F. L. Corcoran, P. N. Keliher, and C. C. Wohlers, lnt. Lab., 52, (1972). W. Kessler and F. Gebhardt, Glastech. Ber., 40, 194 (1967). F. Gebhardt and H. Horn, Glastech. Ber., 44, 483 (1971). R. H. Scott and M. L. Kokot, Anal. Chim. Acta, 75, 257 (1975). K. Govindaraju, Pub/.Group. Av. Methodes Spectrogr., 221 (1960). K. Govindaraju, Bull. SOC.Fr. CBram., 67, 25 (1965). K. Govindaraju, Analusis, 2, 367 (1973). W. Kessler, Glastech. Ber., 44, 479 (1971). M. Roubault, H. de ia Roche and K. Govindaraju, Sci. Terre, 15, 351 (1970). H. de la Roche and K. Govindaraiu, Bull. SOC. Fr. CBram., 100, 49 (1973). (28) P. W. J. M. Boumans, F. J. Dahmen, J. W. de Boer, H. Hoelzel, and A. Meier, Spectrochim.Acta, Part B, 30, 449 (1975). (29) K. Govindaraju. Colloq. C.N.R.S.,No 923 (Nancy), 269 (1970).
(30) G. F. Larson, V. A. Fassel, R. H. Scott, and R. N. Kniseley, Anal. Chem., 47, 238 (1975). (31) K. Norrish and J. T. Hutton, Geochim. Cosmochim. Acta, 33, 431 (1969). (32) R. Tertian and R. aninasca, X-Ray Spectrom., 1, 83 (1972). (33) E. P. FabbiandL. F. Espos, U.S.,&I. SUN. Prof.Pap., 8008, B 147(1972). (34) P. K. Harvey, D. M. Taylor, R. 0. Hendry, and F. Bancroft, %Ray Spectrom., 2, 33 (1973). (35) N. H. Suhr and C. 0. Ingameiis, Anal. Chem., 38, 730 (1966). (36) Y. Besnus and R. Rouault, Analysis, 2, 11 1 (1973). (37) K. Govindaraju, G. Mevelle, and C. Chouard, Am/. Chem., 46, 1672 (1974).
RECEIVEDfor review March 2,1976. Accepted April 27,1976.
Determination of Sulfate by Flame Emission Inhibition Titration J. R. Sand’ and C. 0. Huber* Department of Chemistry, University of Wisconsin-Milwaukee,
Milwaukee, Wis. 5320 1
Flame emission photometric titrations exploiting the signalinhibiting effects of sulfate, provide accurate, rapid, and convenient determinations for real samples at concentrations down to 0.2 ppm. An Improved detection system was built around an interference filter, a photomultiplier tube, and an elementary dc amplifier. Detection and determination limits are, at present, limited by the level of flame noise encountered in the system. Collection and analysis of the refractory particulates formed during the course of these titrations yielded data on the compositionalchanges which give rise to the unusual shapes seen for anion inhibition titration curves. A mechanism involving rate-regulated stolchlometric changes in the refractory inhibition products Is compatible with the data.
onstrate superior performance with relatively simple spectroscopic equipment. During the course of the work, it was also possible to elucidate and apply a newly discovered inhibition signal. An improved method for the collection and analysis of the refractory particles formed in flames used in analytical flame spectroscopic procedures is described. Refractory particulates corresponding to various points during the course of these inhibition titrations were collected and analyzed. The results of these analyses indicate changes in the composition of the particles collected from different regions of the titration curve. These compositional changes are correlated with observed signals to describe a mechanism for the titration process.
EXPERIMENTAL Titration methods based on inhibition effects of anions on flame photometric signals of metals were introduced by Torok ( 1 ) and have since been applied to several determinations (2-4). A method for the simultaneous determination of silicate, phosphate, and sulfate utilizing magnesium atomic absorption has also been reported (5). The inhibition titration experiment involves addition of a metal titrant solution to a stirred solution of anions from which metal cations have been removed. A flame photometer (see Figure I ) , tuned to respond to titrant metal, is used to sample the titration mixture and provide the analytical signal. The type of titration curve obtained is dependent on the flame conditions and the type of anions in the solution to be analyzed. The unusual shapes of these “titration curves” result from complicated processes occurring in the evaporating droplets of sample formed by the burner nebulization process. Thus, the method is a “titration” as to procedure, but there ordinarily is not a stoichiometric reaction in the “titration” vessel. Direct correlations are made between features of the titration curves and sample anion concentrations. Calcium flame emission spectroscopy was examined in order to extend and enhance flame photometric titration techniques by incorporating the instrumental advantages inherent in flame emission. The shape of the analytical signals is such that spectral purity is not a major concern in these analyses. The instrument was therefore redesigned for increased optical throughput and efficiency by sacrificing spectral resolution. In this manner, it was possible to demPresent address, The Trane Company, Lacrosse, Wis. 54601.
