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COLOUR AND CHEMICAL CONSTITUTION. 
Part XL — A Systematic Study of the Beominated Phenolphthaleins 
kegarding the relation between position and colouk. 
By James Moir. 
In Part IX of this work a mathematical formula was developed whereby 
the colour of any halogenated phenolphthalein could be calculated. The 
application of this formula was, however, in practice limited to substances 
halogenated in the phenol rings, it being known that the four phthalic 
positions are very abnormal (although the four taken together are very 
nearly equivalent to four phenolic positions). 
The formula when confined to the halogen bromine alone reads ^ = 1 — 
554 
0-0128m, or since in phenolphthalein \ = 554, A = 1 — 0-0T28w' which 
m is the number of (ortho) bromine atoms present in the brominated 
phenolphthalein the colour of which is to be calculated. For meta-bromine 
554 
atoms m is to be doubled. The expression iZTQ^Ql^STti simplicity 
be written without a divisor as 554 (1 + 0-01287U + 0-000164?>z--), hence 
finally = 554 + 7'091m + O'Odlm^, Terms containing higher powers of m 
are negligible. 
Hence if m is a small number, less than 5, the wave-length may be taken 
to be 7^ units higher than 554 for each ortho-bromine atom present. Thus 
561|-, 569, 576^ and 584 are the wave-lengths of (ortho) mono-, di-, tri- and 
tetra-bromophenolphthalein. This, though not strictly accurate, agrees with 
observation within one unit. 
I have, however, shown in Part II (foot of p. 113) that substitution of a 
second phenolic ring produces less effect than substitution of the first ring, 
the latter producing about 58 per cent (or four-sevenths) of the effect of 
substituting both phenolic rings, and the former 42 per cent, (or three- 
sevenths) of the effect. 
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