582 CONSIDERATION OF CARBON COMPOUNDS. 



oxidation of turpentine and in other ways. It is nearly insoluble in water, 

 alcohol, and ether, and does not yield an anhydride. 



Phenolphthalein. When phthalic anhydride is heated with phenols 

 and concentrated sulphuric acid, a class of substances is obtained 

 known as phthaleins. The simplest of these is phenolphthalein, the 

 composition of which is shown in the following reaction : 



C6Hl \co >0 + 2C6H5 - OH = C 



Phthalic anhydride. Phenol. Phenolphthalein. 



It occurs as a creamy-white powder or crystals, soluble in 600 parts 

 of water and in 10 parts of alcohol. It dissolves in alkaline solu- 

 tions with a beautiful red color, and is used as a sensitive indicator in 

 acidimetry and alkalimetry. Acids destroy the red color by reform- 

 ing the colorless phenolphthalein from its salts. Taken internally, it 

 acts as a purgative, but appears to possess no further physiological 

 action. For adults the average dose is 0.1 to 0.2 Gm., given as 

 powder, in cachets, capsules, or pills. In obstinate cases 0.5 Gm. 

 doses may be given. 



Resordnolphthalein or fluorescein is obtained by heating phthalic anhydride 

 and resorcinol at 210 C. with zinc chloride as a dehydrating agent. It is a 

 reddish-brown substance which exhibits an intense yellowish-green fluores- 

 cence in an alkaline solution, hence its name. By treatment with bromine it 

 forms tetrabromfluorescein, the potassium salt of which is the dye known as 

 eosin, C 20 H 6 O 5 Br 4 K 2 . This is a valuable stain for animal and plant tissues. In 

 dilute solution it shows a beautiful rose tint. 



Phenolsulphonephthalein, C 6 H 4 ^gQX) . This substance is anal- 



ogous to phenolphthalein, and may be obtained in a similar manner 

 by heating together phenol and the anhydride of orthosulphobenzoic acid, 



C H / CO \ 

 6 4 \cjn /^> which is analogous to the anhydride of phthalic acid. The 



source of the anhydride of sulphobenzoic acid is saccharin. 



Phenolsulphonephthalein is a red or brownish-red powder, soluble in alcohol, 

 but not in ether. It is slightly soluble in cold water, giving a deep yellow 

 color to the solution, but readily soluble in alkalies, the mono-sodium salt 

 having a bordeaux red color, while with excess of alkali the solution has a 

 beautiful purple color similar to that of phenolphthalein in alkaline solution. 

 It is used as a diagnostic test of renal efficiency by injecting 6 Mgm. in the 

 form of the mono-sodium salt. The test depends upon the fact that normal 

 kidneys excrete 40 to 60 per cent, of the dose during the first hour after its 

 first appearance in the urine, whereas kidneys not functioning properly 

 excrete a much smaller per cent. The readings are made by means of a 

 colorimeter. 



