ADSORPTIOS BY HGRIIC ACID BY K. KAWANI’RA
Introduction Before defining humic acid, we must first state what humus is. The term “humus” seems to be generally employed t o designate the whole of the black substance which is regarded as an intermediate decomposition product of organic matter in soil. T-an Bemmelenl and later Baumann and Gully2 showed humus to be a colloid complex;. It mzy, however, be simply regarded as a mixture of numerous organic compounds as was thought by old chemists. Many attempts were made to fractionate humus into constituents of definite composition. Among such attempts, n e should cite Schreiner and Shorey‘s investigation3 as a splendid piece of work.4 They succeeded in isolating over thirty organic compounds from humus. However, the yields of these compounds were respectively small as ccmpared with the total amount of humus submitted for the separation. And furthermore, no black colloidal substance was isolated. Accordingly such compounds as found by Schreiner and Shorey are sometimes regarded as impurities of humus.j The characteristic properties of humus are due essentizlly to the groups of humic acid and humin. The term “humic acid” was at first given to the substance which comes donn as a bronn precipitate from alkaline extract of humus on acidifying its6 Later it \\as found by Hopye-Seyler‘ that the precipitate contained 2n impurity nainetl “hj-matomelanic acid” which differs from humic acid in its solubility in alcohol. This was confirmed by Od6n.’ Hence, humic acid is at present defined as one of the humus constituents which is soluble in alkali, and insoluble in water, acid and alcohol. Existence of very much hymatomelanic acicl in natural humus can not be expected. for it is a prcduct derived from humic acid in the course of the fractionationb, “Humin” is designetetl as another portion of humus nhich differs from humic acid in that it is insoluble or difficultly soluble in alkali, h being regarded as derived from humic acid.g w Thus the distinction of humic acid from humin is macle merely by solubility in alkali. Honever, humic acid not only dissolves really in alkali, but also i q apt t o he reptized as in the case nith humin. And hence, we are much
-
J. 11.van Bemmelen: ”Die Absorption” (1910). 2 A. Baumanri. 1Iitt bn?r. 1\1oorkulturaris(alt, 3 , j3 (I9C9 I ; 4 , 31 (1910). Schreiner and Shore\ L-. S.Dept. -4gr. B m . of Soils. Bull. 53 ( 1 9 ~ 9 ) 74 ; (1910). See also Ljon. Fippin and Buckman: ”Soils, Their Properties and Management,” 131-138 (1920). a Ehrenherg und Bahr, J. Land~irtachaft 61. 428 (1913). 6 See J. Rusqell: “Soil Conditions and Plant Growth,” 139 (1921). Od6n: Ber. 35, 6 j 1 (1912); also “Die Huminsauren,” (1919), * OdPn: “Die Huminsauren“ 108 (1919) See Bottomlcy. Biochem. J., 9, 260 (1915).
ADSORPTION BY H U X I C ACID
,
1365
cmfused in deciding whether the colloidal particles come from humin or from humic acid, Accordingly, the so-called humin may be regarded as an irreversible or difficultly reversible gel of humic acid. Xnalogous relation seems to hold in the case of thc discrimination between humic acid and h j inatornelanic acid by treatment with alcohol, because humic acid is also apt to be peptized by alcohol in the presence of a suitable aniount of an electrolyte. For the purpose of preparation of “pure humic acid,” it is cbsolutely necessary t9 treat the precipitate obtained from alkaline extract with hot alcohol in order to make it free from hyniatomelanic acid and from the impurities found by Schreiner and Shore!,. This trea‘ment, hon-ever, seems to cauqe certain changes in the physical or chemical nature of the humic acid, for it iq converted into the substance which is difficultly soluble in alkali resembling humin or O d h ‘ s “difficultly soluble modification”.’ Becklev’ assumed that the alcohd-insoluble humic acid waq still a mixture of pyridinesoluble and -insoluble parts. But he did not give any information as t o whether the whole alcohol-soluble substance submitted to treatment n-ith the weak alkaline pyridine, was entirely soluble in other alkalies. Thus the preparation of “pure humic acid“ is an extremely difficult task. I s huni7c acid n reo1 ncld?-There have been two oppo4te opinions respecting the acidity of hunius in connection TT ith the humic wit1 problem. Old chemists, e. g. Pprengel, Berzclins, ?