DETERMINATION OF THE DECOMPOSITION PRODUCTS OF CALCIUM OXALATE USING THERMAL GRAVIMETRY AND INFRARED SPECTROMETRY

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1 DETERMINATION OF THE DECOMPOSITION PRODUCTS OF CALCIUM OXALATE USING THERMAL GRAVIMETRY AND INFRARED SPECTROMETRY Objective: The objectives of this experiment are: (1) to determine the stability of various decomposition products of calcium oxalate using thermal gravimetry. (2) to identify the evolved gasses resulting from the decomposition of calcium oxalate using infrared spectrometry. Text Reference: H.H. Willard, et. al. Instrumental Methods of Analysis, 7th edition, Wadsworth Publishing Co., Belmont, CA 1988, p Skoog, Holler and Nieman Principles of Instrumental Analysis, 5th edition, Saunders College Publishing, Fort Worth, TX 1997, Ch 31. Introduction: Any technique in which a physical property of a substance and/or its reaction products is measured as a function of temperature can be classified as thermal analysis. While more than ten thermal methods are know, differing is measured properties and temperature profiles, there are three methods in which chemical rather than physical information about samples is obtained. These methods include thermogravimetric analysis (TGA), differential thermal analysis (DTA) and differential scanning calorimetry (DSC). These methods are used for both quality control and research applications on such materials as polymers, pharmaceuticals, minerals, metals and alloys. An example of this type of analysis is the determination of the glass transition temperature of a polymer as a function of its molecular weight. In the present experiment, thermal gravimetry will be used to measure the mass of a sample in an inert atmosphere as a function of time (temperature) as the temperature of the sample is increased. The plot of mass (or mass percent) as a function of time or temperature is called a thermogram or thermal decomposition curve. Although thermogravimetric methods are limited to processes in which the change in mass of the analyte results from a temperature change, (i.e., vaporization, sublimation, decomposition and oxidation reactions) the thermograms provide mechanistic information about the process.

2 The basic components of a modern thermogravimetric instrument are 1) a highly sensitive analytical balance (capable of measuring masses to 0.1 mg), 2) a furnace, 3) a purge gas system and 4) a data control, acquisition and display system. To this system, we will add a gas cell, connected to the purge gas system via a heated transfer line, so that IR spectra of the gasses evolved from the sample can be obtained. Experimental: Use the following procedure to acquire data: 1. From the Main Menu, select Set Up and Run, F2. 2. Select, Recall Method, F2 and using the mouse click on "CaOx" followed by Recall File, F8. 3. Select, Modify Parameter, F3 on the Set Up and Run menu to enter your own information concerning Sample ID, Comment, Operator ID and File Name. In all cases type in each modifications and press Enter to change the entry. To automatically zero the balance and read the weight of your sample, proceed as follows: a. Press the Direct Control (Ctrl-F9) key. A new menu of function key options will be displayed on the screen which permit the positioning of the furnace. b. If the furnace is currently in the run position (that is, the furnace is surrounding the sample pan assembly and is locked on the upper glass ball joint), select Lower Furnace, F3. The furnace will be automatically lowered to allow access to the sample pan. NOTE: When removing or replacing sample pans and stirrups, do not pull on the hangdown wire as it may be broken off from the arm of the balance mechanism. c. Move the sample loading platform directly under the sample pan. Raise the platform up until the pan rests on the platform and the stirrup raises off the bend in the hangdown wire. Using tweezers, remove the sample pan and stirrup assembly. d. Remove any residue that may be in the pan. Place the sample pan and stirrups back on the platform so that the stirrup will hook on the hangdown wire when the sample loading platform is lowered. e. Lower the sample loading platform. The sample pan and stirrup should be hanging on the hangdown wire. Swing the sample loading platform back to the safe position.

