Determination of 

Strontium-90 in Water

GEOLOGICAL SURVEY WATER-SUPPLY PAPER 1696-E

Library Copy 

Quality of Water 

Trenton Office
Determination of 

Strontium-90 in Water

V J. 0. JOHNSON and K. W. EDWARDS

IADIOCHEMICAL ANALYSIS OF WATER

GEOLOGICAL SURVEY WATER-SUPPLY PAPER 1696-E

d description of a method for the 

letermination of Strontium-90 in 

waters free of strontium-89, or for 

letermination of gross radio strontium 

activity as strontium-90

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1967
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY 

William T. Pecora, Director

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402 - Price 15 cents (paper cover)
CONTENTS

Page

Abstract.__--_-_____---___-_-___-_-______-__-_-___-_--_---------- El

Introduction. _____________________________________________________ 1

Reagents. _____-__-_--__-_____-__-______-_________-_____-_-___---- 2

Special apparatus________________________________________________ 3

Preparation of strontium-90 reference standards.____________----_____- 3

Procedure.-----_-_---_--_--__-____-___--__---_-----------_----__- 4

Calculation of results______________________________-____-__-------- 6

Accuracy.________________________________________________________ 8

Discussion.._ _____________________________________________________ 8

References__ _--____________-__-__-_____-___________-_.___-_--_-- 10

ILLUSTRATIONS

Page 

FIGURE 1. Strontium-90 analytical data form_________________________ E6

2. Graph showing counting efficiencies of various radiostrontium

mixtures at 100-hour intervals.____________-______.-.-._ 9

in

RADIOCHEMICAL ANALYSIS OF WATER

DETERMINATION OF STRONTIUM-90 IN WATER

By J. O. JOHNSON and K. W. EDWARDS

ABSTBACT

A method is described for the determination of strontium-90 in waters free of 

strontium-89, or for determination of gross radiostrontium activity as strontium90. The method is suitable for use on natural water samples and other aqueous 

systems in the absence of high concentrations of organic substances which form 

stable complexes with strontium. The minimum detection limit at the 95-percent 

confidence level is 0.55 picocuries per liter for the recommended 500-milliliter 

sample.

INTRODUCTION

Nuclear fission produces two important isotopes of strontium: 

strontium-90, with a half life of 27.7 years, and strontium-89, with a 

half life of 50.5 days. Although strontium-89 is formed in greater 

quantities, strontium-90 is much more important in terms of worldwide distribution because of its longer half life. Except for short 

periods following atmospheric nuclear testing, strontium-90 is the 

predominant radioisotope of this element on the earth's surface. 

This nuclide is now widely distributed in man's environment from 

stratospheric and tropospheric fallout and from disposal of industrial 

waste. It is often present in detectable concentrations in soils, foods, 

water, and biological materials.

The method described in this paper is based on the work of Hahn 

and Straub (1955). Dissolved radiostrontium is first concentrated by 

precipitation as the carbonate, a small amount of stable strontium 

being used as an isotopic carrier. The carbonate precipitate is dissolved in a small amount of acid, and strontium nitrate is precipitated 

by the addition of fuming nitric acid. The nitrate precipitate is 

washed with anhydrous acetone to remove calcium. The remaining 

residue is dissolved in distilled water, the solution is boiled to remove 

acetone, and strontium nitrate is again precipitated with fuming 

nitric acid and washed with acetone.

El
E2 RADIOCHEMICAL ANALYSIS OF WATER

Iron and barium carriers are added and a hydroxide precipitation is 

made. The ferric hydroxide is a suitable carrier for several fissionproduct activities including those of the rare earths, zirconium, 

niobium, and yttrium. The supernatant solution contains strontium90, free of daughter activity; the growth of yttrium-90 is measured 

from the time of this precipitation. Addition of sodium chromate to 

the supernate precipitates the barium, which carries with it any 

barium-140 and isotopes of radium and lead.

Strontium is finally precipitated as the oxalate, and the amount 

of radiostrontium is determined by measuring the beta activity 

after a 21-day aging period. The chemical recovery is determined 

gravimetrically.

The method described is applicable to the determination of strontium-90 in aqueous solutions free of strontium-89 and organics such 

as ethylenediamine tetraacetic acid (EDTA) which form strong complexes with strontium. The method may also be used for total 

strontium radioactivity (Sr89 -fSr90) reported as strontium-90. In 

the latter case, approximately 45 percent of the strontium-89 activity 

is reported as strontium-90.

