The study indicates the dominance of Al in controlling the acidity of acid Peninsular Malaysian

soils. The buffering action of the soils is dominated by Al below pH 5.5 and by the clay minerals, mainly

kaolinite, at pH above pH 5.5. The rate of extraction of Al from the soils depends on the concentration

of the extracting solution. Lime requirement of the soils can be obtained by multiplying the base needed

to raise the pH of the soil to 5.5 by a factor 1.3

Jusop, Shamsuddin

Tessens, Eddy

Universiti Putra Malaysia Institutional Repository

Pertanika 6(1),71-76 (1983)Potentiometric Titration of Acid Soils FromPeninsular MalaysiaJ. SHAMSHUDDIN and E. TESSENSSoil Science Department, Faculty of Agriculture,Universiti Pertanian Malaysia.Key words: Potentiometric titration; acid soils; Peninsular Malaysia.RINGKASANKajian ini menunjukkan bahawa Al memainkan peranan yang penting dalam mengawal keasidantanah-tanah asid di Semenanjung Malaysia. Tindakan tampanan tanah-tanah ini dikawal oleh Al apabilapH kurang dari 5.5 dan oleh mineral-mineral lempung apabila pH lebih dari 5.5. Kadar pengekstrakan Aldari tanah bergantung kepada kepekatan larutan penekstrak. Keperluan kapur tanah-tanah boleh didapatidengan mendarap bes untuk meninggikan pH tanah ke 5.5 oleh faktor 1.3.SUMMARYThe study indicates the dominance of Al in controlling the acidity of acid Peninsular Malaysiansoils. The buffering action of the soils is dominated by Al below pH 5.5 and by the clay minerals, mainlykaolinite, at pH above pH 5.5. The rate of extraction of Al from the soils depends on the concentrationof the extracting solution. Lime requirement of the soils can be obtained by multiplying the base neededto raise the pH of the soil to 5.5 by a factor 1.3.INTRODUCTIONAn important component of humid tropicalsoils is AI, which is either present in the solutionor chemically adsorbed on the clay surfaces. Thebehaviour of this Al can be studied by potentio-metric titration. This study attempts to explainthe behaviour of soils on adding bases and toestimate lime requirement of these soils.MATERIALS AND METHODSSoils for this study were sampled from varioussites throughou t Peninsular Malaysia. The soils areNangka, Bt. Tuku, Kerayong, Lintang, Sg. Buloh,Napai, Awang and Holyrood Series. They arealluvial soils of Holocene age, which have beendescribed by Paramananthan (1981) as the T2terrace soils. The taxonomic classification of thesesoils is given in Table 1. The morphology, physico-chemical characteristics and genesis of these soilswill be pu blished elsewhere.The titration procedure is as follows: I g air-dried sample was equilibrated in 10 ml KCI solutionfor a period of two days, with occasional shaking.The soil was then titrated with 0.05 N NaOH71solution to pH 9.0, using an automatic titratorcoupled to an auto burette and a titrigraph. Thechange in pH with the amount of base added wasrecorded on a paper sheet. Two KCI concentra-tions, viz. I Nand 0.1 N, were used. For eachsoil series, potentiometric titrations were carriedout for both Ap and B horizons.RESULTS AND DISCUSSIONGeneral ConsiderationThe existence of partially hydrolysed Alions on the surface of acid soils and clays wasdemonstrated by potentiometric titration ofextracts with various salt solutions (Cabrera andTalibudeen, 1979).It is well known that the titration curve ofAl depends on Al concentration, ionic strengthand titration speed.An example of a titration curve of Al is givenin Figure 1, where data from the work of Cabreraand Talibudeen (1979) are presented. From thediagram, it is seen that there