Early Stages in the Metabolism of Iron 



199 



Table 2. The mM extinction coEFPiciENT/g atom of 



IRON for EXCESS ITOIC ACID IN THE PRESENCE OF FERRIC 

 CHLORIDE IN 0-05 M-PHOSPHATE pH T'O 



From the results reported in Table 3 for the continuous variations experi- 

 ment, it seems probable that a 3 : 1 structure is the most favoured combination 

 in neutral solution. 



Table 3. Determination of the composition of the ferric complex of 

 ITOIC acid by the method of continuous variation 



All solutes in a final volume of 5 ml of 005 m Na-K-phosphate buffer pH 7-0. Optical density 

 measurements were made after one hour at room temperature 



H moles itoic acid 

 l-i moles FeClj 

 Ratio itoic acid/FeClg 

 Optical density at 560 m/t 



0-524 

 0-524 

 1-0 

 0084 



0-734 

 0-315 

 2-3 

 0-128 



0-784 

 0-274 

 2-9 

 0-139 



0-840 

 0-209 

 4-0 

 0-138 



0-890 

 0-157 

 5-7 

 0-109 



In the titration experiments referred to above the initial pH of both the 

 ferric chloride and itoic acid solutions was 2-6. On mixing the solutions the 

 pH was depressed still further, to a value of 2-1, thus indicating a strong com- 

 plexing of ferric ion even in very acidic media. Two equivalents of base were 

 consumed below pH 7 and between pH 7 and 8 one additional equivalent of 

 base was taken up. The ferric ion remained in solution throughout the entire 

 course of the titration. In the case of cupric ion, no additional production of 

 acid was observed on mixing the reagents. There was no formation of 

 precipitate in these solutions even at pH 10. The cupric complex was green 

 at pH values of 3-8 or greater and the absorption maximum lay at a wave- 

 length above 600 m/<. As with the ferric ion, two equivalents of acid were 

 titrated below pH 5-5 and one additional equivalent was titrated with a 

 p/<:of 6-6. 



In the experiments in which the ferric chelate of itoic acid was equilibrated 

 with EDTA, the colour of the solutions was found to reach a stable value 

 several hours after mixing. If an apparent stability constant of 10"^ is selected 

 for Fe+++ — EDTA, the corresponding value measured for Fe+++ — Itoicg is 



