Chemistry 119: Experiment 4. Titrimetric Determination of Acetic Acid Samples And Preparation of Youden Plot

Transcription

1 Chemistry 119: Experiment 4 Titrimetric Determination of Acetic Acid Samples And Preparation of Youden Plot In this experiment standard sodium hydroxide solution is used to titrimetrically determine acetic acid in two samples. Acetic acid is a weak acid. Since the equivalence point for the titration of acetic acid occurs at about ph 9, phenolphthalein is chosen as the indicator. Each student in the class analyzes the same two samples and a statistical analysis of the data obtained by the class is performed. The results are depicted graphically on a two-dimensional control chart called a Youden plot. A control sample is a sample which has a composition which is known to the person who is evaluating the performance of an analyst or an analytical method, but which is not known to the analyst. The analyst treats the control sample in the same manner in which other samples of the same substance are treated. In most cases the analyst is unaware that a control sample is being analyzed. In most practical chemical analyses an acceptable range of results exists. If the analytical result of a sample is within the range, the sample is judged to be acceptable. If the analytical result is outside the range, the sample is judged to be unacceptable. If a sample is judged to be unacceptable, either a corrective action is taken to make the analyzed sample acceptable, or the analyzed substance is discarded or not used. As an example, if a food product can contain a minimum of 1% and a maximum of 3% acetic acid, and if the food product is found by analysis to contain 5% acetic acid, a corrective action must be taken in order to bring the concentration of acetic acid in the food into the 1 to 3% range. That probably would mean that less of a concentrated acetic acid solution should be added to the food product. The analyst (and/or the analytical method) also can be tested with the aid of a Youden plot. In order to prepare a Youden plot, two control samples are chosen so that one of the samples has a concentration which is near the upper boundary of the acceptable range of results, and the second sample has a concentration which is near the lower boundary of the acceptable range. Concentrations near the upper and lower boundaries are chosen since analytical results in those two regions are especially important in determining whether any future action is required. The two samples are analyzed on a routine basis until at least 10 sets of analytical results are obtained with each sample. The means and standard deviations of the results for the two control samples are calculated and a two-dimensional control chart (the Youden plot) is prepared. 1

2 The x-axis on the chart is assigned values which correspond to the analytical results from one of the control samples, and the y-axis assigned values which correspond to the results from the second sample. A point on the control chart consists of one result from each of the two control samples. Action limits are drawn on the Youden plot which correspond to ±1 and ±2 standard deviations away from the means of the two control samples. The action limits form two boxes which are concentric around the point which corresponds to the means of the two control-sample results. A typical Youden plot is shown in Figure 5.1. Figure 5.1 A Youden plot. The means of the two control samples lie at the origin of the plot. The dashed line indicates the ± 1s limit. The solid line indicates the ±2s limit. After preparation of the Youden plot, it can be used, in a manner analogous to the way in which regular control charts are used, to check for determinate errors in the analysis (see Reference 2). Points which are symmetrically distributed around the means (the origin in the Figure) indicate random error in the analysis. Points which predominantly are found in the +x, +y quadrant indicate a positive determinate error in the analysis. Likewise, points which predominantly are found in the -x, -y quadrant indicate a negative determinate error. Points which predominantly occur in one of the other quadrants indicate a negative determinate error for one control sample and a positive determinate error for the other control sample. Student titrations generally show positive bias. 2

3 Youden plots are primarily used for interlaboratory testing of the same method of analysis and for testing of different methods of analysis for the same substance. In the experiment two acetic acid samples (the control samples) are titrimetrically analyzed with the standard sodium hydroxide solution which was prepared in Experiment 4. The pooled results from the class are used to prepare the Youden plot. Prelaboratory Assignment The sets of data which are listed in the Table below were obtained for the percentages of acetic acid in two control samples. Table 5.1 The Analytically Determined Percentage of Acetic Acid in Two Control Samples Sample Number Control Control Calculate the means and standard deviations (sx and sy) of the results obtained with control sample 1 (x) and control sample 2 (y). Prepare a Youden plot with the x-axis running from 5.0 to 5.5 and the y-axis running from 9.6 to Locate the origin of the two axes at the point given by the means of x and y. Plot the individual data points and draw a box outlining the ±s and a box outlining the ±2s action limits. Is there a noticeable trend in the data? Apparatus 50-mL burette mL Erlenmeyer flasks 25-mL pipette 250-mL volumetric flask 125- or 250-mL bottle Chemicals acetic acid control sample 1 (approximately 5% acid, 1 M) acetic acid control sample 2 (approximately 10% acid, 2 M) phenolphthalein indicator solution (0.1% in ethanol) standard sodium hydroxide solution (0.1 M; Experiment 4) Procedure 1. The standard sodium hydroxide solution which was prepared in Experiment 4 is used in this experiment. If Experiment 4 was not performed or if less than 300 ml of the 3

