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UK CHE 226 - Experiment 8 - Determination of Copper by Electrogravimetry
Course Che 226-
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Department of Chemistry University of Kentucky EXPERIMENT 8 Determination of Copper by Electrogravimetry UNKNOWN Submit a clean, labeled 100-mL volumetric flask to the instructor so that your unknown copper solution may be issued. Your name, section number, and your locker number should be written legibly on this flask. The flask does not need to be dry on the inside, but needs to have been rinsed with distilled water after it has been washed. The flask must be turned in at least 1 lab period before you plan to do the experiment so that the Teaching Assistants will have time to prepare the unknown. Each student will have his or her own unknown to analyze even if you are working in pairs. BACKGROUND In electrogravimetry, a metal ion is quantitatively (>99.9%) electroplated onto a preweighed “working” electrode almost always as the solid metal. This takes place at the negative electrode or cathode, which is defined as that electrode at which reduction takes place. From the gain in mass of the electrode, the amount of metal in the sample can be calculated – assuming no interferants are also electroplated onto the electrode. A few metal ions can be deposited in some chemical form onto the anode, the electrode at which oxidation takes place. For example, under the appropriate experimental conditions, Pb2+ can be deposited onto an anode as PbO2. The two critical factors in performing a good quantitative analysis are finding the conditions so that 100% of the analyte of interest in the sample at hand is plated onto the electrode, and that nothing else is. Electrogravimetry is capable of very high precision and accuracy, + 0.1% or better, because the only measurements made are pairs of weighings, and weighing is readily capable of very high precision and accuracy. All the instrument does is apply a potential to the working electrode that is sufficiently high to force the deposition electrochemistry to proceed to completion. This can be accomplished with equipment as simple as a battery, some wire, and a few resistors. There are two basic variations of electrogravimetric method – controlled-potential or controlled-current. In controlled-potential electrogravimetry, a known constant potential is applied to the electrode for a sufficiently long enough time to plate out 100% of the analyte. The current decreases as the metal is plated out until only the small charging or background current flows. This provides a measure of selectivity if there are two or more metals that can plate out; the metal ion that is more easily reduced can often be plated out quantitatively without any of the CHE 226 – Analytical Chemistry Laboratory 45 Electrogravimetric CuDepartment of Chemistry University of Kentucky other metal also plating out. This procedure generally provides a nice smooth coating, but requires more sophisticated and expensive instrumentation and quite a long analysis time. In controlled-current electrogravimetry, a fairly large voltage is applied to the working electrode in order to force a constant relatively large current to flow through the cell. The current is often on the order of milliamps (mA) instead of microamps (µA), as is typically the case in controlled-potential electrogravimetry. If the concentration of metal ion in the electrolysis solution is insufficient to consume all the current, then other reactions such as hydrogen ion reduction must occur in order to consume all the current that is forced to pass through the cell. Obviously, if any interferents are present, they will also plate out; but the method is significantly faster than using a controlled potential and the required instrumentation is simpler and cheaper. In either electrolysis variation, the solution is stirred or agitated in some manner in order to speed up the analysis. In this experiment, you will plate a dilute, acidified solution of copper ion onto a pre-weighed platinum electrode: Cu2+(aq) + 2e- Æ Cu(s) The determination of copper is one of the easiest, oldest, and most important applications of electrogravimetry. One reason is that cupric ion does not require very negative potentials to be plated out quantitatively. Thus, most common metals do not also plate out and serve as interferents. The Eo for Cu2+ reduction is only +0.34 V, as opposed to the Eo’s for Pb, Sn(II), Ni, and Cd, and which are -0.13, -0.14, -0.25, -0.40, and -0.76 V, respectively. In general, the more positive the Eo for a redox couple, the easier a species can be reduced. Of the more common metals that can plate out under the conditions used in this experiment, only Ag(I), Hg(I), and Hg(II) with Eo’s = +0.80, +0,80, and +0.85 V, respectively, would interfere. INSTRUMENTATION Sargent-Slomin Electrolytic Analyzer. This is a (very old, if not positively ancient) dual-cell, constant-current electrolysis unit. These were first marketed probably before your parents were born, and the units in the laboratory are undoubtedly older than you are, perhaps twice your age. So much for “Waste, Fraud, and Inefficiency” in the CHE 226 Lab. PROCEDURE Preparation of Electrodes 1. Check out one or two pairs of Pt electrodes from the Teaching Assistant. Be very careful with these throughout the entire experiment; they are quite fragile, and extraordinarily CHE 226 – Analytical Chemistry Laboratory 46 Electrogravimetric CuDepartment of Chemistry University of Kentucky expensive. The larger, gauzy electrode is the cathode; the smaller one is the anode, which will rotate during electrolysis to stir the solution. 2. Clean the platinum electrodes by immersing them in the beaker of concentrated nitric acid provided (see the instructor) for about 5 min. This will dissolve any Cu that has already been plated on the cathode as well as removing traces of grease and other materials from the surface. Remove from the acid and rinse well with distilled water into a waste beaker. 3. Rinse off the wet cathode (larger outer electrode) with a few squirts of alcohol from a wash bottle, but not into any container containing acids. 4. Place the electrode on a conventional watch glass in an oven at about 105 °C for 5 min. Cool in a desiccator and weigh accurately. Don't touch the electrode body with your fingers. Grease will prevent copper from adhering properly and can increase the mass of the electrode. Electrolysis 1. Carefully dilute the unknown


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UK CHE 226 - Experiment 8 - Determination of Copper by Electrogravimetry

Course: Che 226-
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