An Experiment to Examine the Phosphate Interference in the Flame Emission Analysis of Calcium Donald C. Jackman Pfeiffer College, Misenheimer, NC 28109
Most instrumental analysis courses discuss flame emission analysis, and many include a simple lab experiment in flame emission (Na, K, and Ca seem t o he popular elements). Textbooks on instrumental analvsis includine the recent editions of two highly regarded texts (1,2) briefly mention interferences in flame emission analysis. The standard treatment is t o mention the phosphate interference in the flame emission analvsis of calcium and t o point out t h a t the interference may heovercome by the addition of a protective agent (e.g., EDTA) or a releasing agent (e.g., lanthanum or strontium). Older editions of these and other texts have been n o more thorough in their coverage. Also, to my knowledge there arc no pulhished experiments that deal wkh t h r phosphate ~nterferencein the tlamr ilnal.vsis of calcium. The exneriment is desiened fur courses that h a w - fnllowinr -~ ~ ~ time for only one flame emission experiment h u t would like t o cover several c o n c e.~ t s(flame emission technioue. cali. hration plots, interferences, indirect flame emissionan&&, and techniaues of removine interferences) in t h a t single lahoratory exeicise. This experhent, of course, could also be used a s a follow-up to a simpler emission analysis.
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Experimental Procedure Prepare a 100-ppm (2.5 X 10-3F) solution of calcium from a IMX)ppm stock solution. Prepare a 0.00250 F solution of NaHzPOa.Hz0 by diluting 0.3455 g of NaHzP01.Hz0 to 11 with distilled water. Preoare a 0.010 F solution of EDTA bv dilutine 3.7245 e of the disodi;m salt to 1 1with distilled water.
Prepare a 0.010 F solution of strontium by diluting 2.6662 g of SrCIz6HzO to 1 1with distilled water. Prepare a 0.40 F solution of EDTA by diluting 37.22 g of the disodium salt to 250 ml with distilled water. Obtain 12 clean 100-mlvolumetric flasks and add 10 ml of 100 ppm calcium solution and 10 ml of pH 10 buffer (prepared by dilutinga mixture of 570 ml of concentrated aqueous ammonia and 70 g of ammonium chloride to 1 1with distilled water) to all 12 flasks. Then (for each numbered flask): 1) Dilute to volume with distilled water. 2) Add 2 ml of the ohosohate -, . . solution and dilute to volume 3) Add 4 ml of the phosphate solution and dilute to volume. 4) Add 6 ml of the phosphate solution and dilute to volume. 5) Add 8 ml of the phosphate solution and dilute to volume. 6) Add 10 ml of the phosphate solution and dilute to volume. 7) Add 15 ml of the phosphate solution and dilute to volume. 8) Add 20 ml of the phosphate solution and dilute to volume. 9) Add 10 ml of the phosphate solution, 2.5 ml of 0.01 F EDTA and dilute to volume. 10) Add 10 ml of the phosphate solution, 10 ml of 0.01 F EDTA and dilute to volume. 11) Add 10 ml of the phosphate solution, 10 ml of 0.40 F EDTA and dilute to volume. 12) Add 10 ml of the phosphate solution, 10 ml of strontium chloride solution and dilute to volume. Adjust the instrument to 100%relative emission intensity with Solution l after determining the most sensitive portion of the flame with the same solution at 4227 A. Determine the relative emission intensity of the other 11 solutions at the same wavelength and under the same flame conditions.
Volume 62
Number 2
February 1985
161
Varlatton of relative fameemissionmtensty of 2 5 X 1 0 4 F(10 ppml ca1ci.m as a fuoctmof pnosphafe concentratfon
Results and Discussion
The solution concentrations and a set of relative emission intensity readings are given in the table. l'he emission readings were obtained on il Jarrell-Auh Model 530 single-beam combination flame emission-atomic absorption spectrophowmeter using manufarturer suggested acetylene-air flow rates and at a flame height of 17 mm. A plot of the data for the first eight solutions is shown in the figure. The plot wan generated using the Scientili~Plotter !copyright, Interactive Microware, and written by Paul Warmel software on an Apple I I t computrr and printed on an Epson hlX-80 printer. The reason for choosing 0.00023 F ('a (lU ppm) is that this is in the concentration range generally used in the uriginal literature (3.41 on inwrferences in flame emission. T h t reawn for using pH 10 ammonia huffer is strictly a pedagogical one-students are taught to use pH 10 buffer in the EDTA
titrimetric analysis of calcium. West and Cooke (3), however, ~ o i nout t that strict o H control of the solutions is not necessary when using EDTA as a protective agent in the flame analvsis of calcium. I And this experiment to be most satisfactory in teaching in a number of wavs. First. the students obtain experience in a flame emission tkchniqu&.Second, they do generate data to prepare a calibration plot, albeit, ultimately, a plot for the indirect determination of phosphate. Third, they are presented with a priori evidence of an interference which they will not forget as readily as they might a single statement in a textbook. Fourth, the experiment examines the two major methods for removing the phosphate interfertnce. l'he use of EIYI'A as a protecti\,e agent is exemplified in Solutions 9-1 1. Note that a stoichiumttric amount of EIYI'A (Sdution 91 shows very little removal oi the phosphate intrrference, a four-fold excess (Solution Iu) of EDTA shows some interferrnce removal, and a 160-foldexcess as used by West and Cooke ( 3 )showscomplete removal of the interference. The use of strontium (four-fold excess] as a releasing agent is shown in Solution 12 where the strontium concentr&n is only slightly below that used by Dinnin (4). A final nicety of this experiment is evidence of the calcium phosphate species present in the solution. The primary phosphate species present a t pH 10 is HPOa2- (99+%) which leads to, a t first blush, a species of formula CaHPOa with a 0 ' . Note that the calibration formation constant (5) of 1 X 1 plot ceases its decline a t a 1:l stoichiometry of calcium phosohate and. thus. lends credence to the formula (given above) at which students should arrive from simplekquilibrium calculations. Literature Cited 11 ,A
(3) (4) (5)
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and U'rrr. Uonald hl . ' Prmr.plr 11 l n i l r m o ~ n t a nl e l n s . ' 2nd .II S n u ! ~ d r b C . ~ l l rPI, a ~ ~ihdrlph~a. . 1Wl. PP ?ILL. w a r d , I I , , I ~ ~ ,Hx , M ~ I.,,,,,~ ~ ~L , 1,~ , r,+av8. T , l ~< h n A , and SWIP, vrdnk .\ , J,.. i n ~ i r u r n e n t n lhlrlhvnr I \n.h*r ' m h r d 1, VrnUw.uandC. .Npa York.1981. pp. 1367. West, A.C.,sndCaaka, W.D.,Anol. Cham.,32,1471(1960). Dinnin, J. I.,Anol. Cham.,32, 1475(1960). Dean,J.A.. "Lange'sHandbookofchemistw,"11th ed., McCray-HillBwkCo,New
York. 1913,54.
The Phosphate Interierence In the Flame Emlssion Analysts of Calcium as a Function of Solution Composltlon Soh. No. I 2 3 4
162
Cat+ ( 0 2.5 2.5 2.5 2.5
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10-~110ppm) 10-'110 ppm) 10-~(10ppm) lo-'(10 ppm)
Journal of Chemical Education
H2PO4-
IR
... 5 X 10F 1.0 x lo-+ 1.5 x lo-4
EDTA IF)
Sr2+( F )
Relative Emission Intensity
