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It should be used in cold, not too dilute, solutions. Its sensitiveness is lessened in the presence of considerable quantities of the salts of the alkalies. A !phenolphthalein solution! is prepared by dissolving 1 gram of the pure compound in 100 cc. of 95 per cent alcohol. This indicator is particularly valuable in the determination of weak acids, especially organic acids.
The presence of this free or excess alkali can be at once detected by rubbing a little phenolphthalein solution on to the freshly-cut surface of a piece of soap; if free alkali be present, a red colour will be produced. Shellac. The resin tribe, of which shellac is a member, comprises vegetable products of a certain degree of similarity.
The chemist was then obliged to resort to certain sensitive coal-tar colours, which did not, as the dyer and printer knew, form lakes with alumina and iron, such as methyl orange, fluorescein, Congo red, phenolphthalein, and so forth.
The comparison of the acid and alkali solutions was made, using methyl orange as an indicator, while the titration of the oxalic acid is made with the use of phenolphthalein. In the oxidation processes of volumetric analysis standard solutions of oxidizing agents and of reducing agents take the place of the acid and alkali solutions of the neutralization processes already studied.
In the following discussion of the principles underlying the behavior of the indicators as a class, methyl orange and phenolphthalein will be taken as types. It has just been pointed out that indicators are bodies of complicated structure. Chem.
Furthermore, if to the colourless solution containing phenolphthalein, and which is acid towards that reagent, a little reddened litmus is added, this is still turned blue, and so still indicates the presence of alkali. We have, therefore, in bicarbonate of soda a salt which behaves as an acid to phenolphthalein and as an alkali to litmus.
A chemist received four different solutions, with the statement that they contained either pure NaOH; pure Na CO ; pure NaHCO , or mixtures of these substances. From the following data identify them: Sample I. On adding phenolphthalein to a solution of the substance, it gave no color to the solution.
The caustic alkali may also be determined by first using phenolphthalein as an indicator, which will show by its change from pink to colorless the point at which the caustic alkali has been neutralized and the carbonate has been converted to bicarbonate, and then adding methyl orange and completing the titration.
However, phenolphthalein, though reacting alkaline with monocarbonate of soda, indicates the acidity of the bicarbonate of soda, a thing which, as I have just said, litmus will not do. We will take two jars containing solution of monocarbonate of soda, and in the first we will put some phenolphthalein solution, and in the second, some litmus tincture.
Sample IV. On titrating with hydrochloric acid, using phenolphthalein, 15.00 cc. were required. A new sample of the same weight required exactly 30 cc. of the same acid for neutralization, using methyl orange. In the analysis of a sample of KHC H O the following data are obtained: Weight sample = 0.4732 gram. NaOH solution used = 24.97 cc. 3.00 cc.
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