DETERMINATION OF NITRATES. 1405 



ture and barometric pressure, or to avoid computation the amount of 

 nitrate may be determined by comparison of the volume of nitric oxid 

 with that evolved by a definite volume (5 or 10 cc) of normal sodium 

 nitrate solution. 



PICRIC ACID METHOD.* 



Weigh 1 gram of the sample into a 100 cc flask, add from 20 to 30 cc 

 of water, and heat on the water bath for fifteen or twenty minutes. 

 Add 3 cc of a saturated solution of silver sulphate b for each per cent 

 of sodium chlorid present, then add 10 cc of lead subacetate and 5 cc 

 of alumina cream, shaking after each addition. Make up to mark 

 with water, and filter through a fluted filter, returning the filtrate to 

 the filter until it runs clear. Evaporate to dry ness 25 cc of the filtrate, 

 add 1 cc of phenol-sulphonic acid, c mix thoroughly with a glass rod, 

 add 1 cc of water and 3 or 4 drops of concentrated sulphuric acid and 

 heat on a steam bath for two or three minutes, being careful not to 

 raise the temperature sufficiently to char the material. Now add 

 about 25 cc of water and an excess of ammonium hydroxid, pour into 

 a 100 cc flask, and dilute to mark with water. Compare depth of 

 color in a Nessler cylinder with an equal volume of a solution pre- 

 pared by drying in a similar manner 5 cc of a solution of potassium 

 nitrate which contains 0.1 gram of nitrogen or 0.72 gram potassium 

 nitrate per liter and dilute to 200 cc. 



Prepare a number of 50 cc Nessler tubes, preferably the long, nar- 

 row tubes, placing in the first 1 cc of this solution, in the second 

 2 cc and so on to 10 cc, then 12 cc, 15 cc, 18 cc, and 20 cc; the com- 

 parison of the solution under examination with these tubes will show 

 directly if it comes within this range, in which case it can be read 

 by direct comparison with the various tubes till the one of the exact 

 shade is found. If the color of the solution is darker than any of the 

 tubes prepared as above it is preferable to dilute as many times. as 

 may be necessary to bring the color within this range by removing 

 25 cc of the solution with a pipette and filling up to the mark again 

 with distilled water. In this case the reading of the diluted solution 

 in cubic centimeters standard solution should be multiplied by 2, 4, 6, 

 etc., according as it has been diluted once, twice, or three times, etc. 



a The determinations of nitrates for this bulletin were made by Mr. A. Given, who 

 adapted the method to the examination of meats. 



b Silver sulphate free from oxids of nitrogen was prepared by fuming the ordi- 

 nary salt for some time with sulphuric acid. The mass, containing an excess of 

 sulphuric acid, is allowed to solidify, is crushed in a mortar, and used directly; 

 excess of sulphuric acid does not interfere. In examining a small number of sam- 

 ples it is often more convenient to proceed without separating the chlorin, and add 

 an equal amount' of salt to the standard solution of potassium nitrate. 



"Prepare by mixing together 550 grams of concentrated sulphuric acid, 25 cc of 

 distilled water, and 25 grams of phenol. 