R e a g e n t s and Solutions. All aqueous solutions were made up in distilled water which had been further purified by passage through a mixed-bed ion-exchange column. Titrant solutions of calcium were prepared by quantitative dilution of a 2000-ppm Ca solution which was made up by dissolving reagent grade CaCO3 in a minimum amount of concentrated hydrochloric acid and diluting to volume. Stock sulfate solutions were prepared by dilutions of reagent grade sulfuric acid and standardization vs. primary standard sodium carbonate. Stock solutions of 1000 ppm Si02 were prepared by fusing the appropriate amount of dried chromatographic grade Si02 with an excess of anhydrous, reagent grade sodium carbonate at 1000 “C for 30 min and dissolving the fusion products to volume. Silicate solutions were deionized with a cation-exchange resin on the same day they were used to prevent silicic acid precipitation. Stock solutions of the cations used for interference studies were made up from the chloride salts, while the sodium or potassium salts were used to make up stock solutions of potential anionic interferences. Interfering cations were removed from sample solutions by treatment in a “batch” process with the hydrogen form of Rexyn 101,16-50 mesh cation-exchange resin. Apparatus. All glassware and polyethylene apparatus used for these determinations were acid-hardened by soaking in 3 N hydrochloric acid overnight. Initial flame emission measurements were obtained on a Jarrell-Ash model 82-516 spectrometer equipped with a JA82-374 laminar flow, slot burner using hydrogen and air. This instrument was used with the 100-Wm entrance slit and 15O-Wm exit slit provided, or with 1-mm slits made from I/lG-in. aluminum stock. The 620-nm interference filter used was 51 X 51 mm with a 10-nm nominal band pass (Optikel Corp., Lexington, Mass.). A solid-state dc amplifier (6) was designed and built for use with the interference filter detection system using integrated circuit operational amplifiers. The flame temperature was approximately 1910 “C. An R 106 photomultiplier tube was used a t the shorter wavelengths and a 1P28A photomultiplier tube was used in conjunction with the interference filter. A commercial electrostatic room air cleaner was used prior to and during sub-ppm sulfate determinations. A Harvard Apparatus, Inc. infusion pump and 50-ml polyethylene ANALYTICAL CHEMISTRY, VOL. 48, NO. 9, AUGUST 1976
e
1331
MONOCHROMATOR
FLAME
I
i SPRAY
Table I. Analytical Results
A M P L I F: I E R
Sample Raw HzO T a p HzO FWQA No. 1 FWQA No. 2 Syn. waste
Determined 20.5 ppm SO4 27.5 7.5 148.1 65.5
Present -220.2" ppm SO4 325.7" 7.8 150
66.7
HzO
FUEL PRINTER OX I DAN
1
T-j
-
Benzene sulfonamide Men om in ee River water Syn. waste HzOb
METER
RECORDER
1.-
19.9%
20.4%
51.1
49." 56c
9.5
Std dev, 0.056 ppm SO4, 5 determinations Detection limit, 0.013 ppm SO1 Determination limit, 0.11 ppm S o d d COMMON S W I T C H
8
CONSTANT RATE I X F U S I O N PUMP
a Gravimetric (8). See Ref. 9 for contents other than sulfate. Turbidimetric (8). See Ref. 10.
Figure 1. Atomization inhibition titration apparatus
Table 11. EDXRA Refractory Particulate Analysis: Stoichiometry Shifts Si Counts/ S Counts I CALCIUM
I
Locations 1. Before peak 2. After peak
0 0
EMISSION INTENSITY
a
C A L C I U M CONC.
Figure 2.
