mL of 7N HC1 by shaking for 1 minute.
8.21 Allow the phases to separate, drain and discard the wash.
8.22 Add 10 mL of 0.3M H2S04 to the separatory funnel and shake for
two minutes.
8.23 Allow the phases to separate and drain the acid layer (bottom)
Into a 40 mL cone bottom centrifuge tube.
8.24 Repeat steps 8.22 and 8.23 two more times.
8.25 Add 1 mL of the aluminum carrier solution and warm in a hot water
bath.
8.26 While stirring, add concentrated NH4OH dropwise until alkaline.
8.27 Centrifuge for five minutes and discard the supernante.
8.28 Wash the precipitate with 20 mL of water, centrifuge, and discard
the wash.
8 29 Dissolve the precipitate In 2 mL of 4£ HC1 and transfer the
solu?1on to a 10 mL volumetric flask, using a Pasteur transfer
pipet.
8 30 Rinse the centrifuge tube twice with 1 mL portions of 4N HC1
adding the rinsing? to the volumetric flask. (Be careful not to
exceed 8 mL.)
8.31 Save the solution for colorlmetric analysis as 1n 7.3.3 to 7.3.5.
8 32 After the colorlmetric analysis, transfer the solution to a 100 mL
beaker and wash cuvette and volumetric flask thoroughly with
water, adding the washes to the beaker.
8.33 Add 1 mL of the lanthanum carrier and 1 mL of concentrated.HNOa
and heat to destroy the color complex.
8.34 Dilute to 60 mL with water, remove from heat, and add 2 mL of 48X
HF with stirring.
8.35 Allow the precipitate/to develop for ten minutes, stirring
occasionally. '•' '•
8.36 Filter through a 47 mm PVC membranle filter, washing once with
water and once with 95X ethanol.
8 37 Transfer the filter to a stainless steel planchet previously
coated with rubber cement and containing 3-5 drops of a 1:1
solution of ethanol and acetone.
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8.38 Place 1n a drying oven at 80<>C for 5 minutes to dry.
8.39 Cool and alpha count In an Internal proportional counter or store
. in a desslcator for later counting.
9. Calculations
9.1 Total Alpha Thorium
PCi/1 ' 2.22 EV '
where: S - gross alpha com
B - alpha reagent blank, cpm
E - Thor1um-230 counting efficiency, cpm/dpm
V « volume of sample, liters
9.2 Thorium-234 (Cover filter with aluminum foil)
where: S - gross betan£pj[lank cpm
E = Th-ZaT^Pa 234m counter efficiency, cpm/dpm
« I ^correSn'fep 8.13 to tl. of count
' 0.0012 hr-'
9.3 Thorium-232 (Natural Thorium)
q 3 1 Using the plot determined from 7.3.6, determine the vg
9 tho?ium content of each sample by Interpolation using the
absorbancy reading for each sample.
^i-ss ssTthSi- f-tt"S (-)
the JSIlSMS.! f tl» absorbancy of each s»ple.
e.tr
thorium found by the volume of the sample aliquot In liters.
10. Accuracy and Precision^,
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Thorium-232 content was determined to be J^Jg-W W/J ml and
total alpha thorium was determined to be 11.25*0.52 pCi/bmL.
Eight 1-Hter tap water samples were each spiked with 5.0 ml of the
stock thorium test solution and analyzed by the prescribed method.
The mean thor1um-232 recovery was found to be 4.52+0.3 yg/L for an
average recovery of 94.9% ± 7%.
The mean total alpha thorium recovery was found to be 11.43+1.24
pC1/L for an average recovery of 101.6+12%.
A Student-t test comparing the two thorium recoveries using a
pooled estimate of the standard deviation of the means Indicated
that the means were not different at the 95% confidence level.
The average recovery for the method Is thus estimated to be 98%.
10.2 Precision
Since precision 1s a function of analyte concentration and
Instrument measurement technique, 1t may vary.
Since Instrumental errors for both the spectrophotometrlc and
radlometrlc determination are not related to the concentration by
the same magnitude, their variance cannot be compared and thus they
cannot be pooled.
However. It can be stated that the precision of method for
thorium-232 at the 5 yg/L concentration level Is ±7.5% and that the
precision of method for total alpha thorium at the 11 pCi/L
concentration level 1s +12%.
11. References
11.1 Cospito M. . L. RigaH, -Determination of Thorium in Natural Waters
After Extraction With AHquot-336," Anal. Chem. Acta, 106.
p. 385-388. (1979).
11.2 Hyde, E. K.. "The Radiochemistry of Thorium" NAS-NS 3004. 1960.
11 3 Moore. F. L.. "Liquid-Liquid Extraction with High-Molecular-Weight
Amine" NAS-NS 3101. 1960.
11.4 White. J. C., W.J. Ross, "Separations by Solvent Extraction with
Tri-n-octyl phosphine Oxide" NAS-NS 3102. 1961.
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