3500-Sr STRONTIUM*
3500-Sr A. Introduction
1.
Occurrence and Significance
Strontium (Sr) is the fourth element in Group IIA of the periodic
table; it has an atomic number of 38, an atomic weight of 87.62, and a
valence of 2. The average abundance of Sr in the earth’s crust is 384
ppm; in soils Sr ranges from 3.6 to 160 ppm; in streams it averages 50
g/L, and in groundwaters it ranges from 0.01 to 10 mg/L. Strontium
is found chiefly in celestite (SrSO
4
) and in strontianite (SrCO
3
). Stron-
tium compounds are used in pigments, pyrotechnics, ceramics, and
flares.
90
Sr is a fission product of nuclear reactor fuels, and was widely
distributed on the earth’s surface as a result of fallout from nuclear
weapons testing.
The common aqueous species is Sr
2⫹
. The solubility of
strontium is controlled by carbonate and sulfate. Some com-
pounds are toxic by ingestion and inhalation. Although there
is no U.S. EPA drinking water standard MCL for concentra-
tion of strontium, strontium-90 measurements are required
when the gross beta activity of a water sample is greater than
50 pCi/L. The U.S. EPA primary drinking water standard
MCL for
90
Sr is 8 pCi/L.
A method for determination of
90
Sr is found in Section 7500-Sr.
2.
Selection of Method
The atomic absorption spectrometric method (3111B) and
inductively coupled plasma methods (3120 and 3125) are pre-
ferred. The flame emission photometric method (B) also is
available for those laboratories that do not have the equipment
needed for one of the preferred methods.
3.
Sampling and Storage
Polyethylene bottles are preferable for sample storage, al-
though borosilicate glass containers also may be used. At time of
collection adjust sample to pH ⬍2 with nitric acid (HNO
3
).
3500-Sr B. Flame Emission Photometric Method
1.
General Discussion
a. Principle: The flame photometric method can be used for the
determination of strontium in the concentration range prevalent in
natural waters. The strontium emission is measured at a wavelength
of 460.7 nm, while the background intensity is measured at a
wavelength of 466 nm. The difference in readings obtained at these
two wavelengths measures the light intensity emitted by strontium.
b. Interference: Emission intensity is a linear function of
strontium concentration and concentration of other constituents.
The standard addition technique distributes the same ions
throughout the standards and the sample, thereby equalizing the
radiation effect of possible interfering substances. A very low pH
(⬍1) could produce an interference, but sample dilution should
eliminate this interference.
c. Minimum detectable concentration: Strontium levels of
about 0.2 mg/L can be detected by the flame photometric method
without prior sample concentration.
2.
Apparatus
Spectrophotometer, equipped with photomultiplier tube and
flame accessories; or an atomic absorption spectrophotometer
capable of operation in flame emission mode. Use of a fuel-rich
nitrous oxide-acetylene flame is recommended.
3.
Reagents
a. Stock strontium solution: Dissolve 2.415 g strontium ni-
trate, Sr(NO
3
)
2
, dried to constant weight at 140°C, in 1000 mL
1% (v/v) HNO
3
; 1.00 mL ⫽1.00 mg Sr.
b. Standard strontium solution: Dilute 25.00 mL stock stron-
tium solution to 1000 mL with water; 1.00 mL ⫽25.0
g Sr.
Use this solution for preparing Sr standards in the 0.2- to
25-mg/L range.
c. Nitric acid, HNO
3
, conc.
4.
Procedure
a. Pretreatment of polluted water and wastewater samples:
Select an appropriate procedure from Section 3030.
b. Preparation of strontium standards: Dilute samples, if
necessary, to contain less than 400 mg Ca or Ba/L and less than
40 mg Sr/L. Add 25.0 mL sample (or a lesser but consistent
volume to keep all standards in the linear range of the instru-
ment) to 25.0 mL of each of a series of four or more strontium
standards containing from 0 mg/L to a concentration exceeding
that of the sample. For most natural waters 0, 2.0, 5.0, and 10.0
mg Sr/L standards are sufficient. A broader range curve might be
preferable for brines. Dilute the brine sufficiently to eliminate
burner splatter and clogging.
c. Concentration of low-level strontium samples: Concentrate
samples containing less than 2 mg Sr/L. Polluted water or
* Approved by Standard Methods Committee, 2004. Editorial revisions, 2011.
