WENGER AND DUCKERT 1 have made a critical study of various organic reagents recommended for the colorimetric determination of iron. In the past few years, a large number of compounds have been suggested for the colorimetric and spectrophotometric determination of iron. Some of these are : 4-hydroxybiphenyl-3-carboxylic acid 2, ethyl a-isonitrosoacetoacetate 3, nitroso R-salt 4, 5,6-benzoquinaldinic acid 5, apicolinic acid and quinaldic acid 6.~, m-methoxy-o-nitrosophenol 8, iodoxine 9, 4,7-dihydroxy-I,IO-phenanthroline 10, 2-fluorobenzoic acid 11, 2-acetyl-pyridine oximO ~, 4,7diphenyl-I, Io-phenanthroline 13, 3-hydroxy-2-naphthoic acid 14, quinolinic acid 15 , picolinedioxime 16 and o-dianisidineXL But all these reagents suffer from the disadvantage that a strict pH control is necessary, and in many cases organic solvents are needed either to keep the complex in solution or to extract the coloured complex; also in many cases foreign ions interfere in the colorimetric determination. SANDELL x8 has described and recommended various reagents for the colorimetric determination of iron. o-PhenanthrolinO 8 is good, but has certain disadvantages: silver and bismuth give precipitates with the reagent, and the method cannot give accurate results when large amounts of metal ions like zinc, tungstate, nickel, cobalt and tin are present with iron. Although the thiocyanate method ~ is extensively used, the method suffers from the following drawbacks: a large excess of the reagent is necessary for complete colour development, the method can be used only within a narrow pH range, fluoride, phosphate, oxalate, vanadate, molybdenum and tungsten interfere and the colour fades rapidly with time.
During the course of our studies of certain derivatives of benzotetronic acid as analytical reagents *0-.2, it has been observed that oximidobenzotetronic acid (or aisonitrosobenzotetronic acid) can be successfully employed for the spectrophotometric determination of iron(II). ANSCHUTZ 28 first prepared this compound and after analysis of its silver salt gave CoH404NAg as the molecular formula for the salt. This author also reported the formation of a blue colour when an aqueous suspension or alcoholic solution of the reagent was treated with an aqueous solution of iron chloride. We have now observed that when a freshly prepared alcoholic solution of this reagent is treated with an aqueous solution of iron(II), a deep blue water-soluble complex is instantaneously produced. Although a similar reaction is seen with iron(III), we have studied only the former reaction, as it is more sensitive and the colour development is instantaneous. The iron(II)-complex is quite stable