\luld~:.settled this problem very simply hy stating that humus contained an acid or acids. Since van Remmelen denionstrated that hunius is a colloid, a theory of non-existence of such acid or zcitls. seemed to prevail for a while. C‘enieronJwrites in his book that “the esi.tence of humic acid is purely hypothetical and xithout experiments1 or scientific verification.” Bauniann and Gully put forth a colloidal theory of the sourness of humus, indicating liy their extensive works a colloidal nature for huiiius. Thiq as, hon-ever, opposed by T a c k and Schuchting4 who advocated the existence of real acid in humus. Soon after their disputes, there apiieared O d h ’ s work. He succeeded in preparing practically pure humic acid and found that the conductivity of an ammoniacal solution of the humic acid v a s greater than that of ammonia nater. On this basis. he aswnied that the humic acid combined n-ith the ammonia, forming a salt whoqe degree of dissociation n-as higher than that of the ammonia water. Ehrenherg and Rahrj studied the discociation-isotherm of the substance resulting from combination of humic acid and ammonia, at different pressures, comparing the results d h those obtained with stearate and arachinate of ammmia; and concluded that humic acid is a real acid. However they, failed t o give exact information as to whether the specimen used was free from h j matomelanic acid. 0di.n: ”Die Huminaauien.” 90 (1919). Beckleg. J. -4lgr. Sei 2 , 66 (1921). Cameron: ‘.The Soil Solution,” j j (1911). Tacke and Schuchting. Landiv-. Jahrbucher, 41, ;17 (1911). Ehrenberg and Bahr. J. Landwirtschaft, 61,42; (1913).
.
a
1366
K. KAWAMURA
Artificial h u m i c acid. As previously stated, the separation and the purification of humic acid from natural humus is not an easy task. It is especially difficult in case a comparatively large amount is required, and, it is therefore preferable to make it synthetically in such case. Studies of artificial humic acid have been made from early times by many investigators. Braconnot and Malaguti prepared a substance resembling humus from cane sugar by decomposing it with hydrochloric acid, and detailed studies were followed by Berzelius, Mulder, and others, Nevertheless, their exhaustive works were not conclusive in identifying the artificial substance with that occurring in nature, although there was no criticism as to their resemblance in appearance, solubility and behavior toward alkali. However, positive verifications have been made by recent investigators in this respect. Bottomley found that, when purified with alcohol, both humic acidsprepared from cane sugar and HC1, and separated from peat-yielded almost the same percentages of carbon, hydrogen and oxygen on analysis. He came to the conclusion that the reason the results of former investigators(Robertson, Irving and Dobson, 190;) indicated considerable difference between the compositions of the artificial and the natural humic acids, was due t o the fact that the samples were not purified by alcohol before the analysis. h3aillardl found that when a mixture of sugar (e.g. glucose), amino acid (e.g. glycin), and water was warmed, it was converted into humus which was soluble in alkali, He accordingly put forth the theory that natural humus was derived from carbohydrates by the action of amino acids during the decomposition of decayed plants. Beckley2 studied along this line and showed, going a step foward, that the formation of humus from carbohydrate and amino acid, proceeds in the following two stages:-
amino acid
Carbohydrate
I
H-C--C-H
! I
HOHZ-C-C
C-CHO
(hydroxyme thylf urf ural) Hyroxymethylfurfural -+ Humus
2
+ Furfural + COZ
Expressed in m-ords, the most part of natural humus is derived from the polymerization of hydrosymethylfurfural which is formed from carbohydrate by the action (probably catalytic) of amino acid. And he explains that ketohexose and keto-hexosans yield a greater quantity of humus than do other carbohydrates3 because their structures resemble that of hydroxymethylfurfural. Futhermore, Beckley detected the intermediate substance , hydroxymethylfurfural, when cane sugar was converted into humus-like sub1
Maillard: Compt. rend., 154, 66; 155, 1554 (1912); 156, 1159 (1913). Sci., 11, 69 (1921). This fact was also noted by Bottomley.
