络合平衡和络合滴定法

第,6,章 络合平衡和络合滴定法,6.1,常用络合物,6.2,络合平衡常数,6.3,副反应系数及条件稳定常数,6.4,络合滴定基本原理,6.5,络合滴定条件,6.6,络合滴定的方式和应用,6.1,常用络合物,以,络合反应,和,络合平衡,为基础的滴定分析方法,简单络合剂,:,NH,3, Cl,-, F,-,Cu,2+,-NH,3,络合物,螯合剂,:,乙二胺，,EDTA,等,乙二胺,- Cu,2+,EDTA,乙二胺四乙酸,(H,4,Y),乙二胺四乙酸二钠盐,(Na,2,H,2,Y),：,：,：,：,EDTA,H,6,Y,2,H,+,+ H,5,Y,+,H,5,Y,H,+,+ H,4,Y,H,4,Y,H,+,+ H,3,Y,-,H,3,Y,-,H,+,+ H,2,Y,2-,H,2,Y,2-,H,+,+ HY,3-,HY,3-,H,+,+ Y,4-,K,a1,= =,10,-0.90,H,+,H,5,Y,H,6,Y,K,a2,= =,10,-1.60,H,+,H,4,Y,H,5,Y,K,a3,= =,10,-2.00,H,+,H,3,Y,H,4,Y,K,a6,= =,10,-10.26,H,+,Y,HY,K,a5,= =,10,-6.16,H,+,HY,H,2,Y,K,a4,= =,10,-2.67,H,+,H,2,Y,H,3,Y,M-EDTA,螯合物的立体构型,EDTA,通常与金属离子形成,1:1,的螯合物,多个,五元环,某些金属离子与,EDTA,的形成常数,Hg,2+,21.8,Th,4+,23.2,Fe,3+,25.1,Bi,3+,27.9,Fe,2+,14.3,Al,3+,16.1,Zn,2+,16.5,Cd,2+,16.5,Pb,2+,18.0,Cu,2+,18.8,Mg,2+,8.7,Ca,2+,10.7,Na,+,1.7,lg,K,lg,K,lg,K,lg,K,1.,络合物的稳定常数,(,K,),M + Y = MY,6.2,络合平衡常数,MY,MY,K,MY,=,逐级稳定常数,K,i, , , ,K,表示,相邻,络合物之间的关系,M + L = ML,ML + L = ML,2,ML,n-1,+ L = ML,n,累积稳定常数,表示络合物与,配体,之间的关系,ML,ML,K,1,=,ML,2,MLL,K,2,=,ML,n,ML,n-1,L,K,n,=,ML,ML,1,=,K,1,=,ML,2,ML,2,2,=,K,1,K,2,=,ML,n,ML,n,n,=,K,1,K,2,K,n,=,2.,溶液中各级络合物的分布,ML =,1,M L,ML,2,=,2,M L,2, ,ML,n,=,n,M L,n, ,M + L = ML,ML + L = ML,2,ML,n-1,+ L = ML,n,c,M,=M+ML+ML,2,+ML,n,=M(1+,1,L+,2,L,2,+,n,L,n,),分布分数,M,=M/c,M,= 1/,(1+,1,L+,2,L,2,+,n,L,n,),ML,=ML/c,M,=,1,L,/,(1+,1,L+,2,L,2,+,n,L,n,),=,M,1,L,MLn,=ML,n,/c,M,=,n,L,n,/,(1+,1,L+,2,L,2,+,n,L,n,),=,M,n,L,n, ,酸可看成质子络合物,Y,4-,+ H,+,HY,3-,HY,3-,+ H,+,H,2,Y,2-,H,2,Y,2-,+,H,+,H,3,Y,-,H,3,Y,-,+ H,+,H,4,Y,H,4,Y + H,+,H,5,Y,+,H,5,Y,+,+,H,+,H,6,Y,2+,K,1,= =,10,10.26,1,=,K,1,=,10,10.26,1,K,a6,K,2,= =,10,6.16,2,=,K,1,K,2,=,10,16.42,1,K,a5,K,3,= =,10,2.67,3,=,K,1,K,2,K,3,=,10,19.09,1,K,a4,K,4,= =,10,2.00,4,=,K,1,K,2,K,3,K,4,=,10,21.09,1,K,a3,K,5,= =,10,1.60,5,=,K,1,K,2,.,K,5,=,10,22.69,1,K,a2,K,6,= =,10,0.90,6,=,K,1,K,2,.,K,6,=,10,23.59,1,K,a1,M + Y = MY,主反应,副反应,OH,-,L,H,+,N,H,+,OH,-,MOH,MHY,MOHY,NY,HY,M(OH),n, , ,ML,ML,n,H,6,Y, , , , ,M,Y,M,Y,6.3,副反应系数和条件稳定常数,副反应系数,:,为未参加主反应组分的浓度,X,与平衡浓度,X,的比值，用,表示。