HOOKER'S METHOD 459 



experience enables a fairly good opinion to be formed of the 

 quantity of nitric acid present. Suppose that the sample be 

 roughly estimated to contain about 0.15 part nitrogen per 100,000 ; 

 in such a case solutions of potassium nitrate containing o.n, 

 0.15, 0.19 part nitrogen are selected from the series. Two cubic 

 centimeters are taken from each and treated, side by side, with 

 a fresh quantity of the sample, precisely as described for the pre- 

 liminary experiment, the various operations being performed as 

 nearly simultaneously as possible with each of the samples, and 

 under precisely similar conditions. Two or three minutes after 

 the carbazol has been added, the intensity of the color of each 

 is observed. If that given by the sample is matched by any of 

 the standard solutions, the estimation is at an end. Similarly, if 

 it falls between two of these, the mean may be taken as repre- 

 senting the nitrogen present in cases in which great accuracy is 

 not required. If this be done, the maximum error will be 0.02 

 part nitrogen, or 0.09 part nitric acid per 100,000. If greater 

 exactness be required, or it be found that the color given by the 

 sample is either darker or lighter than that given by all the 

 standard solutions, a new trial must be made. In such a case 

 the sample must be again tested simultaneously with the solu- 

 tions with which it is to be compared. This is rendered neces- 

 sary principally for the reason that the shade of the solutions 

 to which the carbazol has been added is apt to change on stand- 

 ing. Hence it is desirable that the sample, and the standard 

 potassium nitrate with which it is to be compared, should have 

 the carbazol added at as nearly the same time as possible. When 

 finally the color falls between that given by any two consecutive 

 members of the standard potassium nitrate series, the estimation 

 may be considered at an end, and the mean of these solutions 

 taken as representing the nitrogen present. 



The greatest neatness should be observed in all steps of the 

 analysis. The quantity of nitric acid to be estimated is so small 

 that if the greatest care be not exercised throughout, sources of 

 error may be readily introduced. The test-tube should be rinsed 

 out with nitrate-free water before being used, and dried. The 

 tint is determined by looking through the tube and not through 



