324 ANALYSIS OF URINE. [CH. XIII. 



logarithms of the " acid-equivalent " (acid log.) and of the " alkali-acid ratio " 

 (alkali log.) respectively. 



The following example should be carefully studied. 

 Acid employed was 0-0476 N. sulphuric (acid log. = 1-8237). 

 Alkali employed was 0-0421 N. soda (alkali log. = 1-9467). 



0-5 cc. of urine taken. 



Amount of acid taken was 20-12 cc., and this required 13-6 cc. of alkali 

 for back titration. 



Log. of 13-6 = Jf-1335 



Add the alkali log i -9467 



Anti-log, of 1-0802 is 12-03. 



So 20- 12 - 12-03 = 8-09 cc. of acid have been neutralised by the 

 ammonia formed from 0^5 cc. of urine. 



Log. of 8-09 is '9079 



Add the acid log. i -8237 



Anti-log, of -7316 is 1-539- 



So 0-5 cc. of urine contain 1-539 mg. total Nitrogen. 

 So 100 cc. of urine contain 0-308 gram, total Nitrogen. 



(This example is from an analysis of a very dilute urine. The figure is 

 usually about i per cent.) 



No allowance has been made for the blank determination, but this should 

 not be neglected, especially when using the micro-method. The blank 

 determination is made with all the materials used for an ordinary analysis, 

 distilled water being take instead of urine. Unless the reagents are of very 

 poor quality, the amount of nitrogen found should be very small. This must 

 be deducted from the amount found in the volume of urine taken. An 

 example is given on page 263. 



394. Kjeldahl's method (distillation by boiling). Into a 

 clean, dry, round-bottomed flask of " Duro " glass A (500 cc. 

 capacity, with a narrow neck 8 inches in length) place 5 to 10 

 grams, potassium sulphate, 0-5 cc. of saturated copper sulphate 

 solution, 5 cc. of urine (accurately measured) and 10 cc. of concen- 

 trated sulphuric acid, free from nitrogen. Place the flask in the 

 fume-chamber (or use the fume-absorber, described on page 383), 

 and heat by means of a low flame for 10-15 minutes, then boil briskly 

 for 45 minutes or longer. The solution must be heated for at least 

 15 minutes after it has lost every trace of dark colour. Any particles 

 of carbonaceous matter that adhere to the sides of the flask must be 

 washed down into the acid by carefully shaking the flask. When 

 cool*: add 250 cc. of ammonia-free distilled water, 3 or 4 pieces of 

 broken porous pot, and cool under the tap. Into an Erlenmeyer 



