Determination of the Cation Exchange Capacity of ...

(LntunLlCluyMincrol.r,Vol.2l,pp.2l3-21'7.

PergamonPresslgT3.

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D E T ER MIN A T ION OF T H E CATION EXCHANGE CAPACITY OF CLAYS AND SOILS USING AN A MMON IA E LECTRODE E. BuspNsenc

and C. V. ClevrNcy

D e p a r t m e n t o f G e o l o g i c a l S c i e n c e s .S t a t e U n i v e r s i t y o f N e w Y o r k a t B u f f r r l o . B u J f a l o ,N e w Y o r k 1 4 2 0 7 ( R c c c i Le d 7 D c t t ' n t b c r 1 9 7 2 \ A b s t r a c t - T h e a m m o n i i i e l e c t r o d e s e r v e s a s t h e b a s i s o f a s i m p l e . a c c u r a t em e t h o d l b r d e t e r m i n a t i o n o f c a t i o n e x c h a n g e c a p a c i t y o f s m a l l ( r ' r r .5 0 m g ) s a m p l e so f c l a y s . T h e t e c h n i q u e i s a l s o c a p a b l e o f a c c u r a t e m e a s u r e m e n to f C E C v a l u e so n t h e o r d e r o f 0 ' 0 1 m - e q u i v / I 0 0 g i f l a r g e r( r ' a . 5 0 0 m g ) s a m p l e s are used. The procedure. which requires saturationof the exchangesites with ammonium as in the u s u a l m e t h o d s . u t i l i z e s t h e e l e c t r o d e i n t h e d e t e r m i n a t i o n o f a m m o n i a r e l e a s e db y t r e a t m e n t o f t h e a m m o n i u m c l a y b y s t r o n g b a s e . F o r a W y o m i n g b e n t o n i t e .t h e t e c h n i q u eg a v e a C E C i o f 8 6 m - e q u i v / l 0 0 g w i t h a n S . D . ( f o u r d e t e r m i n a t i o n s )o f 0 8 3 m - e q u i v / l 0 0 g . D u p l i c a t e r u n s o n t h e s a m e s a m p l e b v t h e c o n v e n t i o n a l K j e l d a h l m e t h o d g a v e r e s u l t so f 8 6 . 0 a n d 8 5 . 5 m - e q u i v / 1 0 0 g .

INTRODUCTION MeNv procedures for the determination of the cation exchange capacities (CEC) of clays have been describedin the literature. The most common methods require the saturation of the clays with ammonium ions. followed by determination of the de-sorbedammonium ions by acid-base titrations. by spectrophotometry.by infrared spectroscopyor other methods. Other index ions have also been used in conjunction with othermethods of measurement. For example, a method is described by H i n c k l e y ( 1 9 6 2 ) w h i c h u t i l i z e s t h e e x c h a n g eo f strontium ion followed by measurement of the amount by X-ray fluorescence. The method of Pham Thi Hang and Brindley (1970) utilized the saturation of the clay with methylene blue cations for the determination of the CEC and flocculation of the clay suspensionfor determination of surface area. Some of these procedures suffer from the disadvantages of being time-consuming, or requiring considerable analytical skill or very e x p e n s i v ee q u i p m e n t . Several papers describe the use of sodium and potassium ion selective electrodes for the determination of CEC and for studying rates of cation exchange reactions. Of particular interest are the p u b l i c a t i o n s o f M o r t l a n d ( 1 9 6l ) ; K e n n e d y a n d Brown (1964): Malcolm and Kennedy (1969): M a l c o l m . K e n n e d y a n d J e n n e ( 1 9 6 9 )a n d M a l c o t m and Kennedy ( I 970). The present authors have no experience with these electrodes or methods. ln general.theseelectrodesmay be somewhat affected by ions of similar valence if such are present in the

solution. That is. they are subject to interferences, though perhapsnot in the proceduresreferred to. The method proposed here is simple. rapid, relatively inexpensive,needs only extremely small samples(25 mg) and can be used to measurequite accurately very small cation exchange capacities ( 0 ' 0 1m - e q u i v / 1 0 0 g u s i n g a 5 0 0 m g s a m p l e ) . Briefly, the procedure consists of weighing out a sample of ammonium-saturatedclay, suspendingit in a known volume of water made alkaline with s o d i u m h y d r o x i d e s o l u t i o n , i n s e r t i n ga n a m m o n i a electrode and readingthe potential developed.Tne ammonium concentration is obtained by comparison with standard solutions of known ammonium concentration. The equipment requires only the ammonia electrode and an expanded scale pH meter. An Orion Model 95-10 Ammonia Electrode and the Orion Model 801 Ionalyzer were used in this work. The ammonia electrode was chosen because it has virtually no interferences.in contrast to almost all other electrodes. In addition. since ammonium ion has most often been the basis of CEC determinations. its use enabled comparison with other methods of measurement. ION SELECTIVE ELECTRODES

The most commonly used ion electrode is the 'glass 'pH so-called electrode' or electrode', sensitive to hydrogen ions. In recent years, electrodes sensitive to ions other than hydrogen have been developed. and are capable of measuring activities of these ions. Such electrodes are generally referred to as 'specific ion electrodes'or

