Using Control Charts Early in the Quantitative Analysis Laboratory

Communication Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX

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Using Control Charts Early in the Quantitative Analysis Laboratory Curriculum Dane Scott* and Daniel Firth

J. Chem. Educ. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 04/26/19. For personal use only.

East Tennessee State University, 325 Treasure Lane, Johnson City, Tennessee 37614, United States ABSTRACT: Statistical process control (SPC) is used in the chemical industry to monitor manufacturing and laboratory processes to ensure quality and compliance with regulatory requirements. Control charts are a key tool used in this monitoring. Industrial job postings desire experience with SPC. Most undergraduates entering the workforce have no exposure, let alone experience, with control charts. The few available literature examples of control charts in undergraduate chemistry education involve methods of instrumental analysis at the junior or senior level of an academic program. Educators may improve the student’s preparation for working in industrial and regulatory environments by incorporating components of SPC early in the curriculum. This work provides an example of how to introduce the concept and use of control charts earlier as part of the Quantitative Analysis Laboratory curriculum. The titration of vinegar to determine the weight percent of acetic acid, using the same sample for all students, serves as a platform for this introduction. Using a provided control chart generated from historical student data, students stated in a written laboratory report if their results were within control. The scored laboratory reports and questions on the written final exam assessed student learning and retention of how to use a control chart. Meeting the learning outcomes for the laboratory exercise required the student to report the correct weight percent of vinegar and state whether their result is within control. The learning outcomes on the written final exam were met when the student answered the questions correctly, stating the given result was out of control and suggesting correct experimental changes. The goal was to see 70% or more students meet the learning outcomes. Assessment showed that a simple titration experiment enables the introduction of how to use control charts during the Quantitative Analysis Laboratory curriculum. KEYWORDS: Second-Year Undergraduate, Curriculum, Analytical Chemistry, Acids/Bases, pH, Quantitative Analysis, Calibration

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for hematocrit ratios in blood.13 Data points above or below the control limits are not within control. Chemistry undergraduate students continue to receive rare exposure to control charts.14 At best, they may notice an example of a control chart in their textbook.8 Most examples of introducing control charts in educational curriculum involve instrumental techniques typically covered near the end of the undergraduate curriculum.15−20 Many current job postings for analytical chemists are looking for candidates with skills in statistical and quality-control methodologies. As such, understanding control charts better prepares students for careers in chemistry and industry. Introducing control charts early in the curriculum and as often as possible is advantageous and facilitates deeper understanding. An Undergraduate Quantitative Analysis Laboratory course is an ideal setting to introduce the concept of control charts before students learn about and use instrumentation. Any quantitative laboratory experiment in which all students in the class analyze the same sample provides an opportunity to collect historical data and develop a control chart for the class to use. This work describes a practical example of how to introduce a control chart and assess student learning using the determination of weight percent of acetic acid in vinegar. In this laboratory experiment, the students use a pH probe to

BACKGROUND Analytical methods using statistical monitoring of standards or retained samples is a common and expected practice in commercial, regulatory, and industrial analytical laboratories to ensure quality.1−7 Undergraduates typically receive little instruction on control charts.8 At best, their only exposure is through a side-bar mention in a text book. Control charts may be introduced in undergraduate laboratories without burdening students or distracting from the primary educational objectives of these courses. Students seeking employment in the chemical industry will benefit from greater exposure to control charts and developing of this skill.9 Control charts are an important statistical tool used to monitor and control the quality and performance of processes. First introduced by Walter A. Shewhart at Bell Laboratories in the 1920s, control charts find wide use outside of academics and have been extended to include analysis of continuous variables.10,11 Control charts provide a wealth of information such as performance history, acceptable results, and immediate feedback as to whether or not the process is in control and if a shift in the mean value has occurred.12 Being able to read a control chart enables the analyst to understand and visualize normal variation in the process. This skill avoids unnecessary process changes and identifies the need for potential corrective actions. One type of control chart is generated by plotting the means of measurements (y-axis) against time-ordered observations (x-axis). Upper and lower control limits (UCL and LCL) are determined using eq 1. Any individual result within these limits is within control. Figure 1 shows the control chart © XXXX American Chemical Society and Division of Chemical Education, Inc.

Received: September 28, 2018 Revised: March 27, 2019

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DOI: 10.1021/acs.jchemed.8b00791 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Communication

Figure 1. Example of control chart for hematocrit ratios in blood. Reprinted with permission from ref 13. Copyright 2018 Elsevier.

monitor the titration of a diluted vinegar sample with sodium hydroxide standardized in a previous laboratory session.

