Invisible Ink Revealed: Concept, Context, and Chemical Principles of

Laboratory Experiment pubs.acs.org/jchemeduc

Invisible Ink Revealed: Concept, Context, and Chemical Principles of “Cold War” Writing Kristie Macrakis,† Elizabeth K. Bell,‡ Dale L. Perry,*,§ and Ryan D. Sweeder‡ †

School of History, Technology, and Society, Georgia Institute of Technology, Atlanta, Georgia 30332, United States Lyman Briggs College, Michigan State University, East Lansing, Michigan 48825, United States § Lawrence Berkeley National Laboratory, University of California, Berkeley, California 94720, United States ‡

S Supporting Information *

ABSTRACT: By modifying secret writing formulas uncovered from the archives of the East German Ministry of State Security (MfS or Stasi), a novel general chemistry secret writing laboratory was developed. The laboratory combines science and history that highlights several fundamental chemical principles related to the writing. These include catalysis, redox reactions, kinetics, complex formation−precipitation, and acid−base reactions. After a background historical presentation, students don the mantle of counterintelligence to discover the location of a terrorist bomb on campus. In the process of deciphering the secret message, they pull together the chemical concepts to “save the day”.

KEYWORDS: First-Year Undergraduate/General, Inorganic Chemistry, Laboratory Instruction, Physical Chemistry, Hands-On Learning/Manipulatives, Problem Solving/Decision Making, Acids/Bases, Catalysis, Forensic Chemistry, Oxidation/Reduction, History

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sought to keep and acquire secrets from rival nations. To prevent state secrets and military plans from falling into the wrong hands, governments developed codes, ciphers, and invisible ink to disguise messages so that only the recipient could read it.2 Secret writing is a spy’s classic method of communicating. Thus, the principles of such writing as they are exhibited and discussed here are extremely important from both historical and chemical standpoints. With advances in organic chemistry during the late 19th century, invisible ink became more sophisticated than simply charring organic compounds in fruit juice, milk, or wine to make them visible.3,4 It was not until World War I that government-sponsored laboratories began to experiment with creating and detecting invisible ink.5 Advancing beyond the public image of a “magic pen”, 20th century spy agencies developed sophisticated methods involving impregnating a whole piece of paper with a specific chemical. This sheet was then used like carbon paper in a secret writing “sandwich”; the spy wrote on the top sheet, and the message would appear invisible on the bottom sheet. By the later years of the Cold War, chemists had developed methods using catalysis that only required a tiny quantity of chemical substance making it more difficult to detect. A former Central Intelligence Agency (CIA) scientist likened this method to “uniformly spreading a spoonful of sugar over an acre of land”.6

ntrigue, suspense, and lives at stake. Is this the stuff of college chemistry laboratories? Rarely. Although using lemon juice as a secret writing substance has fascinated children at the elementary school chemistry level for many years, until now no parallel experiments have been developed at the college level, although secret ink experiments for the grade and high school levels exist in the literature.1 This absence is primarily due to secrecy restrictions, but with the recent release of Cold War documents, we obtained a secret writing formula developed by the Stasi, the former East German Ministry for State Security (MfS). From these notes we developed a novel secret writing laboratory combining science and history that highlights several fundamental chemical principles including catalysis, redox chemistry, and acid−base reactions. Even though there are currently more modern electronic methods for the transmission of information in the field of intelligence, the chemical principles discussed here are still important for understanding other interfacial spectroscopic and chemical signatures for the ink/paper document systems such as these here by the Stasi and for learning how to store the material by understanding the degradation processes of the ink/paper interface. Both ink and paper change with time, with both exhibiting variable changes as a function of storage conditions, so any knowledge of these processesespecially involving the inkis valuable. Secret writing is as old as writing itself and can be traced back to the Egyptians. For thousands of years, governments have © 2012 American Chemical Society and Division of Chemical Education, Inc.

Published: February 8, 2012 529

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sulfate monohydrate (MnSO4·H2O), 30% hydrogen peroxide (H2O2), ammonium bicarbonate (NaHCO3), hydrochloric acid (HCl), and sodium hydroxide (NaOH). They were given small samples of different potential catalysts for the reaction, but were not immediately provided with the secret writing sample. Students in each lab section (12−20 students) were divided into smaller groups and asked to cooperatively create a procedure to determine an effective development method. Once these smaller groups created a procedure, they discussed and implemented their ideas. In the laboratory instructions, they were presented with some information about the reaction, but the desirable pH was obscured. The students then determined the relative ratios of solution for the developer. The student groups used the provided potential catalysts to experiment with possible developer combinations leading to one that worked on the actual message. Within each laboratory section, groups of students sometimes discovered different developing techniques that work. This variance led to student discussions within the section about which methods should be applied to the real sample. All the students in the lab section must agree to one method before they are provided with the impregnated paper that contains the imaginary bomb location.

