Flame Spectrophotometric Determination of Micro Concentrations of Strontium in Calcareous Material T. C. RAINS, H.
E.
ZITTEL, and MARION FERGUSON
Analytical Chemistry Division, Oak Ridge National laboratory, Oak Ridge, Tenn.
A rapid and sensitive flame spectrophotometric method was developed for the determination of strontium in calcareous material. It is satisfactory for determining strontium in the concentration range from 0.1 to 1 pg. per ml. in the presence of 0.2 to 5 mg. of calcium per ml. Strontium is determined at a wavelength of 460.7 mp by means of a high-resolution flame spectrophotometer; calcium is used as a radiation buffer. Methods were devised for circumventing some of the common anionic interferences, such as phosphate and sulfate. Also, a previously reported procedure for the preparation of strontium-free calcium carbonate was improved.
T
DETERMINATION of nonradioactive strontium in natural waters and marine organisms is of great interest because of the fallout of Srgo. Naturally occurring strontium has no physiological importance and seems to be nontoxic to humans (10). However, the toxicity of radioactive strontium is important because of the similar behavior of strontium and calcium in the life cycle. Strontium accumulates in bone tissue, to which radioactive strontium causes radiation damage. Horr (5)has made a survey of analytical methods for the determination of strontium that have been reported in the literature over the last 60 years. Wade and Seim (14) separated calcium and strontium by (ethylenedinitrilo) tetraacetic acid complexation-ion exchange, and then determined the strontium flame spectrophotometrically. Their method is laborious. Hinsvark, Wittwer, and Sell (4) determined strontium flame spectrophotometrically in the presence of barium and calcium; however, the minimum concentration of strontium measured was greater than 200 pg. per ml., which concentration is far greater than that of interest for natural TTaters and fresh-water organisms. Previous attempts to determine directly the strontium content of biological material, such as clamshells, by spectrochemical methods have not been entirely satisfactory. I n a t least one investigation (la),the method was abandoned in favor of neutron-activation analysis, HE
778
ANALYTICAL CHEMISTRY
which has some advantages but which requires access to a nuclear reactor and is time consuming. Odum (10) determined strontium in clamshells flame photometrically by use of synthetic standards. However, the limit of detection of his method for strontium was 1.7 pg. per ml.; he was unable to detect strontium in tap water and in calcium-containing reagents. A rapid and sensitive flame spectrophotometric method, described herein, was developed whereby micro concentrations of strontium can be determined in calcareous materials without the separation of strontium from calcium. The effects of experimental variables and potential interferences on the emission intensity of strontium were established. EXPERIMENTAL
Apparatus. A Jarrell-Ash Ebert scanning spectrometer, with flame attachment and the modified ORNL1887A power supply ( 7 ) was used. The characteristics of the monochromator are: focal length, 500 mm.; grating, 1200 lines per mm. over a 52 X 52 mm. ruled area blazed for 5000 A . ; effective aperture, j / 8 . 5 ; linear dispersion a t exit slit, 16 A. per mm.; spectral range, 2000 to 8000 A. in first order. The wavelength drum is rotated by a motor and reduction gear system, which provides a smooth scanning rate a t eight different speeds between 2 and 500 A. per minute. The instrument is provided with fixed slits 50 microns (0.05 mm.) wide; these slits n'ere used in this study. The following instrument settings nere used unless otherwise indicated: sensitivity control, high; multiplier phototube load resistor, 5 megohms; multiplier phototube (RCA 1P28), 80 volts per dynode; slit, 0.05 mm.; spectral band width, 0.08 mp; and scanning speed, 10 A. per minute. Reagents. Strontium-free calcium carbonate was prepared b y a modification of the method developed b y Farmer ( 3 ) and reported b y Mitchell (8). The modified method is as follows. Dissolve 10 grams of reagent-grade CaC03 in 35 ml. of 6M HCI and then neutralize the solution with TU"&H. Add 200 ml. of 95y0 ethyl alcohol and heat the solution on a steam bath. Filter the mixture to remove any periodic group IIIA metals. Make-the