are three bufferingzones. The first zone marks the neutralization ofJ. SHAMSUDDIN AND E. TESSENSTable 1Taxonomic classification of the studied soils.SERIES SUBGROUP FAMILYNangka Oxic Dystropept Coarse loamy, siliceous, non-acid, isohyperthermicBt Tuku PlinthaquicDystropeptFine loamy, mixed, acid, isohyperthermicKerayong Aquic Dystropept Fine clayey, kaolinitic, non-acid, isohyperthermicLintang Oxic Dystropept Coarse loamy, siliceous, acid, isohyperthermicSg Buloh OrthoxicQuartzipsammentCoated, acid, isohyperthermicNapai Typic Paleudult Coarse loamy over clayey-skeletal, kaolinitic, acid,isohyperthermicAwang Aquic Dystropept Coarse loamy, siliceous, acid, isohyperthermicHolyrood Typic Paleudult Fine loamy, mixed, acid, isohyperthermicexcess H + present. The second zone marks theneutralization of Al, while the third zone indicatesthe formation of aluminate. The arrows in thediagram indicate the point of inflection. It has,however, been found also that aluminium hydro-xide particles formed during titration are positivelycharged up to pH of 10.Titration of exchangeable Al in soil can beschematized as follows:some reaction with Fe is also possible. Althoughit is not likely that K will replace much Fe on theclay surface (Dewan and Rich, 1970), reactionmay be possible with Fe, Fe oxides and/or hydro-xides already present either on the surface or inthe solution. This reaction may be viewed asfollows:3K+ + AI-X --)0 K2-X + A1 3+ exchangeIt is possible that this reaction, together withthat of Al provides a strong buffer at a pH levelThe reaction of the added OH- with the soilchemical component is not limited to Al only, butThe first reaction takes place before thetitration in which K replaces Al into the solution.Al in the solution is partly hydrolysed and H+ isthus released:1.50.5 1.0o4OH- meqFig. 1. Base titration curve of acidified 2.9 X10-3 M Al (N0 3 h in 10-2 M NaN0 3 (afterCabrera and Talibdeen, 1979).1012pHaluminateneutralizationAI3+ + 30H --)0 Al (OHhAI(OHh + OH --)0 Al (OH)~Thus, when OH- is added for neutralization partof the added OH- is being used for the neutraliza-tion of H+ released by the hydrolysis of Al. Athigh pH, gibbsite may be formed. Further additionof OH- will raise the pH, and at the same timealuminate is slowly being formed.72POTENTIOMETRIC TITRATION OF ACID SOILS OF PENINSULAR MALAYSIAabove 7. As the pH increases, H+ from the organicfunctional groups, especially the carboxyl, isreleased. This H+ will be neutralized by the addedOH- as well.Buffering Action ofAlA potentiometric titration was carried out forthe Ap and B horizons of all the soils under study.The resultant titration curves were studied indetail. Practically, the change in pH with OH-follows the same pattern. Only the points ofinflection are different. This implies the existenceof some kind of relationship between the shapeof the curves and the chemical and mineralogicalcomposition of the soils.Two distinct parts where the soils showstrongest buffering are noted in all cases. Theseare around pH 5 and above pH 8. On this basis, thetwo parts are discussed separately. To illustratethe significance of this fact, a titration curve ofthe Holyrood series is plotted, which is given inFigure 2. The two parts referred to in this studyare the part of the curve between the pH of thesoil up to pH 5.5 and the part between pH 5.5 and9.0. The reason why the pH of 5.5 is taken as thedividing point is that if one refers to Figure 1,one will immediately see that the second pointof inflection is at the pH between 5 and 6 and thisprobably marks the end of the