Box 1. Experimental-Problem Statement

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You are a new employee in the analytical control laboratory at Golden Sky, a manufacturer of vinegar sold under store brand labels across the country. The weight percent acetic acid in vinegar is a critical parameter in determining whether batches are acceptable to ship. While color indicators can be used to determine the percent acetic acid, they are not precise or accurate enough to meet FDA regulatory requirements. As such, you have been trained to run the method for the potentiometric determination of acetic acid in vinegar. The laboratory maintains a control chart of the average weight percent acetic acid to monitor the stability of the method. Each analyst in the laboratory analyzes an aliquot periodically to verify that the method is in control. As a new employee, you must demonstrate your ability to produce acceptable data before being certified to run production samples. Acceptable performance is defined as producing measured results that fall within the upper and lower control limits of the control chart. Since this method is critical to production, samples are run in triplicate

LEARNING OUTCOMES Students gain an understanding of what a control chart is and how to monitor and control an analytical method. They generate data and compare results to a control chart. If the results are not in control, the student must suggest a corrective action that is expected to result in the method being brought back into control.

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METHODOLOGY The students analyze vinegar purchased locally for weight percent of acetic acid. Previous student data permitted development of a control chart with upper and lower control limits. To introduce the experiment, the students are given a problem statement describing the purpose of the laboratory experiment and background on how to use control charts. A successful outcome occurs when a student reports that their analysis of the vinegar resulted in a weight percent that was within control. If their result is not in control, the student is required to suggest why and what steps are needed to bring the result into control. Assessment of learning is also determined using questions about control charts on the written final exam.

Once the mean weight percent is determined, students compare their result to the provided control chart and assess whether their result is within the control limits. In most cases, student’s results are within the limits. If a result is not within control, the student must suggest changes to obtain a result that is within control. If an error results from calculating the weight percent, or the control statement is omitted or incorrect, students are encouraged to recalculate and submit corrections for some additional points but not full credit as an incentive to foster learning.

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HAZARDS This lab involves the use of solid sodium hydroxide, potassium hydrogen phthalate, vinegar, and pH storage solutions. In performing this lab, proper personal protective equipment should include goggles that are splash proof and a lab coat. Any skin contact with these chemicals should be reported and washed for at least 5 min with running water. Any accident should be reported to your instructor or lab manager.

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PEDAGOGY The problem statement in Box 1 introduces the experiment. In this experiment, students are provided with samples from the same bulk sample of commercial vinegar purchased locally. They dilute 25.00 mL of the vinegar to 250.0 mL in a volumetric flask. Students titrate three 25.00 mL aliquots of the diluted vinegar, recording pH and volume of sodium hydroxide added. The sodium hydroxide solution was prepared in a previous laboratory session. After calibrating the pH meter, students begin by adding 5.00 mL increments of sodium hydroxide to the diluted vinegar, recording the pH after each addition. Smaller-volume increments of sodium hydroxide are added near the equivalence point. The end points are determined by calculating the first and second derivative plots of pH and the volume of sodium hydroxide added.

RESULTS

Control Chart

To construct the provided control chart, actual results for 20 student pairs were collected from three laboratory sections during the Fall 2017 term. The procedure outlined in the fifth edition of Exploring Chemical Analysis by Daniel C. Harris (p 107) was used to develop a control chart.21 The upper control limit (UCL) and lower control limit (LCL) were calculated using eq 1.13,21 3s UCL, LCL = X̅ ± (1) N In eq 1, X̅ is the mean weight percent, s is the standard deviation of the weight percent, and N is the number of student results. Inclusion of warning limits unnecessarily complicates the lesson. Figure 2 shows the control chart B



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Exam Assessment

A written final-exam question assessed retention with respect to the use of control charts. The written final exam included two questions, provided in Box 2, and the control chart in Box 2. Final-Exam Questions 1. You start working for Golden Sky Vinegar, LLC in May of 2018. Your task is to test a new batch of 5.0% vinegar before bottling and shipment. To save the expense of new reagents, your supervisor tells you to use the sodium hydroxide left behind by a former employee. You notice the sodium hydroxide dated 2/30/2018 and with former employee initials. You test a sample of vinegar and determine that the mean weight percent is 5.985%. Given this result and the provided control chart, is your result within control? 2. Given your answer in question 1, justify your answer. If the results are not in control, state what the possible cause is and what necessary steps you would propose to bring the results within control.

