Using a Homemade Flame Photometer To Measure SodiumConcentration in a Sports DrinkChristopher N. LaFratta,*,† Swapan Jain,† Ian Pelse,† Olja Simoska,† and Karina Elvy‡

†Deparment of Chemistry, Bard College, Annandale on Hudson, New York 12504, United States‡Poughkeepsie High School, Poughkeepsie, New York 12603, United States

*S Supporting Information

ABSTRACT: The purpose of this experiment was to create a simple andinexpensive flame photometer to measure the concentration of sodium inbeverages, such as Gatorade. We created a nebulizer using small tubing andsprayed the sample into the base of a Bunsen burner. Adjacent to the flame was aphotodiode with a filter specific for the emission of the sodium D-line. Aftermaking a calibration curve, students could detect the amount of sodium inGatorade to within 10% of the value on the label. The system costs less than $500and can be assembled by students and used in an analytical chemistry laboratorycourse.

KEYWORDS: Elementary/Middle School Science, First-Year Undergraduate/General, Analytical Chemistry, Laboratory Instruction,Hands-On Learning/Manipulatives, Atomic Spectroscopy, Calibration, Food Science, Laboratory Equipment/Apparatus

Atomic emission spectroscopy is one of the oldest analyticaltechniques and it remains relevant today due to its high

sensitivity. First pioneered by Bunsen and Kirchoff, the analysisof light emitted from electronically excited samples helped leadto the quantum revolution in physics and chemistry at the endof the 19th century.1 Flame photometers have beencommercially available since at least the 1940s and continueto be sold because they are simple and robust instruments.2

Sodium and potassium are common clinical analytes and, whilethey are presently measured using ion-selective electrodes, theycan be accurately measured using flame photometry.3 The useof flames in atomic spectroscopy has been replaced byinductively coupled plasmas, as plasmas provide a hotter anda more stable sources of excitation.4 Nevertheless, flameexcitation remains useful as an analytical technique and anexcellent pedagogical method to illustrate a number ofimportant concepts related to atomic spectroscopy, combus-tion, atomization, and instrument design.In this article, we describe an experiment where students

build a simple flame photometer to quantify the concentrationof sodium in a sports drink. It is our opinion that the act ofconstructing an instrument is an important component in ananalytical laboratory course. These courses generally use manysophisticated instruments, the inner workings of which mayseem mysterious to students. This experiment gives thestudents the chance to assemble the instrument used to makethe measurement and exemplifies that at their core allinstruments rely on simple principles of physics and chemistry.The experiment ties together concepts ranging from atomic

structure and the Bohr frequency condition to good practicesfor calibration curve construction and instrument design. Wehave found that students from 8th grade to upper-levelundergraduates enjoy working with the colored flame and areexcited to find that their instrument can make a measurementthat agrees with the value on the bottle.

■ PHOTOMETER

Several other atomic emission experiments have been reportedin this Journal. One experiment is comparable to the presentwork,5 but most either describe the use of a commercialinstrument6 or the development of a spectrometer-basedinstrument,7−9 which while more versatile is also moreexpensive than the photometer described here. A schematicof the photometer is shown in Figure 1 and consists of threemain components: (i) a nebulizer, (ii) a Bunsen burner, and(iii) a filter-based detector. To operate the instrument, thesample is injected from a syringe and is nebulized bycompressed air. The mist is then sent into a natural gas flameand the emission is detected using a photodiode and avoltmeter. The nebulizer is easily constructed from tubing, glue,and a source of compressed air. All other components arecommercially available. The entire apparatus can be assembledfor less than $500. After assembly, students prepare and runstandard solutions to generate a calibration curve and thenanalyze an unknown. Results generally agree within 10% of thevalue on the nutrition fact label.

Published: January 16, 2013

Laboratory Experiment

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© 2013 American Chemical Society andDivision of Chemical Education, Inc. 372 dx.doi.org/10.1021/ed3004758 | J. Chem. Educ. 2013, 90, 372−375

