Determination of anions in water samples by High Performance Ion Chromatography.
In this experiment the concentration of a number of common inorganic anions in tap water and bottled water samples will be determined. This experiment illustrates the utility of high performance ion chromatography (HPIC) as an analytical tool. In ion chromatography, retention is based on the attraction between solute ions and charged sites bound to the stationary phase. Stationary phases for ion chromatography (ion-exchange resins) are frequently made by copolymerization of styrene and divinyl benzene. The divinyl benzene content varies from 1 to 16%; this increases the amount of cross linking of the insolule hydrocarbon polymer. The benzene rings can be modified to produce either a cation-exchange resin (containing negatively charged groups) or an anion-exchange resin (containing positively charged groups).
In HPIC, narrow columns with internal diameters 2-80 mm are used. These columns are packed with particles having an average diameter of less than 50 microns (50 x 10-6m). Resolution is considerably superior to that achieved with an ordinary column, in part because of the tight packing of the stationary phase, which reduces lateral diffusion, and because of the large surface area of the packing. Compared with classical column ion-exchange chromatography, where the columns are gravity fed and a separation can take hours or even days, HPIC can offer analysis times of 5-30 min.
HPIC is suited to the analysis of either anions or cations (depending on whether an anion-exchange or cation-exchange resin is used) and both ionorganic and organic ions can be separated using this technique. In this experiment an anion-exchange column will be used. In HPIC, the separation of analyte ions involves the electrostatic interaction between solute anions and the positively charges sites on the stationary phase (the greater the degree of electrostatic attraction, the stronger the anion is retained on the column).
HPIC has made significant practical impact on the areas of pharmaceutical, clinical, forensic, environmental and industrial research and development analyses, and is an ideal complementary technique to HPLC. Preparative ion-chromatography has also found an important use in the isolation and purification of various compounds.
In this experiment, the amounts of a number of commonly occuring anions in various commercially available bottled waters will be quantitatively determined. A series of anions of known concentration will be injected onto the column. The components can be identified by comparison of retention times to documented standards (see later). The peak area of each component can be calculated; this area is then used to determine the concentration of the unknown samples.
Stock solutions (1000 ppm) of seven anions are provided [F-, Cl-, NO2-, Br-, NO3-, HPO42-, SO42-]. Standard solutions (100 mL) of each anion must be prepared at the concentrations indicated in the table below by diluting the stated volume of stock solution to 100.0 mL with high purity ("MilliQ") water. The standard solutions must be sonicated ("degassed") for 20 minutes prior to injection.
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The standard anions may be identified by comparison with the IC-PAK A Anion Standard Chromatogram shown below.
Water samples must be filtered before being diluted 1:100 (1.00 mL of the filtered water sample diluted to 100.0 mL with MilliQ water). Inject 100 microlitres onto the column and record the trace. Determine the relative peak areas of the individual components. Calculate the concentration of those components present in the water samples by use of the equation below. Express these concentrations also in ppm (don't forget to take the 1:100 dilution into account!).
Compare the anion concentrations found with the Australian Drinking Water Guidelines (below) and with the stated analysis on the labels of the bottles water samples.
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Notes:
| 1. | ALL solutions must be made up with high purity ("MilliQ") water, filtered with the WATERS filter, and sonicated before use. |
| 2. | Pump eluant through the column for at least five minutes before attempting a separation. Thouroughly remove the eluant at the end of the session by pumping pure water through the column. |
| 3. | Be sure to record the injected volumes of standards and sample each time an injection is made. |
| 4. | Data should be presented in the form of a Table which is easily read and which clearly shows how the values were obtained. |
| 5. | Calculations should be clearly shown with an explanation for each step. |
Harris, D.C. "Quantitative Chemical Analysis; 5th Edition"; W.H. Freeman and Company: New York..