20182018年12月2日藥院2017級藥學一小班用pH計測定溶液的pH值及磷酸的電位滴定一、 實驗目的1、 掌握用pH計測定溶液的pH值的方法及原理。2、 通過實驗學會pH計儀器的使用。3、 掌握電位滴定的方法及確定終點的方法。4、 學會用電位滴定法測定弱酸的pKa.二、 實驗原理1、 直接電位法測定溶液的pH，常以pH玻璃電極作為指示電極，飽和甘汞電極作為參比電極，浸入待測液中組成原電池。(-)Ag|AgCl，內充液丨玻璃膜丨試液|KCl(飽和)，HgCl|Hg(+)(s) (s)pHs-25型酸度計使用復合電極，玻璃電極為負極，銀-氯化銀電極為正極(-)玻璃電極|待測溶液|銀-氯化銀(+)在實際工作中，采用兩次測量法，pHs=pHs+(Ex-Es)/0.059實驗操作要求：校準一用一種標準緩沖液按pH計的使用方法校準pH計檢驗一用校準好了的pH計測量另一種與校準時所用緩沖溶液的pH相差3個單位左右的標準緩沖液，測量值與表中所列值相差不應大于0.1pH單位測定一用校準好了的pH計測定待測溶液的pH2、 電位滴定對于容量滴定中的酸堿都能滴定，且對于一些突躍范圍小，無合適指示劑，溶液渾濁，有色的酸堿，都能滴定。此外它還可測弱酸弱堿的解離常數，還可用來尋找合適指示劑或校正指示劑的終點顏色變化。磷酸為多元酸，其pKa可用電位滴定法測定當磷酸的第一個H+被滴定一半時，此時的pH即為第一步的解離常數。三、 實驗步驟(一) 1、用鄰苯二甲酸氫鉀pH=4.00，測定草酸三氫鉀及混合磷酸鹽的pH值2、用混合磷酸鹽定位pH=6.88，測定硼砂及鄰苯二甲酸氫鉀的pH值(二)1、實驗操作、打開電腦上的軟件，用磷酸混合溶液及鄰苯二甲酸氫鉀標準溶液校準pH計，保存校準數據，接著設置滴定條件，準備滴定。、用10.0ml移液管吸取磷酸樣品溶液10.0ml置于100ml燒杯中，加蒸餾水20ml，放入轉子，調節轉速，放入復合電極以及氫氧化鈉滴液管。點擊開始，儀器開始滴定，并自動記錄實驗數據。滴定結束后，停止滴定，保存數據及滴定曲線。2、數據處理，做出一階微商曲線，算出磷酸濃度以及第一第二解離常數。四、 實驗數據

溶液pH值測定結果表鄰苯二甲酸氫鉀定位(pH=4)混合磷酸鹽定位(pH=6.88)次數123次數123草酸三氫鉀1.621.631.60鄰苯二甲酸氫鉀4.074.064.05混合磷酸鹽6.856.866.86硼砂9.239.249.25磷酸的電4立滴定數據記錄體積(mL)pH值一次導數二次導數0.001.84.002.20.0844.602.2-0.025-5.3625.142.30.1853.8755.742.40.1623.5516.242.40.1662.6716.882.50.069-2.9957.532.60.2211.7508.272.80.132-3.1468.923.00.235-2.4089.323.10.5071.9329.423.20.361-5.5859.523.20.6523.7339.623.30.6783.6329.723.40.8366.8299.823.40.9852.2469.873.50.852-2.6719.913.51.1145.2819.963.60.966-2.99510.013.6-0.092-21.26610.063.71.42130.42310.113.82.02412.13110.163.81.075-19.08110.213.92.42727.20510.264.13.38019.17210.314.45.49742.58310.364.52.954-51.15310.414.85.43349.86810.464.92.843-52.10410.515.13.87820.811

10.565.21.286-52.14410.615.32.67928.03510.665.41.691-19.89010.715.51.176-10.35010.765.4-0.431-32.34510.815.51.35435.91610.865.62.17316.48110.915.71.412-15.31810.965.7-0.201-32.43611.015.70.83120.75011.215.90.64211.34111.416.00.6169.30811.606.10.2491.33511.806.10.4073.12612.006.20.3755.21012.206.30.1190.16212.406.30.201-5.96912.656.40.4680.60712.856.40.1923.46013.006.50.297-4.65413.206.50.035-4.74513.396.60.220-1.33513.646.60.2944.51213.846.60.3284.43114.046.70.3043.15714.246.70.2153.31814.396.70.086-3.33914.596.80.074-4.06714.746.80.2924.61314.946.80.2313.72315.236.90.2582.93415.436.90.1833.08615.636.90.2673.21715.837.00.1572.16516.037.00.2603.35916.237.10.4205.94916.437.10.2503.42016.637.10.2723.38916.877.20.344-1.11317.027.20.1515.70617.227.20.2713.62217.427.20.2334.38117.627.30.3364.310

17.827.30.2635.16018.027.40.5125.64518.227.40.3563.71318.417.40.3454.24918.617.50.2422.63018.817.50.6175.93919.017.60.2875.45319.167.60.259-1.36619.317.70.3761.34619.567.70.4393.31819.717.80.160-5.29119.917.90.014-7.49720.10&00.037-6.69820.15&00.73213.98220.20&11.1648.70120.25&10.957-4.15820.30&10.385-11.51320.35&20.5633.57120.40&20.543-0.39520.45&20.339-4.09720.50&21.06314.54920.55&31.2473.71320.60&30.749-10.01620.65&41.2119.29820.70&51.6478.76220.75&62.34614.06320.80&71.527-16.47120.85&71.115-8.28620.90&7-0.882-40.18620.95&93.70292.22921.009.14.84923.06721.059.46.16426.45721.109.62.529-73.12721.159.73.91627.90321.209.81.734-43.88921.259.91.520-4.32021.3510.02.32659.32721.4010.23.32820.14321.5010.30.652-46.80221.6010.4-0.024-16.89621.7010.51.5569.01421.7910.60.524-16.45121.8910.60.322-4.553

21.9410.70.83910.40122.0410.80.572-10.69422.1910.80.9248.23522.2910.90.006-9.05522.3910.90.6016.991由數據所繪制的滴定曲線求得平均濃度為l/2(0.1029+0.1073)=0.1051mol/l由數據查得pKa1=2.3 pKa2=6.9五、實驗結論在pH定位為4的情況下，測得草酸三氫鉀的pH為1.62，混合磷酸鹽的p