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A modified thiochrome procedure for the assay of thiamine is described. It eliminates the use of ion-exchange columns and elutions with hot, saturated potassium chloride solution that are employed in most other methods. The times required for extraction and dephosphorylation have been examined critically and shortened.It was shown by Jansenl that the oxidation of thiamine to thiochrome using akaline potassium hexacyanoferrate( 111) could be used for the determination of thiamine in natural materials and this reaction forms the basis of many assay procedures. In order to remove interfering substances the extracted thiamine was absorbed on an ion-exchange resin, the interfering substances removed by washing and the thiamine eluted with a hot, saturated solution of potassium chloride. In the procedure to be described, thiamine is converted into thiochrome while still on the resin, and the thiochrome then extracted into 2-methylpropan-1-01. The method is simple, reproducible and has been found to be satisfactory in this laboratory. Experimental ReagentsAll reagents should be of analytical-reagent grade. HydrochZoric acid, 0.1 N. Sodium acetate trihydrate.CZarase. This is available from Miles Laboratories Inc., Elkhart, Indiana, USA, and P.O. Box 37, Stoke Poges, Slough, Berkshire.Decalso F (also called Zerozit S / F ) . This resin is manufactured by Permutit Co. (Zerolit) and marketed by BDH Chemicals Ltd. and Hopkin and Williams Ltd. and is a sodium aluminosilicate cation-exchange resin of mesh size 60-85. It is prepared for use as follows. Spread 1 kg of the resin on a tray and run a strong magnet over it in order to remove any contaminating particles of iron or rust. Mix the resin well with a 25 per cent. solution of sodium chloride in dilute hydrochloric acid (2 ml of concentrated hydrochloric acid per litre). After settling, pour off the supernatant liquid together with much of the "fines." Repeat this treatment three times. Next, wash the resin repeatedly with acidified water (1 ml of glacial acetic acid per litre of de-ionised water) until the washings are free from chloride. At the end of this treatment only the coarser particles will be left and these should settle to the bottom rapidly. Finally, dry the treated resin in air. NOTE-We have observed batch variations in the activity of Decalso F; although most batches were satisfactory, some gave high blank fluorimeter readings and did not retain thiamine quantitatively (see under Measurement of $uo1*escence).Cyanogen bromide solution. This solution must be freshly prepared before use. It is convenient to store the two reagents needed, viz., saturated bromine water and 10 per cent.aqueous potassium cyanide solution, in a refrigerator at 0.4 "C. The required amount of bromine water is poured into a flask and the 10 per cent. potassium cyanide solution added dropwise until 1 drop causes the yellow colour of free bromine to disappear completely. Any unused cyanogen bromide solution must be poured down the drain with plenty of running water.

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