CALCIUM AND MAGNESIUM 70 



concentrated hydrochloric acid are added. Then add 10 c.c. cf 

 2.5 per cent oxalic acid. Run in slowly with stirring 8 c.c. of 20 

 per cent sodium acetate. Allow to stand over night at room tem- 

 perature or shake vigorously for ten minutes. Filter off the pre- 

 cipitate of calcium oxalate on a small paper and wash free from 

 chlorides with 0.5 per cent ammonium oxalate solution. The 

 precipitate may then be dried, ignited to constant weight and 

 weighed as calcium oxide or it may be manipulated volumetrically 

 as described below. 



Volumetric Procedure. If free from uric acid, the calcium 

 oxalate precipitate may be washed three times with distilled 

 water, filling the filter about two-thirds full and allowing it to 

 drain completely before adding more. A hole is made in the paper 

 and the calcium oxalate washed into the flask. The volume of the 

 fluid is brought up to about 50 c.c. and 10 c.c. of concentrated 

 sulphuric acid added. Titrate with standard potassium per- 

 manganate solution to a pink color which endures for at least a 

 minute. 



Calculation. One c.c. of N/10 permanganate solution is equiv- 

 alent to 2.8 mg. of CaO. Calculate the daily output of calcium 

 expressed as CaO. 



Procedure for Magnesium. Transfer the filtrate from the 

 determination of calcium as above to a porcelain dish, add about 

 20 c.c. of concentrated nitric acid and evaporate to dryness. Heat 

 the residue over a free flame until the ammonium salts are 

 destroyed and the residue fuses. After cooling take the residue 

 up with water and a little hydrochloric acid and filter if necessary. 

 Dilute to about 80 c.c., nearly neutralize with ammonia and cool. 

 Add a slight excess of sodium acid phosphate, and then ammonia 

 drop by drop with constant stirring, until the solution is alkaline, 

 and then add enough more slowly with constant stirring to make 

 the solution contain one-fourth its bulk of dilute ammonia (sp.gr. 

 0.96). Allow to stand over night. Filter and wash free from 

 chlorides with alcoholic ammonia solution (1 part alcohol, 1 part 

 dilute ammonia, 3 parts water). The precipitate, with filter paper, 

 is incinerated slowly and carefully with good supply of air to pre- 

 vent reduction, in the usual manner, and ignited and weighed as 

 the pyrophosphate. 



Calculation. To obtain the weight of MgO multiply the 

 weight of magnesium pyrophosphate by 0.3624. 



