實驗六-磷酸的電位滴定

實驗六磷酸的電位滴定應化1101夏海琴2014.11.13一、實驗目的1.掌握酸度計測量溶液pH和雷磁ZDJ-4A自動電位滴定儀的操作要點。2.了解電位滴定法的基本原理。3.學習使用Excel或Origin軟件處理NaOH滴定磷酸的數(shù)據(jù)和繪圖。更直觀地觀察滴定曲線中的兩個突躍，學會計算相應的兩級解離常數(shù)。4.掌握用三切線法及一級、二級微商來處理實驗數(shù)據(jù)和分析實驗結(jié)果。二、實驗原理電位滴定法是根據(jù)滴定過程中，指示電極的電位或pH產(chǎn)生“突變”，從而確定滴定終點的一種分析方法。在以NaOH滴定H3PO4時，將飽和甘汞電極及玻璃電極插入待測溶液中，使之組成原電池：Ag|AgCl，HCl(0.1mol/L)|玻璃膜|被測試液||KCl(>3.5mol/L)，HgCl2|HgH+鹽橋玻璃電極甘汞電極被測試液由于玻璃薄膜上的陽離子能與溶液中的H+產(chǎn)生離子交換而產(chǎn)生電勢，因而稱玻璃電極為指示電極，甘汞電極為參比電極，當NaOH溶液不斷滴入試液中，溶液H+的活度隨著改變，電池的電勢也不斷變化，可用能斯特(Nernst)公式表示為E電池=ΔEθ–0.059VlgαH+或E電池=ΔEθ+0.059VpH此處ΔEθ=Eθ(HgCl2/Hg)-Eθ(AgCl/Ag)。以滴定體積VNaOH為橫坐標，相應的溶液的pH為縱坐標，繪制NaOH滴定H3PO4的滴定曲線，曲線上呈現(xiàn)出兩個滴定突躍，以“三切線法”作圖，可以較準確地確定兩個突躍范圍內(nèi)各自的滴定終點，即在滴定曲線兩端平坦轉(zhuǎn)折處作AB及CD兩條切線，在“突躍部分”作EF切線與AB、CD兩線相交于Q，P兩點，在P、Q兩點作PG，QH兩條線平行于橫坐標。然后在此兩條線之間作垂直線，在垂線一半的J點處，作JJ'線平行于橫坐標，J'點稱為拐點，即為滴定終點。此J'點投影于pH與V坐標上分別得到滴定終點時的pH和滴定劑的體積V，見圖8-1。圖8-1三切線法作圖如要求更準確的確定滴定終點，可用一級微商法(dpH/dV-V)和二級微商法(d2pH/dV2-V)。表8-1為數(shù)據(jù)處理示例。如用三切線法求得第一個終點時，Vep1=19.00mL，用一級微商求得Vep1=19.50mL，用二級微商求得Vep1=19.46mL。結(jié)果略有差異，用二級微商法處理準確度較高。1.一級微商計算示例當?shù)稳隢aOH18.80mL到19.00mL時，=0.45表8-1H3PO4電位滴定數(shù)據(jù)記錄和處理示例加入NaOH溶液體積溶液pHΔpH/ΔVΔ2pH/ΔV2V/mLV平均值/mL┆┆18.80…3.51………………18.90……0.4519.00…3.60……2.2519.10……0.9019.20…3.78……6.019.30……2.1019.40…4.20……3.019.50……2.7019.60…4.70……-4.519.70……1.8019.80…5.10………………19.90……0.9020.00…5.28………………┆┆0.45對應的體積V平均值=(V2+V1)/2=18.90mL，其余各點按相同方法計算得到表8-1。以ΔpH/ΔV為縱坐標，V平均值為橫坐標作圖，得圖8-2(b)。2.二級微商法示例先按上述方法求得19.10mL和18.90mL的一級微商后，再求出19.00mL的二級微商。=2.25此即為19.00mL對應的值。以d2pH/dV2為縱坐標，對應的V為橫坐標描點作圖，得到圖8-2(c)。顯然，21.050.2021.103.001.0021.150.3021.203.03-1.0021.250.2021.303.050.0021.350.2021.403.07-0.3321.500.1521.603.100.3321.650.2021.703.120.3321.800.2521.903.17-0.3321.950.2022.003.190.0022.050.2022.103.212.0022.150.4022.203.25-2.0022.250.2022.303.270.3322.400.2522.503.322.3322.550.6022.603.38-2.0022.650.4022.703.42-1.0022.800.2522.903.471.0022.950.4023.003.510.6723.100.5023.203.612.0023.250.8023.303.69-1.0023.350.7023.403.761.0023.450.8023.503.846.0023.551.4023.603.98-0.3323.701.3523.804.253.7523.902.1024.004.67-2.0024.151.6024.305.15-3.2024.400.8024.505.31-0.7524.600.6524.705.44-0.2524.800.6024.905.56-1.5025.000.3025.105.620.5025.200.4025.305.70-1.0025.400.2025.505.740.5025.600.3025.705.80-0.1325.850.2726.005.88-0.2726.100.2026.205.920.0026.300.2026.405.960.0026.500.2026.606.00-0.2526.700.1526.806.030.0026.900.1527.006.060.0027.100.1527.206.090.0027.300.1527.406.12-0.2527.500.1027.606.140.0827.800.1328.006.190.0328.250.1428.506.26-0.0428.750.1229.006.32-0.0429.250.1029.506.37-0.0429.750.0830.006.410.0430.250.1030.506.46-0.0430.750.0831.006.500.0031.250.0831.506.540.0031.750.0832.006.58-0.0132.500.0733.006.650.0033.500.0734.006.720.0134.500.0835.006.800.0035.500.0836.006.88-0.0136.500.0737.006.950.0037.500.0738.007.020.0138.500.0839.007.10-0.0239.500.0640.007.160.0340.500.0941.007.25-0.0141.500.0842.007.330.0243.000.1144.007.540.0344.500.1545.007.690.0445.500.1946.007.880.1146.500.3047.008.180.5847.500.8848.009.060.2948.501.1749.0010.23-0.7849.500.3950.0010.62-0.1650.500.2351.0010.85-0.0951.500.1452.0010.99-0.0352.500.1153.0011.10-0.0153.500.1054.0011.20-0.0254.500.0855.0011.28-0.0255.500.0656.0011.340.0156.500.0757.0011.41-0.0257.500.0558.0011.460.0058.500.0559.0011.510.0059.500.0560.0011.56-0.0161.000.0462.0011.640.0063.000.0464.0011.720.0065.000.0366.0011.790.0067.000.0368.0011.850.0069.000.0370.0011.910.0070.500.0371.0011.94-0.0172.000.0273.0011.980.0273.250.0473.5012.000.0063.000.0464.0011.720.0065.