MxlO'
Peak shape titration curves
Sample solution; (- - -) H20,(- M Si02 (10 ppm), (--)
.-
-
-) 2.0 X M SO4 (20 ppm), (- -) 1.7 X M SO?. 2.0 X M SO4 and 1.7 X
syringes were used for delivery of the CaClz titrant. Titration curves were recorded on a 10-mV, potentiometric recorder. Refractory particulates were collected using a 3-m length of 6.35-cm i.d. flexible stainless steel hose as a cooling train. The end of the hose supported above the flame had a 18-cmdiameter, stainless steel hood, while the filter end was fitted with the outer assembly ring flange of a Nuclepore (Nuclepore Corp., Pleasanton, Calif.) 47-mm membrane filter holder. Nuclepore 0.80-pm-pore size polycarbonate filter disks were used. Vacuum for the filtration was provided through the building vacuum system rated at 240 cubic feet per min a t 20 inches of mercury. Valves and connecting tubes were enlarged to approximately 0.5 in. i.d. to minimize pressure losses due to constricted flow. A 9-mm diameter hole in the flexpipe 30 cm from the filter end provided for injection of cooling, diluent gas. Photomicrographs of filtered samples were made on a Japan Electron Optics Laboratory (JSM-U3) scanning electron microscope (SEM). This SEM was fitted with a Nuclear Diodes (No. 505),energy dispersive x-ray analyzer used to obtain x-ray fluorescence data on the collected particulates. Procedure. The flame photometric titration apparatus is illustrated by Figure 1. Fifty-ml portions of deionized sample solutions or prepared standards containing the appropriate anions are placed in 100-mlglass beakers and stirred magnetically. Titrant delivery and aspirating tubes are inserted into the anion solution through a cover which functions to keep the two tubes separated. Immediately after aspiration has started, titrant flow and titration signal recording are initiated via a common switch. Titrant delivery is continued until the appropriate end-point signals have been recorded. Titrant infusion rates up to 6 ml per min were available and titrant solutions ranging from 10-100 ppm Ca were used. Titrations are or1332
* ANALYTICAL CHEMISTRY, VOL.
48, NO. 9, AUGUST 1976
Ca Counts
0.60 f 0.12" 0.56 f 0.11"
Ca Counts 0.21 f 0.05" 0.42 f 0.04"
Average of four results i std dev.
dinarily completed in 4 min or less. Errors due to aspiration of the titration solution have been shown to be tolerable ( 5 ) . Particulate collectionswere performed on 300-ml portions of anion solutions (-100 ppm) titrated with 2000 ppm Ca, while the titration signal was monitored in the usual manner. The titration was stopped a t the desired point, the flexpipe positioned with the collecting hood over the flame, and vacuum collection of refractory particles was initiated as aspiration of the titrated solution continued. Filtration was continued until air flow through the filter ceased because of plugging of the surface or until 150 ml of solution had been aspirated. Filtered samples were then mounted, gold coated, and analyzed on the scanning electron microscope. Flame temperatures were measured by the line reversal method in a manner similar to the procedure outlined by Craig et al. (7). The 17 X 2 mm tungsten filament background source resulted in observing an area representing a composite of temperatures from different regions. The temperature of the air-hydrogen flame was adjusted using the "auxiliary air" valve of the burner assembly.
RESULTS AND DISCUSSION Sulfate Determination. If inhibition titrations are carried o u t on sulfate a n d silicate solutions at a flame t e m p e r a t u r e near 1910 O C , and t h e formal S042-/SiOz concentration ratio is 2.0 or less, a remarkable peak shaped titration curve is observed. Under such conditions, t h e horizontal position of the peak is linearly related t o the concentration of sulfate i n t h e sample. Titrations performed i n t h e manner described above a r e referred t o a s "sulfate peak" titrations. Figure 2 shows a series of 4 titrations carried out under sulfate peak conditions. W h e n a solution containing S i 0 2 and S042- is t i t r a t e d , the clearly synergetic peak-shaped titration curve is obtained. W h e n obtaining t h e sulfate peak titration curves, a considerable improvement in t h e signal-to-noise ratio was realized by increasing t h e optical collection efficiency of the detection system. T h i s is possible because t h e features of these inhibition titration curves p e r m i t a t r a d e off of spectral resolution for increased optical throughput, i.e., quantitative measure-
d.
C.
