Transition Metal Practicals: Ligands, Colorimetry & Redox Titrations (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching chemistry, and this video is about the inorganic practicals for transition metals including ligand substitution, redox titrations, vanadium oxidation states, colorimetry and autocatalysis.

Transition metal ions are coloured, and their colour depends on the ligand, the oxidation state and the coordination number, so every practical here uses colour as the thing you observe or measure.

Colour is the measurement in all five of these practicals. Because the colour of a transition metal complex depends on the ligand attached to it, on the metal's oxidation state and on its coordination number, a colour change tells you that one of those has changed. And because absorbance depends on the concentration of the coloured complex ion, colour can be measured as well as observed.

The video takes five practicals in turn. Ligand substitution experiments, where test tube reactions show the colour changing with the ligand. Redox titrations, where a colour change marks the endpoint. The vanadium reduction, where each colour is a different oxidation state. Colorimetry, which turns colour into a concentration. And autocatalysis, which follows a colour disappearing to get a rate.

The first practical is ligand substitution. Three things affect the colour of a transition metal ion solution: the type of ligand, a change in oxidation state, and the coordination number. Test tube reactions show all three. Measure two centimetres cubed of a one molar metal aqua ion solution into a test tube, add one molar ammonia solution dropwise, shake gently between drops, and record what you see. Then repeat with a different reagent, such as hydrochloric acid. Copper(II) changes from light blue solution to yellow-green solution with concentrated hydrochloric acid, and gives a deep blue solution with excess ammonia. Change the ligand and the colour changes, so the colour tells you which complex you have.

The second practical is redox titration. Iron has variable oxidation states, and these titrations exploit that. Iron(II) is the reducing agent, and either manganate(VII) or dichromate(VI) is the oxidising agent. With potassium manganate(VII) the mixture is acidified, and the acid used is dilute sulfuric acid because it does not react with the manganate(VII) ions. Manganate(VII) acts as its own indicator: the manganese(II) it forms is present in such a low concentration that it looks colourless, so the endpoint is the first pale pink tinge in the flask. Dichromate(VI) goes from orange to bluish green, and uses sodium diphenylaminesulfonate to give a more definite endpoint. Here the colour change is not identifying a species, it is telling you the reaction is complete.

The third practical is the vanadium reduction. Vanadium is a transition metal with variable oxidation states, and vanadate(V) can be reduced to vanadium species in the two, three and four oxidation states using zinc in acidic conditions. Add a quarter of a spatula of ammonium vanadate(V) to a test tube, half fill it with one molar hydrochloric acid and shake gently, then add a small piece of zinc and note what happens over fifteen minutes. The solid dissolves in the acid to form a yellow solution, which becomes blue, then green, then finally violet. Each colour is a different oxidation state, so reading the colour is reading the oxidation state.

The fourth practical is colorimetry, and this is where colour stops being an observation and becomes a measurement. A colorimeter uses a lamp as a source of white light, which passes through a filter to produce light of one colour. That is the colour the sample absorbs the most, called its complementary colour. Hexaaquacopper(II) solution is pale blue, absorbs red light, and transmits blue. To find an unknown concentration, make up metal aqua ion solutions of known concentration, measure their absorption, and plot a calibration curve of absorbance against concentration. Then measure the absorption of the unknown and read its concentration off the curve.

The last practical is autocatalysis, and it uses the same idea to get a rate rather than a concentration. Autocatalysis is when one of the products catalyses the reaction. Potassium manganate(VII) reacts with acidified ethanedioate ions to give manganese(II), carbon dioxide and water, and the manganese(II) is the catalyst. To follow it, samples of the reaction mixture are removed every thirty seconds into potassium iodide, which stops the reaction, and the iodine released is titrated against sodium thiosulfate using starch as the indicator. The titre is proportional to the manganate(VII) left in the mixture. The rate is slow to start with, then speeds up as more catalyst is released, before slowing down and stopping. Colorimetry works here too, because manganate(VII) ions are purple and the other reactants and products are not coloured.

Two things to remember when you are doing these practicals. In test tube reactions, record the colour, but also record whether a solution or a precipitate has formed. And in the vanadium reduction, vanadium(II) is oxidised in air, so the test tube may need a stopper once the reduction has begun, otherwise it turns back to green because it is oxidised to vanadium(III).

Colour identifies the species: the ligand, the oxidation state and the coordination number all change it. Colour marks the endpoint in a redox titration. Colour measures concentration through a calibration curve in colorimetry. And colour follows the rate in the autocatalysis experiment. In each of these five practicals, the colour of a transition metal species is the thing being measured.