30 



PRACTICE 14. 



PREPARATION OF STANDARD SODIUM HYDROXID AND NORMAL POTASSIUM 

 NITRATE SOLUTIONS.* 



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



(a) Weigh out enough sodium hydroxid (sticks, purified over alcohol, 

 about 75 <J NaOH) to make 3 liters of solution of such strength that I cc. 

 shall be equivalent to 4 mg. of calcium carbonate. Dissolve in ammonia- 

 free water, dilute to exactly 1000 cc., add 100 cc. of a saturated barium hy- 

 droxid solution, shake well, and allow to stand over night, or until clear. 

 Draw off 100 cc. of the clear solution, place in a 250 cc. bottle containing 

 100 cc. of the saturated solution of barium hydroxid. If a precipitate oc- 

 curs, add the 200 cc. to the main solution, shake and repeat the above op- 

 erations until no further precipitate 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 car- 

 bon 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 I cc. of the standard sodium hydroxid 

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



(b) Prepare 5 liters of a normal potassium nitrate solution, assuming 

 the salt to be pure. 



If 100 grams 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 continuing 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 gm. of soil. It has been found by working with a num- 

 ber of different soils that as an average the sum of such a series is 2^2 

 times the first titration. 



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

 equivalent to 4 mg. of calcium carbonate and 125 cc. (which represents 

 50 gm. of soil) are titrated, each o.i cc. required to neutralize corresponds 

 to i mg. of calcium carbonate required by the 100 gm. of soil, or to o.ooi 

 % of calcium carbonate required by the soil tested. 



*To be done by the instructor. 



