14 12 10 8 6 4 20 pka1 pka2 V₁ + V₁ N -V₁ V₂ Volume of base added Figure 9.2: pH versus Volume of titrant plot for phosphoric acid pKa values, i.e. pka1, pKa2 and pKa3. The titration curve would be expected to show three corresponding steep rises. However, in practice only two sharp rises are detected. The shape of the expected pH versus Volume of titrant for the titration of phosphoric acid is shown in Figure 9.2. unpex This plot shows two endpoints V₁ and V₂. As before pKa values can be determined by reading the pH values at the volumes corresponding to half neutralisation. As shown in Figure 9.2, pkai is the pH of the solution at the volume and pKa2 is the pH of the solution at the volume V₁ + V2V₁(= V₁+V/₂). epede Calibra side of Take mark Swirl Rep NaO sults/nTable 2: Change in pH on addition of 0.1 M NaOH to 50 cm³ sample of 0.05 M H3PO4 pH Volume of NaOH PH added (cm³) Volume of NaOH added (cm³) 2.0 H-O 6.0 10-0 12.0 14 O 16-0 18-0 20.0 22.0 24-0 26.0 28-0 30-0 32.0 34.0 From graph: Equivalence point 1: Half-equivalence point 1: pka1: Kat (show calculation): 1.78 179 1.89 2.08 2.21 2.46 3.00 4.91 5.66 6.05 6.29 6.49 682 7.69 9.70 10.19 10.38 10-52 2.0 4.0 6.0 8.0 10.0 12-0 14.0 16.0 16.5 17.0 17.5 18.0 18.5 19.0 F1.5 20.0 20-5 21.0 1.79 2.39 2.25 pka2: Ka2 (show calculation): Equivalence point 2: Half-equivalence point 2: 2.32 2.5H 3.02 3.99 5.72 5.86 5.97 $6.04 6-11 6.18 6.22 6.29 6.35 6.39 6.46/nDATA SHEET Table 1: Change in pH on addition of 0.1 M NaOH to 50 cm³ sample of 0.1 M CH₂COOH Volume of NaOH pH added (cm3) e revoo oneloitt 501 Jor дел 2:0 за но 10.0 12.0 TH O льо en qmets PH 18.0 2010 22.0 26:0 30.0 32.0 From graph: Equivalence point: Half-equivalence point: рка: Ka (show calculation): 2 70 3 90 З'Но 3:66 383 3-98 bio наа h35 тва 483 5 io 5.36 10 31 10.59 10.H9 Volume of NaOH added (cm3) до во 10.0 10 IN.O JH.S 15.0 15:5 16.0 16.5 17.0 17:5 180 19.0 За 3:46 3.68 3:86 3.99 ня наб ная н:33 4.36 4-39 Н.Н3 Н.НЗ 4 50 H.SS 4.58 нь3

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