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Table of Contents EXPERIMENT 18 Gravimetric Determination of the Iodide Ion Content in a Mixture INTRODUCTION The branch of analytical chemistry known as quantitative analysis concerns itself with the amounts of the various components within a sample, and it is often separated into three categories: gravimetric, volumetric, and instrumental. Earlier in the semester instrumental analyses were performed utilitzing the sophisticated device called a spectrometer. In this experiment a gravimetric analysis will be performed, and it will rely on traditional laboratory equipment: balances, beakers, Büchner funnels, and filter flasks. In gravimetric analysis, a substance is treated so that the component of interest is separated either in its pure form or as a constituent in a compound of known composition, which can be weighed. The separation is often accomplished by first dissolving a sample of the substance in a suitable solvent and then adding a reagent that reacts with the desired component to form an insoluble solid. A gravimetric precipitation method must fulfill certain basic requirements in order to provide an accurate analysis. 1. The precipitation reaction should be quantitative so that only a negligible fraction on the component of interest remains in solution after the precipitate is isolated. A moderate excess of the precipitating agent is commonly employed to help achieve this condition. 2. Other constituents in the sample solution should not interfere with the precipitation of the component of interest. 3. The precipitation should be as free of contaminants as possible. 4. The particles of precipitate should be large enough to filter easily and to facilitate the removal of soluble impurities during washing operations. 5. The precipitate should easily be converted to a form suitable for weighing. The conversion may require only a drying operation to remove residual moisture; or, if the precipitate has an indefinite chemical composition, an ignition to produce a solid with a definite composition. 6. In addition to a definite composition, the weighing form should have: stability at the relatively high temperatures required for the drying or ignition operations; little or no tendency to react with substances in the air, and a molar mass significantly larger than that of the compound of interest. The quantity of substance being analyzed may be anywhere from a few micrograms to several thousand kilograms. In the majority of cases, a sample, rather than the entire mass, is selected for analysis. Sampling a homogeneous material is relatively simple since the composition of the system is uniform throughout. In heterogeneous substances, samples must be taken from different sites within the substance. Solid materials must be dried to remove moisture before they are weighed. This operation is usually performed in an oven set at 110C. Once a sample is weighed, it is dispersed in an appropriate solvent system. The solution volume is selected to give the component of interest a concentration suitable for a successful precipitation. Temperature is important because it affects solubility. 179 The following are the major techniques that are used in a gravimetric analysis. 1. DESICCATION. Solid reagents, which must be kept dry, are stored in the low humidity environment of a desiccator. A layer of material, which readily absorbs moisture (a desiccant), maintains the low humidity. Popular desiccants include granular anhydrous calcium chloride, anhydrous calcium sulfate, anhydrous magnesium perchlorate, phosphorus pentachloride, and silica gel. A desiccator made from a plastic container with a top is popular since it is lightweight, easily handled, and inexpensive. Obviously, the cover of the desiccator should be kept in place as much as possible. A desiccant should be replaced at infrequent intervals, or whenever it is apparently caked. 2. PRECIPITATION. The heart of any gravimetric analysis is the precipitation reaction. In most cases the reaction involves the formation of an ionic solid due to cation-anion combination during the mixing of precipitating agent and sample solutions. 3. DIGESTION. A precipitate is seldom ready for filtration immediately after it is formed, because its particles are generally small and retain an unnecessarily large amount of impurities. In order to minimize both these problems, the precipitate is allowed to remain in contact with the liquid phase for a period of time prior to being filtered. The process is known as digestion. An increase in particle size is observed during a digestion, and is attributed to two processes: the dissolution of the very small, more soluble particles to yield ions that then reprecipitate on the surface of the larger crystals; and the coagulation of small particles to form aggregates. As digestion takes place, impurities adsorbed on the surface of the precipitate particles tend to dissolve. Although the impurities may eventually be readsorbed, the decrease in precipitate surface area, which goes along with larger particles, lessens the chance of adsorption. Thus, digestion yields purer as well as larger, more easily filtered particles. 