聊城大学东昌学院,化学与生物系,氧化还原滴定法,*,上一内容,下一内容,回主目录,返回,第,7,章,氧化还原滴定法,7.1,氧化还原反应及平衡,7.2,氧化还原滴定基本原理,7.3,氧化还原滴定中的预处理,7.4,常用的氧化还原滴定法,7.5,氧化还原滴定结果的计算,2024/11/14,7.2.1,氧化还原滴定指示剂,7.2.2,氧化还原滴定曲线,化学计量点，滴定突跃,7.2.3,滴定终点误差,E,/V,突,跃,7.2,氧化还原滴定基本原理,2024/11/14,7.2.1,氧化还原滴定指示剂,1).,自身指示剂,（,self indicator,）,：,是利用标准溶液自身的颜色变化来指示终点。,浓度约为,2,10,-6,molL,-1,，肉眼可见,粉红色,。,37,电对自身颜色变化：,MnO,4,-,/,Mn,2+,KMnO,4,MnO,4,-,（紫红）,+5Fe,2+,+8H,+,=Mn,2+,（近无色）,+5Fe,3+,+H,2,O,1.,指示剂的分类：,氧化还原滴定除用电位法确定终点外，还可用化学指示剂来指示滴定终点，常用的有：,自身指示剂、专属指示剂、氧化还原指示剂,2024/11/14,2).,显色,指示剂,即专用指示剂，指示剂与氧化剂或还原剂发生显色反应。,吸附,络合,淀粉,吸附,I,2,，,蓝色,37,淀粉,与,110,-5,molL,-1,I,2,生,成,深蓝色,化合物,碘量法专属指示剂,SCN,-,+Fe,3+,=,FeSCN,2+,(,110,-5,molL,-1,可见,红色,),如：,I,2,+2S,2,O,3,2-,=2I,-,+S,4,O,6,2-,淀粉（蓝色,),无色,2024/11/14,3),氧化还原指示剂,（,redox,indicator,）,本身发生氧化还原反应的指示剂，其氧化态和还原态具有不同的颜色,.,弱氧化剂或弱还原剂,.,如二苯胺磺酸钠，还原态为无色，氧化态为紫红色。,K,2,Cr,2,O,7,滴定,Fe,2+,，用二苯胺磺酸钠为指示剂，终点：无,紫红,2024/11/14,In(O),+,ne,=,In(R),A,颜色,B,颜色,35,变色点时：,E=E,变色范围：,E,0.059/n,指示剂的选择原则：,P,230,表,7-1,中列出一些指示剂的条件电势及颜色变化,2024/11/14,2.,氧化还原指示剂作用原理：,2024/11/14,3.,常用氧化还原指示剂,指示剂,氧化态颜色,还原态颜色,E,(V),四磺酸基靛蓝,蓝色,无色,0.36,亚甲基蓝,蓝色,无色,0.53,二苯胺,紫色,无色,0.75,乙氧基苯胺,黄色,红色,0.76,二苯胺磺酸钠,紫红,无色,0.85,邻苯氨基苯甲酸,紫红,无色,0.89,嘧啶合铁,浅蓝,红色,1.15,邻二氮菲,-,亚铁,浅蓝,红色,1.06,指示剂选择的原则：,指示剂变色点电位值处于体系的滴定突跃范围内，并尽量与,E,SP,一致。,2024/11/14,氧化还原指示剂,指示剂 还原形,氧化形,颜色变化,次甲基蓝,0.52,无色,天蓝色,二苯胺磺酸钠,0.85,无色,紫红色,邻苯氨基苯甲酸,0.89,无色,紫红色,邻二氮菲亚铁,1.06,红色,浅蓝色,2024/11/14,邻二氮菲,亚铁：,Ce,4+,滴定,Fe,2+,，终点：,红,浅蓝,二苯胺磺酸钠：,K,2,Cr,2,O,7,滴定,Fe,2+,，终点：无,紫红,4.,氧化还原指示剂应用：,2024/11/14,7.2.2,氧化还原滴定曲线,电对电势随滴定剂加入而不断改变,:,E,V,T,曲线,E,/V,突,跃,sp,通常，实验测得。,对于可逆体系也可,理论,计算。,不可逆,体系,，理论计算与实验值相差较大。,2024/11/14,以在,1mol,L,-1,H,2,SO,4,介质中，用,0.1000mol L,-1,Ce,4+,溶液滴定,20.00mL,同浓度的,Fe,2+,溶液为例，说明氧化还原滴定曲线的绘制,。