5-[(3,5-二溴-2-吡啶)偶氮]2,4-二氨基甲苯(简称3,5-二溴-PADAT)可与钴发生灵敏的显色反应,已见报导应用于铜、钯和矿石中微量钴的测定。本文进一步研究了3,5-二溴-PADAT与钴的显色反应条件。在室温下,pH4.0～5.2的乙酸-乙酸钠缓冲溶液中,3,5-二溴-PADAT与钴迅速生成红色络合物。随后加入较浓的无机酸,可形成颜色更深的紫红色络合物,这可能是由于加入酸后,间位的-NH_2基产生双质子化,致使络合物的颜色加深。本法应用于钢铁、废水、维生素B_(12)等试样中微量钴的直接测定,得到了满意的结果。试验部分一主要试剂与仪器钴标准溶液:称取0.5000克纯金属钴,加热溶解 5 - [(3,5-dibromo-2-pyridyl) azo] 2,4-diaminotoluene (referred to as 3,5-dibromo-PADAT) with cobalt sensitive color reaction has been reported application Determination of trace cobalt in copper, palladium and ores. In this paper, the color reaction conditions of 3,5-dibromo-PADAT with cobalt were further investigated. In room temperature acetic acid-sodium acetate buffer solution with pH4.0-5.2, 3,5-dibromo-PADAT rapidly forms red complex with cobalt. Subsequent addition of a stronger inorganic acid resulted in a darker purple complex, possibly due to the di-protonation of the meta-NH2 group resulting from the addition of acid, resulting in a darker complex. The method is applied to the direct determination of trace cobalt in the samples of steel, waste water, vitamin B_ (12) and the like, and satisfactory results have been obtained. Experimental part of a major reagents and equipment Cobalt standard solution: Weigh 0.5000 grams of pure metallic cobalt, dissolved by heating