Bookbinding and the Conservation of Books
A Dictionary of Descriptive Terminology

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double decomposition

A chemical reaction that takes place between two compounds, in which the first and second parts of one compound unite with the second and first parts, respectively, of the other compound. One of the compounds is usually insoluble. The principle of double decomposition has been used in deacidifying paper, as, for example, when a soluble calcium or magnesium salt is dissolved in water and the paper is immersed in it. After the paper has been dried it is then impregnated with a soluble carbonate, such as that of ammonium or sodium. Sodium carbonate and calcium chloride, for example, react to form insoluble calcium carbonate and soluble sodium chloride: CaCl 2 + Na 2 CO 3 → CaCO 3 ↓ + 2NaCl; or, using calcium chloride and ammonium carbonate: CaCl 2 + (NH 4 ) 2 CO 3 → CaCO 3 ↓ + 2NH 4 Cl.

The soluble sodium or ammonium chloride is removed by washing the paper with water, and it is important that the chloride be removed as completely as possible, as its presence in the paper is potentially harmful. The advantage of the double decomposition method is its simplicity, plus the fact that a much greater alkaline reserve can be deposited in the paper than is generally possible with most other methods.

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