The color depth of a red complex which is formed by Mo5 + and thiocyanate is linear positive correlation with content of Mo5 + ,so that we can get the colorimetric determination of molybdenum .Cu2 + and Fe3 + which exist at the same time catalyze the generation of thiocyanate on Mo .At this time ,Cu2 + and thiourea generates colorless complex ,Fe3 + under the catalysis of Cu2 + is reduced to Fe2 + by thiourea under the catalysis of Cu2 + .There is no interference when Fe3 + in the Cu2 + red complex of molybdenum .This method is used to the analysis of mineral and metallurgical slag samples ,and the determination of worth relative standard deviation (n = 6) are between 0.19% ‐8.6% ,the recovery rate is 95% ‐105% .Experimental results show that this method has high sensitivity ,accurate qualitative and quantitative characteristics . This method solves the problem of interference when the solution of Fe3 + and Cu2 + is experimented with analysis colorimetry .The principle of this method is 2MoO42 - + 2CS(NH2 )2 + 10CNS- + 10H+ = 2 [MoO (CNS)5 ]2 - + 2CS(N2 H3 ) + 6H2 O .%M o5＋与硫氰化物作用，生成红色的络合物，颜色深浅与M o5＋含量呈线性正相关，借此进行钼的比色测定。Cu2＋和Fe3＋同时存在，对钼的硫氰化物生成有催化作用。此时，Cu2＋与硫脲生成无色络合物，Fe3＋在Cu2＋的催化下，被硫脲还原为Fe2＋，Fe3＋和Cu2＋对钼的红色络合物无干扰。方法用于矿石和冶金废渣样品分析，测定值的相对标准偏差（n＝6）在0.19％～8.6％之间，回收率在95％～105％之间。实验结果表明该方法具有灵敏度高、定性和定量准确等特点，解决了 Fe3＋和 Cu2＋在分析比色时干扰问题。此方法的原理为2M oO42－＋2CS （NH2）2＋10CNS－＋10H＋＝2[MoO（CNS）5]2 －＋2CS（N2 H3）＋6H2 O
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