* Beckley: J. Agr. 3
.4DSORPTIOK BY HUMIC ACID
I367
stance by the action of HCI, and concluded that the black substance thus obtained, was the same in nature as that prepared from sugar and amino acid. As far as Maillard’s theory holds, Beckley’s results are enough to afford great support for the assumption that the artificial humic acid from cane sugar and HCl is the same substance as the natural humic acid. Thus a11 the results obtained by recent investigators confirm that the artificial humic acid is identical with the natural one, and as a consequence, they show us that it is best to investigate the artificial humic acid prepared from cane sugar for the purpose of obtaining some imformation about the nature of soil humic acid. There are other m e t h d s for preparation of artificial humup substance. Bottomley found that “lactic, acetic, propionic, butyric, citric, tartaric. and oxalic acids when boiled with either cane sugar, dextrose or fructose yield humic acid and humin.” But the yields thus obtained seem to be comparatively small, Eller and Koch’ prepared humic acid from hydroquinone, quinone, pyrocatechol, and phenol by respective oxidation with potassium persulphate or air; and verified the fact that its composition, solubility. etc., are almost the same as those of natural humic acid. Furthermore, he assumes that quinone may be produced as an intermediate substance when hexose converts to humus substance by oxidation and dehydration. Purpose o j the present incestigatzon.-.ls briefly noted before, the humic acid problem still leaves many questions to be further investigated in the fields of both organic and colloidal chemistry. However, so far as the writer knows, nobody has ever attempted t o determine how pure humic acid behaves toward bases and acids-whether the reaction is real13 chemical or entirely physical. I t is needless t o say here much about the great significance of such study for soil chemiFtry. Hence the writer has undertaken to investigate the adsorption-isotherm of pure humic acid prepared from cane sugar. The detailed work and results will be given in the follon-ing.
Experimental
I. Preparatio,? o j Hzc?nic Acid. The details of preparation of humic acid were as follows: 2 0 0 grams of pure cane sugar were placed in a large evaporating dish, and 2 0 cc of 10’7,HCl were poured into it. The mixture was then n-armed on a water bath for over three hours, stirring it occasionally by means of a glass rod. The mixture changed gradually t o yellow.. then to brown, and finally a, black substance was formed, emitting a strong otler of furfural. Thus a mass of crude humus was obtained. X considerable quantity of hot water waq then added to the black mass and the lumps were broken up by means of a porcelain pestle. was then filtered off. The black substance held hehind on the filter paper, was washed with hot water until the wash water showed almost no coloration and almost no trace of the chlorine ion. Thus it was made free from Mulder’s so-called “apocrenic acid.” I
Eller and Koch: Ber. 53, 1469 (1920).
1368
K. KAWAMMURA
The separation and the purification of humic acid from the substance thus obtained, was carried out according t o the methods employed by Oddnl and by Beckley.2 However, some modifications were introduced so as t o make the methods applicable for preparation of the specimens on a comparative11 large scale, That is, the crude humus was then treated with warm (6ooC) solution of XaOH (57) overnight, and then filtered through clean white cotton cloth. The black filtrate obtained in this way, was then transferred tr, a IO-liter bottle and S a c 1 was dissolved in it until the total concentration reached about t v o normal. This was allo~vedt o stand for four days in order t o precipitate most of the solid particles in the solution.
A half portion of the supernatant liquid was then transferred to another bottle by means of a siphon, and was diluted with water until its color was brown but transparent when obFerved in a 4 mm glass tube. Afterwards, a considerable quantity of S a C l was again added so as to make the concentration of KaC1 nearly thc same as before. The solution was again alloFved t o stand for four days. Then a half portion of the alkaline solution of humus was siphoned, and the liquid obtained was filtered through hard fiber paper on a Buchner funnel. I n this way, a practically true solution of humic acid as obtained. However when, examined under the ultramicroscope, it still showed more or less colloidal particles. The solution was then acidified with humic acid-was HC1, and a voluminous dark brown precipitate-crude obtained. After the precipitate had settled t o some extent, the supernatant liquid was siphoned off, and the suspended voluininous precipitate was transferred to a flask and heated to Iioiling: because, otherwise the precipitate was apt t o be peptized again after the most of electrolytes contained were wzshed away in the process given below. The hot ciispended matter was then filtered through hard filter paper by means of a large Buchner funnel connected t o a suction pump, The residue held behind on the funnel was then washed, at first with acidulated hot water and later, with a considerahle amount of pure hot water without stopping the pump. The washing was continued for 2-3 days after the rwsh mater showed no reaction for chlorine ion and no coloration. The residue was then placed in a round-bottomed flask and boiled with 9 5 5 alcohol under a reflux condenser 4-5 hours. Then the alcohol was filtered off by means of a Ruchner funnel. Such treatment was repeated 7-8 times with more alcohol until the wash alcohol showed very faint coloration. The humic acid was then placed in a flask and was shaken up with ether, the latter being removed several times. After it was dried in an oxen a t 60-jo°C t o free it from most 3f the solvent used. The humic acid was &en washed thoroughly with hot water. I t was dried in the ovenagainfor 1-2 days and was then pulverOd6n: Ber. 35, 651 (1912); “Die Huminsknren” (1919). Beckley: J. -1lgr. Sei., 2 , 66 (1921).
