Phenolphthalein

Phenolphthalein, in addition to its use an indicator in analytical chemistry, is a widely used cathartic agent. 

Phenolphthalein is manufactured by adding melted phenol (10 parts) to a cooled solution of phthalic anhydride (5 parts) in concentrated sulfuric acid (4 parts) and heating the mixture 10 to 12 hours at 120°C. The hot condensation product is poured into boiling water and boiled with successive changes of hot water. The condensate is then dissolved in warm, dilute caustic soda and precipitated with acetic acid. It may be purified by crystallization from absolute alcohol after treatment with, and being filtered through activated carbon. 

Other Names Phenolphthalein 3,3-5w(4-hydroxyphenyl)-l(3H)- isobenzofuranone 3,3-Bis (4-hydroxyphenyl)phthalide 3,3-5w(p-hydroxyphenyl)phthalide Euchessina Koprol Laxogen Lilo NSC 10464 NSC 215214 Phthalimetten Phthalin Purga Purgen Purgophen Spulmako-lax Trilax 

CA Index Name l(3H)-lsobenzofuranone, 3,3- w(4-hydroxyphenyl)-CAS Registry Number 77-09-8 Merck Index Number 7243 Chemical Structure 

Chemical/Dye Class Phthalein Molecular Formula C20H14O4 Molecular Weight 318.32 pH Range 8.0-10.0 

Color Change at pH Colorless (8.0) to pink (10.0) pKa 9.10, 9.40, 9.53, 9.70 Physical Form White powder 

Solubility Almost insoluble in water soluble in alcohol, ether UV-Visible (A, ,ax) 552 nm, 553 nm, 374 nm, 205 nm, 229 mn, 276 mn Melting Point 258-262°C 

This body is obtained by action of phenol on phthalic anhydride in presence of strong sulphuric acid. The free phthalein forms colourless crystals, melting at about 350° it dissolves in alkalies with a red colour, and is precipitated by acids as a white precipitate. The alkaline solution is decolorised by excess of alkali. On melting with caustic potash, it yields benzoic acid and dioxy-benzophenone. On account of the change in colour occasioned by free alkalies (not carbonate or ammonia), phcnolphthalein is useful as an indicator in titration. 

Fluorescein is obtained by heating an intimate mixture of two molecules resorcin with one molecule phthalic anhydride to 190°— 200°. The materials used in the manufacture should be as pure as possible, as impure fluorescein is difficult to purify. In the pure state it forms dark yellow crystals, sparingly soluble in alcohol, more easily in glacial acetic acid. It is almost entirely insoluble in water, but dissolves in alkalies, forming a yellowish-red solution, which, when dilute, exhibits a bright green fluorescence. Acids precipitate it from the alkaline solution as a yellow powder. 

Corresponding chlorofluoresceins are obtained by action of di-and tetra-chlorophthaUc anhydride on resorcin. These compounds are entirely different from those formed by direct chlorination of fluorescein. In these the chlorine (bromine, or iodine) always enters the resorcin-rest. The fluoresceins from the chlorinated phthalic acids serve as starting-points for a series of very brilliant phthalem dyestuffs, which were first introduced into the colour-industry by E. Noelting. 

The chlorinated fluoresceins have a somewhat redder shade than the corresponding fluoresceins which contain no chlorine. 

On treating fluorescein with bromine, substitution of the hydrogen atoms in the resorcin-rests takes place. The final result is that four hydrogen atoms of the fluorescein are replaced by bromine, the product being tetrabromfluorescein, CjoHgOgBr [63]. 

In order to obtain the sodium compound, 6 grams of the product are ground in a moitar with i gram of sodium carbonate, placed in a beaker, and moistened with alcohol. Five c.c. of water are added and the nii.xture boiled until the evolution of carbon dioxide ceases. To the sodium salt 25 c.c. spirit are added and the mixture boiled and filtered. On standing for a day or two, the sodium salt crystallises in brown needles. 

The phenol, caustic soda and water are mixed together in zl round flask (i litre) attached to an upright condenser and heated to 50—60°. The chloroform is then added gradually 

Reaction.—kAd a drop of feriic chloride to the aqueous solution of the aldehyde. A deep violet colouration is producecl p-Hydi-oxybcnsaldehyde.—Colouiless needles, m. p. 115—1 16 scarcely soluble in cold water, readily in hot water, alcohol ether. Non-volatile in steam. The bisulphite of sodiutaa compound dissolves readdy in water. 