REAGENTS

Reagent grade chemicals should be used throughout, unless otherwise specified.

Ammonium hydroxide: Concentrated and 6 M (molar solution).

Sodium carbonate: 1 M and 0.1 M.

Nitric acid: Fuming, concentrated, 6 M, and 1 M.

Acetic acid: 6 M.

Ethanol: 95 percent.

Diethyl ether.

Acetone: Anhydrous.

Sodium chromate: 1.5 M.

Ammonium acetate: 6 M.

Ammonium oxalate: Saturated solution.

Phenolphthalein: Indicator solution.

Thvmolphthalein: Indicator solution.

Aluminum foil: 3.5 milligrams per square centimeter or less.

Iron carrier solution (5 mg Fe+3 per milliliter): Dissolve 500 mg of 

standard iron wire in a slight excess of nitric acid and dilute to 100 ml.

Barium carrier solution (3.00 mg Ba+2 per ml): Dissolve 1.33 grams 

of BaCl2 -2H2O in distilled water. Add a few drops of concentrated 

nitric acid and dilute to 250 ml.

Strontium carrier solution (4.00 mg Sr+2 per ml): Dry "anhydrous" 

strontium nitrate overnight at 105° C and cool in a desiccator. Weigh 

out 9.66 g of the dried salt and dissolve in distilled water. Add 2 ml of
DETERMINATION OF STRONTIUM-90 IN WATER E3

concentrated nitric acid and dilute to 1 liter. A more exacting procedure is described by Glendenin (1951, p. 1461), but the added work 

is generally not warranted unless the strontium nitrate contains radioactive impurities which contribute significantly to the reagent blank. 

Strontium-90 standard solutions: Strontium-90 standard solutions 

calibrated to ±1.5 percent are commercially available (Sommerville, 

1963, p. 195). In purchasing standards, it is essential that the concentration of stable isotopic carrier be known. Dilute the standard to 

approximately 50 pc per ml (picocuries per ml) using appropriate 

safety precautions (see Barker and Robinson, 1963, p. A28). It will 

generally be necessary to add both acid and inactive strontium carrier 

at the time of dilution. The final solution should be approximately 

0.1 jV (normal solution) in hydrogen ion (HCL or HNO3) and should 

have a chemical strontium carrier concentration of 0.1 to 10 mg per 1. 

Allowance must be made for the decay of strontium-90 with time. 

The strontium-90 activity, A, at any time, t, expressed in years, is 

A=A0e~°-002502t where A0 is the known activity when time equals zero. 

The decay amounts to approximately 2.5 percent per year.

SPECIAL APPARATUS

Hot plate.

Filtration apparatus for 25-millinieter filters (available from Millipore 

Filter Corp., Bedford, Mass.).

Ring and disk mounting assemblies, 1-inch diameter (available from 

Atomic Accessories Inc., Valley Stream, N.Y.).

Rigid copper backing disks for filters, 0.875-inch diameter by 0.016- 

inch thickness.

Analytical balance sensitive to ±0.1 mg.

Low-background beta counter having instrumental beta background 

of less than 2.0 cpm (counts per minute).

PREPARATION OF STRONTIUM-90 REFERENCE 

STANDARDS

Three or more permanent strontium-90-yttrium-90 standards 

should be prepared by direct precipitation of strontium oxalate. To 

30 ml distilled water in a 100-ml beaker add 1 ml of 6 M acetic acid 

and 2 ml of 6 M ammonium acetate. Mix thoroughly and add 5 ml 

of strontium carrier (4 mg Sr per ml) and 5 ml of strontium-90 

standard solution (approx 50 pc per ml). Stir the solution thoroughly 

and follow steps 13-15 of the procedure. Store the standards in a 

clean, dry place and count periodically on the low-background beta 

counter to determine the counting efficiency. The standards should 

be aged for 21 days before use.

New reference standards should be prepared whenever a new batch
E4 RADIOCHEMICAL ANALYSIS OF WATER

of aluminum foil is used, and the foil itself should be checked for 

radioactive contamination. The standards must also be corrected 

for radioactive decay; strontium-90 has a half life of 27.7 years, and 

its activity decreases by roughly 2.5 percent per year. It is generally 

advisable to prepare new standards at least once a year.

