THE URINE. 209 



acid, and dilute to 1100 cc. The strength of this solution 

 must be empirically determined by titration with a solution 

 of sodium phosphate of known strength. Weigh out accu- 

 rately 10.0944 g. of pure, dry sodium phosphate, Na 2 IIP0 4 -f 

 12H 2 0, which has not effloresced, dissolve in water, and fill 

 up to one liter. The preparation of this solution is, however, 

 often very difficult, sometimes, indeed, even impossible, as 

 the sodium phosphate effloresces in dry air. It is better, 

 therefore, to proceed as follows ; About 12 g. of sodium phos- 

 phate are dissolved in 1100 cc. of water; 50 cc. of the well- 

 mixed solution are evaporated in a platinum or porcelain 

 dish, dried and ignited. The residue, consisting of sodium 

 pyrophosphate, Na 4 P 2 7 , must weigh 0.1875 g. If it weighs 

 more, the solution must be correspondingly diluted. Fifty 

 cubic centimeters of this solution, measured off with a pipette, 

 are run into a beaker, 5 cc. of acetic acid mixture (100 g. 

 sodium acetate, 100 cc. of dilute acetic acid Ph. G. Ill diluted 

 to one liter) added, then a few drops of cochineal tincture, and 

 heated almost to boiling. Now run in about 18 cc. of the 

 uranium solution and observe the color of the mixture. At 

 the same time take out a drop of the solution with a glass rod 

 and bring it into contact with a drop of potassium ferrocya- 

 nide solution (it is advisable to have a number of drops of 

 the ferrocyanide arranged in series on a white porcelain plate). 

 If a faint-brown color appears after a few moments, the end- 

 reaction is attained (the mixture or the precipitate will then 

 also show a greenish color). If the brown coloration does 

 not appear, let more of the uranium solution run in and test 

 each time, after the addition of 0.2 cc. When the end-point 

 is reached heat the mixture for some minutes and test again 

 for the end-reaction, etc. If the solution is of the correct 

 strength, 50 cc. of the sodium phosphate solution will require 

 20 cc. of the uranium solution. Usually this is not the case, 

 but less is required. If, for example, 19.4 cc. were used,, 



