硫酸铜催化重铬酸钾滴定法测定硅铝钙钡合金中铁

Determination of Iron in Si-Al-Ca-Ba Alloy by Copper Sulfate Catalyzing Potassium Dichromate Titration Method

试样用硝酸、氢氟酸溶解,高氯酸冒烟除去硅、氟及氮氧化物。在盐酸介质中,首先用氯化亚锡将大部分三价铁还原为二价铁,然后用TiCl_3定量还原剩余的Fe^3＋为Fe^2＋,以钨酸钠作指示剂指示还原终点,即当Fe^3＋定量还原为Fe^2＋后,过量少量TiCl_3溶液时,可使作为指示剂的钨酸钠中的六价钨（无色）还原成蓝色的五价钨化合物,故溶液呈蓝色。过量的TiCl_3可在硫酸铜的催化下,借水中溶解氧及空气中的游离氧氧化使钨蓝色泽褪去,从而消除过量TiCl_3还原剂的影响。避免了经典法的无汞测定铁分析法用重铬酸钾氧化过量TiCl_3时易引入的误差。在硫磷混酸介质中,以二苯胺磺酸钠为指示剂,用重铬酸钾标准溶液滴定铁量,取得了令人满意的结果。相对标准偏差≤0.30%。

The sample was dissolved with nitric acid and hydrofluoric acid,and the silicon,fluorine,and nitrogen oxides were removed by perchloric acid smoking. In the hydrochloric acid medium,first,the stannous chloride was used to reduce most of Fe^3＋ to Fe^2＋,then the quantitative TiCl3 was used to reduce the residual Fe^3＋ to Fe^2＋,and taking sodium tungstate as indicator to indicate the reducing end,that was to say,after the quantitative Fe^3＋ was reduced to Fe^2＋ and a little of TiCl3 was excess,it would reduce the colourless hexavalent tungsten to blue tungstic compounds,so the solution was blue. The excess TiCl3 could make the tungsten blue fade under the catalysis of copper sulfate and with the help of dissolved oxygen in water and free oxygen in air,and thus the influence of excess TiCl3 was eliminated. In this way,the error introduced when potassium dichromate was used to oxidize the excess TiCl3 in the classical analysis method for mercury-free determination of iron was avoided. In the medium of sulfur-phosphorus mixed acid,taking diphenylaminesulfonic acid sodium salt as indicator,the standard solution of potassium dichromate was used to titrate the content of iron,and a satisfactory result was achieved,the relative standard deviation was less than 0. 30%.