Determination of Zinc and Nickel Concentration by (1) Ion-Exchange Chromatography Followed by Chelometric Titration, and by (2) Atomic Absorption Spectroscopy of the Mixture
Dates of Experiment: 10/14/08 through 10/30/08
Date of Report: 11/7/08
In this experiment, the zinc and nickel contents of unknowns were tested using two methods. In the first method, nickel and zinc were separated through ion-exchange chromatography and analyzed through chelometric titration. In the second method, the unknown was analyzed through the atomic absorption spectroscopy (AAS) of the mixture.
In and ion-exchange column, the ions are separated due to their tendencies to interact with the fixed phase of the column. In this case, the anion-exchange resin is that fixed phase. Nickel passed through the column unhindered and therefore first. Zinc formed chlorozincate anions that reacted with the resin. It could not pass through the column until a neutral aqueous solution was run through.
The elements were determined quantitatively by titration with ethylenediaminetetraacetic acid (EDTA). This method is valued as a good estimate and is used widely because of its use of common lab equipment. Only a single column was run for each metal in the interest of time. To acquire a meaning for the effect that the method has on the outcome, one should split the sample into replicates before the column. However, this may lower the accuracy, reduce the precision, and increase the time of the analysis.
In AAS, ions are distinguished due to the wavelength of light emitted when atomized ions are passed through a flame. Each element has a characteristic wavelength of light emitted. The instrument is calibrated using standard solutions prior to passing samples through. The results are interpreted using a calibration curve. This method is valued due to its efficient speed, ease of use, high selectivity, accuracy, and precision. A disadvantage is that it requires a special piece of machinery to do the experiment.
In order to prepare the ion-exchange column, glass wool was placed above the stopcock of a buret. Thirty to 40 milliliters (mL) of anion-exchange resin were added to the buret. A wash of 100 mL of two molar HCl was passed through the column. The sample was prepared by adding 16 mL of concentrated HCl to 75 mL of unknown solution and diluting that to 100 mL. The 30 mL sample was transferred to the top of the column. Four hundred mL of two molar HCl were washed through the column after the sample at a rate of 5 mL a minute. All of the wash was collected in a 500 mL beaker. The beaker was replaced with another 500 mL beaker in order to catch the next wash of 450 mL of deionized water.
The first beaker contained the nickel sample. It was divided into three aliquots, and the liquid was evaporated. Each residue was dissolved in 100 mL of distilled water and mixed together. Approximately 10 mL of pH 10 buffer were added. The entire solution was diluted to 500 mL. Half a gram of murexide indicator was added to the dilution. Three 50 mL aliquots were titrated with standardized EDTA until the color changed from purple to yellow. All data was recorded in Table 1.
The second beaker contained the zinc sample. It was divided into three aliquots, and approximately five mL of pH 10 buffer were added to each beaker. Two drops of Eriochrome Black T indicator were added to each sample. Each was titrated against standardized EDTA until the color changed from red to blue. All data was recorded in Table 2.
For AAS, two large samples were prepared. For nickel analysis, 33 mL of unknown solution were diluted to 50 mL. For zinc analysis, 2 mL of unknown solution were diluted to 50mL. Three standard solutions of 20, 40, and 60 parts per million (ppm) of nickel were run through the AAS instrument. All absorbencies were recorded in Table 3. A calibration curve was made from that data (Figure 1). The solution for nickel analysis was run through the instrument for three tests. All absorbencies were recorded in Table 4. Three standard solutions of 1.2, 2.4, and 3.6 ppm of zinc were run through the AAS machine. All absorbencies were recorded in Table 5. A calibration curve for zinc was made from that data (Figure 2). The zinc analysis solution was run through the AAS for three tests. All absorbencies were recorded in Table 6.
Table 1: Titration of Nickel in 0.0100 M EDTA
Table 2: Titration of Zinc in 0.100 M EDTA
Table 3: Standard Solutions of Nickel
Table 4: Unknown Absorbencies of Ni
Table 5: Standard Solutions of Zinc
Table 6: Unknown Absorbencies of Zn