,M,a,M,=,M,Y,a,Y,=,Y,MY,a,MY,=,MY,1.,副反应系数,M + Y = MY,主反应,副反应,OH,-,L,H,+,N,H,+,OH,-,MOH,MHY,MOHY,NY,HY,M(OH),n, , ,ML,ML,n,H,6,Y, , , , ,M,Y,M,Y,a.,络合剂的副反应系数,Y:,Y(H),：,酸效应系数,Y(N),：,共存离子效应系数,Y,a,Y,=,Y,H,+,N,NY,HY,H,6,Y, ,Y,M + Y = MY,1,Y,酸效应系数 ,Y(H),：,Y,a,Y(H),=,=,=,Y,=(1+,1,H,+,+,2,H,+,2,+,6,H,+,6,),Y,Y+HY+H,2,Y+H,6,Y,Y,Y+YH,+,1,+YH,+,2,2,+YH,+,6,6,=,a,Y(H),1,Y,a,Y(H),Y,=,lg,Y(,H,),pH,图,lg,Y(H),EDTA,的酸效应系数曲线,共存离子效应系数,Y(N),Y,a,Y(N),=,=,= 1+,K,NY,N,Y,Y,Y+NY,Y,a,Y(N),=,=,= 1+,K,N,1,Y,N,1,+,K,N,2,Y,N,2,+,K,N,n,Y,N,n,=,a,Y(N1),+,a,Y(N2),+,a,Y(Nn),-(n-1),Y,Y,Y+N,1,Y+N,2,Y+N,n,Y,多种共存离子,Y,的总副反应系数,Y,= ,Y(H),+ ,Y(N),-1,Y,=,=,Y+HY+H,2,Y+ +NY,Y,Y,Y,b.,金属离子的副反应系数,M,M(OH),=,1 +,1,OH,-,+,2,OH,-,2,+ +,n,OH,-,n,M(L),=,1+,1,L,+,2,L,2,+,n,L,n,M + Y = MY,OH,-,L,MOH,M(OH),n, ,ML,ML,n, ,M,多种络合剂共存,M(L),=,1+,1,L,+,2,L,2,+,n,L,n,M,=,M(L1),+,M(L2),+,M(Ln),-(,n,-1),c.,络合物的副反应系数,MY,M + Y = MY,H,+,OH,-,MHY,MOHY,MY,忽略不计！,例：,pH=2.0,浓度为,0.01mol/L,的,Mg,2+,溶液,计算,Y(H), ,Y(Mg),Y,解：,= 10,13.51,= 1+10,8.7,0.01 = 10,6.7,= 10,13.51,+,10,6.7,-1 = 10,13.51,lg,K,MY,= lg,K,MY,- lg,M,-,lg,Y,+,lg,MY,lg,K,MY,- lg,M,-,lg,Y,=,lg,K,MY,-lg(,M(A1),+,M(A2),+,M(An),-(,n,-1),-,lg (,Y(H),+ ,Y(N),-1),K,MY,= =,K,MY,MY,M,Y,a,MY,a,M,a,Y,2.,条件稳定常数,金属离子指示剂,6.4,络合滴定基本原理,络合滴定曲线：,溶液,pM,随滴定分数,(,a,),变化的曲线,M + Y = MY,EDTA,加入，金属离子被络合，,M or M,不断减小，化学计量点时发生突跃,1.,络合滴定曲线,例,:,用,0.0100molL,-1,EDTA,标准溶液滴定,20.00mL 0.0100 molL,-1,Ca,2+,溶液，计算在,pH=12,时化学计量点附近的,pCa,值。