One such secret writing method developed by the Stasi was the inspiration for this laboratory.6,7 A historian of science, a chemist, and several honors college undergraduate students working together modified the technique presented by the MfS chemists in the files (Figure 1) by experimenting with the

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HAZARDS The 30% concentration of hydrogen peroxide is used in this lab and can easily damage skin. Students handling this reagent or solutions containing it should wear neoprene gloves. If a developing solution containing 30% hydrogen peroxide and the manganese(II) sulfate becomes basic, an extremely exothermic reaction occurs as the peroxide is catalytically decomposed. The gases rapidly escaping the solution and the significant heat may cause splattering. Cerium(III) oxalate and dysprosium(III) oxalate are irritants, and silver nitrate will stain skin black. Manganese(II) sulfate monohydrate may cause irritation to skin, eyes, and respiratory tract and may be harmful if swallowed or inhaled. The hydrochloric acid and sodium hydroxide solutions are corrosive and cause burns to all body tissue. Material Safety Data Sheets (MSDS) for all reagents used in this laboratory should be read and reviewed by the class before performing the experiments.

Figure 1. Original Stasi invisible ink formula.

chemical process. History and science were woven into the laboratory, while a historical overview of secret writing in lecture provided background and excited the students. In the laboratory, the students donned the mantle of counterintelligence agents to thwart a terrorist attack on a university building.

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EXPERIMENT The students were provided with a torn sheet of paper obtained from the crime scene indicating the reagents present in the development formula (Figure 2), but not the quantities. The

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RESULTS AND DISCUSSION

This secret writing laboratory was designed as a capstone experience for a three-hour second-term general chemistry laboratory. As such, it pulls together a number of fundamental chemical principles including catalysis, redox chemistry, acid− base chemistry, and kinetics. Previous education innovations such as the Project Physics Course8 brought some history into the physical sciences, but the designers of this laboratory have taken this synergy further by weaving together the history and the chemistry in a lab experience that provides students with an intriguing real-life experience. The backdrop of espionage and secret writing capitalize on the “CSI effect” and the growing student interest in forensics to stimulate student engagement and cooperation, both important educational practices.9,10 Although this laboratory exercise emphasizes the concepts of redox reactions and catalysis, one caveat must be made. The precision and detail of these two concepts cannot be completely described. This has a 2-fold origin. First, because much of the overall chemistry of the invisible ink is taken from relatively limited documentation from the East German intelligence community, there are large gaps in the details of

Figure 2. Secret writing formula provided to students.

location of the imaginary bomb was also written on a confiscated piece of paper in invisible ink. The students had access to all the equipment in the chemistry laboratory to create an effective developer, including solutions of manganese(II) 530

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the topic and the requirement that they develop a procedure. Stronger students also often expressed satisfaction with the experience of learning how different concepts are tied to a single phenomenon. Separately, some students expressed frustration and struggled with developing their own procedure. On an end-of-year survey, many laboratory sections rated this laboratory as one of the top two laboratories they most enjoyed, and almost all of the laboratory sections named it as being in their top four laboratories. In general it appeared that smaller laboratories (fewer than 14 students) tended to show the greatest satisfaction with the lab. Notably, the historical lecture resulted in higher quality introductions in the students’ formal laboratory reports.

