Disappearing ink

Whether for a class demonstration, a practical joke, or perhaps a clandestine activity, disappearing ink is a fascinating substance. What is the secret to its action One formulation of disappearing ink contains a common acid-base indicator, that is, a substance that by its color shows the acid or basic nature of a solution. One acid-base indicator that shifts from a colorless hue under acidic conditions to a deep blue color in alkaline solutions is thymolphthalein. If the indicator starts off in a basic solution, perhaps containing sodium hydroxide, the typical blue color of an ink is perceived. How does the ink color disappear This behavior is dependent upon the contact of the ink with air. Over time, carbon dioxide in the air combines with the sodium hydroxide in the ink solution to form a less basic substance, sodium carbonate. The carbon dioxide also combines with water in the ink to form carbonic acid. The indicator solution responds to the production of acid and returns to its colorless acid form. A white residue (sodium carbonate) remains as the ink dries. 

Disappearing ink can be prepared by first dissolving solid thymolphthalein in ethanol, adding water, and then adjusting the pH with sodium hydroxide solution J2 The deep blue color of the basic form of the indicator is readily apparent. Applying the ink to paper increases its exposure to carbon dioxide in air. Two chemical... 

Summerlin, Qiristie L Borgford, and Julie B. Ealy, "Disappearing Ink," Chernknl Demonstrations, A Sourcebook for Teachers, Vol. 2 (American Qiemical Society, Washington, DC, 1988) pp. 176. 

Use waterproof ink Never pencil Pencil will disappear with time, and so will your grade. Cheap ink will wash away and carry your grades down the drain. Never erase Just draw one line through yuor orrcro your errors so that they can still be seen. And never, never, never cut any pages out of the notebook ... 

Ink, as it is usually propared, is disposed to undergo certain ohanges, which considerably impair its velue of these the three following are the most important — its tendency to moulding the liability of the black matter to separate from the fluid, the ink then beoom-ing what is termed ropy and its loss of color, the black first changing to brown, and at length almost entirely disappearing. 

In very stubborn cases, another treatment is used. Inks and pigments are fixed and the artifact is immersed in a bath of polyethylene glycol (PEG 400). An entire codex can be placed in two or three large plastic tubs and kept there until the parchment is soft and pliable. This procedure can take days or weeks. The parchment is removed and blotted and pressed in the same manner as the treatment in ethanol and water. If the parchment becomes translucent while in the bath, the phenomenon will disappear with the blotting and pressing process. 

Lithography is not thwarted by this omni-present, overall thin, aqueous, alcoholic film of fountain solution because it automatically gets out of the way of ink transfer as required, by instantly disappearing into whichever layer, ink or fountain solution, is relatively moisture-starved. 

Whereas inks exist which under the action of light entirely disappear—so far as our ordinary vision are concerned— many such, however, leave a trace on an ordinary photographic plate, and others can be revivified by suitable chemical reagents. 

In the following case, however, this cannot be done. The " ink is made as follows dilute some starch in water to the consistency of a cream and add a few drops of tincture of iodine. Dip the pen in the ink thus made and write on ordinary paper. The writing is plainly visible, and dries very quickly. To obliterate it completely, it is only necessary to rub it with a handkerchief or even with the hand— and it will disappear as easily and completely as chalk from a blackboard. 

Obtained by dissolving cobalt, or any of its oxides, in hydrochloric acid. The solution is pink, and on evaporation yields beautiful red crystals of hydrated chloride, or possibly hydro-chlorate of the oxide. When dried by heat, the chloride is of a deep blue, but is instantly rendered pink by the contact of water. The crystals are either CoCl, HO, or CoO, HCl. When traces are made on paper with a dilute solution of chloride of cobalt, they are invisible when dry but, when warmed, assume a decided blue colour, which disappears agarlr on cooling, as they again absorb moisture from the air. This is the most beautiful of the sympathetic inks. If iron or nickel be present, the traces appear green instead of blue. 

When a drop of ink falls into a beaker of water, the ink diffuses in the water and the spot of ink disappears or becomes invisible to the naked eye after some time. This is a typical diffusion phenomenon that shows the spontaneous movement of particles/ substances from a region of higher concentration to a region of lower concentration. A close examination would confirm that the ink molecules are equally distributed in all directions, i.e., the diffusion of the ink spot is isotropic in the water medium. 

In principle, BCA is simplicity itself add up the benefits of a project, add up its costs, and make approval dependent on whether the difference, total net benefits, is written in black ink or red. Of course, close up this simplicity disappears there are great technical hurdles to be overcome in such areas as valuing intangible or difficult-to-measure costs and benefits, deciding on the rate of discoimt to apply to outcomes that will occur in the future, and, if the analyst is conscientious, adjusting for the effect that income inequality has on the dollar value of outcomes to different social classes. This keeps economists busy. Despite these complications, however, they can appeal to the overall common sense of the benefit-cost framework to gain support in the political process. In the early applications to water policy the technical problems were not important -the most wasteful projects were easily identified - but as the use of BCA widened the problems increased, and different studies of the same proposal worild often come up with wildly divergent numbers. 

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