Phenolphthalein, III

The existence of anode and cathode areas can be seen by the following experiment. A few drops of phenolphthalein are added to a solution of potassium hexacyanoferrate(III) and hydrochloric acid added, drop by drop, until the solution is colourless. (The phenolphthalein turns pink due to hydrolysis of the potassium hexacyano-ferrate(III).) Drops of this solution, about 1 cm in diameter, are now placed on a sheet of freshly abraded steel when pink cathode areas and blue anode areas appear. 

Ammonia is determined by the Kjeldahl method and chromium by the process of decomposing the complex salt with alkali, making the mixture acid to phenolphthalein, filtering, igniting, and weighing as chromium(III) oxide. Anal. Calcd. for [Cr(NH3)6Cl]Cl2 Cr, 21.5 NH3, 35.0. Found Cr, 21.5 NHS, 34.7. 

Methyl orange III, P. 23 Nickel chloride XI Nitric acid, 6mormal Nitric acid, 16-normal Phenolphthalein P. 53 Phosphoric acid, ortho, 85% IV Potassium chlorate (chloride free) III Potassium ferricyanide III, IX, P. 73, Q. 73... 

The correct answer is choice C. First, review the choices. Choices I and II indicate a basic solution. If they were acidic, then the solutions would be red for litmus and clear for phenolphthalein. Only choice III holds true for an acidic solution. In an acidic solution the concentration of hydronium ions exceeds that of hydroxide ion concentration. 

The indicator phenolphthalein, which is colorless in neutral solution, turns reddish-violet in the presence of the basic soda solution. In the third beaker the H30 ions set free in the hydrolysis of the iron(IIl) salt bind the OH" ions from the soda solution, leading to decolorisation of the phenolphthalein at the same time the solution turns yellow because of the presence of the hydrolysed iron(III) species (eqns. la, b) ... 

Although compounds I and IV are satisfactory, they offer no advantages over phenolphthalein. Substances III and V may perhaps be used in place of thymolphthalein. Unfortunately the color transition in the case of the compounds III and V is not especially pronounced. 

Fig. 12. I, tropeolin 0 II, thymol-phthalein III, phenolphthalein IV, thymol blue.

The monovalent ions of phenolphthalein (form III) are known to be colorless. In a titration of an alcoholic phenolphthalein solution, for example, almost an equivalent amount of alkali is added before the red-violet color appears. Rosenstein reports that the apparent first dissociation constant of phenolphthalein is 1.15 X 10 , whereas K, = 2.8 X 10 . Accordingly... 

In an aqueous solution of phenolphthalein the concentration of the lactone form is about 10,000 times greater than that of the hydrate form II.The concentration of the quinoid forms IV and V must be exceedingly small as compared with that of I because the solution is colorless. Addition of hydroxyl ions displaces the equilibrium of II and III through V to the quinone phenolate VI. The equilibrium relationships for phenolphthalein are represented quantitatively by the following expressions ... 

Phenolphthalein [77-09-8] M 319.2, m 263 , pKEst(i) 4.2, pKEst(2) 9.8. Dissolve it in EtOH (7mL/g), then dilute it with eight volumes of cold water, filter and heat on a water-bath to remove most of the alcohol and the phenolphthalein that precipitates is filtered off and dried in vacuo. [Beilstein 18 II 119, 18 III/IV 1945, 18/4 V 188.]... 

Dissolve a package of clear, unflavored gelatin in about 200 ml of warm water. Stir in 2 ml of phenolphthalein solution and 2 ml of potassium hexacyanoferrate(III) solution. Pour the prepared solution into a widemouth glass jar or petri dish to a depth of about 1 cm. 

---