44 
Transactions of the Royal Society of South Africa. 
wave-length of the violet absorption baud is f of that of the visible one the 
formula becomes simply y = x — 7, where y is the visible HgSO^, frequency, 
and X is the violet alkali frequency. Thus for phenolphthalein x is 27'03 
(viz. 10,000-^-370), whence y (calculated) is 20-03 (observed 10,000^499 
or 20-04). For thymolphthalein x = 25-13 (10* of 597), whence y (cal- 
culated) is 18-13 (observed 18-18). The formula is probably rigorously 
exact, but the observations are not capable of a higher accuracy than, say, 
0-2 per cent. The constant 7* in the formula represents the " loading " of 
the molecule with the constant molecular weight of HoSO^, or more probably 
the constant o^tomic- volume of that substance as suggested in Part I. The 
following table indeed shows that the phenomena do not follow the mole- 
cular weights, and consequently that the atomic volumes are probably the 
cause of the position of the absorption-band in each case. 
Position of 
Name. violet alkali 
Molecular 
Position of 
M.W. of 
band in 
oxonium 
Law. 
H2SO4. 
sulphate. 
\ 499 
. 416 . 
= 7 
\ 635 
. 516 . 
■^^^(4TT-"6"5 5") 
= 7 
X 550 
528 . 
^^^{ 'a ~ TTo ) 
= 7 
X 537 
. 920 . 
(391 537) 
= 7 
Phenolphthalein . . X 370 . 318 
A-naphtholphthalein . X 441 . 418 
Thymolphthalein . . X 398 . 430 
Tetriodophenolphthalein X 391 . 822 
The physical interpretation of this is that the frequency 27 of phenol- 
phthalein belongs to the : C : 0 group of the quinonoid part (or to its di- 
valent ion, seeing that it is a sodium salt, viz. : -i-2 Na+). In HgSO^ 
OH 
/ . . 
the configuration s : C\ , and this change, in all the phthaleins, lessens 
the frequency by 7. The particular value 27 belongs to the CO group 
as affected by the rest of the molecule (viz. C|9H;l403). If the rest of 
the molecule is expanded by addition of CgH^^ as in naphtholphthalein, the 
frequency goes down in the same way as it does with HoSO^, but, as the 
Table shows, this addition of molecular weight 100 produces far more effect 
than the substitution of four iodine atoms (increase in molecular weight 504). 
* 6-95 is perhaps a more exact value. 
