Nov. 24, 1923 
Studies on Rancidity 
353 
cc. of concentrated hydrochloric acid and an equal quantity of a 1 per 
cent solution of phloroglucin in ether. The amount of acrolein-hydrogen 
peroxid solution employed was so chosen that the color obtained was of 
suitable intensity for the spectrophotometric examination. The colored 
hydrochloric acid layer was drawn off into a glass cell for observation. 
(b) From epihydrin aldehyde diethylacetal .—One drop of epihydrin 
aldehyde diethylacetal was dissolved in 5 cc. of ether, and varying 
amounts of this solution were treated as described above until a color 
of suitable intensity was obtained. The colored hydrochloric acid 
layer was then drawn off into a glass cell for examination. 
(c) From rancid oleic acid .—About 20 cc. of rancid oleic acid, from 
a specimen which had been originally of a high degree of purity, were 
dissolved in ether and treated with about 20 cc. of concentrated hydro¬ 
chloric acid and a similar quantity of a 1 per cent ethereal solution of 
phloroglucin. After the development of color, the slightly turbid 
hydrochloric acid layer was drawn off for examination. 
(d) From epihydrin aldehyde diethylacetal .—One drop of epihydrin 
aldehyde diethylacetal was dissolved in 5 cc. of ether, and 0.3 cc. of this 
solution were dissolved in 5 cc. of water. The aqueous solution was then 
acidified with 10 drops of concentrated hydrochloric acid and treated 
with a few small crystals of phloroglucin. After about 10 minutes, when 
the color seemed to have reached its full intensity, the aqueous solution 
was transferred to a glass cell for observation. 
(e) From rancid lard. —About 20 cc. of rancid lard were melted and 
shaken with about 10 cc. of water. After separation the water extract 
was drawn off and filtered, and was then brought to an acidity comparable 
to that obtaining in the previous experiment and treated with a few small 
crystals of phloroglucin. After the development of color, the aqueous 
solution was transferred to a glass cell for spectrophotometric exami¬ 
nation. 
The results obtained in the spectrophotometric examination of the 
several solutions are represented graphically in the accompanying charts. 
Figure 1 compares the spectra of the several concentrated hydrochloric 
acid solutions, while figure 2 compares the spectra of the weakly acid 
aqueous solutions. In these graphs the curves are designated by roman 
capital letters corresponding to the letters designating the solutions just 
listed, the abscissas represent wave lengths of light, and the ordinates 
represent the degree of absorption expressed as logarithms of the 
fraction— 
Intensity of incident light of given wave length 
Intensity of transmitted light of same wave length 
For clear solutions of a pure dyestuff in a given solvent, the height of 
the absorption curve at any given wave length will be proportional to 
the concentration of the solution, and will bear a fixed ratio to the height 
of the same curve at any other given wave length. The curves obtained 
for all concentrations will therefore exhibit a family resemblance, in that 
the point of maximum absorption will always correspond to the same 
wave length, and that the curves will exhibit a degree of likeness in con¬ 
tour. A curve obtained from a more concentrated solution may be 
scaled down by dividing each ordinate by the same ratio to form a new 
curve that may be superimposed on a curve obtained from a less con¬ 
centrated solution of the same dyestuff. Such a similarity of absorption 
spectra is generally considered to be sufficient to establish the identity 
