Phenol-sulphonphthalein {^phenol red) has the formula : 



HO OH 



\/\ /\/ 



c_ 

 / o 



so, 



\/ 



Among the other important sulphonphthalein indicators: Brom 

 cresol purple is di-brom-ortho-cresol-sulphonphthalein: 



CH3 CH3 



HO I I OH 



Br C_ Br 



I O 



"SO2 



Thymol blue and Bromthymol blue are thymol-sulphonphthalein 

 and di-brom-thymol-sulphonphthalein respectively; thus: 



CHj CH3 CH3 CH, CH3 CH3 CH3 CH 



3 



CH CH CH CH 



HO I I OH HO I I OH 



. '\ /\/ \/\ /\/ 



11 II and 11 



I Br I \ / I Br 



CH3 C_ CH3 CH3 C_ CH3 



/ o / O 



I I so, \ \ so. 



The formulae are all so similar that there is no need of giving the 

 others. It is enough to state that brom phenol blue is tetra-brom- 

 phenol-sulphonphthalein ; cresol red is ortho-cresol-sulphonphtha- 

 lein ; brom cresol green is tetra-brom-meta-cresol-sulphonphthalein ; 

 chlor phenol red is di-chlor-phenol-sulphonphthalein; brom phenol 

 red is di-brom-phenol-sulphonphthalein; while brom-chlor phenol 

 blue is di-brom-di-chlor-phenol-sulphonphthalein. 



85 