Joint Task Group: 20th Edition—Brian J. Condike (chair), Deanna K. Anderson,
Anthony Bright, Richard A. Cahill, Alois F. Clary, C. Ellen Gonter, Peter M.
Grohse, Daniel C. Hillman, Albert C. Holler, Amy Hughes, J. Charles Jennett,
Roger A. Minear, Marlene O. Moore, Gregg L. Oelker, S. Kusum Perera, James
G. Poff, Jeffrey G. Skousen, Michael D. Wichman, John L. Wuepper.
1
wastewater samples can be concentrated during digestion by
starting with a larger volume (see Section 3030D). For other
samples, add 3 to 5 drops conc HNO
3
to 250 mL sample and
evaporate to about 25 mL. Cool and make up to 50.0 mL with
distilled water. Proceed as in ¶ b. The HNO
3
concentration in the
sample prepared for atomization can approach 0.4 mL/50 mL
without producing interference.
d. Flame photometric measurement: Measure emission inten-
sity of prepared samples (standards plus sample) at wavelengths
of 460.7 and 466 nm. Follow manufacturer’s instructions for
correct instrument operation. Use a fuel-rich nitrous oxide-acet-
ylene flame, if possible.
5.
Calculation
a. Using a calculator or computer with linear regression ca-
pability, enter the net intensity (reading at 460.7 nm minus
reading at 466 nm) versus concentration added to the sample and
solve the equation for zero emissions. The negative of this
number multiplied by any dilution factor is the sample concen-
tration.
b. Plot net intensity (reading at 460.7 nm minus reading at
466 nm) against strontium concentration added to the sample.
Because the plot forms a straight calibration line that intersects
the ordinate, strontium concentration can be calculated from the
equation:
mg Sr/L ⫽A⫺B
C
⫻D
E
where:
A⫽sample emission-intensity reading of sample plus 0 mg/L at
460.7 nm,
B⫽background radiation reading at 466 nm, and
C⫽slope of calibration line.
Use the ratio D/E only when EmL of sample are concentrated
to a final volume DmL (typically 50 mL).
c. Graphical method: Strontium concentration also can be
evaluated by the graphical method illustrated in Figure 3500-
Sr:1. Plot net intensity against strontium concentration added
to sample. If the line intersects the ordinate at Yemissions, the
strontium concentration is where the abscissa value of the
point on the calibration line has an ordinate value of 2Y
emissions due to the two-fold dilution with standards (if
sample and standards are mixed in equal volumes). The
calibration line in the example intersects the ordinate at 12.
Thus, Y⫽12 and 2Y⫽24. The strontium concentration of
the sample is the abscissa value of the point on the calibration
line having an ordinate value of 24. In the example, the
strontium concentration is 9.0 mg/L.
d. Report strontium concentrations below 10 mg/L to the nearest
0.1 mg/L and above 10 mg/L to the nearest whole number.
6.
Quality Control
The QC practices considered to be an integral part of each
method can be found in Section 3020. Also see Part 1000.
7.
Precision and Bias
Strontium concentrations in the range 12.0 to 16.0 mg/L can
be determined with an accuracy within ⫾1 to 2 mg/L.
8. Bibliography
CHOW, T.J. & T.G. THOMPSON. 1955. Flame photometric determination
of strontium in sea water. Anal. Chem. 27:18.
NICHOLS, M.S. & D.R. MCNALL. 1957. Strontium content of Wisconsin
municipal waters. J. Amer. Water Works Assoc. 49:1493.
HORR, C.A. 1959. A survey of analytical methods for the determination
of strontium in natural water. U.S. Geol. Surv. Water Supply Pap.
No. 1496A.
Figure 3500-Sr:1. Graphical method of computing strontium concentra-
tion.
STRONTIUM (3500-Sr)/Flame Emission Photometric Method
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