Flgure 3. Photographs of SEM displays of filter surface (a) Room air. ( b ) Aspiration of CaC12 solution. (c) Before sulfate peak. (d) After sulfate peak
ments are not dependent on the magnitude of the signal so long as the characteristic features of the titration signal are observable. Enhanced calcium sensitivity was obtained initially by widening the monochromator slits to 1 mm. Eventually the monochromator was replaced with a 620-nm interference filter. While using the filter, precautions had to be taken to decrease stray room light to a minimum. To provide enhanced gain and zero suppression over that available on the spectrometer, a dc amplifier was used. Utilizing the filter and a dc amplifier, the system was sufficient ly sensitive so that the sporadic, incandescent flashes caused by entrained room air particulates entering the base of the flame determined the limiting signal to noise ratio. R-C damping time constants larger than 0.10 s in the amplifier circuit tended to obscure the characteristic features of the titration signals. A 12-cm high stainless steel and glass flame shield did not prevent the room air particles from getting into the flame. With the use of the electrostatic air cleaner, however, a reduction in the flame noise sufficient to permit measurements of sulfate down to 0.20 ppm sulfate was observed. This value is near the 0.1 ppm determination limit predicted from precision data in Table I. Analytical results using the sulfate peak titration technique described have been obtained on a number of samples containing sulfate and are summarized in Table I. For most
samples, silicate was added prior to analysis in order to satisfy the conditions necessary for the occurrence of sulfate peaks. Linear calibration results were obtained for sulfate solutions ranging from 0.20-100 ppm. The accuracy indicated by the results shown in Table I shows that low level sulfate measurements are possible in the presence of the various potential interferences found in potable and waste water as well as in urban-industrial river water samples. In addition, the method was used with Schoniger Flask combustion (3%HzOz absorber) of benzenesulfonamide. The results compared well with the theoretical sulfur content of the compound. This technique allows titrations of anions at concentration levels 1 or 2 orders of magnitude less than is common for other titration methods. This sensitivity can be attributed to the inherent sensitivity of flame emission signals and to the complete utilization of discrete portions of the titration solution. The optimum wavelength for observing calcium emission is dependent on the bandpass of the wavelength resolving system used. When the interference filter is used, the 622-Ca0 emission band is the most sensitive for observation of calcium emission. The molecular emission bands are less intense but much broader than the 423-nm atomic emission line. Wider bandpasses permit integration of the emission intensity over ANALYTICAL CHEMISTRY, VOL. 48, NO. 9, AUGUST 1976
1333
a wider wavelength region (11).Well-developed sulfate peak titration curves have been observed at the 554-nm CaOH band emission as well as a t the two wavelengths mentioned above. This indicates an equilibrium distribution between Ca, CaOH, and CaO in the flame (12) and that thy peak corresponds to rapid increase and decrease of all of these species. Particulate Composition and Mechanism. The reaction system operating in these inhibition titrations involves tiny aqueous droplets containing dilute solutions of calcium and refractory anions carried into a flame of nearly 2000 OC. Residence times of the sample species in the flame are on the order of 1ms. Desolvation, complexation, precipitation, and pyrochemical reactions between the chemical components of the droplet must occur on that time scale. The extreme temperature conditions, the large number of reactions possible, and the rate a t which these reactions have to occur all contribute to the complexity of t h e chemical system and its interpretation. It is postulated from the shape of the sulfate peak curve that some refractory compound involving calcium, silicate, and sulfate totally inhibits the formation of free calcium in the flame before the peak is seen. In the region of the peak, some change in the nature of the refractory compound being formed permits a sudden, large release of calcium into the flame which is indicated by a sharp increase in signal. As more calcium is added, however, a new S/Ca stoichiometry is established for a refractory compound which again effectively prevents the formation of free Ca in the flame. The linear variation in the position of the peaks with changing concentration is a strong indication that it is the S/Ca ratio in the refractory particulates which is changing with added Ca and giving rise to the peak-shaped signal. Clearly, Si02 is necessary to obtain the proper