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





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 

 rube with that which has not been treated before treating the second. If the solu- 

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

 trated with KIISO, until pink color disappears. The burette, presumably graduated 

 to tenths of a cubic centimeter, should be estimated to the nearest hundredth just 

 1 tefore 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 con- 

 tinued until the slightest reddish or orange tinge appears. This may besl be discerned 

 against a white background and not in direct sunlight, which sometimes Ltself 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 (NaHCM \ 



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 firsl reaction changed the 

 carbonate to bicarbonate, as follows: Xa 2 C0 3 4-KHS0 4 =K\aS< > 4 -f-NaHCO s . 



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

 bicarbonate present to carbonic acid, as shown by t he reaction: 



NaHC0 3 +KHS0 4 =KNaS0 4 +HC 2 < > 3 . 



For each 0.01 cubic centimeter of KHS0 4 used in the firsl titration, there was present 

 0.00246 milligram of C0 3 in the 25-cubic centimeter solution treated, or 0.0196S milli- 

 gram 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 HC0 3 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 intervals of 5 minutes 

 for 3 hours. Let stand overnight. Draw off 125 cubic centimeters of the supernatant 

 liquid, which in this case is usually quite clear; boil 10 minutes to expel carbon diox- 

 ide: cool, and add a drop of phenolphthalein indicator. Place the beaker under the 

 burette, against a white background, and titrate with XaOH to the appearance of the 

 faintest pink color. 



The amount of the titration being determined by readings of the burette, the amount 

 of acid present in the solution is expressed by the amount of lime which would be 

 required to neutralize it. Each 0.01 cubic centimeter of the sodium hydroxide used 

 in titration is equivalent to 0.04 mg. of calcium carbonate in the 125-cubic centimeter 

 solution used, and while this stands for one-half of the 100 -rams of the soil treated, it 

 really stands for only two-fifths of the acid in that soil, because the first solution does 

 not completely dissolve the acids. Therefore, each 0.01 cubic centimeter of titration 

 indicates 0.1 mg. of lime necessary to neutralize the 100 grams of soil, or the amount 

 required to neutralize is 0.0001 per cent of soil weight. 



In practice, the amount of lime required to neutralize the firsl foot ofsoil is expressed 

 in tons per acre, being computed, of course, on a standard or specific weighl ofsoil per 

 acre-foot. 



Space is provided on "Summary of Physical and Chemical Properties of Soil " form 

 for tabulating the computed results of alkalinity and acidity teste in terms of pi 

 centages of the weight ofsoil, which are as serviceable for scientific comparisons as anv 



other expression. 



