Experimental Techniques

Colorimetry

• Select a filter with the complementary colour to the solution being tested. 
• Zero the colorimeter (calibrate it) using a cuvette of distilled water. 
• Make up solutions of known concentrations both above and below that of the unknown sample.
• Measure the absorbance of each concentration and plot a calibration curve of the results. (absorbance against concentration) 
• Measure the absorbance of the unknown soluton and read off the concentration from the calibration curve once results have been plotted. 

T.L.C.

• Spot mixtures and reference samples on a pencil line 1cm from the bottom of the T.L.C. plate. 
• Place T.L.C. plate in beaker containing solvent. Solvent level must be below 1cm base line. Cover beaker. 
• Remove plate when solvent is near top and mark how far it reached. 
• Allow plate to dry. Locate spots using a locating agent such as iodine or ninhydrin. 
• Find Rf values for each spot and compare these to reference sample Rf values to decide what the unknown is. 

Recrystallisation

• Dissolve solid in a minumum volume of hot solvent. 
• Filter and retain the filtrate. 
• Allow filtrate to cool, which will form crystals.
• Collect the crystals using vaccum filtration and wash them in a minimum volume of cold solvent.
• Dry in an oven set below the melting point of the crystals. 

Potassium manganate (VII) Titration: Used to find the concentration of iron(II) ions or hydrogen peroxide

• Use a pipette to transfer a known volume of the test solution (iron(II) ions for example) into a conical flask. 
• Acidify this solution with dilute sulfuric acid. 
• Slowly add potassium manganate(VII) to the solution in the conical flask from a burette.
• Swirl gently until a pale pink colour persists.
• Repeat the titration until you have two titres within 0.1cm^3.

Iodine-Thiosulfate Titrations: Used to find the concentrations of solutions of oxidising agents

• A known amount of the oxidising agent is reacted with an excess of acidified potassium iodide solution. 
• The iodine produced is then titrated against a standard solution of sodium thiosulfate. 
• Near the end point of the reaction, a few drops of starch solution are added giving an intense blue/black colour which disappears at the end point.