4. Co-HC104

50 ml. 0.5 M Co2Fe7H20 1.0 2.0 528 Boilea cool 50 Ml. 35% HC104 adra 5. Hg-HCl04

1.0 ml. 0.5 MHg (NO) 2 1.0 2.0 555 Read at 15'

50 ml. 35% HC1O4 6. KNO2-HN03. 05 ml. 2.5% Gum Arabic Reagent components Read at 8. 025 ml. 0.5% KNO2 5.0 added individually 527 after add'n. 04 ml. conc. IN03 to IAA aliquot of HNO3 and concentration of reagent added to fixed volumes containing varying amounts ofIAA, the optimal proportions were found to be 2 parts 0.01 M FeCl3 in 35% o HC104 to 1 part IAA solution. Color density reaches its maximum at approximately 20 to 25 minutes and remains virtually constant for at least three hours.

Other metallic salts (1 part 0.5 M to 50 parts 35% o HC104) were tested with IAA: SrC12, TiSO4, SnCl2, Cu (NO3) 2, Ce (HSO4) 4, CsCl, NiC12, U02 (NO3) 2, CdCl2, Hg (NO3) 2, and CoF6. Of these, only Co and Hg gave appreciable color reactions, but neither was as satisfactory as the ferric chloride-perchloric acid procedure given above. In figure 1 are given the concentration-absorbancy curves of those modi-fications discussed, and the procedures followed are detailed in table I. The relative absorbancy of various other 3-substituted indoles in these methods are given in table II. It can be seen that the FeCl3-HCl04 reagent is not only more sensitive within the IAA concentration range studied, but at the same time shows least interference from other indole compounds. We have, therefore, adopted the following colorimetric procedure for routine estima-tions of IAA in aqueous solution in concentrations of 0.2 to 45 jug./ml.: 1. To a 1.0-ml. aliquot add2. 0 ml. of FeCl3-HCl04 reagent 2. Read after 25'at 530 mp.