filtrate just acid (pH s 6) with HC1. Dissolve 15 grams of 8-quinolinol in 200 ml. of 95y0 ethyl alcohol; add the solution to the filtrate. Heat the solution on a steam bath and slowly add with stirring 25 ml. of 1 to 1 NHIOH. After the mixture has stood no more than 5 minutes, filter off the precipitate through a Buchner funnel while the mixture is hot. Wash the precipitate by first mixing it with 50 ml. of 95% ethyl alcohol to form a paste, then mixing the paste with 200 ml. of hot water, and finally filtering the mixture. Dissolve the calcium 8-quinolinolate precipitate in a minimum of concentrated HC1 and repeat the precipitation. (Three precipitations are required to secure calcium carbonate that contains less strontium than 2 pg. per gram.) Ignite the calcium 8-quinolinolate precipitate a t 450" C. for 12 hours. Dissolve the residue in 1M HCl and make the resulting solution alkaline (pH s 8) with concentrated N H 4 0 H . Filter the mixture and then add solid (NHJ2C03 to precipitate the Ca+2 as CaC03. Wash the CaCO3 precipitate several times n-ith a saturated solution of (NH4)2C03and d r y it a t 130" C. for 2 hours. A standard solution of strontium-free calcium, 10.0 mg. per ml., mas prepared by dissolving 2.497 grams of specially prepared CaC03 (see above) in 0.01V HCl, releasing the CO,, and diluting the resulting solution to 100 ml. with 0.01M HCI. A standard solution of strontium, 1.00 mg. per ml., was prepared by dissolving 1.685 grams of SrC03 in 0.01JI HC1, releasing the C02, and diluting the resulting solution to 1 liter with 0.OlJi HCl. hloredilute standard solutions of strontium were prepared from this standard solution by appropriate dilution. All other solutions used were prepared from ACS grade chemicals. PROCEDURE
Preparation of Calibration Curve. Prepare a series of standard solutions of strontium that contain 0.1 to 0.5 pg. of strontium per milliliter and 500 pg, of strontium-free calcium per milliliter. Measure the emission intensity of the atomic strontium liue and of the flame background a t wavelengths of 460.7 and 460.5 mp, respectively. (The background can be measured a t 460.5 mp only if an instrument of high resolution is used.) To obtain the net emission intensity of each standard,
subtract the emission intensity of the flame background from the total emission intensity of the strontium line. (At a wavelength of 460.7 mp, the calibration curve is linear over the strontium concentration range from 0.1 to 10 pg. per ml. If the scanning speed of the Jarrell-Ash Ebert monochromator does not exceed 10 A. per minute and if the response of the recorder is no slower than 1 second per full-scale travel of the pen, the calibration curve passes through the origin.) If sulfate or phosphate is present in the sample-e.g., chicken bones and plant tissue-modify the procedure as follows. Treat the standard solutions of strontium with sufficient 50 v./v. % aqueous solution of glycerol t o give a final concentration of 10% b y volume glycerol and with sufficient concentrated HCIOl to make the final solution 0.1M in HC104. To stabilize the whototube and thus to minimize any drift of its output, open the shutter between the multiplier phototube and the flame at least 15 minutes before aspiration of samples. Analysis of Clamshells and Snail Shells. Ash t h e sample in a muffle furnace a t 800" C. for 2 hours. Weigh out a 1- to 5-gram test portion of the cool ash and transfer i t to a 250-ml. beaker. Dissolve the portion in 2 M IlCl and dilute the solution with water to volume in a 100-ml. volumetric flask. (The final concentration of HC1 should not exceed 0.1M). From this solution, transfer to a 10-ml. volumetric flask an aliquot estimated to contain 1 to 5 pg. of strontium. If the aliquot does not contain a t least 2 mg. of calcium, add 2 mg. of strontium-free calcium. Dilute the solution t o 10 ml. with water. Determine the strontium content b y flame spectrophotometry according to the procedure given above for preparation of the calibration curve. Analysis of Natural Waters. Transfer a 500-ml. test portion of t h e sample of natural water t o a 1-liter beaker. -4dd to the test portion 1 ml. of concentrated HC1. Evaporate t h e solution t o a volume of approximately 25 ml. Quantitatively transfer the solution to a 50-ml. volumetric flask and dilute it to 50 ml. with tripledistilled water. Transfer a 5-ml. aliquot to a 10-ml. volumetric flask. Transfer from a standard solution of strontium-free C a C 0 3 to the flask an aliquot that contains 2 mg. of calcium. Ililute the resulting solution to 10 ml. n-ith triple-distilled water. Determine the strontium content of the final solution according to the procedure given above for preparation of the calibration curve. Analysis of Plant Tissue. Transfer a 1-gram test portion of t h e sample of plant tissue t o a 250-ml. beaker. Add 10 ml. of concentrated H N 0 3 ; then predigest the test portion. Add concentrated HC104 for the final digestion. Cool the mixture and filter off the silica. Dilute the filtrate to a known volume. From the dilute solution of the %!trate, transfer to a 10-ml. volumetric flask an aliquot estimated t o contain 1 t o 4 pg. of strontium. Add
~~
Table I.
