36 METHODS OF INVESTIGATING 



reagent used for effecting the oxidation. Trials carried out by 

 my colleagues, Dr K. W. Dent and Dr E. S. Twyman, indicate 

 that the periodate method is the most satisfactory of these, as 

 regards both simplicity of procedure and accuracy of the results 

 obtained. This experience appears to be fairly general if we are 

 to judge from the fact that this method is the one most com- 

 monly used by recent workers. A number of accounts of the 

 procedure employed in using the method have been published; 

 two of the more recent are those by Coleman and Gilbert (1939) 

 and Cook (1941). Coleman and Gilbert found that the same 

 values for manganese content of tea and coffee were obtained 

 with a wet ashing process and with incineration in a muffle, so 

 it is concluded that no loss of manganese occurs with either 

 ashing process. When dry ashing is employed 1 g. of material is 

 moistened with 1-2 ml. of concentrated sulphuric acid and ashed 

 at a dull red heat in a muffle. The ash is dissolved in 15 per cent 

 sulphuric acid and filtered. The filter paper is ignited and any 

 residue from this dissolved in about 10 ml. of dilute sulphuric 

 acid and filtered into the main filtrate. To the combined filtrate 

 0-1 g. of potassium iodate is added and the mixture boiled for a 

 few minutes and then kept hot for 30 min. for the development 

 of the pink colour. The solution is made up to standard volume 

 and the intensity of the colour determined. 



Amounts of manganese down to about 12/xg. can be estimated 

 in this way. 



For a description of the bismuthate method, which is par- 

 ticularly recommended for the determination of manganese in 

 soils, reference may be made to a paper by Dean and Truog 

 (1935) and for the persulphate method to papers by Majdel 

 (1930) and Olsen (1934). 



A method for the estimation of small quantities of man- 

 ganese which has been described by Sideris (1940) depends on 

 the colour produced when formaldoxime is added to a solution 

 of a manganese salt. The formaldoxime reagent is prepared by 

 dissolving (by boiling) 20 g. of trioxymethylene + 47 g. of hydro- 

 xylamine sulphate in 100 ml. of distilled water. Ten ml. of the 

 hydrochloric acid solution of the ash are neutralized with sodium 

 hydroxide and then acidified with 2 ml. of a 20 per cent solution 

 of acetic acid. Excess phosphate is then removed by adding 