,解：,pH = 12,时,:,K,MY,=10,10.69,，,lg,a,Y(H),= 0.01,lg,a,Ca(OH),= 0,K,MY,= 10,10.68,a,滴定前溶液中钙离子浓度：,Ca,2+, = 0.01 mol,L,-1, pCa = -lgCa,2+, = -lg0.01 = 2.0,b,加入,EDTA,溶液,19.98mL,时，,c,化学计量点时，,Ca,2+,与,EDTA,几乎全部络合成,CaY,2-,络离子：,pH=12,时，,lg,a,Y(H),=0.010,，,Y=Y, Ca,2+, = Y,设为,x mol,L,-1,，, x = Ca,2+, = 3.210,-7,mol,L,-1, pCa = 6.49,d,化学计量点后，设加入,20.02mL EDTA,溶液，此时,EDTA,溶液过量,0.02mL, Ca,2+, = 10,-7.69,mol,L,-1,pCa = 7.69,加入,EDTA,溶液,剩余,Ca,2+,离子溶液体积,V,/mL,过量,EDTA,溶液的体积,V,/mL,pCa,mL,(%),0.00,18.00,19.80,19.98,20.00,20.02,0.0,90.0,99.0,99.9,100.0,100.1,20.00,2.00,0.20,0.02,0.00,0.02,2.00,3.30,4.30,5.30,6.49,7.69,计算所得数据列于下表,可绘制滴定曲线。,pH = 12,时,0.0100molL,-1,EDTA,滴定,0.0100 molL,-1,Ca,2+,的滴定曲线,pM,0 10 20 30 40,EDTA,加入量（,mL,）,化学计量点,(,pM6.5),滴定突跃,(pM5.3-7.7),0,2,6.5,5.3,7.7,浓度,:,增大，突跃增加,K,MY,:,增大，突跃增加,影响滴定突跃的因素,络合滴定准确滴定条件：,lg,c,M,sp,K,MY,6.0,对于,0.0100molL,-1,的,M,lg,K,MY,8,才能准确滴定,HIn,+M,MIn,+ M MY +,HIn,色,A,色,B,要求,:,指示剂与显色络合物颜色不同,(,合适的,pH),；,显色反应灵敏、迅速、变色可逆性好；,稳定性适当，,K,MIn,K,MY,2.,金属离子指示剂,a.,指示剂的作用原理,EDTA,常用金属离子指示剂,指示剂,pH,范围,颜色变化,直接滴定离子,In,MIn,铬,黑,T (EBT),810,蓝,红,Mg,2+, Zn,2+,Pb,2+,二甲酚橙,(XO),K,MY,则封闭指示剂,Fe,3+,、,Al,3+,、,Cu,2+,、,Co,2+,、,Ni,2+,对,EBT,、,XO,有封闭作用；,若,K,MIn,太小，,终点提前,若指示剂与金属离子形成更稳定的络合物而不能被,EDTA,置换，则虽引入大量,EDTA,也达不到终点，这种现象称为,指示剂的封闭。,指示剂的氧化变质现象,指示剂的僵化现象,指示剂与金属离子形成的络合物应易溶于水。若生成胶体溶液或沉淀，滴定时指示剂与,EDTA,的置换作用将进行缓慢而使终点拖长，,即指示剂的僵化。,3.,终点误差,E,t,=, 100%,Y,ep,-M,ep,c,M,sp, pM = pM,ep, - pM,sp,M,ep,=,M,sp,10,- pM,Y,ep,=,Y,sp,10,-pY,Y,sp,10,-pY,-,M,sp,10,-pM,c,M,sp,E,t,= 100%,pM,=-pY,M,sp,=Y,sp,=,c,M,sp,/,K,MY,E,t,=,10, pM,-10,-pM,c,M,sp,K,MY,6.5,络合滴定条件的选择,准确滴定判别式,络合滴定酸度控制,提高络合滴定选择性,1.,滴定判别式,若,pM=0.2,要求,E,t,0.1%,根据终点误差公式,可知需,lg,c,M,sp,K,MY,6.0,若,c,M,sp,=0.010molL,-1,时,则要求,lg,K,8.0,多种金属离子共存 例：,M,，,N,存在时,分步滴定可能性的判断,lg,c,M,sp,K,MY,6.0,，考虑,Y,的副反应,Y(H),K,NY,K,MY,=,K,MY,/,Y,Y,=,Y(H),+,Y(N),-1,H,+,N,NY,H,n,Y,Y(H),Y(N),Y(H),Y(N),时，,Y,Y(H),（相当于,N,不存在）,Y(H),Y(N),时，,Y,Y(N),(,不随,pH,改变,),pH,低,Y(H),=,Y(N),时,的,pH,pH,高,仍以不生成沉淀为限,M,，,N,共存，且,lg,c,K, 