Interfering Element Aluminum Bariuma
Effects of Other Elements on Emission Intensity
Concn. , r g ./ml. 1
10 100
1 10 100 1
Boron
strontium present, 1 pg./ml. Sr Recovered, Interfering Element % 51 Manganese 32 19 Molybdenum 98
100 1000
Calcium
1
10
Chromium
100 1000 1
10 100 1000
Cobalt
1
10 100
1000 1 10 100
Copper
Iron
Lithium Magnesium
a
100 98 100 105 115 99 91 69 45 96 95 89 64 99 80
1
84
100
Nickel Potassium
115
000
Praseodymiuma Rubidium
Sodium Terbium Thorium Titanium
41
Uranium
100 lOlb
1000 1 10
101 107 97 85
55
98b 9.j 99
Concn., pg./ml.
Sr Recovered,
100 1000 1 10 100
1000
1
10
100 1000 1 10 100
so
10 100 000 1 100 000 1 10
100
Manganese
85
92 93 112 >200
1000
Europiuma
101
>300
of Strontium
Yttrium Zinc
1000
100 1000 1 10 100 1000 1 10 100 1000 100 1000 1
10
%
85
50 93 91 97 94 97 92 93 96 104 113 130 128 100
107 102 106 126 85
101 102 128 131 92 84 96 80
100 1000 1
62 57 37
1 10 100
94 90 80 85 99 98 99 96 92 89
10
1000 100 1000
1 10
100
1000
0
Direct spectral interference. Determined by scanning from the longer to the shorter wavelength.
1 ml. of 1 M HC104 and 2 ml. of a 50 v./v. % aqueous solution of glycerol; dilute the resulting solution to 10 ml. a-ith water. Determine the strontium content of the final solution according t o the flame spectrophotometric procedure given above for the preparation of the calibration curve. RESULTS AND DISCUSSION
Interferences. T h e effects of other elements o n t h e emission intensity of strontium are indicated in Table I. Serious spectral interference is encountered from only a few elements if a n instrument with high resolution, such as t h e Jarrell-Ash Ebert, is used. Barium, europium, lithium, and praseodymium produce spectral interference on strontium a t a wavelength of 460.7 mp. The interference of the lithium 460.3mp arc line with the emission intensity of strontium is due to an instrumental difficulty. The response of the recorder and the scanning speed of the monochromator are such that when lithium
is present in high concentration the background is not measurable between the lithium 460.3-mp arc line and the strontium 460.7-mp line. This difficulty can be overcome if the scanning of the strontium 460.7-mp line is approached from the longer wavelength side and if the background radiant intensity is measured a t a wavelength of 460.9 mp. Condensed-phase interference (1) is observed with a number of elements, for example, aluminum, chromium, iron, and titanium. When these elements are present, the use of a radiation buffer-e.g., Mg+2-with glycerol as a releasing agent is recommended (see Table 11). Other types of interferences can be overcome when glycerol and the technique of standard addition are used. The effects of some of the common anions on the emission intensity of strontium are shown in Figure 1. In general, the apparent loss of strontium due to sulfate and phosphate is as expected. The effect of phosphate depends on the form in which it is present; the effect of the monohydrogen phosphate on the VOL. 34, NO. 7, JUNE 1962
779
1-
00CCl
0C?+ 00' c I CC u i E Y T " L - C N 0 ' L\l3N
I
c
2
Figure 1. Effects of anions on recovery of strontium in oxyhydrogen flame Strontium concentration, 1 y g . p e r ml. Wavelength, 460.7 m p
Table II. Recovery of Strontium from a Radiation Buffer-Glycerol Solution that Contained Aluminum or Titanium
