616 STATE BOARD OF AGRICULTURE. 



Unless some means of checking the first solutions made from it are 

 accessible the phosphate should be recrystallized four or five times. 

 This product may tlien be kept as a special reagent for future use 

 and the solutions made from it by carefully weighing out the desired 

 quantities. 



The sodium hydroxide solution is prepared from carbonate-free ma- 

 terial and standardized in the usual manner. It should be kept in a 

 .paraffined bottle and protected from tlie carbon dioxide of the air. 



From these stock reagents solutions may be repeatedly prepared hav- 

 ing reactions so constant tliat no difference in color can be detected 

 between lots made at different times. Tims once the reliability of a 

 given stock is established, the accuracy of the results with it may be 

 regarded as fixed. 



For the purpose in hand the whole series is of course unnecessary. 

 It is well however to prepare two extra solutions, one on either side of 

 the one to which the unknown solution is to be compared. This permits 

 of a check on the accuracy of the standard, whose color should be inter- 

 mediate between the colors of the other tAvo. It also allows a more 

 accurate and rapid adjustment of the reaction of the unknown, the 

 color of which may be roughly made to match that of one of the solutions 

 above or below the one finally sought and then carefully brought to 

 the ultimately desired point. If, on the other hand, tlie endpoint is 

 slightly overreached in the process it may be more easily detected and 

 the magnitude of the error approximately judged by comparison with 

 the third solution. 



In this laboratory the comparisons are made in test tubes 7x7/8 

 inches placed in a comparator similar to that described by Dernby and 

 Avery. (13). 



Method of procedure: The desired quantity of citric acid is dissolved 

 in water, the solution being kept (luite concentrated, and almost neutral- 

 ized with concentrated ammonia. The neutralization is controlled 

 roughly by withdrawing a few cc. of the citrate solution from time to 

 time, dilutiug with water to a convenient volume mixing with the 

 proper amount of indicator (which may be measuied in drops from a 

 dropping bottle) and comparing with a standard solution several degrees 

 lower in reaction than the solution ultimately sought. We have found 

 it convenient for instance to use a G.G or (5.8 standard when the final 

 reaction is to be 7.0 or 7.4. When the acidity of the solution has been 

 reduced to this point, a measured sample is transferred to a test tube 

 with a calibrated pipette, the correct amount of indicator added, and 

 standard ammonia solution run in from a burette, first adding the 

 alkali rapidly until the color of the solution approximates that of the 

 proper standard. This coniparison need not be made in the comparator. 

 The process is finally com])loled by carefully adding small amounts 

 of ammonia nnlil llic color of the citrate matches perfectly tliat of the 

 standard having tlie leaclion finally sought. 



During the ])rrliminary titraticm the tube containing the standard 

 solution is sniipoiicd in sonic convenient fashion just behind that into 

 which the ammonia is being lun from the buret'e. a white background 

 being arranged lor botii. In this way the jirogress oi' the titration may 

 be easily controlled and followed almost up to the tinal endpoint. The 

 final adjustment is however made by means of the com])arator, after 

 bringing the volume of the citrate solution up to that of the standard. 



