Invisible Inks With Household Ingredients: Mechanisms and Properties Ben Artin Northeastern University College of Professional Studies CMH1011: Chemical Principles 1 Instructor: Patricia Brandl June 24, 2011 Abstract Invisible inks | colorless substances usable for writing that can be turned to color using a chemical process | have both practical value in covert communications and educational value in chemistry teaching. Publicly available invisible ink recipes are numerous and varied, but are often published without any elaboration as to their utility and usability. This paper evaluates approximately a dozen different invisible ink recipes, with focus on those that can be prepared from common household ingredients. Safety, cost, availability, shelf-life, and dif- ficulty of use of the different invisible inks are considered. Based on this evaluation, a general recommendation is made in favor of heat-based inks for casual covert communications and chemistry education, and precipitation-based inks for more secure covert communications and chemistry education in a lab setting. Additionally, two proposed mechanisms for action of invisible inks based on organic acids and revealed by heating are evaluated. Oxidation of ink by heat is confirmed to play a role in revealing such inks. 1 Introduction ing mathematics2, computer science3, quan- tum physics4, and chemistry5. Parents want to know about the activities of In the realm of chemistry, a popular tech- their children; generals want to know about the nique for covert communication | seen in sci- activities of their troops. Throughout human ence education6, intelligence operations5, and history, on scale from personal to international, mass-market fiction7 | are invisible inks. knowledge has been a key ingredient in power. An invisible ink is any substance that can In struggle to establish and maintain power, be used for writing (typically on paper) and people often desire to communicate covertly is not easily detected by the naked eye. The and intercept others' covert communications. process of rendering the ink visible (ideally by As a result, numerous technological advances the intended recipient) is known as developing have been developed primarily to deal with the ink; the apparatus or substance used to de- covert communication | both to enable it, velop the ink is the developer. and to intercept it. Recorded accounts of such This paper reviews several substances that technology go as far back as 1500 BCE1, and can be used as invisible inks, as well as touch nearly every scientific discipline, includ- their corresponding developers, with focus on 1 { those that can be manufactured from common species, Ind1 and Ind2 , of which one can be household products and other readily available obtained by protonation of the other: reagents. − + Ind1 Ind2 + H 1.1 Classification of inks by mechanism of action and development and those two species have different colors be- cause their different bond and electronic struc- Many ink/developer pairs share the same tures result in their having different absorption underlying chemical mechanism of action (on spectra in visible frequencies of light. writing surface) and development. Such re- When Ind1 is exposed to an acidic or basic lated pairs are often similar to each other on environment, the equilibrium concentrations of several of the evaluation criteria. It is, there- { Ind1 and Ind2 depend on the pH of the envi- fore, convenient to group their evaluation by ronment, and therefore so does the color of the mechanism. { 15 mixture of Ind1 and Ind2 . Mechanisms present among the inks and de- Substances containing acids or bases (such velopers evaluated in this paper include: as citric acid or ammonia) can be used as in- visible inks and developed by exposing them to Sugar solutions developed by heat a suitable pH indicator. To develop a pH-based invisible ink, a pH Substances containing sugars (such as honey indicator in aqueous solution is applied both solution and sugar solution) can be developed to the marked regions of the writing surface by heating. When applied to paper and then and the unmarked ones. The indicator has to heated, these solutions turn brown8. be chosen so that the ink produces a different The reaction responsible for this change of indicator color than the unmarked writing sur- color is presumably caramelization (a non- face does, which depends both on pH of the ink enzymic reaction that results in browning of and the pH of the writing surface. sugars9). Organic acids developed by heat Inorganic salts developed by a precipitation reaction Substances containing organic acids (such as citric acid in lemon juice) can be developed Water-soluble salts can be developed using by heating. When applied to paper and then a precipitation reaction, if the salt solution is heated, these inks turn brown10. colorless and a colored precipitate can be pro- The mechanism of action and development duced. is unclear; two different possibilities are com- The ink, containing