Absorption Spectra of Dilute Solutions. 145 



d = 0-00163 1 55'5 K^ = 0-0000085 



0*00196 f 0-0000072 



0*00205 J 0-0000069 



0-0000075 

 Mean used = 0'002 



Mean = 0'0000075 



The numbers now found are approximately constant. As 

 d = 18-5 ??, we get 5? = 9250. 



That is, the dissociation constant of ferric hydroxide is about 9000 

 times that of water. Ferric hydroxide is thus an extremely weak 

 base. From 55 Kid = 0*0000075 we get also K : = 0-000068. For 

 comparison,* the dissociation constants of KC1 = 0'00958, that of 

 CuS0 4 = 0'0001898, so that ferric chloride must be regarded as one 

 of the exceptionally slightly dissociated salts. 



A number of photometric measurements were made of the absorp- 

 tion spectra of solutions of ferric chloride, but their peculiar properties 

 make it rather difficult to obtain accurate results. The solutions 

 were made by diluting a concentrated solution of ferric chloride, 

 which had been made as clear as possible by filtration through paper. 

 They were then allowed to stand till no further change took place in 

 their colour; if they were then without visible turbidity, they were 

 used for measurement, but, owing to the peculiar fluorescent appear- 

 ance of solutions containing colloid ferric hydroxide, it was not easy 

 to see whether they were really free from solid suspended particles 

 or not. 



The . numbers which are contained in Table VI of the Appendix, 

 and from which the following table is calculated, give results which 

 agree approximately with the results of the filtration experiments 

 given above. The calculation of the amount of ferric hydroxide in 

 the solutions was made as follows : The most dilute solution of 

 FeCl 3 which was used (XXV, Table XII), and another one to which 

 a small quantity of ammonia had been added (XXVI, Table XII), 

 gave nearly the same numbers. The decomposition in these solutions 

 appeared to be complete, as neither of them gave a red colour on 

 addition of excess of ammonium sulphocyanide. Solutions of colloid 

 ferric hydroxide, obtained by dialysis, also give no red colour with 

 sulphocyanide. I took the absorptions of these solutions as corre- 

 sponding to complete hydrolysis. The smaller absorptions of the 

 other solutions will then correspond to a proportionately smaller de- 

 composition, if we assume that the whole absorption is due to the ferric 

 hydroxide, an assumption which is quite allowable, as the filtration 



* The numbers are from van Laar, loc. cit., p. 154. To convert them into the 

 units used by Ostwald they must be divided by 0*018. 



