252 PHYSIOLOGICAL CHEMISTRY 



through clear, add a pinch of bismuth subnitrate and refilter 

 through the same filter paper. Repeat if necessary until clear. 

 The bismuth subnitrate, which is extremely insoluble, closes up 

 the larger pores of the filter paper, and thus holds back the fine 

 calcium oxalate crystals. To the clear filtrate add a few c.c. of 

 ammonium oxalate and allow to stand for some time, preferably 

 over night, to make sure that all calcium has been removed. 

 After complete removal of the calcium, add ammonium hydrate 

 and ammonium phosphate to the clear filtrate and allow to stand 

 for 15 minutes. The precipitate is magnesium ammonium phos- 

 phate. Write the equation. Save the filtrate for the sodium and 

 potassium tests. 



17. Iron. Add hydrochloric acid and potassium ferrocyanide 

 to the liquid to be tested. In the presence of iron "Prussian 

 blue" is formed. A small amount of Prussian blue in the solu- 

 tion may result only in a green color, due to a combination of 

 blue with the yellow already present in the solution. A green 

 color may thus be taken to indicate a trace of iron. If more 

 than a trace of iron is present a precipitate will form. Write 

 the equation. Compare with a blank test on the reagents used. 

 If iron is found, remember that it may have come from the 

 hemoglobin of the blood present in traces in the tissues. 



18. Sodium. Use one-half the filtrate from the magnesium 

 test. Make sure that all magnesium has been removed by add- 

 ing a few drops of ammonium phosphate. Divide the filtrate 

 into two portions and reserve one for the potassium test. 



To test for sodium, add potassium hydrate a few drops at a 

 time and warm in an evaporating dish until all the ammonia has 

 been driven off. (Test by holding over the dish a piece of red or 

 neutral litmus paper moistened with distilled water.) Pour into 

 a watch glass and concentrate on the steam bath to about % c.c. 

 There should be no solid material present when the process is 

 stopped. If solid material has appeared in the liquid, the con- 

 centration has been carried too far, and the solid should be dis- 

 solved up by the cautious addition of water drop by drop, stir- 

 ring the mixture with a glass rod. Add a few drops of potas- 