L4DSORPTIOh- BY HUMIC A C I D
I369
ized by nieansof porcelainniortar and 1:estle. The powder which passed through 160 mesh sieve was allon-ecl to dry in the air for 2 4 hours, and thus the sample used for the present investigation wa,s obtained.
11. A c t i o n of Bases on Stearic A c i d . Humic acid is still a substance of uncertain composition. purity. and properties. It is assumed! however. to be an insoluble acid which forms practically insoluble salts with certain bases. Before studying t'he property of humic acid, therefore, it seemed desirable to find out what, a substance will do Tvhich is really an insoluble acid, in order tq have some basis for determining whether humic acid behaves normally or not. Stearic acid seems to satisfy this request. The action of BatOH)? and KaOH on stearic acid was therefore studied. To prepare the specimen to be ins-est'igatecl, commercial stearic acid wis taken up and recr>-stallizetl from alcoholic solution four times in swcession. The scale-like crystals (m.p.69'C') thus obtained were pulverized iJy means of a porcelain mortar and pestle: and the part xhjch passsed through an 80mesh sieve was used for the folloning experiment,: Procedure:--One grzm of stearic acid was placed in a 2 0 0 cc bottle, and a IOO cc portion of Ba(OH)?Folution of varying normalities wat: atldetl to it. Since t,he reaction \-elocit\- between stearic acid and Ba(OH)? vas very slow ,at room temperature, the treatment n-as at first carried out at,h i p temperature hut just below the melting point of stearic acid. That is ,the ;bottle was immersed in a water bath of 65-6 jot‘ without a stoprer for the fi:.st fen- minutes and t,hen a rubber stopper n-as inserted. The treatment TYER continued for eight hours shaking vigorously hy hand 3-4 times an hour. The white film of stearic acid which had coyered the inside wall of the empty part as well as the cir-liquid surfe.ce of the hottle, disappeared when the Ba(0H)Z in the bottle was totally taken up by t'he stecric acid. Such a phenomenon, hon-el-er, never occurred when free Ba(OH)? remained in the bottle. FrQm this we could find approsirnately the aniount of Ba(OH)? equivalent to the stearic acid added. The bottle was then taken out of the hath. 2nd Tvas cooled t o room temperature ( 2 2 O C 1 ) . I t was ol:ened once and quickly stoppered again: anti was then allon-ed to stand a.t rcom temperature for 48 hours, wcasionally shaking by hand. After the reaction ree,ched equilibrium, the contents Tvere filtered through three sheets of piled, dry filter papers about 2-3 times ir succession. Thus a perfectly transparent filtrate n-as obtained. However, ivhen the filtrate was neutra'l, it was rat'her difficult t,o make the liquid entirely free from ivhite particles. Then a I O cc portion of the filt,rate was pi:ietted into en Erlenmeyer flask and was titrated with 0.1or 0.01 normal HC1, using phenolphthalein as an indicator. From the number securec! in the titration, the concentration as well as the amount of the base taken rLpby the humic acid was calcurated. The results are summarized in Table I.
I370
K. KAWAMCRA
Kext, the examination of the NaOH adsorption was carried out. The procedure of the treatment was almost the same as used in the previous case but that the preliminary treatment was made a t 62-63'c for 5.5 hours. However, at the final titration, some modification was found necessary in the this case, because, first, the sodium stearate formed, swelled up so that only a small quantity of the filtrate was obtainable ; (such swelling was especially remarkable when the initial concentration of KaOH solution mas equivalent t o the amount of stearic acid brought into contact, or a little less than the
TABLE I Xdsorpt'ion of Ba(OH)?by Stearic Acid (One gram stearic acid and I O O cc Ba(OH), solution) so.
