1、1Exp. Potentiometric Titration of Phosphoric AcidObjectivesEquipments and chemicals A pH meter with a compound electrode made of a glass and a calomel electrode. Magnetic stirrer with a Teflon- coated stirring bar. The titration of acid with NaOH is followed by measuring pH of the solution after eac

2、h addition of the titrant. The calculation of the ionization constants from the potentiometric titration curve is illustrated.uEquipmentsuChemicals Standard NaOH (0.1 mol/L); Phosphoric acid (approximately 0.1 mol/L); Potassium hydrogen phthalate solution (0.05mol/L), pH 4.00 2Principles1. Determina

3、tion of pKa H3PO4+ H2O = H3O+ + H2PO4- H2PO4-+ H2O = H3O+ + HPO42- HPO42-+ H2O = H3O+ + PO43- 43421 POHHPOHKapHpKHKPOHPOHVVWhenaaeqNaOH114342121 32242 POHHPOHKapHpKHKPOHPOHVVVWhenaaeqeqNaOH22422421)(21 32. Determination of end point The curve of potentimetric titration pH/V 2pH/V2 41.The standardiza

4、tion of the pH meterProcedures The glass electrode is activated by soaking it in deionized water for 24hr Wash the electrode thoroughly Calibrate the pH meter using a potassium hydrogen phthalate solution (0.05mol/L), pH 4.00 Wash the electrode thoroughly .2.The potentiometric titration of phosphori

5、c acid pipette a 10.00ml portion of 0.1mol/L phosphoric acid into a 100ml flask dilute with 20ml distilled water Place the electrode and a magnetic stirring bar into the beaker Add 0.1mol/L NaOH solution from burette and take readings after each addition. 5Noticesu1. After each addition of NaOH, wai

6、t a few seconds till the reading of pH is stable, and then record the data.u2. To add accurate volumes of NaOH ( 0.1 ml ) near the stoichiometric point, use a glass-stirring rod for help. u3. Choose a proper speed of stirring to avoid the splash of titrate.Calculations1. Calculate 2pH/V2 vs. millili

7、ters of NaOH at the two equivalence points and determine the V1 and V22. Plot pH vs. milliliters of NaOH. Determine pKa1 and pKa2 from the titration curve according to milliliters of NaOH at the two equivalence points.3. Determine the concentration of phosphoric acid by V1. 6三、確定電位滴定終點(diǎn)的方法：三、確定電位滴定終點(diǎn)

8、的方法：電位滴定數(shù)據(jù)電位滴定數(shù)據(jù): :7三、確定滴定終點(diǎn)的方法三、確定滴定終點(diǎn)的方法1E V曲線法曲線法 滴定終點(diǎn)滴定終點(diǎn)：曲線上轉(zhuǎn)折點(diǎn)（斜率最大處）對(duì)應(yīng)V 特點(diǎn)特點(diǎn)：應(yīng)用方便； 但要求計(jì)量點(diǎn)處電位突躍明顯。 2E/V 曲線法（一階導(dǎo)數(shù)法）曲線法（一階導(dǎo)數(shù)法） 曲線曲線：具一個(gè)極大值的一級(jí)微商曲線 滴定終點(diǎn)滴定終點(diǎn)：尖峰處（E/V極大值）所對(duì)應(yīng) 特點(diǎn)特點(diǎn)：在計(jì)量點(diǎn)處變化較大，因而滴定準(zhǔn)確； 但數(shù)據(jù)處理及作圖麻煩。 32E/V2 V曲線法（二階導(dǎo)數(shù)法）曲線法（二階導(dǎo)數(shù)法） 曲線曲線：具二個(gè)極大值的二級(jí)微商曲線 滴定終點(diǎn)滴定終點(diǎn)：2E/V2由極大正值到極大負(fù)值與 縱坐標(biāo)零線相交處對(duì)應(yīng)的V。 特點(diǎn)特點(diǎn)：準(zhǔn)確度高，可用內(nèi)插法求滴定終點(diǎn)VPVPVP8三、確定電位滴定終點(diǎn)的方法：三、確定電位滴定終點(diǎn)的方法：電位滴定數(shù)據(jù)處理電位滴定數(shù)據(jù)處理: :E-V曲線 曲線VVE/2E/ V2-V曲線92E/V2-V曲線例如：例如：V1=11.30mL 2E/V2=5600 V2=11.35mL 2E/V2=-400mLmLxx35.11347.1156000560040030.1130.1135.11解得：注意：注意：利用上述三種作圖法確定滴定終點(diǎn)，主要是根據(jù)化學(xué)利