Determination of the Solubility Product of Copper (11) Tartrate Marcus W. Thomsen Franklin and Marshall College, Lancaster, PA 17604
One of the most important topics in the general chemistry course is aqueous equilibria. The formation and dissolution of "insoluble" salts provide some of the simplest introductions to these equilibria. Although the general chemistry laboratory provides an excellent opportunity to reinforce the defmition and determination ofK.,, many experiments on this topic are unfortunately often relegated to upper-level cours& in analytical chemistry or physical chemistry. The spectrophotometric determination of the solubility product of cupric tartrate is an appropriate experiment for the introductory chemistry course. The Experiment The objective of the experiment is the determination of the K., for copper(I1) tartrate, CuC4H406, by analysis of a saturated solution of the salt. The concentrationofthe species can be determined by spectrophotometricanalysis and the aoolication of Beer's Law because the c u ~ r i cion absorbslvkible light. Also, since the stoichiornet& for the dissolution of the salt requires that the cation and anion be present in a 1:l ratio, the determination of the cation's concentration also eives the wncentration of the anion. Once the concentratio& are known, the K , is readily calculated.
CoppedII) tartrate is an appropriate salt for this type of experiment for several reasons. First, the concentration of 'Milio, F. R.; Debye, N. W. G.; Metz, C. Experiments in Chemistw Harcourt, Brace, Jovanovich: New York, 1989; p 363. 'Material Safety Data Sheet, Johnson Matthey Chemicals, Danvers, MS. 3MaterialSafety Data Sheet, ICN Biornedicals, Cleveland, OH
the cupric ion in the saturated solution of this salt is hich enough to give reliable absorbance values as measured with a Spectronic 20. At the same time the concentrations are low enough to avoid the complications associated with i o ~ strength. c Also, simultaneous equilibria, if present, do not interfere simificantlv with the determination of K.. . Unlike other s z t s that Gave been used for spectropho& metric determinations (e.e.. . - . strontium chromate.' which is carcinogenic2),copper(I1) tartrate has little or no t~xicity.~ Finally, this salt is commercially available at moderate cost. This ex~erimentalso reinforces several fundamental experimentHl techniques in addition to the concept of the solubility product. The procedure requires the students to prepare standard cupric ion solutions by dilution using volumetric glassware. The Spectronic 20 is calibrated with the standard solutions through the preparation of a Beer's Law plot. Then the instrument is used as an analytical tool. Furthermore, the preparation of the copper(I1) tartrate solution emphasizes the meaning of a saturated solution. Also, the sp~ctrophotometricandysis provides an opportunity to review the electronic structure of ions. Procedure Saturated Cupric Tartrate Solution
A 0.10-0.15 g sample of CuC4H4O8(K and K Laboratories, Division of ICN, Cleveland, OH) is placed in a large test tube, and approximately 10 mL of distilled water is added. The mixture is heated in a hot water bath (4045 OC) for a t least 15 min, during which it must be stirred several times. The mixture is removed from the water bath and set aside to cool to room temperature. A filter crucible with n ~ackedbed of Celite (about 8 mm a small amount of Celite in thick) is prepared by the filter crucible and gently tamping it with the bottom of a small vial. The saturated wpper(I1) tartrate solntion is filtered through the Celite via vacuum filtration, and the fdtrate is transferred to a clean, dry test tube. The test tube is stoppered to minimize evaporation. Standard Cupric SulfateSolutions
The primary stock solntion of copper(I1) sulfate is prepared by dissolving CuS04 5H20 (5.0 x lo3 mol, i n which mass is known to the nearest 1mg) in water in a 100-mL volumetric flask. Three additional standard solutions are prepared by dilution of three separate aliquot8 of the stock solution. To prepare the second solution, an aliquot of 4.00 f 0.20 mL of the primary stock solution is transferred by buret to a clean test tube (or flask) and diluted with 5.00 mL of distilled water. The test tube is stoppered, and the resulting solution is gently mixed. [cuZ+I, M This procedure is repeated to dilute ali?er's Law plot for the absorbance of cupric ion as a functionof concentration (M). Corre- quots of 8.00 mL and 12.00 mL of the stock lion coefficient (?) = 0.991 solution with 5.00 mL of water to prepare
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the remaining standard solutions. Again, the volumes should be within 0.20 mL of the sueeested values, and the actual volume used should be measured to the nearest 0.01 mL. The concentrations of these four solutions are then calculated.
calibration curve. The K,, for cupric tartrate and the soluELof solution) is then determined. bilitv of this salt (in -
Results and Discussion A typical Beer's Law plot for this experiment shows good linearity (see the figure). In a typical laboratory section (14 Calibration Curve and Determination of the Kip students) the average correlation coefficient (12) for these plots was 0.96. Although a least-squares analysis of the The percent transmittance of each of the four standard data may be desirable, it is not essential for the reliable solutions is measured using a Bausch and Lamb Spectmdetermination of^,,. ~h~ mostsignificant sources oferror nic 20 that has been set to 675 nm. Each tube should be in this part of the experiment were the inaeeurate prepsrinsed twice with a small amount of the solution being ofstandard solutions by dilution and the extested before completely filling the tube for measurements. perimental emr associated with reading the scale on the The percent transmittance of the saturated copper(I1)tarSpectronic 20. trate solution is measured in the same manner. The average value ofK., as determined by 40 students in The Beer's Law plot (calibration curve) is ~ n ~ t ~byd e d severallaboratory sedions was 6 104at mom tempera. plotting absorbance for the standard solutions as a fumtwe (approximately24 ' C ) .This result compares favorably tion of cupric ion concentration. The slope of the best with the literature value of4 104 at 20 0c.4~ h , in straight line through the data points is then determined. this portion ofthe experiment was most commonlycaused The cupric ion concentration of the copper(II)tartrate soby improper filtration of the saturated copper(I1) tartrate lution is determined from the absorbance value using the solution. If undissolved tartrate salt were suspended in the solution, the scattering of light by this particulate matter 4~ange'sHandbook of Chemistry, 13th ed.; Dean, J. A., Ed.; would increase the apparent absorbance of the solution McGraw-Hill: New York, 1985. and thus increase the determined value ofK,,.
,,,,
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