PROCEDURE

Two blanks should be run with each set of samples to check for 

contamination of reagents and to permit an accurate blank correction 

to be made. Occasional standards and "spiked" samples (samples 

containing a known amount of added standard) should also be run 

through the entire procedure as checks, but need not be run routinely 

unless an estimate of strontium-90 is desired in less than 21 days.

1. To 500 ml of clear water sample in an 800 ml beaker, add 20 mg 

of strontium carrier (5 ml of 4 mg Sr+2 per ml). Place on a hot 

plate and heat to boiling. Add a few drops of thymolphthalein 

indicator. Make the solution basic to the indicator (blue color) 

by dropwise additions of concentrated ammonium hydroxide; 

add an additional 6 ml of ammonium hydroxide.

2. Add 15 ml of 1.0 M sodium carbonate. Stir thoroughly, cover 

the beaker with a watchglass, and digest on a steam bath for 

1 hour. Add more ammonium hydroxide, if required, to maintain the blue color of thymolphthalein. (Add more indicator 

if the color fades.)

3. Remove the beaker from the steam bath and allow the precipitate 

to settle while the solution cools to room temperature.

4. Carefully decant or draw off as much as possible of the supernatant 

solution without disturbing the precipitate. Transfer the insoluble material to a 50-ml graduated centrifuge tube and police 

the beaker with 0.1 M sodium carbonate. Centrifuge and 

discard the supernatant liquid.

5. Cautiously add 1 N nitric acid dropwise to the centrifuge tube 

until the carbonate precipitate is completely dissolved. Bring 

the volume up to 5 ml with distilled water. (Note: The solids 

in the centrifuge tube may not completely dissolve because of 

the presence of silica or other insoluble matter.) Add 25 ml of 

fuming nitric acid. (Caution! Use rubber gloves and face 

shield, and work in a hood. Do not inhale vapors.) Cool, stir, 

and centrifuge.

6. Using the above safety precautions, pour off the nitric acid as 

completely as possible, cool the tube below room temperature 

in a cold water or ice bath, and cautiously add 25 ml of anhydrous acetone to the residue. Stir thoroughly and centrifuge, 

again discarding the supernate.
DETERMINATION OF STRONTIUM-90 IN WATER E5

7. Repeat step 6.

8. Dissolve the nitrates in 5 ml of distilled water and place the tube 

in a boiling water bath until the odor of acetone is gone. 

(Caution! Be sure acetone is completely removed.)

9. Cool the tube below room temperature and again precipitate 

strontium nitrate by adding 25 ml of fuming nitric acid. Swirl, 

cool, and centrifuge. Discard the supernate.

10. Dissolve the precipitate in 10 ml of distilled water and add 5 mg 

of ferric carrier (1 ml of 5 mg Fe+3 per ml) and 15 mg of barium 

carrier (5 ml of 3 mg Ba+2 per ml). While stirring, add concentrated ammonium hydroxide dropwise until ferric hydroxide 

begins to precipitate, then add several drops excess. Centrifuge 

and decant the supernatant liquid into a clean centrifuge tube. 

Discard the ferric hydroxide precipitate (which contains the 

yttrium daughter of Sr90). Note the time and date.

11. Add a drop or two of phenolphthalein indicator solution, followed 

by dropwise addition of 6 N nitric acid until the pink color 

disappears; then add 1 ml of 6 M acetic acid and 2 ml of 6 M 

ammonium acetate. Place the sample tubes in a boiling water 

bath and add, while agitating the sample, 1 ml of 1.5 M sodium 

chromate. Agitate for 1 minute and continue heating for 5-10 

minutes.

12. Cool the samples to room temperature, centrifuge, and decant the 

supernate into a 100-ml beaker. Discard the precipitate.

13. Add 2 ml of concentrated ammonium hydroxide and heat to 

boiling. Then add 5 ml of saturated ammonium oxalate solution and stir. Allow the precipitate to settle and the solution 

to cool to room temperature.

14. Filter through a tared 25-mm filter paper (Whatman No. 42) 

supported on the sintered glass disk of a microanalytical 

filtration assembly. Wash the precipitate with three 5-ml 

portions of distilled water and then with small volumes of 95 

percent ethanol and diethyl ether. Dry at room temperature 

in a desiccator. Weigh as strontium oxalate monohydrate 

(SrC2O4 -H2O) and determine the gravimetric yield.

