Objective: Visually determine the molar concentration of a dichromate ion in an unknown solution of Cr 2 O 7 using color bracketing. Background: Many ions impart a color to the aqueous solutions in which they are dissolved. The intensity of the color is directly proportional to the concentration of the ion. When dissolved in water, the dichromate ion imparts a faint yellow color to the solution. The concentration may be determined by visual color comparison to a set of prepared standard solutions. Filtering solutions would be necessary if any turbidity was present, to eliminate interference.

The concentration of the dichromate ion in the solution can be determined stoichiometrically. Materials: Unknown, dichromate solution Standard stock solution, 0. 100 M Cr 2 O 7 Volumetric pipes, 5 ml and 10 ml Graduated cylinders, 100 ml Test Tubes, 50 ml Para film Test tube rack Method: Serial dilutions were made by diluting 5 ml of stock solution to a 100 ml final volume, then dilutions of 10: 100, 82: 100, 50: 100 and 70: 100 were made in successive order of one another. Each dilution was transferred to a test tube, filling the tube to a depth equal to the unknown to preserve constant path-length when viewing down through the tube against a white background. All solutions were free of turbidity. Concentration of dichromate was determined by estimation of the color intensity as compared to two standards, one of higher and one of lower concentration. The final molar concentration was determined stoichiometrically from this estimated comparison.

Discussion: The standard solutions used for comparisons are shown in the table below. Dilution: Gustar Concentration, Cr 2 O 7: Share 0. 100 Deb 1 0. 005 D 2 zero. 0005 D 3 0. 00041 Deb 4 0. 000205 D 5 zero. 0001435 Concentration of the unknown was estimated by both members of the lab group to be 0. 0004 M. It was done by inserting it between D 3 and D 4. The color of the unknown was slightly less intensive than the Deb 3 standard. Computations: Make reference to page several of lab notebook.

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