576 PHYSIOLOGICAL CHEMISTRY 



Interpretation. The daily excretion of magnesium by way of the 

 urine usually amounts to between o.i and 0.3 gram (expressed as MgO). 

 The amount depends mainly upon the diet. Usually 50 per cent or 

 more of the excreted magnesium is eliminated by the kidneys, the re- 

 mainder passing out in the feces. The proportion varies, however, and 

 it is impossible to draw any conclusions from the urinary output alone. 

 There may be a retention of magnesium in certain bone disorders ac- 

 companying a loss of calcium; in osteomalacia for example. Thus the 

 excretions of calcium and magnesium do not necessarily run parallel. 



Determination of Calcium in Ash of Foods or Feces. Ignite the material in a 

 crucible to a white ash and dissolve the ash with the aid of a little hydrochloric acid. 

 Bring the volume of the ash solution to 75-150 c.c. Make just alkaline with strong 

 ammonia added drop by drop (using litmus paper or alizarin as an indicator). Add 

 concentrated HC1 drop by drop until just acid to litmus. Then add 10 drops of 

 concentrated HC1 (sp. gr. 1.20), and 10 c.c. of 2.5 per cent oxalic acid. Either 

 of two procedures may then be followed, (a) The solution is boiled until the pre- 

 cipitated calcium oxalate is coarsely crystalline, and then an excess of 3 per cent 

 ammonium oxalate is slowly added to the boiling solution and the boiling continued 

 until the precipitate is coarsely crystalline. (If but little calcium is present nothing 

 will precipitate at this point and it is not necessary to add oxalate.) Or (6) the 

 flask closed with a rubber stopper is shaken vigorously for ten minutes. An excess 

 of 3 per cent ammonium oxalate is then added. Cool to room temperature. Add 

 8 c.c. of 20 per cent sodium acetate solution. (In case of ash of feces add 15 c.c.) 

 The solution may either be (a) allowed to stand over night or (b) stoppered and 

 vigorously shaken for ten minutes. The calcium oxalate is filtered off on a small 

 ash^free paper and washed free from chlorides with 0.5 per cent ammonium oxalate 

 solution. Either of two procedures may next be followed, (a) The precipitate 

 and filter are dried, burned in a platinum or porcelain crucible to constant weight as 

 CaO. (b) The precipitate is washed three times with cold distilled water, as given 

 under the method for urine and the oxalate titrated with potassium permanganate. 

 Magnesium is determined in the filtrate from calcium just as given above. 



Sodium and Potassium 



Determination of Combined Sodium and Potassium. From 50 to 100 c.c. of 

 urine, depending upon the specific gravity, are oxidized in a Kjeldahl flask with 

 nitric and sulphuric acids as in the Neumann procedure for total phosphorus (see 

 page 570). To remove the sulphuric acid as completely as possible transfer with 

 the aid of a little water to a platinum dish and evaporate to dryness over & free 

 flame. (The alkalies are in the form of sulphate and do not volatilize.) Dissolve 

 the residue in hot water with the aid of a little dilute hydrochloric acid. Heat to 

 boiling and add barium chloride solution until no more precipitate forms. While 

 still hot add an excess of ammonia and ammonium carbonate. The barium chlo- 

 ride precipitates the sulphates and part of the phosphates: the ammonia in the 

 presence of excess barium precipitates the rest of the phosphates, and the carbonate 

 precipitates the calcium and most of the magnesium, as well as the excess barium. 

 Filter and wash the precipitate well with hot water containing a few drops of am- 