'trontium present, 1 pg./nil. ht rontium IZiwveretl, Froin Concen-
emission intensity of strontium is more pronounced than its effect on the emission intensity of calciuni (11). Glycerol was studied as a releasing agent to overcome the inhibition effect of phosphat'e and sulfate on strontium. The results of this study are prmented in Figure 2. Even with a 10% by volumc glycerol solution, complete recovery of strontium in 0.000lM phosphate as the nionohydrogen phosphate is not accomplished until the solut'ion is made to have a pH of 1 to 2 . Of the mineral acid sohtions tested, 0.1-11 HClO, was the most satisfactory for adjusting the pH. K h e n it is used, complete rec'overy of strontium is obtaincd from a 0.01M monohydrogen phosphate solution that cont'ained 1 pg. of st'rontiuni pcr milliliter. Effect of Calcium on Strontium Emission. T h e emission intensity of strontium in the concentr a t'ion range from 0.1 t'o 1 p g . per ml. incwased with increasing calcium concentration up to 100 pg. per ml. Data for a solution of 0.3 bg. per ml. strontium concentra. nil. calcium tion and 0 to 1000 ~ g per concentration :ire shonx in Figurp 3. The strontium emission intensity hccomes constant over the, mnge of rnlcium concentration from 200 to 5000 pg. pcr ml. 'The cnhnncemcnt, of the. ~niiwionint'cnsity of strontiiim 1)y p a l -
,GI! I 00001
' I f ' "
I
,
,
,
'
oc0j 501 COLCE\-SATi\
I
1
'
.I
8
01
C-C
I /
-
Figure 2. Effect of 10% b y volume glycerol on recovery of strontium in presence of various anions (compare with Figure 1 ) Strontium concentration, 1 f i g . per mi. Wavelength, 460.7 mp
cium has been studied by Hultlt ( 6 ) :iiid by S u g a m r a , Koyania, and Kawasaki (13) a t higher concentrations of strontium. The background e m i s h i intensity due to calcium m s observed to i n c i ~ a s e with increasing calciiim conccntmtion. A background correction can bc applied easily if the emission intensity is nicisured by sloidy scmining (10 A. per minute) the 360.7-mp -trontium line.
~
tration of
Element .Ilurninum
ritanium
radiation buffer-
Froin
Element, aqueous glycerol pg./LIl. solution solutiona 1 51 100 3 ... 97 10 20 40 1
32 , . .
96 $11 8T
37
99
...
99
3
10 20 40
97
'0 ..
94 87
...
Composition: L\Ig+2, 500 pg./ml.; HC104, 0.lM; and glycerol, 10% (v./v.).
Table IV.
Results of Flame Spectrophotometric Determination of Strontium in Various Substances
Relative Standard
Sr Found, Wt. %
Saiiiple Limestone
0.181 0.181
( NBH-la )
Dolomite
Clamshellsc
Table Ill. Strontium Impurity in Reagent-Grade Calcium Compounds
Strontium Content, ('dcium Conipound Calcium Citrate, Gal( CeH607)~. 4Hd3
.Inodonta corpulenta hlegnlonaias gigantea
CaC17
140
345
Chicken bones (ash)
Range of results obtained on eight lots of CaC03 from three manufacturers. a
e
ANALYTICAL CHEMISTRY
430 453
4-19 av. -140
188
-1
308
-2
144
1
2
3
3
194
188
2
av. 188 306 av. 307 I50 av. 147
3d
0 066 0 065
1
pg./lll.
Ran water (Clinch River)
Si7
174 to 1500a 346
Day 14 147 145 av. 149 430
Day 1 147 136
185
285 500 1613
CaHPO,
780
pustulosu
pg./Grani of Cn'2 in
('ompound
c
fig. / G rain
0
Quadrula.
0,181 0.183 av. 0.182' 0,0055 0,0056 av. 0 . O05jb
0,0054 0 0056
(SBS-88)
L)eviation,
a
b c
0 065 0 065
av. 0.065
SBS-certified value, 0.19 v t . co Sr. ?;US-certified value,
