246 PRACTICAL PHYSIOLOGY 



In the case of a tribasic acid such as phosphoric a third of the 

 molecular weight would be taken. 



To Titrate a Solution. An accurately measured quantity say 50 

 c.c. is measured into an Erlenmeyer's flask, and mixed with about 

 two drops of a solution of methyl orange. This gives a deep red colour 



ty\ 



with acids and a faint yellow with alkalies. Suppose 50 c.c. y^ acid 

 had been taken, the red of the indicator would change to yellow 

 when exactly 50 c.c. of yx alkali had been added. Litmus solution 



might also be employed, but it is not so delicate. (For standardising 

 the decinormal solutions, see Advanced Course.) 



The application of the above to the determination of the liberated 

 ammonia in Kjeldahl's method will now be readily understood. A 



measured quantity say 50 c.c. of y^ acid is placed in an Erlen- 

 meyer's flask, and in contact with the surface of this is the distillation 

 tube. As the NH 3 comes over, it at once combines with the acid to 

 form ammonium sulphate. After distilling for half-an-hour, test to see 

 if all NH 3 has come over. This may be done by raising the distilling 

 tube a little and removing a drop of the distillate with a clean 

 glass rod and placing it on sensitive red litmus paper ; if no blue 

 stain results all NH 3 is over ; if a blue stain results wash the glass rod 

 and litmus paper with a jet of distilled water, allowing the washings 

 to run into the receiving flask, and continue the distillation. 



When all the NH 3 is distilled remove the receiving flask and titrate 



n 

 with yx alkali. The number of c.c. of this which it is necessary to 



employ in order to produce neutralisation corresponds to the number 

 of c.c. ofy^ acid which has not combined with NH 3 , and if we deduct 

 this from the quantity of y^ acid originally employed, we obtain the 

 number of c.c. of y^ acid which has been neutralised by NH 3 . This 



result multiplied by OO014 1 gives the amount of nitrogen in grammes 

 contained in the urine. 



EXPERIMENT V. Determine by the above method the amount of 

 nitrogen contained in an acid solution of ammonium sulphate. 2 



1 Because the molecular weight of nitrogen is 14. Each c.c. of an N solution 



14 n 



equals JQQQ= '014 j or of a JQ solution '0014. 



2 A suitable solution for the purpose is made by dissolving 1'32 grammes of 

 ammonium sulphate crystals in 100 c.c. of 1 per cent, sulphuric acid; 5 c.c. 

 of this solution contains 0*014 grammes N. 