signal, but a 10-fold variation in the Si02 concentration does not affect the position of the peak as long as a formal S042-/Si02 concentration ratio of less than 2 is maintained. T o study further the refractory particle formation, the particles formed in the flame at several points during the course of these titrations were collected and analyzed. Preliminary calculations indicated that particles of approximately 2-pm diameter would form from the dehydration of 10-pm diameter droplets containing 100 ppm CaS04. Inertial collection devices (e.g., cyclones) are not suitable for these collections because they require high total gas flow rate and because of collection efficiencies which decrease with particle size. Recently, however, commercial filtering materials have become available which employ uniform cylindrical pores in a thin polycarbonate substrate. These pores result in true sieve-like filtration and give superior strength and thermal stability. Filtered material is mostly deposited on the surface rather than imbedded in tortuous channels. Refractory particulates were collected at points before and after the peak on a sulfate peak titration. A collection was also made while only 50 ppm Ca (CaC12) solution was being aspirated into the burner. Room air was sampled as a blank. When portions of these filters were viewed with the SEM (Figure 3), an array of collected material was seen. Blanks obtained by sampling room air for 30 min (Figure 3a), showed essentially empty filters with an occasional very large particle (cf., Figures 3b and 3c). Filters used to collect samples during the course of an inhibition titration had surfaces virtually littered with particles. The larger crystalline particles were approximately 10 pm in diameter and therefore much larger than what would be anticipated from a 10-pm droplet. These larger particles were present on samples collected before the peak. Energy dispersive x-ray analysis (EDXRA) of these larger crystals indicated that they were composed primarily of calcium chloride. Close examination of these larger crystals a t high electron beam currents resulted in disruption of their 1334
ANALYTICAL CHEMISTRY, VOL. 48, NO. 9, AUGUST 1976
Figure 4. Titration at various temperatures
sharp crystalline features, presumably due to loss of water of crystallization. The smaller particles on the filters ranged from 0.05 to 0.4 pm and were presumed to be the refractory particles of interest. Their size indicates that either all of the solute from a nebulized droplet does not go into a single particle or the droplets formed as a result of the aspiration nebulization processes are smaller than 10 pm. Wherever possible x-ray fluorescence data were collected from regions of the filter where primarily small particulates were seen. EDXRA data collected from these refractory crystallites was in the form of counts per channel in a multichannel analyzer (MCA). While a TV display on the spectra was available, quantitative measurements could best be made from the teletype print-out of the MCA contents. EDXRA data were collected on all samples, including blanks, under the same beam current conditions until approximately 1000 counts accumulated for the gold M a peak (2.12 keV). In this manner contributions of the bremsstrahlung radiation measured from the blank can be subtracted from the Si; K a (1.74 keV), S; Koc (2.31 keV), C1; K a (2.62 keV), and Ca; K a (3.69 keV) peaks observed for the refractory particles. Chlorine counts presumably arise from the presence of the unexpected CaC12 particles (see discussion below) in the sampled area. Pure CaClz gave twice as many C1 counts as Ca counts. Accordingly, error due to the presence of CaCl2 was compensated by subtracting one half of the chlorine counts from the total calcium counts. After these corrections, the Si and S counts were “normalized~’with respect to calcium and are presented in Table 11. X-ray fluorescence data in Table I1 indicate that approximately twice as many sulfurs are present per calcium in the refractory particles collected after the peak as compared to those collected before the peak. It had been anticipated that the calcium rather than the sulfur content of the refractories would increase as more calcium titrant was added. Calibration data obtained from titrating known concentrations of sulfate indicate a S/Ca formal ratio in the solution of 1.1a t the point where this peak occurs (see Figure 2). The deviation of this value from 1.0 is consistent and larger than what would be
expected from experimental errors. The Si/Ca ratio of the collected particles remains constant before and after the peak. The stoichiometries observed suggest a series of reactions shown in Equations 1through 3 below. Before peak: xCa2+
+ ySiO2 + s04'-
ki
-
xCaO ySi02 SO3 At peak: Ca2+
+ Si02 + S042-
After peak: xCa2+
+ ySi02 + 2S042-
-
-
ki'
N.R.
Ca
(1)
(2)
kz
-
xCaO ySiO2 2S03 e
k 2'
N.R.