Reaction.—same as above but the colouration is less intense. See Appendix., p. 297. 

Colourless needles 111. i. 155 — 156 soluble in ilcohol raiad hot water, too parts water dissolve o 225 part at 15 and 7 y3 5 jiaits at loo.  

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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. 

Example. Dissolve 0 3 g. of />-chlorobenzoic ncid in a small quantity of warm ethanol (about 10 ml.), and ctlrefully add 5 o aqueous sodium hydroxide drop- wise until the solution is just pink to phenolphthalein. Evaporate to dryness on a water-bath. Dissolve the sodium -chlorobenzoate in a minimum of water, add a solution of 0-5 g. of phenacyl bromide in ethanol (about 5 ml.), and boil the mixture under reflux for i hour, and then cool. The phenacyl ester usually ciy stallises on cooling if it does not, add water dropnise with stirring to the chilled solution until separation of the ester just begins. Filter the ester, wash on the filter with water, drain and recrystallise from ethanol m.p. 90 . The /)-bromophenacyl ester is similarly prepared, and after recrystallisation from aqueous ethanol has m.p. 128 . (M.ps., pp. 543-545.)... 

Phthalein reaction. Fuse together carefully in a dry test-tube a few crystals of phthalic acid or of a phthalate and an equal quantity of ph tol moistened with 2 drops of cone. H2SO4. Cool, dissolve in water and add NaOH solution in excess the bright red colour of phenolphthalein in alkaline solution is produced. 

Hydrolysis of methyl oxalate. The exceptionally rapid hydrolysis of rnethyl oxalate can be followed thus Dissolve 0 2 g. of finely powdered methyl oxalate in 10 ml. of water, and add i drop of phenolphthalein. Then add very dil. NaOH solution (1%) drop by drop until the solution just turns pink it will be noticed that the colour rapidly fades, but is restored on the Further addition of 1-2 drops of NaOH solution. The colour fades again and the addition can be repeated until hydrolysis is complete. Oxalic acid (with which methyl oxalate may be confused) gives a precise end-point when treated with NaOH solution in this way. 

Physical Properties. Glycine is a colourless crystalline solid soluble in water. Owing to the almost equal opposing effects of the amino and the carboxylic groups. its aqueous solution is almost neutral (actually, slightly acidic to phenolphthalein) and glycine is therefore known as a neutral ampholyte. f It exhibits both acidic and basic properties. 

Sorensen s reaction. First read carefully the Estimation of Glycine, p. 463. Dissolve 0 2 g. of glycine in a few ml. of water in a test-tube A, add 2 drops of phenolphthalein and then very dilute NaOH solution drop by drop until the solution just turns pink. In a second test-tube B place 2 ml. of 40% formalin solution, add 2 drops of phenolphthalein solution and then the dil. NaOH solution until the solution just turns pink. Pour the contents of B into A and note the immediate decolorisation of the phenolphthalein, the solution now being acid. Observe also that several drops of dil. NaOH solution can now be added before the pink colour is restored. 

Tyrosine and cystine are insoluble in water therefore place about 0 2 g. in the test-tube A, dissolve in the dil. NaOH solution, add phenolphthalein as before and then add dil. HCl until pink colour is iust not discharged then proceed as above. 

The molecular weight of many carboxylic acids which arc freely soluble in cold water (i.e., chiefly the aliphatic acids) can readily be obtained by titrating a known weight of the acids in aqueous solution with standard sodium or potassium hydroxide solution, using phenolphthalein as an indicator. To avoid the use of unduly large quantities of the acid, it is advisable to use Mj2 caustic alkali solution, and in order to obtain a sharp end>point, this alkali solution... 

Weigh out accurately about 2-5 g. of pure powdered succinic acid, transfer to a 100 ml. graduated flask, dissolve in distilled water, make the solution up to the graduation mark and mix well. Now, by means of a pipette, transfer 25 ml. of the solution to a 150 ml. conical flask, add a drop of phenolphthalein solution and titrate with A/ 2 NaOH or KOH solution to obtain consistent results. 

Cool the flasks in cold water, and then titrate the contents of each with /i/.t a()H solution, using phenolphthalein as an indicator. Shake the contents of the flask A repeatedly during the titration in order to keep the fine crystals of acetanilide dispersed in the aqueous solution. 

