186 
Transactio7is of the Boyal Society of South Africa. 
benzoic acid is blue in alkali with central wave-length at X 598. It differs 
from thymolphthalein in giving an orange solution in HoSO^^ in which there 
is no band at X 550, the absorption being in the violet. 
(d) Thymol-^-cresol-'phthalein also resembles thymolphthalein in alkaline 
solution, its central wave-length being at about X 595 ; in H2SO4 it is also 
orange (thymolphthalein purplish-pink) with only violet-absorption. The 
additive property may again be illustrated here : 
phenolparacresolphthalein 572 ^ phenolthyniolphthalein 578 
phenolphthalein 554' phenolphthalein 554 
572 X 578 
. • . calculated value for thymol-jj-cresol-phthalein is = 597. 
The agreement is better if the differences are merely added together : 
572 + 578 - 554 = 596. 
(e) Thymol-^-najjhthol'phthalein is also indefinite : it is bluish-green 
in alkali with absorption-centre about X 700, and like the rest of the class is 
orange in H2SO4 with violet absorption and no fluorescence. 
(/) Phenol- f^-anthrolphthalein is olive-green in alkali with a band at the 
treme red : its centre is about \ 730. 
c. Derivatives of Phenylphenolphthalein (Oxydiphenylphthalide). 
Phenylmetacresolphthalein. — Condensation of benzoylbenzoic acid with 
metacresol yielded a phthalein which, although giving a salmon colour 
in H2SO4 (central-wave-length 506), gave a perfectly colourless solution 
in alkali whatever strength of the latter was used. To explain this is 
difficult : one can only assume that since oxydiphenylphthalide itself is 
easily bleached by excess alkali, the methyl group in the new substance 
has so exaggerated this property that the quinonoid phase only exists 
momentarily, passing immediately into the colourless carbinol phase. 
Fhenyl])aracresol]phthalein and phenylparaoxyhenzoic-acid-phthalein were 
both yellowish -orange in alkali. The former had no band and the latter 
a faint, broad one at about X 486. 
The absorption-band of the orange H0SO4 solution of anisoylbenzoic acid 
CH,0<^ \_C0— <^ ^ 
COolT^ 
was observed to be at X 466 about 7 units higher than that of the 
unmethylated substance (Part II). These almost unique sharp bands in 
the blue are due to the formation of phthalein-like sulphates, thus — 
SO4H 
