POTASSIUM 283 



Intake Levels. The daily intake of potassium by the human adult 

 averages 2 to 3 g. Rats require 8 to 15 mg/day, and 0.5 per cent in the 

 diet is normally adequate. Chicks and swine require 0.15 to 0.24 per cent. 

 A recommended nutrient solution for plants contains about 236 ppm, and 

 crop plants may have 1 to 5 per cent K on the dry-weight basis. Animal 

 tissues have about 3 mg K per gram fresh weight; plasma, about 0.2; and 

 urine, about 2 mg/ml. 



Radioassay. Routine hard-beta or gamma counting of K^^ is entirely 

 satisfactory on account of the energetic radiations emitted. If the target 

 material is of low Na assay, there will usually be no significant contam- 

 inating Na-^. Otherwise the K^- can be purified by reprecipitation as 

 perchlorate in the presence of carrier Na (K-1). There is always the 

 question of the contribution of naturally occurring K^° (beta, 1.35; K) to 

 the radioactivity measurement. It has been calculated that 1 mg of 

 natural potassium contains about 0.75 X 10~^ nc K^" (K-2). If the 

 amount of total potassium in the sample is known, then the amount of 

 K"*" can be calculated and its contribution estimated from sensitivity 

 values. For example, a 10-ml sample of urine might contain 20 mg K 

 and therefore 1.5 X 10~^ nc K^". Even if the sensitivity for K""^ were as 

 high as for K*-, it can be seen that this sample would probably have a net 

 count less than the background of the instrument. 



Chemistry. Wet-ashing, and dry-ashing with H2SO4 are satisfactory. 

 The flame photometer may be conveniently used to determine potassium 

 in ash solutions. A method for ashing plant samples is as follows (K-3) : 

 The sample was weighed into a porcelain crucible, wetted down with 1 : 1 

 H2SO4 plus distilled water, dried on a steam bath and then in an oven, and 

 ashed at 500 to 550°C overnight. The ash was digested in 3 ml concen- 

 trated HCl, made to volume, filtered, and used for direct flame-photom- 

 eter measurement. Animal tissues have been prepared for flame photom- 

 etry by digestion in HNO3, use of mild heat to avoid charring, and 

 removal of excess nitrates with formic acid (K-1). Plasma was diluted 

 1:25 and compared with standard solutions containing 1 ml of 0.145 M 

 NaCl to correct for the sodium effects in the plasma. 



