354 AGRICULTURAL ANALYSIS 



by redistillation in a large glass retort with the addition of a few 

 drops of sulfuric acid. All vessels used in the determination 

 are rinsed out beforehand with this water. 



(4) Ammonia-free Soda-lye is most conveniently prepared by 

 adding 270 grams of common sodium hydroxid in sticks, little 

 by little, to one liter of distilled water which is kept continually 

 boiling, by means of a small flame, in a good-sized silver dish. 

 The dish is kept covered with a glass plate. Care has to be exer- 

 cised not to add the alkali too rapidly, nor in too large quantities 

 at a time, for in this case the fluid will boil too violently at every 

 addition of the alkali. After the operation is finished the lye is 

 at once siphoned into a glass flask, and when cold is poured into 

 a glass-stoppered bottle. 



(5) One-twentieth Normal Sulfuric Acid is prepared from sul- 

 furic acid and water, both absolutely ammonia-free, and is kept 

 ii) a place where no fumes of ammonia can reach it, in a well- 

 stoppered glass bottle, the stopper being smeared with vaseline. 



(6) Sodium Thiosulfate Solution. This should be of the same 

 strength as the one-twentieth normal sulfuric acid. It is pre- 

 pared by dissolving the salt in ammonia-free water and is com- 

 pared with the acid, to which has been added potassium iodid 

 and iodate, using starch as an indicator, in the manner described 

 above. The solution is kept in a well-stoppered bottle, in the 

 dark. When the salt and water used are perfectly pure, it will 

 keep unchanged for a long time. 



(7) Potassium Iodid. Dissolve five grams of chemically pure 

 potassium iodid in ammonia-free water and make the volume 100 

 cubic centimeters. Keep the solution in the dark and in a well- 

 stoppered bottle. Ten cubic centimeters bf this solution are used 

 for each determination. 



(8) Potassium Iodate. Dissolve four grams of chemically pure 

 potassium iodate in ammonia-free water and make the volume 

 100 cubic centimeters. Use two cubic centimeters of this solu- 

 tion for each determination. 



(9) Starch Solution. Digest pure starch for about a week 

 with dilute hydrochloric acid, wash perfectly free from chlorin 