significance of Alin the solution. In fact the buffering zone endssomewhere around pH 5.3.Y :: 0.2]2 + O.829Xr = O.9)JVl~~g'"8~•-5Iili0 3A1 3+(meq/lOO 9 soil)2. Neutralization of AP+3. Neutralization of organic functional groups.OH- = 0.232 + 0.829 (AI), r = 0.93**(pH 5.5)The amount of base needed to raise the pH to5.5 is estimated from the buffer curves (Table 2).From this, it is possible to determine the relation-ship between the base and AI. The regressionequation is given by:The plot of the base against Al is given inFigure 3. It is thus confirmed that Al plays animportant role in the buffering of the soil belowpH 5.5. The reaction with Fe is not expected aspKa of Fe (III) is 3.0 and Fe (II) is 9.5.B23tApIepH ,2. I7jFig. 3. The relationship between the base neededto raise the pH to 5.5 and the AI3+ presentin the soils.Buffering Action of ClaysIn part 2, one or more of the followingreactions may have taken place:1. Reactions with broken edges of colloids2. AI (OHh + OW --+ Al (OH)~3. Fe 2 +.H2 0 + OH- --+ Fe+.OH + H2 04. RCOOH + OW --+ RCOO- + H2 0OH-Cmea/IOO. 9 5011)Fig. 2. Potentiometric titration curves of soils ofHolyrood Series.In part 1, the following reactions may havetaken place:1. Neutralization of H+ in the solutionThe four mentioned reactions can providesome buffering action in the pH range of 5.5-8.0.To find out which one of them plays a dominantrole, the amount of base needed to change thepH from 5.5 to 9.0 were estimated from the buffercurves (Table 2). The base needed was thencompared to the amount of clay content, ironoxide, organic matter and Al present in the soil.73J. SHAMSUDDIN AND E. TESSENSTable 2The amount of OH- needed to raise the pH of the soils to 5.5,9.0 andbetween pH 5.5 and 9.0 for samples equilibrated in IN KC1OH- meq/100 g soil AISERIES HORpH 5.5 9.0 5.5-9.0CLAY %meq/100 gNangka (I) ApB22Bt Tuku (3) ApB22Kerayong (4) ApB22Lintang (9) ApB23Sg. Buloh (II) ApAC3Napai (18) AllB23tcnAwang (20) ApB3/BCHolyrood (21) ApB23t0.280.651.582.430.304.030.700.680.750.551.403.350.382.001.152.171.001.273.213.536.628.013.282.501.341.614.3810.591.014.062.355.230.720.621.631.106.323.982.581.820.591.062.987.240.632.061.203.068.512.117.931.553.554.414.923.15.99.312.253.14.225.221.234.60.210.290.361.041.772.290.570.930.529.521.254.890.130.500.881.441.270.120.810.061.740.090.850.191.640.081.680.320.420.031.920.150.430.902.143.600.114.210.580.551.080.711.173.360.111.170.941.50The regression equation for the relationshipwas determined and is given by:OH- (pH 5.5-9.0) = -0.28 + 0.116 (% Clay) - 0.068(% Fe203) + 0.396 (roOC) -0.217(% AI)F4.11 = 9.76** R2 = 0.77,,determines the reaction, the base was comparedto the amount of kaolinite, mica/chlorite, gibbsiteand iron oxide (represents mainly goethite)present in the soil. The regression equation is givenby:OH 0.114 + 0.055 (% Kaol) + 0.015 (% Mica/Chlo)+ 0.449 (% Gibb) + 1.09 (% Fe203)Fl;14 = 35.26**, r=0.85This relationship is illustrated in Figure 4a.It is thus known that clay minerals play adominant role in the adsorption of base abovepH 5.5. In order to know which of the mineralsThe best relationship is obtained for the % ofclay, with r = 0.85**. The r values for the restin order of decreasing importance are OC > Al >Fe203.. The relationship between bases and clay %is given by:OH- (pH 5.5-9.0) -0.016 + 0.099 (% clay)74F4;9 11.23**, R2 = 0.81The highest r is obtained for kaolinite withr = 0.83 **. The relation between the base andthe % of kaolinite (Figure 4b) is given' by theequation:OH- (pH 5.5-9.0) = -0.067 + 0.186 (% Kaol), r = 0.83**The reaction with kaolinite is taking placeat the broken edges. Kaolinite develops a smallamount of positive charges at low pH, around theedges of the crystal (shown below), due to protonacceptance by the aluminol group (Greenland and7POTENTIOMETRIC TITRATION OF ACID SOILS OF PENINSULAR MALAYSIA...7 •6(a) ...6(b) •5.... 