200 BULLETIN 1059, IT. S. DEPARTMENT OF AGRICULTURE. 



12 hours and allow to settle. The test rnayv best be started early in the day, 

 shaking jars occasionally during the day and leaving them to settle overnight. 

 Turbidity of the solution is difficult to eliminate in this test, but time is saved 

 if complete settling occurs. 



Draw off with pipette 50 cubic centimeters of the supernatent liquid ; filter, if 

 not fairly clear, into an evaporating dish ; and evaporate over Bunsen flame, 

 continuing the drying almost to red heat, so that residue is devoid of humus 

 and will cling together in flakes when the dish is scraped. When dish is cool 

 add 50 cubic centimeters of distilled water ; allow to stand for two hours ; then 

 pour half of solution into each of two 50-cubic centimeter Nesslar tubes. If 

 fusing of residue has been complete, filtering at this stage would be unnecessary, 

 as the flakes of solid matter will remain in evaporating dish. 



Add to each Nesslar tube a drop of phenolphthalein indicator, comparing the 

 first tube with that which has not been treated, before treating the second. If 

 the solution in tube 1 is colored pink, carbonate (Na 2 C0 3 ) is present, and the 

 solution is titrated with KHS0 4 until pink color disappears. The burette, pre- 

 sumably graduated to tenths of a cubic centimeter, should be estimated to the 

 nearest hundredth just before beginning to titrate, and again as soon as the 

 color disappears. The comparison tube may then be similarly treated, reading 

 the burette for the second treatment, as a check on the first. This second tube 

 is carried simply as a color comparator. 



A drop of methyl orange indicator is now added to each of the above tubes, 

 whether or not the titration for carbonate has been made. This will give to 

 each a yellow color, and the second tube will be kept alongside the one which 

 is titrated so that change of color in the latter will be quickly apparent. The 

 titration of the first tube is continued until the slighest reddish or orange tinge 

 appears. This may best be discerned against a white background and not in 

 direct sunlight, which sometimes itself imparts a reddish tinge to the yellowish 

 solution. A final reading of the burette is obtained to compute the quantity 

 of KHS0 4 used in titrating for the bicarbonate (NaHC0 3 ). 



The amount of the second titration, less the amount of the first, gives the 

 amount necessary to neutralize the HC0 3 originally present, since the first 

 reaction changed the carbonate to bicarbonate, as follows : Na 2 Co 3 +KHS0 4 = 

 KNaSo 4 +NaHC0 3 . 



The second titration, or the first if no carbonate was originally present, 

 reduces all bicarbonate present to carbonic acid, as shown by the reaction : 



NaHC0 3 +KHS04=KNaS04+H 2 C0 3 . 



For each 0.01 cubic centimeter of KHS0 4 used in the first titration, there 

 was present 0.00246 milligram of C0 3 in the 25 cubic centimeter solution 

 treated, or 0.01968 milligram in the solution representing 100 grams of soil, 

 or 0.00001968 per cent of the soil weight. 



For each 0.01 cubic centimeter of KHS0 4 in the differential titration, there 

 was present in the original solution 0.0025 mg. of HCOs in the 25 cubic centimeter 

 solution used, or 0.02 mg. in the solution representing 100 grams of soil, or 

 0.00002 per cent of the weight of the soil. 



Acidity Test. 



Place 100 grams of air-dried soil of the sample to be tested in a quart jar; 

 add 250 cubic centimeters of normal KN0 3 solution and stopper ; and shake at in- 

 tervals of 5 minutes for 3 hours. Let stand overnight. Draw off 125 cubic centi- 

 meters of the supernatant liquid, which in this case is usually quite clear ; boil 

 10 minutes to expel carbon dioxide ; cool ; and add a drop of phenolphthalein 