5,a.,络合掩蔽法 加络合掩蔽剂,降低,N,Y(N),Y(H,),lg,K,MY,=lg,K,MY,-lg,Y(N),Y(N),1,+,K,NY,N,K,NY,c,N,/,N(L),Y(,H,),Y(,N),lg,K,MY,=lg,K,MY,-lg,Y(H),N,被完全掩蔽,M + Y = MY,H,+,N,NY,H,n,Y,L,H,n,L,NL,n,Y(H),Y(N),N(L),L(H),H,+,络合掩蔽注意事项：,2.,掩蔽剂与干扰离子络合稳定,:,N(L),=1+L,1,+L,2,2,+,大、,c,L,大且,1.,不干扰待测离子,:,如,pH10,测定,Ca,2+,、,Mg,2+,用,F,-,掩蔽,Al,3+,， 则,CaF,2,、,MgF,2,CN,-,掩蔽,Co,2+, Ni,2+, Cu,2+, Zn,2+, F,-,掩蔽,Al,3+,;,3.,合适,pH,F,-, pH4; CN,-, pH10),b.,沉淀掩蔽法,例：,Ca,2+,Mg,2+,混合溶液中,Ca,2+,的测定,lg,K,CaY,=10.7, lg,K,MgY,=8.7,p,K,sp,: Ca(OH),2,=4.9, Mg(OH),2,=10.4,加沉淀剂,降低,N,Ca,2+,Mg,2+,Ca,2+,Mg(OH),2,CaY,Mg(OH),2,pH12,Y,Ca,指示剂,c.,氧化还原掩蔽法,FeY,lg,K,BiY,=27.9,lg,K,FeY,-,=25.1,lg,K,FeY,2-,=14.3,红,黄,抗坏血酸,(Vc),红,黄,pH=1.0,XO,EDTA,EDTA,pH5-6,测,Fe,测,Bi,Bi,3+,Fe,3+,Bi,3+,Fe,2+,BiY,Fe,2+,d.,其它滴定剂滴定,Mg,2+,Ca,2+,Sr,2+,Ba,2+,lg,K,M-EDTA,8.7,10.69,8.73,7.86,lg,K,M-EGTA,5.21,10.97,8.50,8.41,EGTA:,乙二醇二乙醚二胺四乙酸,1.,直接滴定法,:,6.6,络合滴定方式及应用,例 水硬度的测定,: Ca,2+,、,Mg,2+,lg,K,CaY,=10.7 lg,K,MgY,=8.7,在,pH=10,的氨性缓冲溶液中，,EBT,为指示剂，测,Ca,2+,、,Mg,2+,总量；,pH12,，,Mg(OH),2,用钙指示剂,测,Ca,2+,量,lgc,K,6,反应速率快,有合适指示剂指示终点,被测离子不水解,封闭指示剂 被测,M,与,Y,络合反应慢 易水解,例,Al,3+,的测定,2.,返滴定法,Al,3+,AlY,Y,过量,AlY,ZnY,pH3.5,过量,Y,加热,pH5,6,冷却,Zn,2+,黄,红,XO,置换金属离子：被测,M,与,Y,的络合物不稳定,例,Ag,与,EDTA,的络合物不稳定,lg,K,AgY,=7.3 lg,K,NiY,=18.6,2Ag,+,+Ni(CN),4,2-,2Ag (CN),2,-,+Ni,2+,Y,紫脲酸铵,MgY,+EBT,+Ca,2+,CaY+,MgEBT,+Ca,2+,例,间接金属指示剂,Y,CaY+MgY,+EBT,3.,置换滴定法,置换出,EDTA,Al,3+,Pb,2+,AlY,PbY+ Y(,剩,),AlY,PbY,ZnY,AlF,6,3-,PbY+Y(,析出,),ZnY,AlF,6,3-,PbY,ZnY,pH3,4,Y,过,加热,Zn,2+,F,-,加热,Zn,2+,(,测,Al),(,测,Al,Pb,总量,),pH5,6,冷却,XO,黄,红,冷却,黄,红,测非金属离子,: PO,4,3-,、,SO,4,2-,待测,M,与,Y,的络合物不稳定,: K,+,、,Na,+,4.,间接滴定法,总结：,络合平衡,副反应系数,Y(H),Y(N),M,条件稳定常数,K,MY,pM,sp,和,pM,ep,(pM,t,),及,Et,滴定酸度控制,共存离子分步滴定,滴定方式及应用,