The likely impurities of (CAS NO. ) are H₂O and amines such as the picolines and lutidines. Pyridine is hygroscopic and is miscible with H₂O and organic solvents. It can be dried with solid KOH, NaOH, CaO, BaO or sodium, followed by fractional distn. Other methods of drying include standing with Linde type 4A molecular sieves, CaH₂ or LiAlH₄, azeotropic distn of the H₂O with toluene or benzene, or treated with phenylmagnesium bromide in ether, followed by evaporation of the ether and distn of the pyridine. A recommended method dries pyridine over solid KOH (20g/Kg) for 2 weeks, and fractionally distils the supernatant over Linde type 5A molecular sieves and solid KOH. The product is stored under CO₂-free nitrogen. Pyridine can be stored in contact with BaO, CaH₂ or molecular sieves. Non-basic materials can be removed by steam distilling a soln containing 1.2 equivalents of 20% H₂SO₄ or 17% HCl until about 10% of the base has been carried over along with the non-basic impurities. The residue is then made alkaline, and the base is separated, dried with NaOH and fractionally distd.

Alternatively, pyridine can be treated with oxidising agents. Thus pyridine (800mL) has been stirred for 24h with a mixture of ceric sulfate (20g) and anhydrous K₂CO₃ (15g), then filtered and fractionally distd. Hurd and Simon stirred pyridine (135mL), water (2.5L) and KMnO₄ (90g) for 2h at 100 °C, then stood for 15h before filtering off the ppted manganese oxides. Addition of solid KOH (ca 500g) caused pyridine to separate. It was decanted, refluxed with CaO for 3h and distd.

Separation of pyridine from some of its homologues can be achieved by crystn of the oxalates. Pyridine is ppted as its oxalate by adding it to the stirred soln of oxalic acid in acetone. The ppte is filtered, washed with cold acetone, and pyridine is regenerated and isolated. Other methods are based on complex formation with ZnCl₂ or HgCl2. Heap, Jones and Speakman added crude pyridine (1L) to a soh of ZnCl₂ (848g) in 730mL of water, 346mL of conc HCl and 690mL of 95% EtOH. The crystalline ppte of ZnCl₂.(pyridine) was filtered off, recrystd twice from absolute EtOH, then treated with a conc NaOH soln, using 26.7g of solid NaOH to 100g of the complex. The ppte was filtered off, and the pyridine was dried with NaOH pellets and distd. Similarly, Kyte, Jeffery and Vogel added pyridine (60mL) in 300mL of 10% (v/v) HCl to a soln of HgCl₂ (405g) in hot water (2.3L). On cooling, crystals of pyridine-HgCl₂ (1:1) complex separated and were filtered off, crystd from 1% HCl (to m 178.5-179 °C), washed with a little EtOH and dried at 110 °C. The free base was liberated by addition of excess aq NaOH and separated by steam distn. The distillate was saturated with solid KOH, and the upper layer was removed, dried further with KOH, then BaO and distd. Another possible purification step is fractional crystn by partial freezing.

Small amounts of pyridine have been purified by vapour-phase chromatography, using a 180 cm column of polyethyleneglycol-400 (Shell 5%) on Embacel (May and Baker) at 100 °C, with argon as carrier gas. The Karl Fischer titration can be used for determining water content. A colour test for pyrrole as a contaminant is described by Biddiscombe et al.