ANALYTICA L ED1 TION
144
This type of furnace is so eminently suited to experimental work and the furnace are so made that lack Of the correct size of coil should not be allowed to prevent or delay the use of this valuable new aid to Scientific investigation.
VOl. 1, KO.3 Literature Cited
(1)Northrup, Trans. A m . Eleclrochem. SOL.,311, 69 (1919); Gen. Elec. Rev., 656 (1922), (2) Schuette and Maier, Trans. A m . Electrochem. SOL.,54, 155 (1928).
Determination of Total Replaceable Bases in Soils' R. H. Bray and F. M. Willhite DEPARTMENT OF AGRONOMY, UNIVERSITY OF ILLINOIS, URBANA, ILL.
HEN a soil sample is leached with a neutral normal ammonium acetate solution, the bases present in a replaceable form are replaced by the ammonium ion and are found in the filtrate. I n using this method, it was noticed that when this filtrate was taken to dryness and heated, carbonates of these bases were formed, as a result of heating the acetates. This suggested the idea that perhaps the total amount of replaceable bases could be determined by titrating the carbonates or oxides thus formed. Ignition and titration of different acetates confirmed this idea and showed in the procedure the possibility of a quantitative determination. Known amounts of sodium, potassium, calcium, magnesium, barium, and manganese acetates were ignited and titrated. All the titration results were within 1 per cent of the theoretical values. Iron and aluminum acetates gave no titration value. Procedure
A 10-gram sample of 20-mesh air-dried soil is leached with 500 cc. of neutral normal ammonium acetate solution. The filtrate is placed in a 600-cc. Pyrex beaker and evaporated to dryness on the steam bath. The residue is heated gently on a silica plate over a Bunsen burner for a few minutes, then a t full heat for 20 minutes. After cooling, an excess of standard acid is added. The solution is warmed and the bottom of the beaker is rubbed with a rubber policeman. The back titration is made with standard base using methyl red as an indicator. I n testing the method from 3 to 40 cc. of acid were neutralized by different samples and duplicates usually checked within 0.2 cc. The blank from the evaporation of 500 cc. of ammonium acetate was 0.4 cc. Results
A number of soils were analyzed for replaceable potassium, sodium, calcium, and magnesium by the usual methods. Table I gives the results compared with those obtained by the titration method. All results are given in milligram equivalents of replaceable base per 100 grams of soil. The blanks for the different determinations were approximately equal. The principaI replaceable bases found in mature soils of a humid region are calcium and magnesium with small amounts of potassium and sodium. All the soils listed in the table belong in this category and include samples from the various horizons and subhorixons. The sum of the four principal bases, as determined by the regular methods, is here compared with the total value as found by titration. Some samples show a wide variation, but in general the agreement is good. 1 Received February 28, 1929. Published with the approval of the Director of the Illinois Agricultural Experiment Station.
Table I-Total Replaceable Bases in Soils Determined b y Titration Compared w i t h t h e S u m of Individual Replaceable Bases Determined by Separate Analyses TOTAL REPLACEABLE BASES SOIL CAL- MAG- POTASDeterBy DIPFERNUMBERCIUM NESIUM SIUM SODIUM mined titration ENCE
Mg.
equzv.
13051 13052 13053 13054 13055 13056 13057 13080 13081 13082 13083 13084 13085 13086 11159 11215 11229 56102 56130 S6138 S6122 56123 S6124 S6125 56126 13063 13063 Bianks
Mg.
equzv.
Mg.
equzv.
Mp: equzv.
0.34 0.35 0: 39
0.33 0.24 0.19 0.17 0.18 0.14 0.13 0.14 0.25 0.44
.. .. .. ..
.. ..
.... ..
..
..
Mg.
Mg.
Mg.
equzv.
equzv.
equzv.
14.59 10.56 8.03 9.98 16.91 14.62 12.30 9.46 11.64 14.67 14.95 13.24 13.02 11.06 7.35 1.24 7.24 5.78 9.33 9.05 9.84 9.35 8.46 8.44 8.80
14.56 10.52 9.48 10.71 18.11 14.84 12.39 9.35 11.55 14.63 14.63 13.04 13.75 11.31 8.62 1.78 7.19 5.94 8.97 9.02 9.91 9,08 8.79 8.45 8.74 27.17 27.46 0.28
-0.03 -0.04 t-1.45 +0.73 +l.ZO
0.26
$0.22 +0.09
-0.11 -0.09 -0.04 -0.32 -0.20
$0.73 $0.25 +1.27 +O. 54 -0.05 +O. 16 -0.36 -0.03 $0.07 -0.27 $0.33 $0.01 -0.06
Value in Limestone Experiments
The soil numbers preceded by S represent soil from a limestone experiment in which an acid soil very low in replaceable bases was treated with calcium carbonate. Here the differences between the quantitative determinations and the titration values are small. A great deal of time can be saved by using this titration method on experiments of this kind, obtaining the value for the increase in replaceable calcium by subtracting the titration value of the untreated soil from that of the treated. The method is suitable, since the limestone additions cause no appreciable increase in replaceable bases other than in calcium. Use in Base-Exchange Studies
Another use for this method has been found in base-exchange studies in this laboratory. A soil leached with a sodium or potassium acetate solution retains in a replaceable form the base of the wash solution. The amount of base retained as replaceable base can easily be determined by leaching with ammonium acetate and proceeding as above, thus avoiding time-consuming sodium or potassium determinations. Sample 13,063 shows results obtained when the soil was thus saturated with sodium and potassium. Where total replaceable bases only are wanted, as may be the case in soil type horizon studies, acidity studies, and in determining the degree of saturation of the soil with bases, the titration method gives quick and accurate results.
