An Introduction to the Chemistry of Paints (Paperback)

Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: SULPHATE PIGMENTS You are all familiar with the smell of burning sulphur. As I explained in my last lecture, it is due to the combination of the sulphur with the oxygen of the air, whereby a gas known as sulphur di-oxide is produced. When dissolved in water this gas yields sulphun acid, which only differs from sulphuric acid, or oil of vitriol, in that it possesses rather less oxygen in proportion to the sulphur. When a dilute solution of sulphurous acid is exposed to the air, it very gradually takes up the necessary oxygen, and becomes converted into dilute sulphuric acid. But a second oxide of sulphur is known, namely, sulphur tri-oxide, which contains half as much oxygen again as the di-oxide, and which, when dissolved in water, yields sulphuric acid direct. Here is a sample sealed up in this flask, and you notice its fine silk-like crystals, which are perfectly colourless. When heated strongly it decomposes into sulphur di-oxide and oxygen. It is not formed in any appreciable quantity when sulphur is burned in air, but if a mixture of sulphur di-oxide and oxygen is heated in contact with platinum or with rouge, either being in a very fine state of division, sulphur tri-oxide is readily obtained. As already stated, when this oxide is dissolved in water, sulphuric acid is obtained. In the highly concentrated condition, this acid is somewhat oily in appearance, and is hence frequently known as oil of vitriol, although it is a true oil in no sense of the word. The majority of metals and their oxides, on being warmed with either concentrated or dilute sulphuric acid, yield compounds known as " sulphates." Thus, to quote a familiarexample, zinc dissolves in the dilute acid, as we saw during the last lecture, evolving hydrogen gas, while zinc sulphate is left behind in sol...