4 solution is available, prepare 1 liter of the solution as directed in steps 1 through 6 of Experiment Transfer about 50 ml of the stock solution of control sample 1 to the clean, dry storage bottle. Use a pipette to deliver 25 ml of acetic acid control sample 1 into a labeled, 250 ml volumetric flask. Dilute the solution to the mark with distilled or deionized water which has been boiled for about 5 minutes and cooled to room temperature. After thoroughly mixing the solution, use a pipette to deliver 50 ml of the solution into each of three, 250-mL Erlenmeyer flasks which are successively labeled 1, 2, and Add 2 drops of phenolphthalein solution to each of the three Erlenmeyer flasks. Fill a 50-mL burette with standard sodium hydroxide solution, and titrate the solution in each of the flasks to the end point (colorless to pink color change). Record the end-point volumes to the nearest 0.01 ml. Clean the Erlenmeyer flasks for use in step Use a pipette to deliver 25 ml of acetic acid control sample 2 into the cleaned 250-mL volumetric flask. Dilute the solution to the mark with boiled and cooled water. After the solution has been thoroughly stirred, use a pipette to deliver a 25-mL aliquot to each of the three 250-mL Erlenmeyer flasks which are successively labeled 4, 5, and Repeat step 3 with these three samples of control sample 2. Calculations 1. Use the concentration of the standard sodium hydroxide solution, the end-point volumes for each of the six titrations, the volume of the diluted sample (50.00 or ml) which was used in each titration, and the dilution factor which was used while preparing the titrand (25 ml diluted to 250 ml) to calculate three values of the molarity of acetic acid in control sample 1 and three values of the molarity in control sample Calculate and report the mean, standard deviation, and relative standard deviation of the results for control sample 1 and the mean, standard deviation, and relative standard deviation of the results for control sample 2. Report the means to your instructor as soon as you have calculated them.. 3. Use the pooled class data and the following instructions to prepare a Youden plot. Each set of data (mean concentration of sample 1, mean concentration of sample 2) from each student is used as a single data point. At least 10 sets of data should be used to prepare the plot. 4. Calculate the class average of the concentrations of control sample 1, and the class average of the concentrations of control sample 2. Calculate the standard deviation 4

5 (sx) for the results which were obtained with control sample 1, and the standard deviation (sy) for the results which were obtained with control sample Divide a sheet of graph paper into quarters by drawing the x and y axes so that the origin is in the center of the paper. Assign the coordinate of the origin the values of the class means which were obtained for the two control samples; i.e., the abscissa of the origin is assigned and the ordinate of the origin is assigned. Assign scales to the x and y ordinate axes values which make it possible to plot all of the class data points on the same sheet of graph paper. 6. Draw four vertical lines on the paper which correspond to ±sx and ±2sx away from x - for control sample 1. Similarly draw four horizontal lines on the paper which correspond to ±sy and ±2sy away from ȳ for control sample 2. Connect the vertical and horizontal lines so that they form two concentric rectangles as shown in Figure Plot the data sets obtained by each student so that the result which is reported for control sample 1 corresponds to the x value and each result which is reported by the same student for control sample 2 corresponds to the y values of a point on the plot. From the results determine whether any of the data points are suspect. LABEL them. The Youden plot prepared in the experiment can be used to check the analytical results of future analyses of the same two control sample solutions. 8. Report your results on the form provided. Further Reading 1. W. J. Youden, Anal. Chem., 20, 1136 (1948). 2. R. D. Braun, "An Introduction to Chemical Analysis," McGraw-Hill, New York, 1982, pp This experiment has been adapted from a laboratory manual authored by Professor S. D. Brown 5

6 Chem 119 Report Sheet: Experiment 5 Analysis of Vinegar Samples Name: TA: Section: (NaOH Molarity = M) Control Sample vol. NaOH M, HOAc Avg. M, HOAc Std. dev. (M) RSD % Control Sample vol. NaOH M, HOAc Avg. M, HOAc Std. dev.(m) RSD % Attach your Youden plot to this report sheet. Graph quality will be graded! 6