Sulphonephthalein indicator

Fenn, J. (1995). The cellulose nitrate time bomb using sulphonephthalein indicators to evaluate storage strategies. In Postprints Tate Gallery conference From Marble to Chocolate . The Conservation of Modem Sculpture London, 18—20 September 1995 (J. Heuman, ed.) pp. 87-92, Archetype Books. 

Table 5.1 summarizes the details of some useful acid-base indicators. Exact agreement with the pH range expressed by equation (5.5) is by no means always observed. This is because some colour changes are easier to see than others and so the general approximation made in deriving equation (5.5) is not uniformly close. Structurally, the indicators form three groups phthaleins (e g. phenolphthalein) sulphonephthaleins (e.g. phenol red) and azo compounds (e.g. methyl orange). 

S.S) is not unifonnly close. Structurally, the indicators form three groups phthaleins (e.g. phenolphthalein) sulphonephthaleins (e.g. phenol red) azo compounds (e.g. methyl orange). 

This method has been extensively exploited for the synthesis of a large group of 3H-2,1-benzoxathiole 1,1-dioxide derivatives known as sulphonephthaleins, many of which, such as phenol red, p-cresol red and bromothymol blue are well known acid-base indicators. There is excellent coverage of this subject in Breslow and Skolnik s monograph8 and equation 60 merely serves to illustrate two representative syntheses81,82. The sulphonephthaleins can also be prepared from 3ff-2,l-benzoxathiole-l-dioxide derivatives with reactive substituents (such as dichloro or carbonyl) at the 3-position (see Section III.D.3). 

The extracting solvent can be diethyl ether or carbon tetrachloride (tetrachloromethane). The former is advised in the method given here, because of its lower toxicity. ASTM Method D 4224-89 [3] is similar, but uses a 13.6% solution of the reagent, permits only carbon tetrachloride and directs that the initial extraction should be done by shaking for 10 min. It uses ortho-cresol red (o-cresol sulphonephthalein) as indicator, and adds the correction for extracted p-toluidine hydrochloride (step 10), which was not part of the original method. 

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