■ EXPERIMENTAL DETAILS

Instrumental Assembly

The instrument assembly starts by securing the Bunsen burnerand the detector setup to the lab bench using tape. Thedetector is mounted on a post holder to facilitate alignmentwith the flame. The sample nebulizer is then mounted in amodified centrifuge tube that acts to guide the sample into theBunsen burner (Figure 2). The nebulizer can be made by the

instructor to save time. The nebulizer consists of two pieces ofpolyethylene tubing glued to the edge of a glass slide slightlyoffset from one another (Figure 3). Compressed air is sentthrough the shorter tube, which acts to create a mist from liquidsamples injected into the other tube. The nebulizer is theninserted into a slot cut into the opening of the disposablecentrifuge tube that also has a hole cut into its base. Thecentrifuge tube confines and aims the mist into the air inlet ofthe Bunsen burner. The other air inlets of the Bunsen burnernot receiving the mist are covered with labeling tape. A Petri

dish is placed under the centrifuge tube to collect mist thatcondenses from the walls of the centrifuge tube.After the natural gas flame is ignited, the compressed air is

turned on and the position of the nebulizer is adjusted until aninner cone forms in the flame indicating a good fuel-to-air ratio.An amplified photodiode with an attached filter is placedapproximately 10 cm from the flame. The bandpass filter iscentered at 589 nm and has a pass band 10 nm wide. Thedetector is connected to a low-pass RC filter made using a 15.8kΩ resistor and a 100 μF capacitor. The voltage across thecapacitor is measured using a voltmeter. A data point isrecorded while steadily injecting 1 mL of sample over thecourse of 10 s. The low-pass RC filter acts to suppress voltagefluctuations due to the flame flickering or inconsistent sampleinjection. The voltage for a sample is recorded approximately 8s after starting the injection. Alternatively, a data acquisitionsystem, such as a National Instrument DAQ card or a VernierLabQuest, could be used instead of the filter−voltmeter toaverage the raw voltage signal over the 10 s injection.A calibration curve is made by injecting standard solutions

having different concentrations. Each sample is run five timesand the average value of each solution is used to make thecalibration curve. The unknown is then also run five times.

Preparation of Standard and Unknown Solutions

Standard solutions were prepared by diluting a stock solution ofNaCl in water that contained 125 ppm of Na, in volumetricflasks. Seven standards, 25 mL each, were made having thefollowing concentrations: 0, 5, 10, 20, 30, 40, and 50 ppm Na. Asample of Gatorade G-Series 02 Frost Glacier Freeze flavorhaving a concentration of 457 ppm Na, according to its label,was diluted by 20× using a 25 mL transfer pipet and 500 mLvolumetric flask.

■ HAZARDS

The open flame poses a fire hazard and care should be taken toavoid getting clothing or hair too close to the flame. Also, careshould be taken when optimizing the nebulizer position toavoid blowing out the flame. If this occurs, shut off the gassupply and relight the flame after waiting a minute. The smallneedles used with the tubing also can be dangerous, as theysometime need to be reinserted into the tubing. This insertionshould be done by the instructor to avoid students prickingthemselves.

Figure 1. A schematic of the flame photometer.

Figure 2. Photograph of the flame photometer apparatus. The insetshows the RC-filter circuit.

Figure 3. Photograph of a nebulizer inserted into a slot cut in acentrifuge tube. The inset shows the air (“A”) and sample (“S”) tubesand how these tubes are slightly offset from one another to create afine mist.

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■ RESULTSTable 1 shows a typical data set collected by a pair of upper-level chemistry students after they assembled the photometerand ran their samples. The average background signal for thisdata set was 0.0438 V, which was subtracted from each raw datapoint (not shown).These data are graphed in Figure 4 and show a linear trend

up to 50 ppm Na. Using the equation of the best-fit line, the

concentration of sodium in the Gatorade was calculated to be21.6 ± 0.6 ppm. This value agrees within 6% of the expectedvalue from the label, which was 22.9 ppm following 20×dilution.

■ DISCUSSIONThe photometer gives results that confirm the nutrition factslabel and the students enjoyed building the apparatus to makethe measurement. Students work in pairs to assemble thecomponents (1−2 h), prepare the solutions (1 h), and measurethem (1 h). The entire experiment can be completed in 3−4 hdepending on how much guidance is given to the studentsduring assembly. Alternatively, using assembled equipment andsolutions made in advance, 8th grade students have performedthis experiment in 30 minutes. The calibration curve tends tohave a worse regression coefficient for the 8th grade databecause they do not perform multiple trials for each solution.Nevertheless, their results are often within 20% of the label.The data were collected using both a computerized data

acquisition system (e.g., Vernier Labquest) and a low-pass RCfilter and voltmeter. The data were recorded from bothsimultaneously for a given calibration curve and found not to besignificantly different. Thus, the low-pass filter was chosenbecause of its simplicity and cost and because it provides an

opportunity to teach how simple circuits are used ininstrumentation.The main source of error in the experiment is inconsistency

