Question

Sodium thiosulphate may be determined by indirect titration against high purity potassium dichromate using the method of iodometry. A standard solution was prepared by dissolving 1.234 g $K_2Cr_2O_7$ (RMM = 294.18) in distilled water (250 cm³). An excess of acidified potassium iodide was added to a 25 cm³ portion of this standard $K_2Cr_2O_7$ solution, and the liberated iodine was titrated against a sodium thiosulphate solution (18.7 cm³) to the starch end point. The concentration of the thiosulphate is: $Cr_2O_7^{2-} + 14H^+ + 6e^- \rightleftharpoons 2Cr^{3+} + 7H_2O$ $E^o: +1.36 V$ $I_2 + 2e^- \rightleftharpoons 2I^-$ $E^o: +0.54 V$ $S_4O_6^{2-} + 2e^- \rightleftharpoons 2S_2O_3^{2-}$ $E^o: +0.08 V$ 0.002692 mol dm?³ 0.1346 mol dm?³ 0.06729 mol dm?³ 0.03365 mol dm?³ 0.07530 mol dm?³

          Sodium thiosulphate may be determined by indirect titration against high purity potassium dichromate using the method of iodometry. A standard solution was prepared by dissolving 1.234 g $K_2Cr_2O_7$ (RMM = 294.18) in distilled water (250 cm³). An excess of acidified potassium iodide was added to a 25 cm³ portion of this standard $K_2Cr_2O_7$ solution, and the liberated iodine was titrated against a sodium thiosulphate solution (18.7 cm³) to the starch end point. The concentration of the thiosulphate is:
$Cr_2O_7^{2-} + 14H^+ + 6e^- \rightleftharpoons 2Cr^{3+} + 7H_2O$  $E^o: +1.36 V$
$I_2 + 2e^- \rightleftharpoons 2I^-$  $E^o: +0.54 V$
$S_4O_6^{2-} + 2e^- \rightleftharpoons 2S_2O_3^{2-}$  $E^o: +0.08 V$
0.002692 mol dm?³
0.1346 mol dm?³
0.06729 mol dm?³
0.03365 mol dm?³
0.07530 mol dm?³

Sodium thiosulphate may be determined by indirect titration against high purity potassium dichromate using the method of iodometry. A standard solution was prepared by dissolving 1.234 g K2Cr2O7 (RMM = 294.18) in distilled water (250 cm³). An excess of acidified potassium iodide was added to a 25 cm³ portion of this standard K2Cr2O7 solution, and the liberated iodine was titrated against a sodium thiosulphate solution (18.7 cm³) to the starch end point. The concentration of the thiosulphate is:
Cr2O7^2- + 14H^+ + 6e^- ⇌ 2Cr^3+ + 7H2O E^o: +1.36 V
I2 + 2e^- ⇌ 2I^- E^o: +0.54 V
S4O6^2- + 2e^- ⇌ 2S2O3^2- E^o: +0.08 V
0.002692 mol dm?³
0.1346 mol dm?³
0.06729 mol dm?³
0.03365 mol dm?³
0.07530 mol dm?³

Added by Brooke P.

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