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The determination of inorganic anions was very difficult until the development of ion chromatography (IC) (1). IC is an HPLC version of ion exchange that has become the method of choice for routine anion analysis (2). In IC, ions are usually detected by measuring the conductivity of eluate (3). The two techniques for conductivity detection are "single column" or "nonsuppressed" ion chromatography (NSIC) and "suppressed" ion chromatography (SIC). Although SIC presents the advantage of a signal-to-noise ratio about one order of magnitude better than NSIC, NSIC is often selected because of its simple instrumentation. With the addition of a conductivity detector, any standard HPLC system can perform NSIC. Moreover, NSIC presents several analytical advantages: calibration from borate-gluconate is more linear than with eluents commonly used in SIC (4); and anion exchange and conductivity detection of very weak acids (e.g., borate, cyanide, sulfide, silicate) can be performed only by NSIC (5).
Because of the importance of IC, we felt a need for developing a series of IC experiments that could be integrated into existing laboratory courses in analytical chemistry. In the present paper we report one of these experiments: The determination of phosphate in cola beverages by NSIC.
Phosphate, when present at minor or trace levels, is currently determined spectrophotometrically, preferably as molybdenum blue or using the phosphovanadomolybdate method (6). These standard procedures are tiresome compared with recent procedures based in IC (7-9).
Colas are actually relatively rich in phosphoric acid, having concentrations of about 5 × 10^sup -3^ M (10), and may be considered as very suitable laboratory samples for the experiments (Coca-Cola®, Coca-Cola Light®, Pepsi-Cola®, Pepsi Diet®, Crystal Pepsi®, Schweppes Black Cola®, Tab®, etc.).
Experimental Procedure
Apparatus
A Waters 501 HPLC pump is used with a Waters IC-Pak A...