Phenolphthalein, color reactions preparation

Standard solutions are prepared by dissolving dry phenolphthalein in ethanol and diluting with about two volumes of water. Ethanol does not interfere in the color reaction, provided that its final concentration does not exceed 5%. 

In the meantime, a sodium sulfite solution is prepared by dissolving 890 g. of sodium hydroxide, of about 90 per cent purity, in about 1 1. of water and then diluting to 6 1. A few drops of phenolphthalein solution are added and sulfur dioxide passed in, first until an acid reaction is indicated and then for two or three minutes longer. During the addition of the sulfur dioxide, the solution is cooled with running water. On account of the strong alkaline solution, the original color produced by... 

An ethanolic solution of hydroxylamine is prepared, in a fashion similar to that described by Hurd, as follows To 300 ml. of absolute ethanol in a 500-ml. Erlenmeyer flask is added 12.5 g. (0.545 mole) of metallic sodium. After completion of the reaction (2-3 hours), the sodium ethoxide solution is added drop by drop to a suspension of 35.0 g. (0.50 mole) of finely ground hydroxylammonium chloride in 200 ml. of absolute ethanol which contains about 0.01 g. of phenolphthalein in a 1-1. flask. A magnetic stirrer keeps the mixture stirred while the sodium ethoxide is added the base is added at such a rate that the mixture does not turn pink. (If the addition of all the base causes the suspension to turn pink, some more solid hydroxylammonium chloride is added to discharge the pink color.) The solid is quickly removed by suction filtration on a 60-ml. medium-porosity sintered-glass funnel the filtrate is collected in a 2-1. filter flask. The residual sodium chloride is rinsed twice with small portions of absolute ethanol. 

A specific identification test for urea is the enzymatic reaction with urease. 50 mg of powdered resin or 0.1 ml of the resin solution is carefully heated in a test tube with a Bunsen burner until all formaldehyde has been removed (check odor ). After cooling and neutralizing with 10% sodium hydroxide using phenolphthalein as an indicator, 1 drop of 1 N sulfuric acid and 0.2 ml of a freshly prepared 10% urease solution are added. A moist piece of litmus paper is then attached to the upper rim of the test tube. After a short time, the blue coloration of the indicator paper demonstrates the presence of ammonia which is formed only by urea-containing resins and not by melamine resins. Hexamethylene tetramine is the only substance that may interfere with this reaction. 

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