Colorimeters

ENERGY CORRESPONDING TO THESE COLOURS IS ABSORBED 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 split d orbitals 3) when the e- falls back light is EMITTED ions with d10 (full) Cu+,Ag+ Zn2+ or d0 (empty) Sc3+ configuration are colourless no d split e.g. titanium(IV) oxide TiO2 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. ligands approach along the x, y and z axes 2 of the orbitals point directly at the ligands and e- repel, so greater energy while the other three point between them, less energy of repulsion.

Ligand field theory With no ligands the d-orbitals are of equal energy (degenerate) When the d-orbitals are surrounded by ligands their energy is split, due to the 2 negative charge repulsion

SPLITTING OF 3d ORBITALS Electron repulsion of approaching ligands makes the d orbitals split to change In an Octahedral 2 d orbitals gain energy (z2 and x2-y2) 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 3d 3d Degree of splitting depends on the type of LIGAND approaching The energy difference between governs the colour of light we see.

Use these 6 pictures to work out the three factors that affect the colour of a transition metal compound… MnCl2 FeCl2 FeCl2 FeCl3 FeCl3 Fe(NO3)3

Use these 6 pictures to work out the three factors that affect the colour of a transition metal compound… (1) Metal MnCl2 FeCl2 FeCl2 FeCl3 FeCl3 Fe(NO3)3

Use these 6 pictures to work out the three factors that affect the colour of a transition metal compound… (1) Metal MnCl2 FeCl2 (2) Oxidation state FeCl2 FeCl3 FeCl3 Fe(NO3)3

Use these 6 pictures to work out the three factors that affect the colour of a transition metal compound… (1) Metal MnCl2 FeCl2 (2) Oxidation state FeCl2 FeCl3 (3) Ligand FeCl3 Fe(NO3)3

So why is an octahedrally hydrated cupric ion is blue? An artist’s wheel. Each color has a opposite complementary color An object has a color because it absorbs some colours of light, while others escape. What we see is the colours that escape. 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 to our eye and red and orange stayed in the solution. 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   Observed Color of Compound Color of Light Absorbed Green Red Blue-green Orange-red Violet Yellow Red-violet Yellow-green Orange Blue Dr. Zahra Afrasiabi 2003

Green and yellow light are absorbed while other wavelengths are transmitted. This blends to a purple color. The color of [Ti(H2O)6]3+. The hydrated Ti3+ ion is purple. When the ion absorbs light, electrons move from the lower energy to the higher 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 NOTICE HOW STONG CO is! 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- < H2O < SCN- < NH3 < en < NO2- < 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

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 Suppose this graph is generated by many dilutions 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 can catch and hold 2 tennis balls We throw 10 balls into the area We measure what comes out (Trans, so say 4 balls out) But we find the number of people by what doesn’t come out (Abs, 3 people behind screen) # People is related to number of balls caught (Abs)