3500-Sr STRONTIUM* 3500-Sr A. Introduction 1. Occurrence and Significance pounds are toxic by ingestion and inhalation. Although there is no U.S. EPA drinking water standard MCL for concentration 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 90Sr is 8 pCi/L. A method for determination of 90Sr is found in Section 7500-Sr. 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 (SrSO4) and in strontianite (SrCO3). Strontium compounds are used in pigments, pyrotechnics, ceramics, and flares. 90Sr 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 Sr2⫹. The solubility of strontium is controlled by carbonate and sulfate. Some com- 2. Selection of Method The atomic absorption spectrometric method (3111B) and inductively coupled plasma methods (3120 and 3125) are preferred. 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. * 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. 3. Sampling and Storage Polyethylene bottles are preferable for sample storage, although borosilicate glass containers also may be used. At time of collection adjust sample to pH ⬍2 with nitric acid (HNO3). 3500-Sr B. Flame Emission Photometric Method 1. General Discussion 3. Reagents a. Stock strontium solution: Dissolve 2.415 g strontium nitrate, Sr(NO3)2, dried to constant weight at 140°C, in 1000 mL 1% (v/v) HNO3; 1.00 mL ⫽ 1.00 mg Sr. b. Standard strontium solution: Dilute 25.00 mL stock strontium 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, HNO3, conc. 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. 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 instrument) 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 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. 1 STRONTIUM (3500-Sr)/Flame Emission Photometric Method 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 HNO3 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 HNO3 concentration in the sample prepared for atomization can approach 0.4 mL/50 mL without producing interference. d. Flame photometric measurement: Measure emission intensity 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-acetylene flame, if possible. 5. Calculation a. Using a calculator or computer with linear regression capability, 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 concentration. 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 ⫽ Figure 3500-Sr:1. Graphical method of computing strontium concentration. 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. A⫺B D ⫻ C E where: 6. Quality Control 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. 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 Use the ratio D/E only when E mL of sample are concentrated to a final volume D mL (typically 50 mL). c. Graphical method: Strontium concentration also can be evaluated by the graphical method illustrated in Figure 3500Sr:1. Plot net intensity against strontium concentration added to sample. If the line intersects the ordinate at Y emissions, 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 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. 2