Colorimeters COLOURED IONS Observed colours of a solution

Colorimeters

COLOURED IONS Observed colours of a solution depends on the wavelengths absorbed Copper sulphate solution appears blue because all the other colours are absorbed by the copper ions but blue is NOT. Out it comes and we see it…ta da! WHITE LIGHT GOES IN SOLUTION APPEARS BLUE ENERGY CORRESPONDING TO THESE COLOURS IS ABSORBED

COLOURED IONS A characteristic of transition metals is their ability to form coloured compounds Partially filled d-orbitals tend to be coloured, it is caused by: 1) ABSORPTION of light energy 2) which PROMOTES electrons into higher ligand split d orbitals 3) when the e- falls back light is EMITTED ions with or d 10 (full) Cu+, Ag+ Zn 2+ d 0 (empty) Sc 3+ configuration are colourless e. g. titanium(IV) oxide Ti. O 2 is white colour depends on. . . Being a transition element oxidation state ligand type (water or chlorine) coordination number (number of ligands)

OCTAHEDRAL ORBITIAL ORIENTATION The five d-orbitals (2 e- each) in a field of ligands approach along the x, y and z axes 2 of the orbitals point directly at the ligands and erepel, so greater energy while the other three point between them, less energy of repulsion.

SPLITTING OF 3 d ORBITALS Electron repulsion of approaching ligands makes the d orbitals split to change In Octahedrals 2 d orbitals gain energy (z 2 and x 2 -y 2) and the other 3 have less energy In a tetrahedral complex it is exactly reversed because the orbitals are orientated in space in differently OCTAHEDRAL TETRAHEDRAL 3 d 3 d Degree of splitting depends on the type of LIGAND approaching The energy difference between governs the colour of light we see.

An artist’s wheel. Each color has a complementary color opposite it. An object will have a particular color because it absorbs some colours of light, while others escape. What we see is the blend of colours that escapes. Example: If Red Orange is absorbed, Green Blue is what we see So why is an octahedrally hydrated cupric ion is blue? Answer: We mean it is blue because blue escaped (along with the other colours of ROYGBIV. Red and orange stayed in the solution. The cupric ions absorbed photons of 600 to 650 nm (red and orange light). The left over light, what the ions do NOT capture escapes and we see it with the red removed. It appears blue to our eyes.

Observed Color of Compound Color of Light Absorbed Approximate Wavelength of Light Absorbed Green Red 700 nm Blue-green Orange-red 600 nm Violet Yellow 550 nm Red-violet Yellow-green 530 nm Red Blue-green 500 nm Orange Blue 450 nm Yellow Violet 400 nm

The color of [Ti(H 2 O)6]3+. Green and yellow light are absorbed while other wavelengths are transmitted. This blends to a purple color. The hydrated Ti 3+ ion is purple. When the ion absorbs light, electrons can move from the lower energy level to the higher eg level. The difference in energy between the levels (Δ) determines the wavelengths of light absorbed. The visible color is given by the combination of the wavelengths transmitted.

Crystal Field Theory Although the d-orbitals are degenerate (equal energy); still their shapes differ. The bonding of the ligands to the metal ion cause the energies of the metal ion d-orbitals to split. The splitting of the d-orbitals depends on the relative the ligand type. I- < Cl- < F- < OH- < H 2 O < SCN- < NH 3 < en < NO 2 - < CN- < CO LARGER DE SMALLER DE Weak field ligands lead to a smaller splitting energy. Strong field ligands lead to a larger splitting energy. NOTICE HOW STONG CO is!

Colorimetric Analysis Colorimeter is an instrument which compares the amount of light getting through a solution with the amount which can get through a sample of pure solvent. Photometric measurement visual comparison using colour standards Eye 10

How Do We Do This? • We make several solutions with known concentrations and graph them all • We determine which wavelength or color of light the unknown solution most looks like. Absorbs the same amount of light as one of our known concentrations

Finding the unknown We make several solutions with known concentrations and graph them all. We determine which wavelength or color of light the unknown solution most looks like…. . follow below • Suppose this graph is generated by many dilutions • The unknown is measured to have an absorbance of 0. 500 • Across and down • The concentration is 0. 775 (or so)

Why Not Transmittance? • Imagine an area enclosed by a curtain containing a mystery number of people • Each person catch and hold 2 tennis balls • We throw 10 balls into the area • We measure what comes out (Trans) • But we find the number of people by what doesn’t come out (Abs) • # People is related to number of balls caught (Abs)