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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