Absorption Spectra of Dilute Solutions. 



133 



solutions of copper acetate are much larger than those of the other salts. 

 The concentrations of the two solutions, the measurements on which 

 are there given, were O003748 and 0-004203 gram molecules per litre 

 respectively. The following numbers (Table Y) show that the ex- 

 tinction coefficients diminish rapidly when the concentration becomes 

 smaller. The most dilute solution used still gives larger extinction 

 coefficients than those found for the other copper salts, but it was 

 unfortunately not possible with my apparatus to make accurate 

 measurements with still smaller concentrations, so that I was unable 

 to find out whether the absorption spectrum of copper acetate 

 becomes ultimately identical with that of the others or not ; it 

 appears, however, to tend to do so.* 



Table V. Copper Acetate (e). 



Two explanations of the abnormal behaviour of copper acetate 

 solutions offer themselves : (1) The electrolytic dissociation into Cu 

 ions may be incomplete, and the still undissociated part of the salt 

 may have a different colour from the Cu ion ; (2) the salt may be 

 partially hydrolysed, acetic acid being a so much weaker acid than 

 the others used. In the latter case, however, one would expect the 

 difference between the spectrum of copper acetate and that of the 

 other salts to become greater with increasing dilution, as in the more 

 dilute solutions the hydroly tic decomposition would be most advanced. 

 The opposite, however, is the case. The addition of acetic acid to 

 the solutions should, by diminishing the amount of hydrolysis, bring 

 the copper acetate spectrum nearer to that of the others, if the 



* The uncertainty of observations of absorption spectra made without the help 

 of photometric measurements is illustrated by the following remark of Knoblauch 

 ('Wied. Ann.,' vol. 43, p. 754). He says: "Ebensowenig wie bei Kupfersulfat zeigte 

 sich bei Kupferacetat mit der Verdiinnung eine Veranderung im Character des 

 Spectrums. Die Beobachtung der Absorption s-Grenze im Roth war auch bei 

 diesem Salze fur verdiinnte Losungen durch die Absorption des Wassers beein- 

 traohtigt." Table Y shows that a not very considerable dilution (about one to 

 five) reduced the absorbing power of a copper acetate solution by about one-half. 



