CERIUM 



231 



Intake Levels. Cerium does not occur naturally to any significant 

 extent^in biological materials. Its metabolism in animals has been of 

 interest primarily because of the production of radiocerium in the fission 

 process. Intravenous injection in rats of 50 to 60 mg/kg of cerium chlo- 

 ride was fatal. 



Radioassay. Ce^^^-Pr^^^ appears to be the preparation of choice. The 

 contaminating Ce^^^ can be allowed to decay out if necessary, and the 

 samples can stand for about 175 min after being taken from the biological 

 system to permit attainment of Ce^''^-Pr^'** equilibrium. The Ce-141-P 

 preparation should also be useful if for some reason a short half-life is 

 desired for the major part of the activity. Routine beta or gamma count- 

 ing can be employed, and it is noted that the Ce^^^-Pr^^"* energies are 

 greater than those of the Ce^^^ 



Chemistry, Procedures for estimation of cerium in biological material 

 have not been developed. Dry-ashing and presumably wet-ashing of 

 tissues will be satisfactory. Various colorimetric methods are described 

 in standard texts which are based on oxidation to eerie ion or reaction 

 with brucine or 8-hydroxycjuinoline. 



Typical Methods. Rats were injected intravenously with about 5 ^c 

 of carrier-free Ce^^'^-Pr^*'' in 0.25 ml of a 0.01 per cent sodium citrate solu- 

 tion, pH 6 (Ce-1). The tissues and excreta were dried and then ashed at 

 650°C for 10 hr. The ash was dissolved in 2 V HCl, and aliquots up to 

 5 ml were placed in j^-oz metal ointment capsules and dried for counting 

 with a thin-mica-window tube. Self-absorption corrections were made 

 when necessary. Barley plants were placed in a bentonite suspension 



