MAGNESIUM IN BLOOD 169 



principle which every chemist will recognize as theoretically incor- 

 rect, viz., the determination of magnesium by the measurement 

 of the phosphate combined as ammonium magnesium phosphate, 

 the precipitation of which latter compound must be carried on 

 in the presence of a large excess of phosphate. It is obvious that 

 extremely careful work is essential in the washing of the precipitate 

 in order to avoid contamination with residual traces of ammonium 

 phosphate; furthermore, as is well known, it is possible to over- 

 wash an ammonium magnesium phosphate precipitate, so that 

 low results are obtained. 



After the removal of the last portion of wash liquid, the tube 

 and beaker are allowed to stand, preferably on a water bath or 

 register until the ammonium has evaporated, and the ammonium 

 magnesium phosphate is then dissolved in 10 c.c. of 0.1 N hydro- 

 chloric acid and transferred to a 100 c.c. volumetric flask by means 

 of distilled water. The solution is then made to volume with 

 distilled water, mixed, and the phosphate determined by means 

 of the strychnine molybdate reagent (see Bloor's Method for 

 Phosphoric Acid in Blood, this Manual, page 171). 



For the determination of magnesium in normal plasma or 

 serum 25 c.c. of the above solution are usually the most convenient 

 to use, but larger or smaller amounts are sometimes called for. 



To the amount of solution taken is added a quantity of dis- 

 tilled water sufficient to bring the volume to 50 c.c., then 25 c.c. 

 of the strychnine molybdate reagent are added. After standing 

 for a period of five minutes the suspension is read against a standard 

 containing 0.01 mg. of magnesium in a volume of 50 c.c. 1 to which 

 have been added 25 c.c. of the strychnine molybdate reagent, and 

 which has been allowed to stand for the same length of time as 

 the unknown. 



1 This standard is prepared from ammonium magnesium phosphate by 

 dissolving 1.02 gms. of the pure salt in 100 c.c. of N hydrochloric acid and 

 diluting to a volume of 1 liter with distilled water; 1 c.c. of this solution is 

 equivalent to 0.10 mg. of magnesium. For the preparation of this dilute 

 standard a portion of the strong solution is diluted to 50 times with 0.1 N 

 hydrochloric acid so that 5 c.c. of the resulting solution will be equivalent to 

 0.01 mg, of magnesium. 



