Absorption Spectra of Dilute Solutions. 

 Fm. 4. 



121) 



(1.) From 0'5 to 1 litre of the solution was evaporated to dryness 

 (with a little hydrochloric or acetic acid) on the water bath, the 

 residue dissolved in a small quantity of water, excess of potassium 

 iodide added, and the liberated iodine titrated with sodium thio- 

 sulphate solution. In order to obtain accurate results by this method 

 the cupric solution must be concentrated, otherwise the reaction with 

 potassium iodide is incomplete, and the thiosulphate solution must be 

 standardised by means of pure CuS0 4 in exactly the same way as it 

 is subsequently to be used in the analyses. 



(2.) The solutions, without evaporation, were titrated with potas- 

 sium cyanide solution in presence of ammonia. Here also the KCN 

 solution was always standardised by means of a solution of CuS0 4 of 

 known strength before using it, as its strength changes on keeping. 

 The results of the two methods agreed satisfactorily. 



The results of the photometric measurements are contained in Table 

 II Appendix. In order to be able to compare the absorption spectra 

 of the different solutions, the extinction coefficients e for the dissolved 

 substance for 1 cm. layer, and for a concentration of 1 gram molecule 

 per litre, have been calculated from the values of e x given by equation 2 

 (p. 125) . The extinction coefficient is proportional to the leiigtfy of layer 



(Q, and to the concentration (c), so that e = J-. From these values of 



c.l. 



c curves were drawn (not smoothed), and from them the numbers con- 

 tained in the following table (Table III) were interpolated. 



The Roman numerals at the head of each column indicate that the 

 figures are derived from the experiment similarly numbered in the 

 Appendix. 



