The Flame Spectrometric Determination of Calcium in Fruit Juice by Standard Addition Arthur N. StrOhll Niagara University, Niagara University, NY 14109
The determination of analytes in real-life samples is always
an interesting and exciting project for students. In this experiment, students measure the calcium concentration in fruit juice by atomic absorption. Fruit juice is an excellent choice because it is an important consumer product, a large selection of sample type i i available, and calcium e x i s t s h juice a t concentrations that do not require excessive dilution or preconcentration prior to measurement. But more importantly, calcium is a necessary mineral in the human diet. Reflecting this im~ortance.the nroximate amount of calcium in a varietv of fruit'juices is readily found in nutrition hooks.2,3This ex"periment also utilizes the much neglected method of standard addition. Bader4 examined the methodology of standard addition and renorted that there are nine distinct ~rocedures. He included'the design characteristics, the mathematical manipulations, and in several cases the error treatment for the deterhination of the analyte in the sample. Bader titled the procedure that is used for this experiment as "continuous variation of standard a t constant total volume." A series of solutions is prepared each containing the identical amount of unknown-(except for the blank), but also containing incremental amounts of standard, starting with no added standard. After the addition of necessary reagents, each solution is then diluted to the identical final volume. The inf solution is then measured and the strument r ~ s p o n s e oeach results are graphed as instrument response against amount t line is drawn from the of standard added. The h e ~ srraieht data points, and the %-interceptis numerically equal to the amount of the analvte in the unknown. If a more mathematical approach is desired, the x-intercept can be computed from the slope and y-intercept (x-int = -(y-intlslope)). Procedure Obtain at least 200 mL of a fruit juice and filter or centrifuge it. (Somejuices that haw a lot of pulp may hed~fficult to filter., Determine the density of the juice if needed or instructed to do so. Solutions toprepare 1) 500 pg/mL Ca: Add 1.2489 g of dry calcium carbonate to 50 mL of water in a l-L volumetric flask. Add 6 M HCI dropwise until the solid is dissolved. Dilute to 1L. 2) 100pg/mL Ca: Dilute 50 mL of 500 pg/mL Ca to 250 mL. 3) 50 g/L La: Dissolve 15.58 g lanthanum nitrate hemhydrate (flame enhancement AA grade) in water and dilute to 100mL. Lanthanum is added to prevent the formation of calcium phosphate (fruit
juices contain significant amounts of phosphorus) and the resultant decrease in ahsorbance. 4) Prepare a series of five solutions in 100-mL volumetric flasks and label them A through E. To each flask add 10 mL of 50 g L La solution and 20 mL of 6 M HCI. To flasks B through E add 20 mL of filtered fruit juice. Add portions of 100pg/mL Ca solution to the flasks as follows: A (OmL),B (OmL),C (5 mL), D (IOmL),and E (15 mL). Dilute each solution to 100 mL Set up the atomic ahsorption spectrophotometer for the iletwnination of calcium. Use an air.scetylcne flame and set the wavelength at 422.7 nM. Set zero absorbance with the blank (Solution A). Starting with Solution B, aspirate each in turn into the flame. Take five replicate ahsorbance readings for each solution. Recheck zero ahsorbance periodically. Calculations I ) Usea data rejection method to rest the valdity of any suspect absorbance reading and compute [he awragc absorhsnce reading for each solution from the valid data. 21 Plot the awrage abnsl~anrtreading uf each solution against the pa of calcium oddpd t o it. 31 ilse a h e a r regreraion prwedure to determine the best line fur thedata. The x-intercea is numericallvmual . to the wrieht ofcalcium in the orieinal - undiluted iuice samole. 4).~ Com~ute the .... bire. . the concentration iue/mT.) of.enleinm in~.. 51 Compare your experimental answer in 4 to the average amount of ralrium listed in nutritim h>oksandtor listed on the lruit-juice container. Use the density of the fruitjuice if needed. Comment on the possible causes for any differencebetween the experimental and average value. 6) (Optional)Since this experiment eenerates a lot of data ouicklv. manv statistical Drocedures can be incohorated into the exoe;iment: These ~ncludeerror analysis, confidence intervals, and, if replicate drvrmmarions a r t performed, smt~sticalcornpanson of result^. ~
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'Present address: U S . Pharmacopeial Convention Inc.. 12601 Twinbrook Parkwav. MD 20852. ~ Rockville. .. ~enninglon.Jean A. 7.. and church. Helen Nichols. -Food Values of Portions Commonly Used." 13th ed.. Harper and Row Puolishers, .N.e w.. Vnrk , -..., 1980 .- - -. Wan, Bernice K.. and Merrill, Annabel L.. "Handbook of the Nutritional Contents of Foods," Dover Publications. Inc., New York, ~~
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1975.
'Bader, Mwris, J. CHEM. EDUC., 57, 703 (1980)
Volume 62
Number 4
April 1985
343