with the sample injection. Injecting the sample too quickly ortoo slowly causes the signal to fluctuate, and although thevoltage is filtered, these fluctuations can cause a deviation fromlinearity in the calibration curve. Students usually get better atcorrectly timing the injection after a few practice runs. Injectioncould be automated using a syringe pump, but this would costmore than the apparatus itself.Other sources of error include drafts, which cause the flame

to flicker and variations in ambient light. These issues can beavoided by proper placement of the apparatus in the lab and byavoiding sunlight or shadows being cast on the detector.Fluorescent room lights generally add slightly to the back-ground but are stable as long as nothing in the line of sight ofthe detector changes. For example, if the detector is aimed at anaisle and people walk by it, the signal may increase as lightscatters off of them.Some of these sources of error may be negated using an

internal standard, such as lithium, which is a commontechnique in atomic emission spectroscopy.4 An internalstandard was tried and the results were not significantly betterthan without the internal standard. Although it is a goodanalytical method to include an internal standard, the cost ofanother detector and filter for only a slight improvement in theresults was not justified.

■ CONCLUSIONWe have developed a simple flame photometer than can beconstructed by students and can be used to measure theconcentration of sodium in sports drinks. The system costs lessthan $500 and gives results that agree within 10% of the valuegiven by the manufacturer of the beverage. We have used thesystem with 8th grade students and with upper-level analyticallaboratory students and their response was overwhelminglypositive.

■ ASSOCIATED CONTENT*S Supporting Information

Instructor notes and student handouts. This material isavailable via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author

*E-mail: clafratt@bard.edu.

Notes

The authors declare no competing financial interest.

Table 1. Student Calibration Curve and Unknown Data

Background Corrected Data/V

[Na] (ppm) Trial #1 Trial #2 Trial #3 Trial #4 Trial #5 Average/V Std Dev/V

0 −0.0018 0.0002 −0.0018 0.0002 0.0032 0.0000 0.00205 0.0202 0.0182 0.0252 0.0202 0.0232 0.0214 0.002810 0.0472 0.0492 0.0392 0.0512 0.0522 0.0478 0.005220 0.0912 0.0882 0.0842 0.0782 0.0862 0.0856 0.004930 0.1182 0.1242 0.1232 0.1142 0.1282 0.1216 0.005540 0.1562 0.1462 0.1582 0.1602 0.1572 0.1556 0.005550 0.2032 0.1952 0.1832 0.1992 0.1882 0.1938 0.0081Gatorade 0.0892 0.0812 0.0892 0.0912 0.0812 0.0864 0.0048

Figure 4. Calibration curve generated by students. The red point iscalculated concentration of Gatorade based on the calibration curve.

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■ ACKNOWLEDGMENTSThe authors would like to acknowledge the Bard students inCHEM 350 who performed this experiment. K.E. would like toacknowledge support from the ACS-SEED program. S.J.,C.N.L., I.P., and O.S. thank the Bard Summer ResearchInstitute for funding.

■ REFERENCES(1) Rigden, J. S. Hydrogen: The Essential Element; Harvard UniversityPress: Cambridge, MA, 2003.(2) Cole Palmer Home Page. www.coleparmer.com (accessed Dec2012).(3) Tietz Fundamentals of Clinical Chemistry, 6th ed.; Burtis, C. A.,Ashwood, E. R., Bruns, D. E., Eds.; Saunders Elsevier: St. Louis, MO,2008.(4) Skoog, D. A.; Holler, J. F.; Crouch, S. R. Principles of InstrumentalAnalysis; 6th ed. Brooks/Cole: Belmont, CA, 2006.(5) Fragale, C.; Bruno, P. J. Chem. Educ. 1976, 53, 734−735.(6) Goodney, D. E. J. Chem. Educ. 1982, 59, 875−876.(7) Seney, C. S.; Sinclair, K. V.; Bright, R. M.; Momoh, P. O.;Bozeman, A. D. J. Chem. Educ. 2005, 82, 1826−1829.(8) Smith, G. D.; Sanford, C. L.; Jones, B. T. J. Chem. Educ. 1995, 72,438−440.(9) Goode, S. R.; Metz, L. A. J. Chem. Educ. 2003, 80, 1455−1459.

■ NOTE ADDED AFTER ASAP PUBLICATIONThe toc/abstract graphic contained an error in the versionpublished ASAP on January 16, 2013, the correct versionreposted on February 4, 2013.

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