PRACTICE VIII 



PREPARATION OF STANDARD SODIUM HYDROXID AND NORMAL 

 POTASSIUM NITRATE SOLUTIONS 1 



These solutions are to be used in the determination of soil acidity. 



1. Weigh out enough sodium hydroxid (sticks, purified over alcohol, 

 about 75 per cent NaOH) to make about 3 liters of solution of such 

 strength that 1 cc. shall be equivalent to 4 nig. of calcium carbonate. 

 Dissolve in ammonia-free water, dilute to exactly 1000 cc., add 100 cc. 

 of a saturated barium hydfoxid solution, shake well, and allow to stand 

 over night, or until clear. Draw off 100 cc. of the clear solution, and 

 place in a 250-cc. bottle containing 100 cc. of the saturated solution of 

 barium hydroxid. If a precipitate occurs, add the 200 cc. to the main 

 solution, shake, and repeat the above operations until no further pre- 

 cipitate occurs. (If no precipitate is formed, throw away the 200 cc.) 

 Always keep an exact record of the amount of liquid remaining in the 

 stock bottle. When the solution is free from carbon dioxid, draw off 

 100 cc. and place in a 100-cc. bottle. Fill the burette with this and 

 titrate against 10-cc. portions of standard hydrochloric acid. Add 

 ammonia-free water so that 1 cc. of the standard sodium hydroxid 

 solution shall be exactly equivalent to 4 mg. of calcium carbonate. 



2. Prepare 5 liters of a normal potassium nitrate solution, assum- 

 ing the salt to be pure. 



If 100 g. of an acid soil are placed in 250 cc. of normal potassium 

 nitrate solution and shaken for three hours, a reaction takes place 

 between the potassium nitrate and the acid constituents of the soil, 

 giving, as one of the products, soluble acid salts, and so making the 

 acidity determinable. An equilibrium is reached, however, before 

 this reaction runs to an end ; and if, after having drawn off 125 cc. 

 to titrate, 125 cc. of fresh potassium nitrate are added to the bottle 

 and the bottle again shaken for three hours, 125 cc. drawn off will 

 give a titration which is more than one half of the first. By continu- 

 ing this process until the last 125 cc. shows practically no acidity, 

 we have a series of titrations the sum of which represents the total 

 acidity of the 100 g. of soil. It has been found by working with a 

 number of different soils that, as an average, the sum of such a series 

 is two and one-half times the first titration. 



Consequently, when the sodium hydroxid is made up so that 1 cc. 

 is equivalent to 4 mg. of calcium carbonate, and 125 cc. (which repre- 

 sent 50 g. of soil) are titrated, each 0.1 cc. required to neutralize cor- 

 responds to 1 mg. of calcium carbonate required by the 100 g. of soil, 

 or to 0.001 per cent of calcium carbonate required by the soil tested. 



1 To be done by the instructor. 



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