Fluorescein test paper

Fluorescein test paper Is prepared by dipping filter paper into a dilute solution of fluorescein in ethyl alcohol it dries rapidly and is then ready for use. The test paper lias a lemon yellow colour. 

This solution may also be employed in the test for bromine. If iodine has been found, add small amounts of sodium nitrite solution, warm shghtly and shake with fresh 1 ml. portions of carbon tetrachloride until the last extract is colourless boil the acid solution until no more nitrous fumes are evolved and cool. If iodine is absent, use 1 ml. of the fusion solution which has been strongly acidified with glacial acetic acid. Add a small amount of lead dioxide, place a strip of fluorescein paper across the mouth of the tube, and warm the solution. If bromine is present, it will colour the test paper rose-pink (eosin). 

Fluorescein test Free bromine converts the yellow dyestuff fluorescein(I) into the red tetrabromofluorescein or eosin(II). Filter paper impregnated with fluorescein solution is therefore a valuable reagent for bromine vapour since the paper acquires a red colour. 

Procedure, (a) The apparatus shown on page 54 (Fig. 26) is used. It is charged with a drop of the test solution and a few particles of lead peroxide and acetic acid, or a few drops of chromic acid. The apparatus is closed with the funnel stopper and filter paper moistened with fluorescein solution is laid over the funnel. The apparatus is gently warmed, and, according to the amount of bromine present, a circular red fleck is formed more or less rapidly on the yellow test paper. The apparatus of Fig. 27 may alternatively be used. 

Bhatia and coworkers established synthetic urinary biomarkers for detection of disease-specific protease activity with independence from the need for expensive machinery and an ease of interpretation [173,174]. They used activatable nanoprobes, producing lu-inary biomarkers readily detectable using a paper strip assay similar to a pregnancy test. Ihe luinary biomarkers are PEG-coated iron oxide nanoworms densely conjugated with fluorescein-labeled thrombin and MMP substrates, with high peptide density both for sensitivity and photoinduced fluorescence quenching. 

Fluorescein provides a specific reaction for bromine and iodine chlorine can also be detected if the other halogens are absent. The yellow dye reacts with free bromine to produce red eosin (tetrabromofluorescein), whereas iodine yields red-yellow erythrosine (tetraiodofluorescein) (see page 145). Chlorine is without direct action on fluorescein. If, however, a mixture of potassium bromide and fluorescein is used, the bromine set free by the chlorine immediately forms eosin. Thus fluorescein also provides a test (indirect) for chlorine. All these tests can be carried out as spot reactions on fluorescein and fluorescein-potassium bromide paper. 

Procedure. A drop of the neutral test solution is placed on fluorescein paper. If a red spot appears, free bromine or iodine is present. It is not possible to differentiate between these halogens by this means. 

If the response is negative, a drop of the test solution is placed on fluorescein-potassium bromide paper. If chlorine is present, a red spot results. 

Procedure, (c) One drop of the test solution is placed on filter paper, dried, and spotted with acetic acid-hydrogen peroxide solution (2 parts 6 % hydrogen peroxide + 1 part glacial acetic acid). It is important that the oxidizing mixture cover the fleck completely. The fleck is dried and if iodine is still visible the treatment with the oxidant and drying must be repeated. The spot is then treated with a drop of 1 % alcoholic fluorescein solution. A red ring or stain develops on warming. 

Procedure. A small amount of the solid test material or a drop of its solution is treated in a micro test tube with several drops of chromic-sulfuric acid (preparation see Vol. II page 65). The open end of the test tube is covered with a disk of filter paper moistened with 2 N sodium hydroxide. The test tube is kept for 5 minutes in boiling water. The disk of reagent paper is then placed on a glass plate, spotted with several drops of permolybdic acid solution (for preparation see Vol. II page 68) and then dried with a current of cold air. If iodine is present, a brown fleck will appear but it fades within a few minutes. The stain is then treated with a drop of 0.1 % alcoholic solution of fluorescein. A positive response is indicated by the development of a red spot. 

Remove the aqueous layer and boil it until nitrous fumes are no longer evolved. Cool, add about 0-5 g of pure lead dioxide and heat to boiling. The presence of bromine is indicated by the appearance of brown fumes which cause a filter paper strip soaked in an ethanolic solution of fluorescein to become pink or red. Boil the solution to remove bromine, cool, filter off excess lead dioxide, and test a portion of the filtrate for residual bromine by adding carbon tetrachloride and chlorine water. A brown colouration in the lower layer shows that removal of bromine is incomplete, and this mxist be rectified by fxirther treatment with lead dioxide. 

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