Colour and Chemical Constitution. 
115 
forward, but it is probably due to the hig-li ionisation of the ~ SO.-H group 
as compared with the — COoH group in the ordinary phthaleins. Thus 
neutral phenol- sulphonephthalein is probably 
j^-SO,-0' 
\/ 
i. e. a substance which is still quinonoid ; whereas neutral phenolplithalein 
is a non-quinonoid lactone, viz. : 
\ X 1-0^3 — / 
and it should be remembered that neutral benzaurine, the parent substance 
of both, is yellow, like the sulphonephthalein. 
It is therefore probable that the deeply coloured alkaline solutions of 
all the phthaleins of every class have the same sort of constitution and one 
different from the above, viz. : 
where X is either H, or GO3', or SO/. 
This is the only theory of phthalein colour which explains all cases, 
including those in which there is only one phenolic ring, such as mono- 
xydiphenylphthalide and phenol-phthalein-monomethyl ether. 
As regards the difference in alkaline colour between any phthalein and 
the corresponding sulphone-phthalein, it will be seen from the table that 
the change of CO into SOo makes little difference, the frequency of the 
absorption-band being lowered by about per cent., which is about the 
same difference as is produced by the entrance of one ortho- methyl group 
into ordinary phenolplithalein to form plienolcresolphthalein. 
Other New Phthaleins and their Spectra. 
These were made from 3-nitroplithalic acid (itself made from a-nitro- 
iiaplithalene) . 
A table giving the position of their absorption bands is appended : 