3 f. Select Raise Furnace, F2. The furnace will be automatically raised to the run position and locked in place. Press Exit, F1. g. Wait several seconds for the sample pan to stop swinging. When the weight signal displayed in the status window stabilizes, select Sample Zero and press Insert The weight signal will be automatically set to zero. h. Select Direct Control (Ctrl F9) and Lower Furnace, F3. The furnace will be automatically lowered to allow access to the sample pan. i. Using tweezers, remove the sample pan and stirrup assembly and place approximately 25 mg (almost a full pan) of calcium oxalate into the sample pan. Place the sample pan and stirrups back on the platform so that the stirrup will hook on the hangdown wire when the sample loading platform is lowered. j. Lower the sample loading platform. The sample pan and stirrup should be hanging on the hangdown wire. Swing the sample loading platform back to the safe position. k. Select Raise Furnace, F2. The furnace will be automatically raised to the run position and locked in place. Press Exit, F1. l. Wait several seconds for the sample pan to stop swinging. When the signal in the status window stabilizes, select Sample Weight and press Insert The exact sample weight will be automatically read and stored in memory. m. Press Exit, F1 to exit from the Sample Weight menu. 4. Connect the heated transfer line to the gas outlet. Use care in making this connection since both the transfer line and gas outlet are fragile (and expensive). DO NOT tighten the knurled nut beyond finger tight. 5. Turn on the power to the transfer line and sample cell. Wait until both reach 120 o C. 6. When the transfer line and sample cell temperatures to reach 120 o C, acquire a Abackground@ spectrum on the IR spectrometer. Save this file as bkg.spa using Save As under File. Under Collect, select Collect Setup and then select AUse specified background file@. Click on OK. This spectrum will be used as the background during the course of the experiment. 7. Prior to the start of the run, press Go to temperature, F11, and enter 50 o C. Wait a few minutes for the system to attain this temperature and to stabilize. Both the temperature and the weight are displayed as well as an "X" on the y-axis.

4 8. To start the run, select Start Run, F8, on the Set Up and Run menu. The analyzer will run according to the parameters in the method, and the curve will be drawn in the graphics window of the screen as the data is collected. [In this experiment, the temperature will initially by ramped from 50 to 950 o C at a rate of 10 o C/min.] 2 A curve similar to the one below should be generated. As the weight losses 2 occur, collect a series of IR spectra of the evolved gasses. 9. To stop a run in progress, select Stop Run, F7, in the displayed menu. Press Enter to continue. Use the following procedure to analyze data: 1. Select Display Segments, F1. To change the display limits, select Optimize Data, F5, followed by Rescale, F2. Use the Type in X Limits and Type in Y Limits function keys and type in the limits. Pressing Rescale, F2, after entering the limits will rescale the display. Select Exit, F1,(twice) to return to the Run menu. 2. Select Select Calc, F6, followed by Multi-calc, F8, followed by Delta Y, F5; two X- shaped cursors will appear at the first and last points displayed on the screen. The X and Y positions of the cursors will also be displayed above the screen, and a new menu will be displayed.

5 3. Move the mouse pointer to select the left height of the first weight-loss step and leftclick. Move the mouse pointer to select the right height of the first weight-loss step and right-click. 4. Select Calc, F2; two lines appear on the screen tangent to the curve along with the % wt change, Y, for that step. 5. Select Exit, F1 followed by Delta Y, F5 and now position the X-shaped cursors at the next weight-loss step as in 3 above. Select Calc, F2 and a second set of lines with the corresponding Y value are displayed. 6. Repeat the calculation of Y for each step. Obtain a print out of the display by pressing the Print Scrn key. 7. Using the Exit keys, (F1, twice) return to the Set Up and Run menu. Results: Include in your report the thermogram and representative IR spectra for the various evolved gasses. Provide an interpretation of the IR spectra and the identification of the evolved gasses. From the thermogram, determine the weight of the stable decomposition products. Relate the percent weight losses of the various decomposition products of calcium oxalate to the molecular weights of the evolved gasses.