(3)
Before the peak, calcium, silicate, and sulfate react to form a stable refractory compound. As the ionic calcium content of the droplets increases, the tendency for more s04'- to be incorporated into the lattice goes up. When the S0d2-/Ca ratio is 1.1(the peak in the titration curve), nucleation rate or other rate processes prevent a stable refractory from forming so calcium is released to the flame. After the peak, a different heat-resistant material is being formed embodying twice as many sulfates per calcium as that formed before the peak. Literal interpretation of the magnitude of the ratios in Table I1 combined with the relative x-ray intensities for the elements would indicate an approximate lOCa0-5Si02.SO3 stoichiometry before the end point and one of 10Ca06Si022S03 after the peak. Intensities of fluorescent x-rays from a surface are modified by a series of factors which would make such an interpretation somewhat hazardous. The changes in relative elemental intensities compared in this study are valid, however, because all of the samples were of the same nature and contained the same elements. Ample evidence for the complexity of the calcium-silica system can be found in cement chemistry studies. Phase diagrams for CaO-Si02 systems show that a large number of Si/Ca stoichiometries and crystal modifications exist (13). During an inhibition titration, the relative abundance of calcium continuously changes making any one of the stoichiometries or crystal changes along an isotherm a possibility. It is assumed that the presence of sulfate in these titration mixtures adds to the complexity of the chemical reactions. The presence of the large CaCl2 crystals among particles collected before the peak-shaped end point is surprising. No
appreciable calcium flame emission signal is seen in these regions so that these crystals do not represent calcium and chlorine that have recondensed after having been initially dissociated by the flame. It is unlikely that these particles resulted from droplets large enough to survive the flame since they are seen neither in collections after the peak nor for solutions containing only calcium chloride. In light of the above considerations, it appears that hydrochloric acid and water vapor from the flame gases react with calcium containing refractory particles already on the filter. This reaction at favorable sites on the filter results in calcium chloride crystal growth. An example of the importance of flame temperature in controlling the type of signal obtained from these titrations is illustrated by Figure 4. This diagram shows the curves obtained for titrations of solutions containing 2 ppm Si02 and 6 ppm S042- as the flame temperature is increased from 1795 to 1910 "C in five steps. The diagram clearly indicates that the intricacies of the flame inhibition titration curve shapes are dependent on flame temperature. The features of the curves at ca1cium:sulfate ratios larger than unity for the lower flame temperatures correspond to similar features for atomic absorption inhibition signals reported earlier ( 5 ) .
ACKNOWLEDGMENT The authors thank I. Rubeska for helpful correspondence.
LITERATURE CITED (1) D. J. Currough, "Treatise on Titrimetry: New Developments in Titrimetry", J. Jordan, Ed., Marcel Dekker, New York, 1974, pp 172-175. (2) K. C. Singhal, R. P. C. Sinha, and B. K. Banerjee. Tecbnology(Sindri,hdia), 6. 219- (19691 R. W. Looyenga and C. 0.Huber, Anal. Cbem., 43,498 (1971). W. E. Crawford, C. I. Lin, and C. 0.Huber, Anal. Cbim. Acta, 64, 387 (1973). C. I. Lin and C. 0.Huber, Anal. Cbem., 44, 2200 (1972). A. J. Diefenderfer, "Principles of Electronic Instrumentation", W. B. Saunders Co., Philadelphia, Pa., 1972, Chapter 13. N. C. Craig, T. S. Carlton, and R. C. Schoonmacker, J. Cbem. Educ., 51, 54 (1974). M. J. Taras, A. E. Greenburg, R. D. Hoak, and M. C. Rand, Ed., "Standard Methods for the Examination of Water and Waste Water", 13th ed., American Public.HealthAssociation, New York, 1972. "Cleaning Our Environment, the Chemical Basis for Action", American Chemical Society, Washington, D.C., 1969, p 109. M. L. Parsons, J. Cbem. Educ., 46, 290 (1969). J. Duorak, I. Rubeska. and 2. Razac, "Flame Photometry: Laboratory Practice", Chemical Rubber Co. Press, Cleveland, Ohio, 1971, Chapter 14. D.R. Jenkins and T. M. Sagden, "Flame Emission and Atomic Absorption Soectrometry". J. A. Dean and T. C. Rains, Ed.. Marcel Dekker, New York, 1969, Chapter 5. H. F. Tayler, Ed., "The Chemistry of Cements", Vol. 1 and 2, Academic Press, New York, 1964.
-.-
1
- - - I
RECEIVEDfor review November 19, 1975. Accepted March 22, 1976. Work supported by the University of WisconsinMilwaukee Center for Great Lakes Studies.
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