;2SdLYTICdLE D I T I O S
338
Vol. 2. s o . 3
Use of Barium Sulfate for Clarifying Soil Suspensions with Particular Reference to Colorimetric pH Determinations' L. D. Baver and C. J. Rehling ALABAMA A G R I C U L T U R A L
M
EXPERIMENT STATION,
AUBURN, &.A.
ANY methods for measuring the hydrogen-ion
Experimental
concentration of soils have been suggested. There appears, however, to be no universal method that is satisfactorily applicable to all soils. Hydrogen electrode determinations are usually inaccurate in the presence of nitrates or ferric iron. The quinhydrone method is unsatisfactory with soils containing oxidizing or reducing agents. Colorimetric methods are generally limited by the turbidity of the soil suspensions. The recent use of the collodion bag method for obtaining clear soil extracts has been a decided improvement in colorimetric pH determinations. This, however, requires the making of collodion bags and a long period of time before readings can be made. Equilibrium is often not obtained until after several days. This is especially true in the titration of clay soils with a base in determining their lime requirement.
Kineteen soils representing various soil types, textures. and reactions mere used in this study. Barium sulfate (x-ray purity) in different amounts was added to 25 cc. of a 1:s suspension of these soils in order to determine the quantity necessary to produce flocculation. The BaS04soil suspension mixtures were thoroughly shaken and centrifuged 5 minutes in order to hasten flocculation. The suspensions will clarify without centrifuging if allowed to stand about an hour. The pH values of the clear supernatant liquid were determined colorimetrically. These values were compared with those obtained with the hydrogen electrode and with the collodion bag method. The results of this study are given in Table I. It is evident that a yery satisfactory agreement exists between the three methods. These data clearly show that barium sulfate
Table I hIg.BaSO~P E R 25 SOIL
202 210 319 384 423 425 673-1 673-2 673-3 674 675 677 678 680 776-A 671-1 . . _ 671-2 671-3 671-4 ~
SOILTYPE
Putnam silt loam Hagerstown loam Black clay loam (Aledo) Houston clay Carrington silt loam . Wisconsin sandy peat Delta lizht silt loam Delta light silt loam b Delta light silt loamc Cecil clay loam Norfolk sandy loam Susquehanna clayd Cecil clay Greenville fine sandy loam Norfolk fine sandy loam Corv silt loam Cor, silt loam b Cory silt loam b Cory silt loam h
CC.
1:5 S u s P E N s I o X
PH ~
LOCATION
MO.
Ala. 111. Ala. Wis. \Vis. Miss. Miss. Miss. Ala. Ala. Ala. .41a. Ala. Ala. Ill. Ill. Ill. Ill.
lO0Q
200a
4005
600"
800"
. . . ...... . . . .
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+ +++ ++ + Clear +++ Clear +++ +
+++ + Clear
Clear
Clear Clear
Clear Clear Clear Clear Clear Clear
+ ++ +++++ +++
......
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+. . . . . . + -+++ ++ +++ $+++ +. . . . . . ...... .....
+.".". '. . .
......
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Clear
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. . . . . . . .
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$+
+
Clear Clear Clear
..... ..... ,
..
Clear
++ ++ -I++ Clear Clear
....
.,.. ....
+
+ Clear
++ + Clear Clear
+
Clear Clear Clear Clear Clear ciiar ,, ,,
..
1000 Clear Clear Clear Clear Clear Clear Clear Clear Clear Clear Clear Clear Clear Clear ciiar Clear Clear Clear
H Bas04 electrode
Collodion bag
5.3 5.6 4.9 7.7 5.6 4.6 6.7 7.3 4.8 5 . 69
5.5 5.7 5.1 7.6 5.6 4.7 6.6 7.3 4.8 6.6 5 0
5.7 5.7 5 0 7.7 5.6 4.5 6.7 4:9 6.7 5 1
4.7 5.3 5.6 4.8 4.6 5.3 6.0 6.4
4.9 5.4 5.6 4.9 4.6 5.3 6.0 6.5
4.7 5.6 5.8 4.9 4.5 5.4 6.1 6.6
+
a Denotes amount of turbidity. b Titrated with 0 1 N Ba(0H)z. e
Acidified with H6Oa.
d Contains a large amount of sand.
Kuhn (1) reported that soil suspensions can be clarified with barium sulfate, provided this compound contains no soluble barium salts. Such a compound is sold under the name of "x-ray purity BaS04." As colloidal barium sulfate carries a positive, and colloidal clay a negative charge, the two mill mutually flocculate when shaken together. Because of the extreme insolubility of barium sulfate no cations or anions are added to the soil suspension. As soil acidity depends upon the amount of exchangeable hydrogen present, the acidity of the suspension should not be changed by the barium sulfate because further exchange should not occur. Kuhn does not present data showing the amount of barium sulfate that is required to flocculate different soils. Moreover, he does not compare his results with those obtained with the hydrogen electrode. It is the purpose of this paper to report the results of a study of his method for clarifying soil suspensions in colorimetric pH determinations. 1
Received April 28, 1930.
can be used for clarifying soil suspensions without materially affecting their pH values. The amount of barium sulfate required to produce flocculation depends upon the texture and the degree of dispersion of the soil. All suspensions of sandy soils were clarified with about 0.5 gram per 25 cc. of a 1:5 suspension. Clays and silt loams required about 1 gram. This amount will vary with the reaction of the soil. Clay soils that are very acid may require only 0.5 gram, while those that are alkaline may require more than 1 gram. This method was not satisfactory in determining the reaction of the highly colloidal Putnam silt loam (Soil 202) after it had been peptized with barium hydroxide. This soil contains an unusual amount of colloidal material, however, and should therefore be taken only as an exception. With most soils this method should prove very useful because of the rapidity with which determinations can be made. Literature Cited (1) Kuhn, Z . Ppansenernahr. Dungung,
A M , 13 (1929).