Concentrat,ion of Ba(OH)*in Equivalent millimols of Ba(0H)n equivalent millimols per IOOCC. takenup by one gram of stearic acid Initial conFinal concentracentration tion a t equilibrium
I.
0.09.;
0.00
0.095
2.
0.503
0.00
0.jog
3.
0.96 2.96 3.62
0.00
0.96 2.96
5.0.7
I .46
6.70 8.30
3 . IO
4. 3'
6. 7. 8. 9. IO.
10.20 1.5.20
0.00
0.1:
4.70
6.45 11.30
3.47 3.59 3.60 3.60 3.75
3.95
equivalent) ; and the second, the sodium stearate formed dissolved partly in the solution, givirg an alkaline reaction, so that we could not simply determine the XaOH concentration by such titration. To overcome sich difficulties 2 0 grams af XnCl were added t3 the bottle before the filtration of its contents, thus making the sn-elled stearate coagulate thoroughly ant the dissolved stearate salt out entirely. Then the mixture was filtered out through dry filter paper as before. I O cc of the filtrate were then pipetted out and titrated with standard HC1 using phenolphthalein as indicator. Since the addition of S a C l causes volume change of the solution, I O cc of the filtrate do not correspond t o I O cc of the initial solution. I n order to determine this relation, I O cc of all stock XaOH solutions used were titrated before and after addition of 2 0 grams of S a c 1 per IOO cc.; and the concentration change caused by the salt addition was found in this way. Then the number secured in the titration of the filtrate was checked up wjth that found when the filtrate did not have FaC1. Table 11 gives the simplified result thus obtained showing the relation between the concentration and the amount of NaOH taken by up one gram of stearic acid.
1371
ADSORPTION BY Ht-RlIC B C I D
Result and discussion.-The results given in Tables I and I1 are shown graphically in Fig. I . ,As shown by the tables and the chart, it is obvious in every case that the whole of the Ba(OH)? or XaOH is taken u p when the initial concentration of the base is below 3 . j equivalent millimols per roo cc. Beyond this point the amount taken may be regarded as approximately constant. X o w applying the phase rule t o the following three component system :
+
+
2 CliH3j COOH Ba(OH)?= (C1;HXS COO)?Ra 2 H2O n e will find two solid phases and one liquid phase so long as there is any free
TABLEI1 Adsorption of S a O H by Stearic zicid so.
Concentration of S a O H in milliniols per I O O c c . Initial conFinal Concencentration tration at equilihrium
1Iillimols of S a O H taken up by one gram of stearic acid
I
0 . 0 9 1j
0.00
0.091;
2
0,443
0.00
0.443 0.96;
0.00
0.00
2.86;
3 50
0.00
3 50
j.00
I
.4I
6.55
2.81
8.00
4.30
3.59 3.74 3 . ji
IO.00
6 .j o
3 . 50
IO
j o .oo 100.00
'$5 83 95.70
4.17
I1
'
4.30
stearic acid left. I n such case the system has two degrees of freedom-temperature and pressure. Therefore the concentration of Ba(C,H)z should be constant as long as the salt formation is not completed. On the other hand, when the stearic acid is converted completely into stearate, there d l be one solid phase and one liquid phase at equilibrium, and in turn the systerr will have three degrees of freedom. If so, the concentration of Ba(OH)? can be altered without altering the composition of the solid phase. In other m r d s , the amount of Ba(OH)?taken up by the stearic acid is to be eonstant at any concentration. Therefore, the adsorp tion-concentration diagram should yield a right-angled curve. I n a similar way, the rule is applicable t o thesysten of stearic acid and KaOH solution. The results secured above indicate that the rule holds fairly well in every case; the vertical portion of the curve, the portion of zero concentra ion in the liquid, represents the formation of a chemical conipouncl, the salt, while the horizontal portion denotes zero adsorption. But strictly speakng,
1372
K. KAWAJIUR.4
we cannot neglect the fact that the amount taken up increases a t a slight rate with increase of concentration, indicating slight adsorption by the stearate formed. The point where the stearic acid is changed completely into stearate, is found in the place where 3.5 equivalent millimok of base is taken up. This
hdsorption--Coneelitration
FIG.I Diagram of Bases by Stearic - k i d .
