References:  G. P. Haight and V. Paragamian, Analytical Chemistry,   pp. 32, 642 (1960). 
H. Onishi and E. B. Sandell, Photometric Determination of Trace Metals, 4th Edition, Part 1, p. 295 (1978).

Molybdate is used throughout the industrial water treatment and power generation industries as a corrosion inhibitor in both open and closed loop cooling water systems.  In solution, molybdate anions complex with oxidized iron to form a protective film of molybdate and ferric-oxide.  Molybdate is considered an effective, environmentally acceptable alternative to chromate treatment.  Unlike many other transition elements, molybdenum exhibits low or even negligible toxicity.

The Colorimetric Method.  The molybdate test method employs the catechol chemistry.  In a mildly reducing alkaline solution, catechol reacts with hexavalent molybdenum to form a yellow-orange colored chelate in direct proportion to the hexavalent molybdenum concentration.  Test results are expressed in ppm (mg/L) molybdenum Mo.