4. TESTING FOR COMPLETE PRECIPITATION. After the precipitate has settled, a drop of the precipitating agent is added to the clear supernatant liquid. If the drop produces a cloud of new precipitate, additional precipitating agent must be added, and digestion continued. If the drop produces no visible precipitate, it may be assumed that the precipitation is complete. 5. SUCTION FILTRATION. To hasten the separation of a solid and a liquid, a filtration can be performed with the use of a vacuum. The funnel used in this filtration is called a Büchner funnel. Fitted with a piece of filter paper, the funnel sits on a filter flask, which is just an Erlenmeyer flask with a sidearm that can be connected to a vacuum with rubber tubing. 6. WASHING THE PRECIPITATE. After the removal of the supernatant liquid, the small amount of solution that remains along with a precipitate may serve as a source of contamination in later operations. In addition, foreign ions are likely to be absorbed by a precipitate as it forms. To remove these contaminants, the precipitate is washed. Although the wash liquid is often deionized water, occasionally a very dilute solution of the original precipitating agent is used. A precipitate can also be washed with acetone to remove water from the precipitate in order to decrease the drying time. The liquid is added directly to the precipitate container and is removed by suction filtration. 7. DRYING. Solid materials must be dried to remove moisture before they are weighed. This operation is usually performed in an oven set at 110C, and the solid is dried from several hours to overnight. 180 A traditional gravimetric analysis for iodide ion features the use of the reagent lead (II) nitrate to bring about the precipitation of lead (II) iodide from an aqueous solution containing a known mass of sample. The precipitate reaction is - 2+ 2I (aq) + Pb (aq) PbI2 (s) This reaction satisfies the important criteria for a satisfactory gravimetric method: namely, a quantitative reaction yielding a product of definite composition, essentially free from impurities. After a period of digestion, the lead (II) iodide is isolated by filtration, washed and dried to a constant mass. Since the composition of lead (II) iodide is known, a comparison of the precipitate mass with the mass of the original sample yields a value for the mass percentage of iodide in the substance being analyzed. STATISTICAL ANALYSIS In this experiment up to seven students will be analyzing the same unknown phosphate ion sample. The students with the same unknown will compile their data and summarize it in a useful manor. A complete summary of a data set must include a measurement of central tendency, and a measurement of dispersion. 1. CENTRAL TENDENCY. The prediction of a central or typical value for a measurement in a data set is called central tendency. The three most common measurements of central tendency are mean, median, and mode. The most familiar measurement of central tendency is the average, or mean. We usually denote the mean value of some variable by placing a bar over its symbol: x = x ____ n where x is the mean of n observations of x. As an example, if four students determined the percentage of phosphate ion in the sample to be 51.3%, 55.6%, 49.9%, and 52.0%, the mean percentage of phosphate ion in the sample would be: x = 51.3% + 55.6% + 49.9% + 52.0% = 52.2% __________________________________________ 4 Although we will not calculate them in this experiment, the other measurements of central tendency are median, the middle value of a data set, and mode, the value that appears most often in a data set. 181 2. DISPERSION. The prediction of a spread or distribution of measurements in a data set is called central tendency. The three most common measurements of dispersion are range, variance, and standard deviation. Range is the difference between the highest and lowest values in the data set. However, it is a poor indicator of the distribution of all the data points because it only depends on the highest and lowest values. This leads to a measurement of dispersion called the variance. The variance is the mean square deviation of all data points: 2 Vx = (x - x) ___________ n - 1 where Vx is the variance of the n observations of x, and the deviation is the difference between an individual data point and the mean: x - x. The demoninator in the definition is of some interest. Because x is calculated from the n individual x values, only n – 1 deviation calculations are independent, so the sum of the square deviations is divided by n – 1, not n. Consider a sample with 5 data points. First, we calculate x and then we begin to calculate the 5 deviations from the mean, (x - x). When we reach the fifth data point, the deviation is no longer independent, because the value of the fifth data point could be calculated from the mean and the four other data points before we even saw it! Therefore, Vx is the variance of the n – 1 independent calculations.
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