the chemical processes involved. Second, with respect to both the catalysis and redox reactions, there were no existing data retrieved from the East German Stasi files to quantitatively support an understanding of their roles in the present invisible ink exerciseno rate studies for the catalysis or detailed knowledge of the redox concept at work in the exercise or of mechanisms involved. Indeed, a complete understanding of the role of several of the chemical components in the context of their involvement in many other chemical components is not forthcoming. As a result, there is no ability to unequivocally have a complete knowledge of the chemistry involved. Most experimenters in secret intelligence laboratories do not know why invisible ink works; they just want to get it to work. Several plausible reaction schemes and reaction mechanisms are contained in the Supporting Information that accompanies this paper. Cerium(III) oxalate plays the starring role as the secret writing substance and catalyst for a redox reaction. The salt’s role prompts a discussion of catalysts and how they increase the rate of a reaction. The reaction that is occurring when the reagent is applied to the invisible writing is a redox reaction that produces a colored solid, presumably MnOx with the cerium(III) oxalate catalyzing the oxidation of Mn(II). Once the students realize this is a redox reaction, they will typically balance the reaction. If they balance the reaction as if it were in a basic solution, the conditions where the reaction works best, they discover that the hydroxide ion is a reactant, and the importance of the pH is made clear. The dramatic reactivity difference based on pH can then be explained through the variation of hydroxide concentration. A highly basic developer solution will react in the absence of the catalysts to produce either MnO(OH)2 or MnO2. If MnO2 is produced, it will catalyze the decomposition of the hydrogen peroxide, creating an exothermic reaction that bubbles vigorously. By contrast, an acidic solution will yield no observable reaction. A relatively neutral to slightly basic solution will react to form the desired product, but it will do so only in the presence of a catalyst. This laboratory can easily be adjusted to students’ capabilities and skill levels. The false catalysts provide flexibility. By including materials that react with either the manganese(II) ion or the hydrogen peroxide, students encounter a more complex set of results as they attempt to determine the correct developer solution. Substances that react relatively independently of the pH provide an interesting source of discussion for the students. Silver nitrate, for example, will react with the peroxide to yield a black solution of silver oxide. Some of the students will realize that they do not actually have to determine which potential catalyst is correct, but rather only determine what conditions are necessary to make it work. Thus, if it is observed that three different potential catalysts all show reaction at the neutral pH, but only one reacts when acidic, then the students will conclude that the neutral pH would successfully develop the message no matter the catalyst. Our experience with this lab over several years and over 1800 students suggests that the first-year students need gentle prodding by the lab instructors to make a genuine connection between the concepts and the phenomena. Although the lab would ideally inspire students to make these mental connections on their own, it is useful to have instructorspurred discussions at the end of the lab. In general, chemistry students responded positively to this laboratory. Anecdotally, many students were enthusiastic about

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CONCLUSION In an attempt to braid science with history, the authors developed this laboratory combining chemical concepts with historical context. The students enjoyed the secret writing experiment and learned about catalysis, redox reactions, kinetics, complex formation−precipitation, and pH. The storyline engaged them, and solving the invisible ink puzzle intrigued them. After the first year of the laboratory, the authors implemented a historical background lecture that enhanced the student experience and heightened their interest in the laboratory and the quality of their final written report.

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ASSOCIATED CONTENT

S Supporting Information *

Instructions for the students; notes for the instructor; the original Stasi document; comments on the chemistry. This material is available via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

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ACKNOWLEDGMENTS The two principal investigators (K.M. and R.D.S.) of this work and D.L.P. are especially grateful to the anonymous reviewers of the manuscript who provided detailed and stimulating comments. Thanks also to Robert L. LaDuca for providing the X-ray diffraction analysis. K.M. wishes also to acknowledge an NSF Grant No. SES9811494 & Fulbright Commission Award. One of the authors (D.L.P.) wishes to acknowledge support of the U.S. Department of Energy under Contract Number DEAC02-05CH11231.

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REFERENCES

(1) Wright, S. W; Folger, M. R.; Rice, M. A. J. Chem. Educ. 2006, 83, 1437−1439. (2) Singh, S. The Codebook; Anchor Books: New York, 1999. (3) Kahn, D. The Codebreakers: the Story of Secret Writing; Scribner: New York, 1996; pp 522−525. (4) SOE Syllabus: Lessons in Ungentlemanly Warfare in World War II; Introduction by Rigden, D.; The National Archives: Surrey, U.K., 2001; p 234. (5) The Production and Detection of Messages in Concealed Writing and Images; Publication No. 50; Riverbank Laboratories: Geneva, IL, 1918; p 5. The publication is copyright by George Fabyan the benefactor and director of the lab. We would like to thank H. Keith Melton for providing us with a copy of this rare publication. 531

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(6) Macrakis, K. Secret Writing Revealed. In Seduced by Secrets: Inside the Stasi’s Spy-Tech World; Cambridge University Press: New York, 2008; Chapter 9, p 224. (7) The formula was obtained from the archives of the former East German Secret Police BStU, MfS. Procedure 41601-002. MfS 218. Nr. B 38/77. (8) Holton, G., On the Educational Philosophy of the Project Physics Course. In The Scientific Imagination; Harvard University Press: Cambridge, 1998; pp 284−299. (9) Umbach, P. D.; Wawrzynski, M. R. Res. High. Educ. 2005, 46, 153−184. (10) Astin, A. W. What Matters in College? Jossey-Bass Publishers: San Francisco, 1993.

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