Box 9-2 The Secret of Carbonless Copy Paper

Back in the days when dinosaurs roamed the Earth and I was a boy, people inserted a messy piece of carbon paper between two pages so that a copy of the upper page would appear on the lower page. Then carbonless copy paper was invented to perform the same task without the extra sheet of carbon paper. Alas, now even carbonless copy paper is gone. 

Carbonless copy paper has an acid-base indicator inside polymeric microcapsules stuck to the underside of the upper sheet of paper. When you write on the upper sheet, the pressure of your pen breaks the microcapsules on the bottom side to release indicator. 

The indicator becomes adsorbed on the upper surface of the lower page, which is coated with microscopic particles of an acidic material such as the clay 

Several indicators are listed twice in Table 9-3, with two different sets of colors. For example, thymol blue loses one proton with a pA of 1.7 and a second proton with a pK of 8.9  

At pH = 1.7 (= pKi), there is a 1 1 mixture of yellow and red species, which appears orange. We expect the solution to be red when [R]/[Y ] 10/1 and yellow when [Y ]/[R] 10/1. From the Henderson-Hasselbalch equation 9-5, we expect to see red when pH pK — 1 (= 0.7) and yellow when pH pK + 1 (= 2.7). Table 9-3 states that thymol blue is red below pH 1.2 and yellow above pH 2.8. There is another transition, from yellow to blue, between pH 8.0 and pH 9.6. In this range, various shades of green are seen. 

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