隐蔽剂在螯合滴定中的应用(Ⅹ)——丙二酸可作为铝的专属性隐蔽剂

THE USES OF MASKING AGENTS IN CHELOMETRY(Ⅹ)——MALONIC ACID AS A SPECIFIC MASKING AGENT FOR ALUMINIUM

本文从理论和实践上证明了以环己烷二胺四乙酸(DCTA)为滴定剂,丙二酸在pH5.5可作为铝的专属性隐蔽剂,在以DCTA-丙二酸隐蔽法间接滴定铝时,锆、钍、锡、钛、铁、镓、稀土、铜等卅种离子不干扰。试以此法测定硅酸盐或铝青铜的铝时,进一步证明丙二酸的良好性能。

The masking properties of malonic acid in chelometric titration have been investigated.Theoretically when EDTA is used as titrant,it can mask Fe(Ⅲ),Ti(Ⅳ)and Al,but if DCTA is used instead,it can mask Al only at pH 5-6.It was found experimentally that when DCTA is used as the titrant,malonic acid can mask Al up to 80mg or 60mg at pH5.3 or 5.7.In order to indirect titration of Al,we had used malonic acid to displace the DCTA from the Al-DCTA complex at pH5.5,but the displacement reaction was not complete even when the solution was heated or put aside for a long time.So we used the following procedure;Take two equal aliquots of Al solution A and B.Only in solution A,add 1 to 2 g.of malonic acid to mask the Al.Then equal volumes of 0.02 M DCTA(over 2 ml in excess)are added to both solutions.After adjusting to pH 5.5 and using xylenol orange as indicator,titrate the remaining DCTA with 0.02M Zn solution.The difference between the volumes of Zn solution in titrating the A and B solutions multiplying the molarity will give the content of Al.In this way we found that:(1)in the determination of 5,8 or 10 mg Al,1,1.5,or 2 g malonic acid should be used with accurate results;(2)in determining 5 mg of Al,1 to 2 folds of the following metals do not interfere:Zr,Th,Sn,Bi,Fe,Ti,Ga,Ce,La,Y,Gd,Cr,Hg,Cu,Ni,Co,Pb,Zn,Cd,Mn,V,Sb,W,Mo,Ca,Mg,Ba,Sr,Ag.The selectivity of the titration is very high.It can be said that when DCTA is used as the titrant,malonic acid is a specific masking agent for Al.