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COLOUR AND CHEMICAL CONSTITUTIOF. 
Parts V and YI. 
By James Moir, M.A., D.Sc, FJ.C. 
Part V. — The Yellowness op certain Phthaleins when Acid. 
Attention has already been called to the fact that phenolsulphonephthalein, 
which is constitutionally an orthosulphonic acid of benzaurine, resembles the 
latter substance in being yellow when neutral or slightly acid, whereas 
phenolphthalein is colourless under the same circumstances, although it also 
is an ortho-acid of benzaurine. Two more examples of yellow derivatives of 
benzaurine have now been investigated. The first is benzaurine-parasulphonic 
acid, which is one of the products obtained when benzaldehyd is condensed 
with phenol and sulphuric acid at a fairly high temperature, benzaurine being 
first formed by oxidation of leucobenzaurine CgH^ . CH(CgHj^OH).„ and sub- 
sequently sulphonated. The product, the constitution of which is — 
Hso3<^ )>-<i^ = <( y •■ ^ 
is yellow when slightly acid. It is salmon-pink in fairly strong acid, the 
band-centre of its absorption being at A 512 (value for benzaurine a 507). 
It is violet in alkali, with band-centre about a 568 (the value for benzaurine 
is A 570 with weak alkali, increasing to about A 580 in concentrated alkali). 
Its behaviour is very similar to that of phenolsulphonephthalein (acid band 
A 507, neutral only violet absorption, alkali band A 563), and as the 
new substance is obviously not a phthalein it becomes a fair inference that 
phenolsulphonephthalein also is not a phthalein either, in other words the 
ortho — SO3H group of the latter compound never joins up with the central 
carbon, for if it did the product would be a colourless non-quinonoid lactone 
like phenolphthalein. This difference is, of course, due to the high ionisation 
of all sulphonic acids. Phenolsulphonephthalein, then, should be re-named 
benzaurine orthosulphonic acid. 
The third of these yellow compounds is phenolphthalein-monomethyl 
ester, first described by A. G. G-reen and King (BericMe, 1907, 3727). This 
also resembles benzaurine closely, being yellow when neutral or faintly 
acid, salmon-pink in acid, and violet-pink in alkali. The acid solution very 
