Our solid chlorophyll has been thoroughly dried for several days
over PzOr, in vucuo. Chlorophyll is ordinarily very retentive of
traces of water, and the dried product, when dissolved in anhydrous
ether, is apparently solvated to a different extent, or in a different
way than is the case when the ether contains traces of alcohol,
or the chlorophyll traces of water. In the case of the pigment
hypericin (7), there even appear to be two distinct molecular
species capable of coexistence in a mixed solvent. The effect of
solvent on maxima for both chlorophylls a and b may also be noted
in the change from anhydrous acetone (2) to 80 per cent aqueous
acetone (Columns 2 and 3, Table II).
Another explanation involves the possible adsorption of pigment
on the walls of the absorption cell, noted by Dr. Strain with xanthophyll
spectra. It may therefore be an irony that with highly
purified preparations errors will be introduced by the use of highly
purified solvents particularly when the solutions so prepared are
to serve as standards for comparison with extracts. It becomes
of utmost importance that, if crude extracts are to be analyzed,
the standards for comparison shall be in the same solvent with the
same solvent contaminants as in the extracts, and with the same
moisture content.
In Table I are given a few values of k, the absorption coefficient,
at selected wave-lengths for samples of chlorophylls a and b from
spinach, in methanol (99.8 per cent) and in ether (Merck’s u.s.P.),
and anhydrous values recalculated on the same basis. Solid
chlorophyll a is unfortunately insoluble in aqueous methanol.
Even with 99.8 per cent methanol it was necessary to dissolve the
sample in 1 ml. of acetone, and subsequently to make to volume
with the methanol.
In Table II, Columns 2 and 3, are given the k values for the
same preparation of chlorophylls a and b in aqueous acetone (20 ml.
of distilled water per 80 ml. of redistilled anhydrous C.P. acetone).
To bring the chlorophyll into solution, 2 ml. of acetone were used,