Implementing Logic Gates with Acid–Base Neutralization: Focusing on the Reproduction of the XOR Gate
This study began from the observation that litmus paper takes one of two colors, red in acid and blue in base, much like the 0 and 1 of binary numbers, and asked whether acid–base neutralization can perform logic operations. We built an ion-counting model with two rules, that H⁺ and OH⁻ react one to one to form water and that the current of a solution is proportional to the number of remaining ions, and computed it with a Python program and a web simulator. With litmus, NOT, OR, NOR, AND and NAND gates could be built, but an XOR gate never appeared while the amount of base in the background solution was varied from 0 to 3 units. Litmus shows only which kind of ion remains, and that kind changes in one direction only as acid is added. This is the same structure that prevents a single-layer perceptron from computing XOR. Reading the same reaction by electric current reveals how many ions remain. With sulfuric acid and barium hydroxide as inputs, the ions disappear as water and precipitate when the two inputs are equal, so the bulb behaved as an XOR gate. Hydrochloric acid and sodium hydroxide, in contrast, leave Na⁺ and Cl⁻ after neutralization and behaved as an OR gate. Finally, when two XOR beakers were chained, pouring the first solution directly into the second gave a wrong answer in one of eight cases, while signal restoration, reading only the result and adding a fresh solution, gave correct answers in all cases. This is the role shared by transistors, activation functions in neural networks, and action potentials in neurons.