Figure 2. Initial control chart generated from Fall 2017 class data.

provided to the students for the laboratory exercise and for assessment of learning on the written final exam. Future class data permits updating the control chart. Summer and Fall 2018 Student Results

Students in the summer and fall of 2018 were asked to use the provided control chart to assess the quality of their results with respect to the control limits. Table 1 summarizes actual Table 2. Summer and Fall 2018 Student Final-Exam Results and Their Determination of the Method Being in Control

Table 1. Summer and Fall 2018 Actual Student Results and Control Statements Student

Reported Weight Percent (%)

1 2 3 4

5.0 5.22 5.212 5.059

5 6 7 8 9 10 11 12 13 14

5.276 5.14 5.372 5.343 5.011 5.29 4.921 5.201 5.14 5.35

Uncertainty, ± (%)

Summer 2018 0.1 0.00 0.034 0.0268 Fall 2018 0.000 0.00 0.01863 0.000 0.015 0.00297 0.134 0.44 0.09 0.14

Control Statement In control In control Not stated Not stated In control In control Not stated In control In control Not stated In control In control In control In control

student results and their control statements. Because these are actual student results, the significant figures of answers and uncertainties vary. Points were deducted for having the wrong number of significant figures.

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Student

Results in Control

1 2

Yes No

3 4

No No

5

No

6 7 8 9

No No No No

10 11 12

No Yes No

13

No

14

No

Cause

Suggested Action

Summer 2018 None None NaOH Remake and standardize the NaOH NaOH Restandardize the NaOH NaOH Restandardize the NaOH Fall 2018 NaOH Remake and standardize the NaOH NaOH Restandardize the NaOH NaOH Restandardize the NaOH None Error is too high NaOH Remake and standardize the NaOH NaOH Prepare a new NaOH solution None None NaOH Remake and standardize the NaOH NaOH Remake and standardize the NaOH NaOH Remake and standardize the NaOH

Figure 2. Table 2 lists the results from the final-exam questions. The expected answers are that the results are not in control, and appropriate corrective action involves preparing a new solution of sodium hydroxide or restandardizing the existing solution. To meet the learning outcome the student must have stated that the result is not within control. The suggested actions considered correct were to restandardize the existing sodium hydroxide solution or prepare and standardize a fresh solution. On the basis of the scored final exams, 79% of the students answered the final-written-exam questions correctly, meeting the learning outcomes. More than 70% of students met the learning outcomes, which was the desired outcome in

ASSESSMENT

Control Statement

Students 3, 4, 7, and 10 did not state whether their determined weight percent was in control or not. These students chose not to submit a corrected version of their report. This resulted in 71% of students achieving the learning outcomes for the laboratory exercise determining the weight percent of acetic acid in commercial vinegar. C



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introducing the development and use of control charts early in the curriculum.

AUTHOR INFORMATION

Corresponding Author

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*E-mail: [email protected].

DISCUSSION Data from previous classes was used to generate a control chart for the students to use as an example, to expose students to control charts and teach them how to use the charts. The laboratory exercise resulted in 71% of the class stating that their results were within control, which is acceptable. From Table 2, all students except for three correctly determined that the results were out of control and suggested the correct action. This corresponded to 79% achieving the learning outcomes for understanding control charts. These results indicate that 70% or more of the students achieved the learning outcomes for the laboratory exercise and written final exam. There is key information that must be taught during lecture or the prelaboratory discussion to ensure that students meet the learning outcomes. Instructors teaching this laboratory session and class need to include a discussion of control charts and of what standards are and how they are used. Additionally, students must learn that the concentration of sodium hydroxide decreases over time because of reaction with carbon dioxide. One method of emphasizing this is to have students use the previously standardized sodium hydroxide near the end of the laboratory sequence and perform a titration with potassium hydrogen phthalate. Students obtain a lower concentration for sodium hydroxide. This exercise reinforces the idea that standardized solutions have a concentration that can change over time and may require restandardization. The students then have the necessary information for the written final exam without being given the answer directly. Overall, using a simple titration for determining weight percent of acetic acid in vinegar is a way to introduce the use of control charts for the first time early in the Quantitative Analysis Laboratory curriculum. Later applications of control charts in instrumental laboratory courses reinforce students’ abilities to use and implement control charts in future positions.

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Communication

ORCID

Dane Scott: 0000-0003-0018-7189 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the East Tennessee State University Department of Chemistry for the purchase of pH meters for the laboratory exercise.