15. Put the filter on a copper disk, 0.875-inch diameter, in the base of 

a ring and disk assembly, 1-inch diameter. Cover with a small 

piece of aluminum foil (3.5 mg per sq cm or less) and secure 

the ring in place. Store the sample for at least 21 days to 

allow yttrium-90 to reach more than 99.5 percent of its equilibrium activity.

16. Count the sample for 100 minutes on a low-background beta 

counter and compute the strontium-90 concentration of the 

sample.
E6 RADIOCHEMICAL ANALYSIS OF WATER 

CALCULATION OF RESULTS

A suggested data sheet for tabulation and calculation of results is 

shown in figure 1. Although this form is intended for use when 

strontium-89 is absent from the sample, it may be easily modified to 

report results of gross radiostrontium (Sr90+Sr89) as strontium-90.

Strontium-90 

Analytical Data Form

Sample Ho._______________ 

Pretreatment___________________________

Sample vol., V_____ml (Sr) in sample______ppm Sr in sample, FS______mg

Sr carrier added, Wc_________mg

Total Sr, WQ+ Ws _______mg

Sr-Y separation: Date________ Time________

C ount ing: Dat e________T ime________

Net growth time of Yyu ________days 

Recovery:

Weight of filter paper: Weight of filter paper + 

Gross _________ Gross__________ 

Tare Tare

Net W, Net

Theoretical weight of SrCO H0 _______ = W = 2.210 (W + Wg )

Recovery (g)

2 1

Counting efficiency (e) of Sr - Y' standards e = ______cpm per pC Sr'

00 Overall efficiency (E) = e g E = _______cpm per pC Sr

Counting data;

Observed count rate, R, for counting

time of ______min. R = _________cpm

Blank B = _________cpm

-** Net count rate, A, of sample = R-B A = _________cpm

Concentration of Sryu in sample = i^ . | =_______ + _________pc/1

Date _______ Checked by _______ Date_

FIGURE 1. Strontium-90 analytical data form.
DETERMINATION OF STRONTIUM- 90 IN WATER E7

The concentration, in picocuries per liter, of strontium-90 in the 

sample is given by the equation:

where

V= volume of sample in milliliters, 

A net counting rate,

e= effective counting efficiency from strontium-90, and 

g= chemical recovery.

The counting efficiency, e, is the number of counts per minute (of 

the aged precipitate) per picocurie of strontium-90 in the sample. 

Since both strontium-90 and its daughter, yttrium-90 are counted, 

this is not a true counting efficiency for strontium-90 only, but 

rather an effective counting efficiency which must be used in the 

calculations. It is determined by counting an aged strontium-90 

standard for 100 minutes or longer and dividing the count rate by the 

calculated amount (picocuries) of strontium-90 present in the standard 

after any necessary corrections for decay are made.

The chemical recovery, g, is obtained by dividing the experimental 

weight of strontium oxalate monohydrate by the theoretical weight 

calculated for complete recovery. The latter must take into account 

both the amount of strontium carrier added and the weight of strontium initially present in the water sample aliquot. Since stable 

strontium is frequently found in natural water, a preliminary analysis 

for this element should be made. Ws, the weight in milligrams of 

strontium in the sample, may be calculated from the equation:

where

Sr= concentration, in parts per million, of strontium in the 

sample, and

V= sample volume in milliliters. 

The recovery is then calculated from the equation:

W2 -Wl

y 2.2lQ(We+W.)

where

(Wz Wi) = experimental weight of strontium oxalate found, and 

Wc=weight of strontium carrier added.
E8 RADIOCHEMICAL ANALYSIS OF WATER

ACCURACY

The accuracy of this method for low levels of strontium-90 in 

water free of strontium-89 is normally limited by counting statistics. 

Another major source of error is in the determination of the gravimetric yield of strontium oxalate. Error in the standards is normally 

small «3 percent) but should not be neglected. A small error may 

also be caused by uncertainty in the chemical concentration of 

dissolved strontium in the sample.

The minimum detection limit (MDL) obtainable by using the 

equipment available at the Denver radiochemical laboratory (U.S. 