Since formaldehyde solutions almost invariably contain formic acid, and amino-acids themselves are seldom exactly neutral, it is very important that both the formaldehyde solution and the glycine solution should before mixing be brought to the same pH (see footnote, p. 509), and for this purpose each solution is first madejWl alkaline to phenolphthalein by means of dilute sodium hydroxide solution. This preliminary neutralisation must not be confused with... 

Glycine itself is almost neutral, and requires very little sodium hydroxide to give a pink colour with phenolphthalein some other amino-acids, e.g., glutamic acid, aspartic acid, etc., are definitely more acidic and consequently require more alkali for this purpose cf. footnote, p. 380). 

Place about 50 ml. of 40% formalin solution in a conical flask and add at least 10 drops of phenolphthalein solution. No add ery carefully from a burette dilute sodium hydroxide solution (il/ 10 will serve the purpose) until the solution is just faintly pink. 

It must be emphasised that these experiments are only roughly quantitative, for owing to the alight opalescence of the digesting mixture, accurate end-px>ints using phenolphthalein can be obtained only by using an instrument known as a comparator. 

Remove 25 ml. by means of a pipette, add a few drops of phenolphthalein the colour is pink. Now add very cautiously, drop by drop, dilute acetic acid (say Mjioo) until the pink colour has just not disappeared. It is important not to add too much acid, otherwise the casein will be precipitated. Now add 5 ml. of neutralised formalin (see p. 464) and then titrate with Mj 10 NaOH solution. Note the amount required. 

To the remainder of the casein solution add 0 5 to o 8 g. of finely powdered commercial trypsin, shake to dissolve, and place in a thermostat (or in an incubator) at 40 . After 15 minutes, remove 25 ml. and add a few drops of phenolphthalein it will now be found that the solution remains colourless. Run in carefully Mj 10 NaOH solution until the colour of the solution is just pink, add 5 ml, of neutralised formalin and then titrate against Mj 10 NaOH solution until the pink colour is just restored note the amount required. Remove fiirther quantities (rf 25 ml. at intervals which must be determined by the speed of the reaction. The following will probably make a suitable series i, 2, 3,... 

Dissolve 6 g. of gelatin in about 250 ml. of warm water. Carry out the formaldehyde titration on 25 ml. of the solution note in this case that the solu> tion is acid to phenolphthalein. 

The estimation. Label two 250 ml. conical flasks A and B, and into each measure 5 ml. of urine solution (or about o i g. of solid urea, accurately weighed). Add to each about 20 ml. of water and bring the temperature to about 60°. To A add 3 drops of phenolphthalein solution and to B add i ml. of 0-5% mercuric chloride solution. Now to each solution, add 10 ml. of the urease solution and mix well. The mixture A soon turns red. 

Make up an approximately 5% solution of acetaldehyde in water, add a few drops of phenolphthalein and then add dilute NajCOj until the solution just turns pink, thus removing any free acid. 

Dissolve or suspend 0 - 5 g. of the acid in 5 ml. of water in a small conical flask, add a drop or two of phenolphthalein indicator, and then 4-5 per cent, sodium hydroxide solution until the acid is just neutrahsed. Add a few drops of very dilute hydrochloric acid so that the final solution is faintly acid (litmus).f Introduce 0-5 g. of p-bromophenacyl bromide (m.p. 109°) dissolved in 5 ml. of rectified (or methylated) spirit, and heat the mixture under reflux for 1 hour if the mixture is not homogeneous at the boiling point or a solid separates out, add just sufficient alcohol to produce homogeneity. [Di- and tri-basic acids require proportionately larger amounts of the reagent and longer refluxing periods.] Allow the solution to cool, filter the separated crystals at the pump, wash with a little alcohol and then with water. Recrystallise from dilute alcohol dissolve the solid in hot alcohol, add hot water until a turbidity just results, clear the latter with a few drops of alcohol, and allow to cool. Acetone may sometimes be employed for recrystallisation. 

Dissolve (or suspend) 0-25 g. of the acid in 5 ml. of warm water, add a drop or two of phenolphthalein indicator and neutralise carefully with ca. N sodium hydroxide solution. Then add 2-3 drops of ca. O lN hydrochloric acid to ensure that the solution is almost neutral (pale pink colour). (Under alkaline conditions the reagent tends to decompose to produce the evil-smelling benzyl mercaptan.) If the sodium salt is available, dissolve 0-25 g. in 5 ml. of water, and add 2 drops of ca. 0 -hydrochloric acid. Introduce a solution of 1 g. of S-benzyl-iso-thiuro-nium chloride in 5 ml. of water, and cool in ice until precipitation is Dibasic and tribasic acids will require 0-01 and 0-015 mol respectively. 

