MAGNESIUM OF THE BLOOD 165 



Calculations. From the permanganate equivalent of 0.3 c.c. 

 of 0. 1 N oxalic acid is subtracted twice (since half the total quantity 

 is titrated) the number of c.c. of 0.01 N permanganate used in the 

 final titration, the latter number, however, having been diminished 

 by the amount of permanganate (0.04 c.c.) necessary to give a 

 permanent pink color to water. The result is multiplied by 0.2, 

 since 1 c.c. of the 0.01 N permanganate is equivalent to 0.64 mg. 

 of calcium oxalate or 0.2 mg. of calcium. 



Preparation of Reagents. 0.1 N Oxalic Acid. 0.1 N sodium 

 oxalate solution (Sorenson) is made up and a potassium per- 

 manganate solution standardized against it. This solution then 

 serves to standardize the N solution of oxalic acid. This N solu- 

 tion is quite permanent if kept in a cool place in the dark, and 

 from it 0.1 N oxalic acid in 0.05 sulfuric acid may be prepared 

 whenever needed. 



0.01 N Potassium Permanganate. An approximately 0.01 N 

 solution of potassium permanganate is made by diluting a N or 

 0.1 N solution and the factor determined by titrating against 

 the known 0.1 N oxalic acid solution. Filter before using. 



MAGNESIUM IN BLOOD 



Method of Marriott and Howland x 



Principle. After precipitation of the calcium (see page 160, 

 this Manual) magnesium is precipitated as ammonium magnesium 

 phosphate. This precipitate is dissolved in HC1 and estimated 

 colorimetrically against a standard ammonium magnesium phos- 

 phate solution. 



Procedure. After precipitation of the calcium oxalate, as 

 previously described, see page 161, this Manual, the contents of 

 the beaker are rinsed into a conical centrifuge tube and centrifuged 

 for about ten minutes. The clear liquid is syphoned off 2 into a 



1 Marriott and Howland: Jour. Biol. Chem., 1917, 32, 233. 



2 Syphoning is accomplished by means of a small glass tube, one end of 

 which is drawn to a short capillary point and bent upwards. This form of 

 tip prevents the sucking up of any precipitate and permits the removal of 

 almost all of the liquid. The syphon tube is passed through a small double- 

 bored rubber stopper, which fits the centrifuge tube. A short glass tube 

 passes through the other hole in the stopper. The syphon is started by, 

 blowing into this tube or by the use of an atomizer bulb. 