an ion (IonI ) in aque- monly cited | that the organic acid itself ous solution is applied to a writing surface and browns with heating11,12, and that the acid re- allowed to dry. To develop the ink, aqueous acts with the polymers of the paper, converting solution of a different ion (IonD) is applied to them into compounds that brown on exposure the writing surface. If the two ions combine to heat13,14. into a salt (SaltP ) that has low solubility in water and forms a colored precipitate: Acids and bases developed by a pH Ion + Ion Salt # indicator I D P pH indicators are substances whose color then the appearance of colored precipitate of changes on exposure to acidic or basic solu- SaltP on the writing surface makes the writ- tions. Specifically, an indicator exists as two ing visible. 2 2 Methods R 22: Harmful if swallowed. 2.1 Ink Evaluation R 23/25: Toxic by inhalation and if swal- lowed. Inks and developers were assessed on the fol- lowing criteria: R 25: Toxic if swallowed. Availability R 36/37: Irritating to eyes and respira- tory system. Availability of a reagent is determined to be \common" if it is readily available from com- R 36/38: Irritating to eyes and skin. mon brick-and-mortar and internet stores, such as grocery stores; and \specialty" if it is only R 36/37/38: Irritating to eyes, respira- available from specialty stores, such as garden- tory system and skin. ing stores. R 37/38: Irritating to respiratory system Cost and skin. Except where otherwise specified, cost is R 41: Risk of serious damage to eyes. based on retail prices during the week of Jan 20{26 2001 at Walgreens and Stop and Shop in R 46: May cause heritable genetic dam- Malden, MA. age. Preparation R 62: Possible risk of impaired fertility. Preparation is classified as \simple" if it involves nothing beyond mixing with water Contrast at room temperature, \medium" if common Contrast of the writing was classified as household processes, such as boiling water, are \high" if an ink yielded strokes with high required, and “difficult” if specialized equip- contrast against the background, \low" if the ment (other than safety equipment) is required. strokes had low contrast against the back- ground, and \invisible" if strokes were com- Shelf-life pletely invisible. Shelf-life was estimated based on prior ex- perience with household products in question, Stroke width assuming that the ink is not kept refrigerated. Stroke width of the writing was classified as Safety \broad" if only broad strokes (applied with a cotton swab) were discernible, and \thin" if Risks of inks and developers were deter- thin strokes (applied with a nib pen) were dis- mined and summarized by the following stan- cernible. dard16,17 risk phrases: R 8: Contact with combustible material Sharpness may cause fire. Sharpness of the writing was classified as R 20: Harmful by inhalation. \sharp" if strokes of ink with food dye were not visibly distorted or blurred by developing, R 20/22: Harmful by inhalation and if and \blurred" if they were blurred by develop- swallowed. ing. 3 2.2 Choice of inks and developers & Shop 100% Pure Corn Starch, UPC 688267070365) In order to be useful for development of pH- based inks, pH indicators have to be available Sugar, 10 g dissolved in 100 ml H2O in aqueous solution. Common aqueous labo- (Domino® Premium Granulated Pure ratory indicators (such as phenolphthalein and Cane Sugar, UPC 049200045701) methyl red) are deemed unsafe for consumer 18 products in various world regions , and there- Honey, diluted to 20 % by H2O (Gunter's fore are typically only available in specialized Pure Honey, UPC 021273100129) stores, such as online lab supply stores. They are toxic, often carcinogenic, and irritants, as White onion water (filtrate of 50 g well as being relatively expensive. Such devel- chopped jumbo white onion boiled in opers were not evaluated. 500 mL H2O) A viable alternative to these is preparation Lemon juice, diluted to 20 % by H O (Si- of aqueous solution of flavin, an anthocyanin 2 cilia Lemon Juice, UPC 030849000053) present in red cabbage19. Red cabbage is commonly and cheaply available in brick-and- ® Milk, diluted to 20 % by H2O (Hood mortar stores. An extract of flavin from red Fat Free Milk, UPC 044100169250) cabbage can be prepared with a simple stove- top process. Its shelf life is unknown, and it is along with the following developers: not hazardous beyond risks involved with boil- ing water on a stove. pH indicator: Red cabbage water (fil- Numerous ionic compounds exist that par- trate of 170 g chopped red cabbage boiled ® ticipate in precipitation reactions and there- in 500 mL H2O; Stop & Shop Fresh fore could be used as invisible inks, but many Red Cabbage, UPC 02112041383) of them are not readily available in consumer Iodine: Iodine tincture diluted to 5 % products due to their toxicity (such as cop- by H O (Swan Iodine Tincture, UPC per(II) sulfate and potassium ferricyanide); 2 30869385110) they can typically only be obtained from spe- cialty online stores.