Indicator universal. Color plate

Universal bH indicator oaDer, Add a droD of Nesslet s reagent (to 5g of Kl in a min of cold water add a satd aq Hjgl solo until a ppt forms add 40 ml of 50 aq KOH, let settle, dilute to 100 ml with water, let settle and decant) and note color of ppt. Place 50 mg of sample in each of three depressions on a spot-test plate. To one add 2—3 drops of a 65—68% aq ethylene diamine soln, stir and note color of soln to another add 3—4 drops of a solution of Ig diphenylamine in 100ml of ooned sulfuric acid, stir, and note color of soln after 1 min to the last sample add 50mg of thymol and 3 drops of coned sulfuric acid, stir and note color after 5 min... 

Measurements should be carried out with two different indicators, unless routine analyses are involved. When nothing at all is known of the acidity of the solution, the approximate pH must first be found in order to permit a choice of proper indicators. Of course only those indicators may be used which impart a distinct intermediate color to the unknown solution. An indicator is of no value if its color in the unknown solution is due to the pure acid or pure basic form. The preliminary examination for acidity may be performed with the use of various indicator papers such as congo, litmus, phenolphthalein, and turmeric papers, or by treating small portions of liquid (on a spot plate) with various indicators. If it happens, for example, that a solution remains colorless in the presence of phenolphthalein and is alkaline towards methyl orange, its pH must lie between 8 and 4.5. Should further tests show the unknown to be alkaline also to methyl red, we would know that the pH being measured lies between 6 and 8, and that a suitable indicator could be chosen from the group which includes bromthymol blue, phenol red, neutral red, and cresol red. Universal indicators are especially useful for making such estimates of pH values. 

Procedure. A drop of the test solution and a drop of a 10 % solution of sodium thiosulfate are mixed on a spot plate and tested with a piece of universal indicator paper. The presence of mercuric cyanide is indicated by a color change. 

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