50 Point Lab Report

10/17/11 OUTLINE FOR REPORT ON EXP V: CHLORIDE ISE (50 points max.) I. Purpose: Use one or two complete sentences to give a concise statement of the purpose of the experiment. Experimental Parameters: Theoretical Equations: Give the theoretical equations which relate the potential of the ISE/SCE pair to the activity of chloride and the concentration of chloride. What must be true if the latter equation is to be valid? What is the ideal slope of a plot of potential versus the logarithm of the concentration? Standard Additions Method: Give the parameters which correspond to the x and y axes of the standard additions plot and the equation used to obtain the concentration of the unknown from the plot. Be sure to define all symbols used in the equations
and parameters, and give their units. Sample Calculations: Include sample calculations with suitable headers for each of the following: Concentration of chloride in the calibration solution.
Use data obtained after the addition of 10 mL 0.1 M KCl. (Be sure to use the exact titrator reading for the 10 mL addition.) Slope, intercept, and the equation for the calibration line. (Be sure to use the actual variables, not y and x, in the equation.) The equation for the line must be calculated by the Linear Least Squares (LLS) method (Appendix V). All work for the LLS calculation must be shown in a separate table in the Sample Calculations section. Be sure to plot the data first, so that points below the LOD (and any other obviously stray points) can be eliminated from the calculation. Be sure to indicate which points have been eliminated. Theoretical electrode slope and the relative error of the measured slope from the theoretical value. Concentration of the Cl- unknown in the 250 mL volumetric flask in moles per liter and in ppm chloride using the equation of the calibration line. Wt% Cl- in the solid unknown. Y axis variable for the standard additions plot using data for the 5 mL addition. (Use your observed slope from the calibration plot when calculating the value of the y axis
variable.) Value of Vs' using the LLS equation for the standard additions line. Be sure to include the y-value for the original celery salt solution (Vs=0.000 mL) on the plot and

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in the LLS calculation.
All work for the LLS calculation must be shown in a separate table in the Sample Calculations section. Concentration of chloride in the 250 mL celery salt mixture in moles per liter and in ppm chloride. Calculation of the wt% NaCl in the solid celery salt. IV. Data and Results: The report must include two graphs (a calibration plot and a standard additions plot) and the data tables for each. (Note: These are not the same as the LLS tables, which belong in the Sample Calculations section.) Use graph paper ruled 10 sq/cm. Note that section C-2 on page 69 of the laboratory manual gives instructions for preparation of a table to aid in data analysis. As usual, in the report other necessary raw data (e.g., concentration of KCl, unknown weight and volume, temperature, etc.) and final calculated values (e.g., equation of the line, [Cl-] in the unknown, etc.) should appear in the table. The equation for each line should appear on the corresponding graph, and the limit of detection (in concentration units, not log [Cl-]) should be indicated on the calibration plot. Note that the number of significant figures in the concentration (number of decimal places in log C) changes as C increases. To make the graphing uniform use three places in all log C axis labels. Conclusion: State your results: wt% Cl- in the solid unknown; wt% NaCl in the celery salt. Then discuss any problems and notable sources of error pertinent to your results. Also, give a complete
discussion of the following possible systematic errors (even though they probably did not happen in your own experiment). a) You failed to quantitatively transfer your unknown (step 4) from the beaker to the volumetric flask. b) In the measurement of E for your unknown, you immediately recorded the voltage after pressing START on the Metrohm 702 automatic titrator; but you waited for the voltage to equilibrate, when you ran the standards. For b, include a sketch of a calibration plot to explain how the error would affect the calibration line and the unknown. (On the sketch, use a solid line to represent the correct calibration line and a dotted line to represent the line with the systematic error.) For both a and b, state the effect on the wt% Cl- in the white solid unknown (falsely high, falsely low, or unaffected) and clearly explain your reasoning. Also discuss the advantages and disadvantages of the use of ISE's and direct potentiometric methods of analysis.
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