,对电对,Ce,4+,/Ce,3+,：,对电对,Fe,3+,/Fe,2+,：,Ce,4+,滴定,Fe,2+,的反应式为,:,Ce,4+,+Fe,2+,=Ce,3+,+Fe,3+,24,E,Ce,4+,/Ce,3+,=1.44V,E,Fe,3+,/Fe,2+,=0.68V,滴定过程中两个电对,:,Fe,3+,e,-,=Fe,2+,Ce,4+,e,-,=,Ce,3+,2024/11/14,整个体系只有一个电位值，在滴定的各个阶段，可选用两电对中任一便于计算的电对来计算体系的电极电位值。,滴定过程中电位的变化可计算如下,：,25,滴定过程中，达平衡时有：,E,=,E,Fe3+/Fe2+,=,E,Ce4+/Ce3+,滴定过程中电势的计算：,4,个过程,2024/11/14,1),滴定前,，,Fe,3+,未知，不好计算,以被滴定者计算,sp,前，,Ce,4,+,未知，,2,）滴定开始到化学计量点前：,用滴定剂，还是用被滴物质来计算电势？,滴定反应为,:Ce,4+,Fe,2+,=,Ce,3+,Fe,3+,2024/11/14,25,当滴加,19.98mLCe,4+,（,Er,=-0.1%,）时，溶液中,99.9%,的,Fe,2+,被氧化为,Fe,3+,，,未被滴定的,Fe,2+,只有,0.1%,：,2001,年南开大学：,用,Ce,4+,Fe,2+,，当体系电位,E=0.68v,时，求,a.,2024/11/14,Ce,4+,+Fe,2+,=Ce,3+,+Fe,3+,3),化学计量点时,sp,（,a=1.00):,均很小，用单一电对不易计算,E.,两式相加得,:,化学计量点时，,2024/11/14,推广：,对于可逆、对称的任一氧化还原反应：,n,2,O,1,+n,1,R,2,n,1,O,2,+n,2,R,1,2024/11/14,当滴加,20.02mL Ce,4+,（,Er,=+0.1%,）时，即,Ce,4+,过量,0.1%,：,以滴定,剂,计算,E,sp,后，,Fe,2+,未知,4,）化学计量点后：,2024/11/14,V,Ce,4+,mL,滴定分数,电势,V,说 明,0.00,0.0000,不好计算,1.00,0.0500,0.60,E,=,E,Fe,3+,/Fe,2+,=0.68+0.059lgc,Fe,3+,/c,Fe,2+,10.00,0.5000,0.68,12.00,0.6000,0.69,19.80,0.9900,0.80,19.98,0.9990,0.86,20.00,1.000,1.06,E,sp,=(,E,Fe,3+,/Fe,2+,+,E,Ce,4+,/c,Ce,3+,)/2,22.02,1.001,1.26,E,=,E,Ce,4+,/Ce,3+,=1.44+0.059lgc,Ce,4+,/c,Ce,3+,30.00,1.500,1.42,40.00,2.000,1.44,突跃,-0.1%E,=,E,Fe,3+,/Fe,2+,+0.059,3,0.1%E,=,E,Ce,4+,/Ce,3+,-0.059,3,化学计量点前后,0.1%,相对误差范围内电位变化范围，称为氧化还原滴定的电位突跃范围。,2024/11/14,E,/V,突,跃,1.26,1.06,1.06,邻二氮菲亚铁,0.86,0.85,二苯氨磺酸钠,电势突跃的范围是选择氧化还原指示剂的依据,Ce,4+,滴定,Fe,2+,的滴定曲线,电位突跃：,1.26V-0.86V=0.4V,n,1,=n,2,=1,E,SP,=1.06V,正好处于突跃中间。,整个滴定曲线基本对称。,2024/11/14,滴定突跃范围,:,设滴定剂为,Ox,，,待测物为,Red,。,对反应：,n,2,O,1,+n,1,R,2,=n,1,O,2,+n,2,R,1,，,滴定突跃范围上下限的电势为化学计量点,0.1,的电势：,滴定,不足,0.1,时电势,：,2024/11/14,滴定过量,0.1,时电势,滴定剂为,Ox,，,待测物为,Red,。