Adsolption-Concentration
FIG.2 Diagram of Bases by Humic Acid.
agree very well with the point theoretically found; for, the molecular weight of stcaric acid is 284.33, and as a consequence, the equivalent amount of base shouid be 3. j 1 6 equivalent millimols per one gram of the acid.
III. Action of Bases on Humic Aczd. The preceding experiments show how a real insoluble acid behaves t o w r d bases. TYe can nom- determine whether humic acid behaves in the same way that stearic acid does. Frocedure. The experiment was carried out similarly to that employed in tle case of Ptearic acid. The details were as follows:-
I373
ADSORPTION BY HUMIC ACID
One gram of air-dry humic acid (moisture content1 7.05) obtained by the method described before, was treated with IOO cc of Ba(0H)z solution of various concentrations in a 2co cc rubber stoppered bottle. The treatment was carried out at 60-62'c for the first three hours, hhen the bottle was allowed t o stand at room temrerature for 48 hours. shaking occasionally by hand. -After equilihricni was thus esteklished. the mixture was filtered
TABLE I11 =Idsorption of Ba(OH)2 by Humic Acid ConcPiitr:ition of BatOH)?
in equivalent millimols aer I O O cc. SO.
1 1
I
2
3
i ,
4 3
6 7
~
I
8 9 10
I ~
11
I2
I
I3 14
i
I5
I
16 17
I8
i
I
Initial Conc.
Concentration at equilibrium C loe C
1 I
1
I
0 00
0.186
0.00
0.98 I.96 2 . 51 2.78
0.00
1
0.00
I
3.013 3.25
~
1 '
0.00
trace 0.01;
-1,8239
3.76 3.96
0.070
5.04
0.265
4685 2518 -1.1.;49 -0..;~68
6.07
0.623
-0.20;;
8.11
2.02
13.25
3 70 6.40 9 . IO
3.50
16,30 20.40
0.034 o.o.;~
13
.oo
I
-I.
-I.
0.30.54 0..j682 0 . So62 0.9590 1.1139
1
by I pram of dry humic acid
'I,?. I gram of air-dr>-humic acid l
0.095
0.00
Equivalent millimols of BafOH)? taken up
,
1
'
,
x
0.09.; 0.186 0.98 1.96
0.10
2.51
2 . 70
109: s
0 . 2 0
I.0j Z . S I
2.78
2.99
3.01;
3.23
3.24
3.48
0 .j 4 1 6
3 73 3.98
0 . .oiiivnient millimols aer Ion cc. so.
1 '
,
Initial cone.
PH
Concentration 1 a t equilibrium 1 (C)
by I gram of air-dry
~
by I gram of dry humic
I
I .02
8.0
I .02
I . IO
0.0414
2
I . j0
8.2
I . j0
0.209j
3 4
2.02
8.3
2.02
1.62 2.18
2.46 2.64
8.j 8.7 8.7 8.7 8.8 8.8
2.46 2.64
3
6 I
2.73 2.88;
8
2.98
r
3.06 3.16
9 10
I1
,
3.52
trace trace trace 0.04
.
,
'
8.9 9.1
2.73 2.885 2.98
3.06 3.16
,
3.48
2.6j
0.3385 0.4232
2.84 2.94 3.11 3.21
0,4533
3.30 3.40 3.73
0 . j18j
0,4683 0.4928 0.506;
0.531 j 0 . jj 4 0
The 1011-er line leans slightly t c the right side from the vertical position respecting the abscisqa, pH. However, it may be regarded as "practicdly vertical." The upper line may be represented as the following equation: log s
=
R
+ n pH
nhere x is the amount taken and I< and 71 are constants, both of which can be found from the figure. Therefore, we may say that the upper line represents adsorption while the lower line shows chemical conillination. Detailed discussion will be given later. Humic *Acid and S z O H . In thip cme, the treatment of the two substances n-as carried out as usual. The filtrate had a light hen-n color. The intensity of the color n e s measured by means of a colorimeter using an alkaline solution of about 0 . 0 7 ; ~ ; humic acid as a standard color. The rec.ults are given in
1382
K. K.4W.4MUR.4
Table XII. In this table. the term “relative intensity of color” represents that figure which is c3mpared with the most intense color of the filtrates, assuming the latter as 100. As shown in the results, the color intensity increases as the initial concentration of S a O H , or pH a t equilibrium, increases. The colored filtrates were diluted. before the titration, with distilled water until a yellow color of definite intensity vas produced so as to overcome
TABLE XI1 Relative Solubility of Humic Acid in XaOH Solution so.
