244 PHYSIOLOGICAL CHEMISTRY 



small beaker containing distilled water and a drop of alizarine. 

 If the indicator turns pink, replace the Woulff bottle and con- 

 tinue the distillation until the liquid coming over no longer 

 contains ammonia. When the ammonia is all over, discontinue 

 the distillation, rinse down the glass pearls with distilled 

 water, and titrate the excess of N/10 HC1 with standard alkali. 



Subtracting the amount of alkali used from the total acid 

 (50 c.c.) gives the volume of acid neutralized by the ammonia. 



Multiply this figure by the weight of nitrogen in 1 c.c. of 

 N/10 ammonia (1.401 mg.). This gives the weight of nitrogen 

 in 5 c.c. urine. 



In accurate work, a blank determination should be made to 

 estimate the amount of nitrogen in the reagents used. 



9. Indicators. Very carefully titrate 2-20 c.c. portions of 



N N 



HC1 with p- NaOH, using phenolphthalein. as indicator in 



one titration and methyl orange in the other. 



10. Repeat the process using as acid 20 c.c. portions of oxalic 

 acid 0.1 N. Explain. 



Why was methyl orange used in standardization of acid by the 

 sodium bicarbonate method above? Why could not phenol- 

 phthalein have been used? 



By adding properly chosen indicators to solutions of known 

 pH which are made up to form a series, it is possible to obtain 

 a series of graduated tints, each corresponding to a definite pH. 

 By making up an unknown solution with the same amount of 

 indicator, it is possible to determine its pH by comparing it 

 with the colors in the standard tubes. 



11. Catalysis, Into each of two Erlenmeyer flasks intro- 

 duce 5 c.c. of urine and 10 c.c. cone. H 2 S0 4 . To one add two 

 or three crystals of copper sulphate. Boil both in the hood or 

 in a fume flue. Which clears first? Explain. 



The catalytic action of enzymes is treated at a later point in 

 the laboratory work. 