,对反应：,n,2,O,1,+n,1,R,2,=n,1,O,2,+n,2,R,1,2024/11/14,从上式可知：,（,1,）,E,与平衡常数,K,有关，,K,越 大，,E,越大。,（,2,）,E,与条件电势差有关。,（,3,）,E,与电子转移数有关。,（,4,）,E,与被滴物质的浓度无关，这一点与酸碱滴定、络合滴定不同。,2024/11/14,突跃范围（通式）：,E,2,+,0.059,lg10,3,n,2,E,1,+,0.059,lg10,-,3,n,1,化学计量点（通式,):,n,1,E,1,+n,2,E,2,n,1,+n,2,E,sp,=,n,2,O,1,+,n,1,R,2,=,n,2,R,1,+,n,1,O,2,可见，突跃范围、,Esp,仅取决于两电对的条件电势与电子转移数，与滴定剂或被滴物的浓度无关。,（小的）,（大的）,2024/11/14,Ce,4+,滴定,Fe,2+,的滴定曲线,E,/V,突,跃,1.26,1.06,1.06,邻二氮菲亚铁,0.86,-0.1%,时,E=,0.68+0.059lg10,3,=0.86,Sp,时：,E=1.06,+0.1%,时,E,=1.44+0.059lg10,-3,=1.26,2024/11/14,例,9,（,P234,）：计算在,1 molL,-1,H,2,SO,4,介质中，,Ce,4+,与,Fe,2+,滴定反应的平衡常数及滴定至化学计量点时的电势。,2024/11/14,例如,在,1molL,-1,HCl,介质中,用,0.2000molL,-1,Fe,3+,滴定,0.10molL,-1,Sn,2+,试计算化学计量点时的电位,E,sp,和突跃范围。,解：,（查表）,滴定反应式为,2Fe,3+,+Sn,2+,=2Fe,2+,+Sn,4+,突跃范围：,即,0.230.52V,，其中点为,0.38 V,，,Esp,偏向于,Sn,4+,Sn,2+,电对,(,电子转移数较大的电对,),一方。,2024/11/14,对于两电对电子转移数不同：,Fe,3+,滴定,Sn,2+,/V,突,跃,0.52,0.23,0.33,n,2,:,n,1,=2:1,E,sp,偏向,n,大的电对一方,.,2Fe,3+,+Sn,2+,=2Fe,2+,+Sn,4+,-0.1%,E,=0.14+(0.059/2)x3=0.23,sp:,E,=(0.14x2+0.70)/(2+1)=0.33,+0.1%,E,=0.70+0.059x(-3)=0.52,2024/11/14,29,E,/,突跃,；,n,1,、,n,2,突跃,b.,计量点的位置,n,1,=n,2,时，,Esp,正好位于突跃范围的中点,（突跃以,sp,为中心，前后对称）。,n,1,n,2,时，偏向电子转移数大的电对一方,。,讨论：,a.,影响突跃范围的因素,滴定突跃的大小与,E,/,和,n,有关，与,c,无关。,例如：,Ce,4+,+Fe,2+,=Ce,3+,+Fe,3+,突跃范围：,0.86,1.26V,，,Esp,=1.06V,。,例如,:2Fe,3+,+Sn,2+,=2Fe,2+,+Sn,4+,突跃范围：,0.23,0.50V,，,sp=0.33V,中点为,0.38 V,偏,E,/,Sn,4+,/,Sn,2+,2024/11/14,对于不对称的氧化还原反应,(,自学了解,),34,说明：,E,/,0.2 V,才有明显的突跃；,E,/,为,0.2,0.4V,之间，可用电位法确定终点；,E,/,0.4 V,时，用电位法和指示剂法,2024/11/14,7.2.3,滴定终点误差,=,O,1,ep,-R,2,ep,C,2,sp,100%,E,t,=,100%,被测物质的物质的量,滴定剂过量或,不足的物质的量,滴定反应进行完全，指示剂灵敏,计算值与实测值不符合,不可逆电对,2024/11/14,n,1,=n,2,1,n,1,n,2,O,1,+R,2,=R,1,+O,2,2024/11/14,请珍惜青春年华,En