Initial concentration of SaOH in millimols per I O 0 cc
pH a t equilihrium
8.6 8.6 8.7 8.94 9 .o
I
I .O
2
1.5
3 4
2
5 6 7
8 9
.o
2 .jI
2.6 2.63 2.79 2.9 3.OI
IO I1 I2
I3 I4 15 16
Length of column of unknown compared with 20 mm. stsndard in millimeters 27.0
16.8 15.8 13.6 13 .o 10.6 10.6 10.6 10.6
9.24 9.3 9.3
9.7
-_
3.8 4.06
I O .06 10.8
4.95
11.j
6.04 7.97 9.95
11.9 12.3 12.jq
Relative intensity of color
8.6 7 .O :. . 6
&
4.0
46
57 71 80
4.0
IO0
4.0
IO0
obscurity of color change of the phenolphthalein used as an indicator. The writer knew, by preliminary esperiment xvith the filtrate of the most intense color, that it was necessary for such purpose to dilute I cc to 40 cc. Hence the dilutions of the other filtrates were carried oui according t o the following equation : 40 X 1.T’. Final volunie after dilution = IO0
where I is rel.*tive intensity of color, and l7 is volunie in cc t o be diluted. Then it was titrated as usual. The hydrogen ion concentration Ti-as determined electrometrically with the original filtrate, that is, iyith one which was not diluted. The results are summarized in Teble SI11 and are charted in Fig. 7 . In ‘he case where comparatively strong SeOH was added, the amount of S a O H taken up by humic acid n-as found to be a little lower than that obtained with Ba(OH)?. This seems t o be due t o the fact that in the present work no
I383
ADSORPTIOK BY HUMIC A C I D
S a C l was added before filtration, and futhermore that the sample was finer than the previous specimens so that a greater quantity of humic acid might have been dissolved or peptized than was in the previous case. In the case where ccamperztively dilute SaOH \$-asadtied t o the reaction, the pH value was a little higher than thet obtained nith Ba(OH)?. GenerzJly speaking, however, the results cbtained herein with S a O H , are similar to those secured with Ba(OH)?. The logarithmic curve of adsorptionpH diagram bends at the point where 3.0 millimols cf the base n-ere taken
-
TABLE XI11 Relation between Hydrogen Ion Concentration and Adsorption of KaOH by Humic Acid Millimols of S a O H taken up
Concentration of S a O H I in millimols per IOO cc. Initial Cone. a t cone. equilibrium1
1 ~
so.
(C)
~
.oo
I
I
2
I .j o
3 4
2
I I
~
,
___,
PH
I
I 1
trace do d0 do d0
894
0.04
9i
I1
3 .OI 3.IO 3.38
0.I 1
IO
I2
4.06
0.40
IO
I3
4.95
0.90
I1 j
14 16
9.95
1.6; 3.18 4.90
I1
15
6.04 i.97
3
6 7 8 9 IO
2 . jI
2.60 2.68 2.79 2.90
0.06;
I
gm.
humic acid
j
by I gm. of dry humic acid
I ~
Y
-~
86 8 h 8 7
.oo
by
of air-dry
9 0 9 24 9.3 9 4
12
' I
~
I
9 7
12
~
06 8 9 3 54
I
' I
,
log x
I .00
I .08
0,0334
I .j o
1.62
2.00
2.1;
0.209; 0.3324
2.51
2 .io 2.80 2.89
0.4314 0.4472
2.60 2.68 2.79 2.9
1.OI
0.4609 o 4786
3
12 3.20
0.3942
2.79 3 03
3.26
0.5132
3.52 3.92
0.
3.27
3.64 4.05
4.36
4.37
.+.TI
4.i9
j.16 5.44
j.0j
0.50;'
j46;
0.5933 0.6395
0.6730 0.i126 0,7356
up by the humic acid. Hence the curves Peem t o consist of two straight lines of which the adjecent parts are connected with a gradually bended curve. The loner line runs at a very small angle to the ordinate; and the ui-rer line is undoubtedly reprecented by the equation : log x
=
I