Acidify the residue in the flask with dUute sulphuric acid and distil off 10-15 ml. of the solution. Test a smaU portion of the distillate for acidity, and also observe the odour. Neutralise the main portion with sodium hydroxide solution (add a drop of phenolphthalein to act as indicator), evaporate to smaU bulk, and convert the sodium salt into the p-bromophenacyl ester or into some other suitable derivative (Section 111,85) determine the m.p. of the derivative. 

Drop 1 g. of sodium into 10 ml. of ethyl alcohol in a small flask provided with a small water condenser heat the mixture until all the sodium has dissolved. Cool, and add 1 g. of the ester and 0-5 ml. of water. Frequently the sodium salt of the acid will be deposited either at once or after boiling for a few minutes. If this occurs, filter oflF the solid at once, wash it with a little absolute ethyl alcohol (or absolute methylated spirit), and convert it into the p-bromophenacyl ester, p-nitro-benzyl ester or S-benzyl-tso-thiuronium salt (for experimental details, see Section 111,85). If no solid separates, continue the boiling for 30-60 minutes, boil oflF the alcohol, allow to cool, render the product just neutral to phenolphthalein with dilute sulphuric or hydrochloric acid, convert the sodium salt present in solution into a crystalline derivative (Section 111,85), and determine its melting point. 

The residue in the flask will contain the sodium (or potassium) salt of the acid together with excess of alkali. Just acidify with dilute sulphuric acid and observe whether a crystalline acid separates if it does, filter, recrystallise and identify (Section 111,85). If no crystaUine solid is obtained, the solution may be just neutralised to phenolphthalein and the solution of the alkali salt used for the preparation of a crystaUine derivative. This wiU confirm, if necessary, the results of hydrolysis by method 1. If the time factor is important, either method 1 or the product of the caustic alkali hydrolysis may be used for the identification of the acid. 

It is essential to standardise the alcoholic potassium hydroxide solution immediately before use by titration with standard 0-5N or 0-25N hydrochloric or sulphuric acid using phenolphthalein as indicator. 

The excess of alkah is then neutralised with dilute hydrochloric acid (phenolphthalein) and the solution is evaporated to dryness on the water bath. The acid may then be characterised as the S-benzyl-tao-thiuronium salt or as the p-bromophenacyl ester (Section 111,85). In many instances the derivative may be prepared directly from the neutralised solution. 

If crude anthranilic acid is employed, it should be titrated against standard alkali with phenolphthalein as indicator, and the weight adjusted in accordance with the purity. 

The sodium sulphite solution may also be prepared by dissolving 100 g. of pure (or a corresponding quantity of commercial) sodium hydroxide in about 125 ml. of water, and then diluting to 750 ml. The flask is cooled in running water, a few drops of phenolphthalein indicator are added, and sulphur dioxide passed in until the pink colour just disappears (it is advisable to add a further 1-2 drops of the indicator at this point) and then for 2-3 minutes longer. It is best to remove a sample for test from time to time, dilute with 3-4 volumes of water, and test with I drop of phenolphthalein. 

Equip a 3 litre three-necked flask with a thermometer, a mercury-sealed mechanical stirrer and a double-surface reflux condenser. It is important that all the apparatus be thoroughly dry. Place 212 g. of trimethylene dibromide (Section 111,35) and 160 g. of ethyl malonate (Section 111,153) (dried over anhydrous calcium sulphate) in the flask. By means of a separatory funnel, supported in a retort ring and fitted into the top of the condenser with a grooved cork, add with stirring a solution of 46 g. of sodium in 800 ml. of super dry ethyl alcohol (Section 11,47,5) (I) at such a rate that the temperature of the reaction mixture is maintained at 60-65° (50-60 minutes). When the addition is complete, allow the mixture to stand until the temperature falls to 50-55°, and then heat on a water bath until a few drops of the liquid when added to water are no longer alkaline to phenolphthalein (about 2 hours). Add sufficient water to dissolve the precipitate of sodium bromide, and remove the alcohol by distillation from a water bath. Arrange the flask for steam distillation (Fig. this merely involves... 

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