Geological Survey's Water Eesources Division) is 0.55 pc per 1. This 

limit is based on a counting efficiency of 1.8 cpm per pc of strontium-90 

and a statistical blank (background plus reagent blank) of 2.0 cpm. 

Variations is these quantities will increase or decrease the MDL. 

Experimental results, whether less than or greater than the MDL, 

should be reported as obtained.

The accuracy of the method for concentrations of strontium-90 greater 

than about 1 pc per 1 is limited by the accuracy of standards and 

chemical recovery, as well as by counting statistics. On the basis of 

the work of Hahn and Straub (1955) and experience in the Denver 

radiochemical laboratory, the accuracy of the method is taken to be 

± 10 percent for concentrations greater than 5 pc per 1. For weaker 

concentrations, counting statistics exceed 10-percent error, and 

results should be reported as ±0.55 pc per 1. It is desirable for each 

laboratory to run random samples with and without added strontium90 "spikes," so that an experimental evaluation of the error at concentrations in excess of the MDL can be obtained.

Interference due to other fission products (with the exception of 

Sr89) is negligible within the accuracy of the method (Glendenin, 

1951). Natural sources of radioactivity should also cause negligible 

error. Because of the significant fission yield of strontium-89, 

interference due to this nuclide can be expected at times of recent 

atmospheric nuclear testing and in areas of underground nuclear 

testing. The presence of this nuclide will cause results to be high 

by approximately 45 percent of the strontium-89 concentration 

when the recommended 21-day aging period is used. If the method 

is used under conditions where such interference may occur, results 

should be reported as the strontium-90 equivalent of the gross radiostrontium activity.

DISCUSSION

This method is recommended for use on clear water samples free 

of suspended material. Other samples should be clarified by suitable 

means before the analysis is performed.
DETERMINATION OF STRONTIUM-90 IN WATER E9

A rough estimate of the amount of strontium-89, when present, may 

be obtained by plotting a growth-decay curve of the activity in the 

final precipitate. The curve is then compared with figure 2. Sample 

counts made in the first 250 hours are most useful because the beta 

activity of yttrium-90 changes most rapidly in that interval. A 

similar graph must be prepared for a counter that has efficiencies 

differing from those shown in figure 2.

100 200 300 400 500 

ELAPSED TIME IN HOURS FROM STRONTIUM PURIFICATION

Explanation

A=fraction of strontium-90 activity 

.B=fraction of Strontium-89 activity

The following counting efficiencies were assumed:

Strontium-90=0.26

Yttrium-90= .58

Strontium-89 = .51

Counting efficiencies were obtained on a low-background beta counter. 

The radiostrontium was carried with 36 mg of strontium oxalate.

FIGURE 2. Counting efficiencies of various radiostrontium mixtures at 100-hour intervals.

600
E10 RADIOCHEMICAL ANALYSIS OF WATER

If time is of no consequence, strontium-89 may be estimated by 

counting the sample at the end of the 21-day aging period and again 

after a total aging time of 51 days. After 21 days, any change in 

activity will be due almost entirely to strontium-89 decay; the activity of this nuclide is reduced by one-third for each 30 days of decay. 

(The activity of Sr89 after 51 days is therefore two-thirds of the Sr89 

activity at the end of 21 days.)

It should be recognized that the statistical uncertainties in strontium-89 and strontium-90 activities may be considerable when these 

quantities are determined from decay data on low-activity samples. 

A preferable method for strontium-90 in the presence of strontium-89 

involves the separation of yttrium-90 and measurement of its activity. 

This procedure has been used by many investigators and will be 

desciibed in a separate report.

REFERENCES

Barker, F. B., and Robinson, B. P., 1963, Determination of beta activity in 

water: U.S. Geol. Survey Water-Supply Paper 1966-A, 32 p.

Glendenin, L. E., 1951, Determination of strontium and barium activities in 

fission, in Coryell, C. D., and Sugarman, N., eds., Radiochemical studies: 

The fission products: New York, McGraw-Hill, Natl. Nuclear Energy 

Ser., div. 4, v. 9, book 3, paper 236, p. 1460-1469.

Hahn, B,. Z., and Straub, C. P., 1955, Determination of radioactive strontium 

and barium in water: Am. Water Works Assoc. Jour., v. 47, p. 335-340.

Sommerville, J. L., ed., 1963, The isotope index 1963-1964: Scientific Equipment Co., Indianapolis, 296 p.

o