Manual of qualitative analysis / by Robert Galloway.

  • Galloway, Robert.
Date:
1861
Catalogue details

Licence: Public Domain Mark

Credit: Manual of qualitative analysis / by Robert Galloway. Source: Wellcome Collection.

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    turns red litmus paper blue. After the caustic soda has been thoroughly mixed with it by agitation, in the way described in the first exercise (par. 9), oxide of copper, which is of a black colour, will precipitate, owing to its being deprived of the sulphuric acid by the soda. If the oxide of copper is precipitated under the boiling point, it combines with water, forming hydrate of the oxide, which is of a blue colour. 11. Dissolve two or three grains of sulphate of zinc in a test-tube half filled with water; add ammonia, drop by drop, until a white precipitate, which is hydrate of zinc, is formed; then continue to add the ammonia until the precipi- tate disappears. The precipitate disappears, because the hydrate of zinc is soluble in ammonia ; it 1s also soluble in the other alkalies, soda and potash; therefore, if caustic soda had been added, a precipitate would have formed, as in the case of the ammonia, which, on a further addition of soda, would have disappeared. 12. When substances are employed in analysis in the way ammonia and caustic soda have been employed in the above experiments, they are called tests, or reagents, and the phe- nomena produced by them are termed reactions. The re- agents are either acids, bases, or salts; and the reaction consists sometimes in a change of colour, sometimes in the production of effervescence, but most frequently by the formation of a precipitate, which, in many cases, possesses a characteristic colour. Reagents may be divided into two classes—general and special. The general reagents are employed to divide substances into families or groups, and the special reagents to detect the individual members of a group. A special reagent is said to be characteristic, when it produces a reaction so decisive that it admits of no mis- take. A reagent is called sensitive, or delicate, if its