A system of instruction in qualitative chemical analysis / by Dr. C. Remigius Fresenius .. ; edited by J. Lloyd Bullock, F.C.S.
- Carl Remigius Fresenius
- Date:
- 1855
Licence: Public Domain Mark
Credit: A system of instruction in qualitative chemical analysis / by Dr. C. Remigius Fresenius .. ; edited by J. Lloyd Bullock, F.C.S. Source: Wellcome Collection.
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![§ 24.] water, and filter the solution; divide the intensely blue filtrate into two equal parts; saturate the free alkali in the one part, by stirring with a glass rod, dipped in very dilute sulphuric acid, and repeating this process, until the color of the fluid just appears red ; add now the other part of the fil- trate, pour the fluid into a dish, and draw slips of fine unsized paper through it; suspend these slips over threads, and leave them to dry. The color of litmus paper must be perfectly uniform, and neither too light nor too dark. Uses.—Litmus paper serves to detect the presence of free acid in fluids, since acids change its blue color to red. It is to be borne in mind, how- ever, that the soluble neutral salts of most of the heavy metallic oxides produce the same effect. /3. Reddened litmus paper. Preparation.—Stir blue tincture of litmus with a glass rod dipped in dilute sulphuric acid, and repeat this process until the fluid begins to look distinctly red. Steep slips of paper in the tincture, and dry them as in a. The dried slips must look distinctly red. Uses.—Pure alkalies and alkaline earths, and the sulphides of their metals, restore the blue color of reddened litmus paper; alkaline carbonates and the soluble salts of several other weak acids, especially of boracic acid, possess the same property. This reagent serves therefore for the detection of these substances in general.* y. Georgina paper (Dahlia paper). Preparation.—Boil the violet-colored petals of Georgina purpurea (pur- ple dahlia) in water, or digest them with spirit of wine, and steep slips of paper in the tincture obtained. The latter should be neither more nor less concentrated than is necessary to make the paper, after drying, appear of a fine and light violet blue color. Should the color too much incline to red, this may be remedied by adding a very little ammonia to the tincture. Uses.—Georgina paper is reddened by acids, whilst alkalies impart a beautiful green tint to it. It is therefore an extremely convenient sub- stitute both for the blue and the reddened litmus paper. This reagent, if properly prepared, is a most delicate test both for acids and alkalies. Concentrated solutions of caustic alkalies turn Georgina paper yellow, by destroying the coloring matter. 8. Turmeric paper. Preparation.—Digest and heat one part of bruised turmeric root with six parts of spirit of wine ; filter the tincture obtained, and steep slips of fine paper in the filtrate. The dried slips must exhibit a fine yellow tint. * Mr. A. S. Taylor has suggested that a very delicate test paper for detecting alkalies may be prepared by steeping slips of paper in an acid infusion of rose petals.](https://iiif.wellcomecollection.org/image/b28080361_0051.jp2/full/800%2C/0/default.jpg)