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REFERENCES

(1) Albazzaz, H.; Wang, X. Z. Statistical Process Control Charts for Batch Operations Based on Independent Component Analysis. Ind. Eng. Chem. Res. 2004, 43 (21), 6731−6741. (2) Mukundam, K.; Varma, D. R. N.; Deshpande, G. R.; Dahanukar, V.; Roy, A. K. I-MR Control Chart: A Tool for Judging the Health of the Current Manufacturing Process of an API and for Setting the Trial Control Limits in Phase I of the Process Improvement. Org. Process Res. Dev. 2013, 17 (8), 1002−1009. (3) McAvoy, T. Model Predictive Statistical Process Control of Chemical Plants. Ind. Eng. Chem. Res. 2002, 41 (25), 6337−6344. (4) Sitoe, B. V.; Mitsutake, H.; Guimarães, E.; Gontijo, L. C.; Santos, D. Q.; Borges Neto, W. Quality Control of Biodiesel Content of B7 Blends of Methyl Jatropha and Methyl Crambe Biodiesels Using MidInfrared Spectroscopy and Multivariate Control Charts Based on Net Analyte Signal. Energy Fuels 2016, 30 (2), 1062−1070. (5) Sistare, F.; St. Pierre Berry, L.; Mojica, C. A. Process Analytical Technology: An Investment in Process Knowledge. Org. Process Res. Dev. 2005, 9 (3), 332−336. (6) Gejdoš, P. Continuous Quality Improvement by Statistical Process Control. Proc. Econ. Fin. 2015, 34, 565−572. (7) Abbas, N. Homogeneously weighted moving average control chart with an application in substrate manufacturing process. Computers & Industrial Engineering 2018, 120, 460−470. (8) Laquer, F. C. Quality control charts in the quantitative analysis laboratory using conductance measurement. J. Chem. Educ. 1990, 67 (10), 900. (9) Dickey, D. A.; Dickey, M. D.; Stewart, M. D.; Willson, C. G. An Automated Statistical Process Control Study of Inline Mixing Using Spectrophotometric Detection. J. Chem. Educ. 2006, 83 (1), 110. (10) Brownlee, K. A. Construction and Use of Statistical Control Charts on Continuous Variables. Ind. Eng. Chem. 1951, 43 (6), 1307− 1310. (11) Wernimont, G. Use of Control Charts in Analytical Laboratory. Ind. Eng. Chem., Anal. Ed. 1946, 18 (10), 587−592. (12) Skibsted, E. T. S.; Boelens, H. F. M.; Westerhuis, J. A.; Smilde, A. K.; Broad, N. W.; Rees, D. R.; Witte, D. T. Net Analyte Signal Based Statistical Quality Control. Anal. Chem. 2005, 77 (22), 7103− 7114. (13) Pereira, P.; Seghatchian, J.; Caldeira, B.; Xavier, S.; de Sousa, G. Statistical methods to the control of the production of blood components: principles and control charts for variables. Transfus. Apheresis Sci. 2018, 57 (1), 132−142. (14) Quintar, S. E.; Santagata, J. P.; Villegas, O. I.; Cortinez, V. A. Detection of Method Effects on Quality of Analytical Data. A Statistical Exercise. J. Chem. Educ. 2003, 80 (3), 326. (15) Carter, D. W. Testing Boyle’s law: A context for statistical methods in the undergraduate laboratory. J. Chem. Educ. 1985, 62 (6), 497. (16) Libes, S. M. Learning Quality Assurance/Quality Control Using U.S. EPA Techniques. An Undergraduate Course for Environmental Chemistry Majors. J. Chem. Educ. 1999, 76 (12), 1642.

CONCLUSIONS

By using the pH titration of vinegar to determine the weight percent of acetic acid, students receive initial exposure to control charts and why they are important tools that aid in assessing analytical-method quality. Assessment involved scoring student results for the weight percent of acetic acid in commercial vinegar and their statements of whether or not their results were within control limits. The written final exam also contained questions that assessed whether the students learned how to properly use a control chart and could suggest corrective actions. Overall, 71% of students determined a mean weight percent and correctly interpreted their results using a provided control chart for the laboratory exercise. On the basis of the final-exam assessment, 79% correctly interpreted the given weight percent using the control chart and suggested the correct action to obtain results within control. As the majority understood what control was, how to determine if a method was in control, and appropriate corrective actions, these results were acceptable. Overall, this exercise successfully implements instruction on how to use a control chart using a simple titration method early in the Quantitative Analysis Laboratory curriculum. D



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(17) Perone, S. P.; Englert, P.; Pesek, J.; Stone, C. Transforming traditional quantitative analysis into a course on modern analytical science. J. Chem. Educ. 1993, 70 (10), 846. (18) Vitha, M. F.; Carr, P. W.; Mabbott, G. A. Appropriate Use of Blanks, Standards, and Controls in Chemical Measurements. J. Chem. Educ. 2005, 82 (6), 901. (19) Bell, S. C.; Moore, J. Integration of Quality Assurance/Quality Control into Quantitative Analysis. J. Chem. Educ. 1998, 75 (7), 874. (20) Schazmann, B.; Regan, F.; Ross, M.; Diamond, D.; Paull, B. Introducing Quality Control in the Chemistry Teaching Laboratory Using Control Charts. J. Chem. Educ. 2009, 86 (9), 1085. (21) Harris, D. C. Exploring Chemical Analysis; W. H. Freeman: New York, 2013.

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Using Control Charts Early in the Quantitative Analysis Laboratory

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