4....oIIIIJI8 3H';,..§'0 2o5y-0.OI6 + 0.099x Y = -0.067 + 0.I86Xr = 0.85 lflf r = 0.83 lflf•4•3 •...•...•... •2...•... •...... •... I •...•IO 20 30 40 50 0 IO 20 30clay (glIOO g soil) kaol(g/IOO g soil)Fig. 4. The relationship between the base needed to raise the pH from 5.5 to 9.0 and clay (a) and betweenthe base and kaolinite present in the soils(b).Matt. 1978) Note that below the edge pHo there isno negative charge present on the edges.The pH at which the net charge of the edgesis zero (pHo) is 7.3 (Rand and Melton, 1975).Note that the pHo of the whole kaolinite crystalis much lower, i.e., between 4 and 5 (Dixon,1977).The more kaolinite present in the soil, thegreater the buffering action it will give to thesoil. This probably explains why in Figure 2 thetitration curve of the Ap horizon of HolyroodSeries is steeper than that of the B23t aroundpH 5.5-8.0. More kaolinite is present in theB23t than in the Ap (resp. 21.86% and 12.64%).This phenomenon has also been noticed in othersoils.(acid) (pH. 7.3) (alkaline)75Extraction of Al by KCl solutionIn this study, the samples were equilibrated ineither 1 N KCl or in 0.1 N KCl before they weretitrated. It is found that for 1 N KCl equilibratingagent, the amount of OH- needed to raise thepH to 5.5, 7.0 and 9.0 is slightly higher thanthe values obtained by using 0.1 N KCl. This issimilar to the findings of Dewan and Rich (1970)(Table 3).This difference arises as a result of the dif-ferent rate of extraction of Al by the differentconcentration of KCl solution (Amedee andPeech, 1976).J. SHAMSUDDIN AND E. TESSENSTable 3Comparison between titration curves of soils equilibrated in IN KCI andthose equilibrated in 0.1 N KCl solution.SERIES HORKerayong B22Lintang ApSg. Buloh AC3Awang ApHolyrood B23tIN KCI 0.1 N KCIpH 5.5 7.0 9.0 5.5 7.0 9.0OH- (meq/IOO g soil) (meq/lOO g soil)4.03 5.40 8.01 4.00 5.25 7.450.70 0.93 3.28 0.50 0.80 3.200.55 0.85 1.61 0.50 0.80 1.600.38 0.55 1.01 0.18 0.38 0.902.17 2.75 5.23 2.10 2.73 5.15Table 4Lime requirement (ton/ha) of the studied soils by the methods of Sanchez (1976),(Lathwell, 1979) and the buffer curves.SERIES AIx1.65 Alx2(Sanchez, 1976) (Lathwell, 1979) OHx1.3Nangka 0.71 0.86 0.37Bt Tuku 3.53 4.28 2.05Kerayong 0.18 0.22 0.39Lintang 0.96 1.16 0.91Sg. Buloh 1.78 2.16 0.97Napai 1.93 2.34 1.82Awang 0.18 0.22 0.50Holyrood 1.55 1.88 1.50of CaC0 3 0 50 mg). Therefore, the lime require-ment is:Estimation of Lime RequirementOne important practical application of thetitration curves obtained in this study is theestimation of the lime requirement. Liming is acommon agronomic practice in tropical soils,which are usually limed to the pH of 5.5 on thebasis that the effect of Al toxicity is minimalabove this pH.100 g soil need 1.15 X 50 mg CaC031.15 X 50 X 2.6 X 10'2.6 X 10' g soil need100 X 10·1.5 ton/haton/haThe theoretical calculation of the lime re-quirement is done in the following way. Theweight of 1 ha of soil to a depth of 20 em. withan average density of 1.3 gjcm 3 , is 2.6 X 10' g.Taking the case of Ap horizon of HolyroodSeries, the amount of OH- needed to raise thepH to 5.5 is 1.15 meq/lOO g soil (Figure 2).1 meqof OH- is equivalent to 1 meq of carbonate (1 meq76For simplicity, a factor of 1.3 can be derivedfrom the above calculation. To calculate the limerequirement (tonjha), one can just multiply 1.3with the base. The amount of the base (OH) canbe obtained directly from the titration curves.In this way, a simple method to estimate limerequirement of soil is thus proposed.POTENTIOMETRIC TITRAnON OF ACID SOILS 0 F PENINSULAR MALAYSIASoils with variable charge colloids often havepH values of 5 or less. Under this conditions, Al ispresent in significant amounts and Al toxicity is acommon cause of infertility in this soils. Limingsoils with variable charge colloids leads to anincrease of negative charges, hence the soils canretain more K, Ca and Mg.Theoretically, it is possible to lime the soilto any pH value, depending on the physiologicalrequirement of the crop. One of the reasons forthe choice of pH 5.5 is that at higher pH, theeffect of Al is considered minimal. A comparisonof the lime requirement obtained by this methodwith those of other methods is shown in Table 4.Q t-rest on paired observations shows thatfor the comparison between Sanchez methodand the present one, the teal = 1.378, to be com-pared with t 7 ;0.2 = 7.415. For the comparisonwith the Lathwell .method the teal = 2.02, to becompared with t7 ;0.05 = 2.365.It is thus seen that the lime requirement ofthe studied soils obtained by potentiometrictitration is comparable to that obtained by themethod of Sanchez (1976) and by the method ofLathwell (1979).CONCLUSIONSPotentiometric titration provides evidenceon Al as a dominant component of acidity inacid tropical soils below pH 5.5. Above pH 5.5,kaolinite proivdes a strong buffering action to thesoils. The lime requirement of these soils can beobtained by multiplying the amount of baseneeded to raise the pH of the soil to 5.5 by afactor 1.3.77ACKNOWLEDGEMENTThe authors wish to record their gratitude toUniversiti Pertanian Malaysia and ABOS Brusselsfor financial support. "REFERENCESAMEDEE, G. and PEECH, M. (1976): The significanceof KCI extractable Al (III) as an index to limereq uirement of soils of the humid tropics. Soil Sci.I2l: 227-233.CABRERA, F. and TALIBUDEEN, O. (1979): The releaseof aluminium from aluminium silicate minerals.II. Acid-base potentiometric titration. Clays andClay Minerals. 27(2): 113-118.DEWAN, H.C. and RICH, C.l. (1970): Titration of acidsoils. Soil Sci. Soc. Amer. Proc. 34: 3844.DIXON. J.B. (1977): Kaolinite and serpentine groupminerals. In: Minerals In Soil Environments. Dixon,J.B. and Weed, S.B. (Ed.). A mer. Soc. Soil Sci.,Madison, pp. 357403.GREENLAND, D.J. and MOTT, C.J.B. (1978): Surfacesof soil particles. In: ',The Chemistry of Soil Con-stituents. Greenland, D.J. and Hayes, M.H.B. (Eds.). -New York. John Wiley and Sons. pp.: 321-354.LATHWELL, D.J. (1979): Crop response to liming ofUltisols and Oxisols. Cornell International Agricul-tural Bull. 35. University of Cornell. Ithaca.PARAMANANTHAN, S. (1981): A simplified key to the-identification of Peninsular Malaysian soils. (Inpress.)RAND, B. anJ MELTON, I.E. (1975). Isoelectric pointof the edge of kaolinite. Nature. 257: 214-216.SANCHEZ, P.A. (1976): Properties and Managementof Soils In The Tropics. New York. John Wiley andSons.(Received 14 September 1982).

Potentiometric Titration of Acid Soils From

Peninsular Malaysia

RINGKASAN Kajian ini m enunjukkan bahawa SUMMARY The stu d y indicates the dom inance o f A l in controlling the acidity o f acid Peninsular Malaysian soils. The buffering action o f the soils is dom inated b y AI below p H 5.5 and by the clay minerals, m ainl

Potentiometric Titration of Acid Soils From Peninsular Malaysia

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