2t3

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214

'ion

selective electrodes'. General discussions of t h c i r p r i n c i p l e s a n d L l s e sm a y b e f c r u n di n D u r s t ( 1 9 6 9 )a n d i n M o o d y a n d l ' h o m a s( l 9 7 l ) . M o s t i o n s e l e c t i v ee l e c t r o d e sr e s p o n dd i r e c t l yt o t h e a c t i v i t y o f t h e d e s i r e di o n i n a q u e o u ss o l u t i o n . bLrt this responsenray be affected by the presence o f o t h e r i o n i c s p e c i e s .e s p e c i a l l yt h o s e o f s i m i l a r c h a r g e .T h e a b i l i t y o f t h e e l e c t r o d et o d i s c r i m i n a t e b e t w e e n t h e d e s i r e di o n a n d o t h e r si s a m e a s u r eo f its elliciency and freedom from interferences. -l-he a m m o n i a e l e c t r o d e .w h i c h o p e r a t e so n i l somewhat diflerent principle. merits discussion. W h a t f b l l o w s i s t a k e n ( w i t h p e l m i s s i o n )l a r g e l y from the Orion Instrut'tiott Mutruul (Anonymous, 1972).l'he Oricln:immoniaelectrodeis a gas-detecti n g e l e c t r o d e ,w h i c h i s a b l e t o s e n s et h e l e v e lo f d r s s o l v e d a m m o n i a ( n o t a m m o n i u m ) i n a q u e o u ss o l u t i o n . N e v e r t h e l e s s i.t i s u s e d a s i f i t w e l e a s p e c i f i c iorr electrode. The electlode can be used to meas u r e a m m o n i u m b y a d d i t i o n o f s t r o n g b a s et o c o n vert ammonium to ammonia according to the = NH.]+H:O. Nitrate equation: NHr*+OH can be measured afler reduction to ammoniar.and organic nitrogen can be detelmined afler Kjeldahl digestion of the sample and addition of strong base. The ammonia electrode is free of interferencesby cations, anions and other dissolved species. e x c e p t v o l a t i l e a m i n e s .S a m p l ec o l o r a n d t u r b i d i t y do not afl-ectthe measurement. A l t h o u g h t h e a m m o n i a e l e c t r o d ei s f u n c t i o n a l l y s i m p l e ,t h e m e c h a n i s mo f i t s o p e r a t i o ni s c o m p l e x . Like other electrodes it fbllows Nernst's l-aw and r e s p o n d sl o g a r i t h m i c a l l yt o t h e a c t i v i t y o f d i s s o l v e d a m m o n i a i n a s o l u t i o n .1 ' h e a c t i v i t y i s p r o p o r t i o n a l t o t h e a m m o n i a c o n c e n t r a t i o n .w h i c h . i n t u r n . i s proportional to the concentration of ammonium i o n t h a t w a s o r i g i n a l l yp r e s e n t . -l-he electrode containsa hydrophobic membrane w h i c h i s p e r m e a b l et o a m m o n i a . b u t n o t t o a n y i o n i c s p e c i e s .D i s s o l v e d a m m o n i a i n t h e s a m p l e solr,rtion diffuses into the electrode until the a c t i v i t y o f a m m o n i ai s t h e s a m eo n b o t h s i d e so f t h e m e m b r a n e . A m m o n i a w h i c h p a s s e st h r o u g h t h e m e m b r a n e d i s s o l v e si n s o l u t i o n c o n t a i n e d w i t h i n t h e e l e c t r o d ea c c o r d i n gt o t h e e q u a t i o n : N H . r + H r O= N H , ' + O H

(l)

T h e p o t e n t i a lo f a n i n t e r n a ls e n s i n gc l e m e n tv a n e s i n a N e r n s t i a nm i t n n e t w ' i t h c h a n e e si n t h e h v d r o x idelevel: E:

E,

S log fOH I

w h e r e . l i s t h e s l o p eo f t h e c l e c t r o d er e s p o n s e .

lZt

T h e r e l a t i o n s h i pb e t w e e na m m o n i a .a n d a m m o n i u m a n d h y d l o x i d e i o n s i s g i v e n b - vt h e e q u a t i o n : I N H , ] l O H: r :l lNHr]

constant'

(3)

T h e i n t e r n a l f i l l i n g s o l L r t i o nc o n t a i n s a m m o n i u m chloride at a sr-rfficiently high concentration so that I N H , * ] c a n b e c o n s i d e r e df i x e d . E q u a t i o n ( 3 ) thenbecomes: [OH ] :

l N H , l . c o n s t a n t e' q u a t i o n s . @ )

C o m b i n a t i o no f ( 2 a n d 4 ) s h o w s t h a t t h e e l c c t r o c r e r e s p o n s et o a m m o n i ai s a l s o N e r n s t i a n . E a c h t e n - f o l di n c l e a s ei n a m n r o n i ac o n c e n t l a t i o n g i v e s r i s e t o a m i n u s - 5 t l ' 5m V c h a n g ei n e l c c t r o d e potential at room temperature. The potential developed under stiindard conditions is replodr,rci b l e t o b e t t e r t h a n + 0 . - 5m V . o r t 2 p e r c e n t a c c o r d i n gt o t h e O l i o n m a n u a l( A n o n y m o u s . 1 9 7 2 ) . -l'he ammonia electrode can acculately mcasure c o n c e n t r a t i o n sa s l o w a s l 0 p a r t s p e r b i l l i o n . O r d i n a r y d i s t i l l e dw a t e r c o n t a i n sl e v e l so l ' a m m o n r : r higher than this. It is also important. therelirre. to u s e a m m o n i a - f r e ew a t e r i n a l l e x p e r i m e n t s .T h r s standard can easily be prepaled by passing drstilled watel through a commercialion-exchanger which reduces the concentration of ammonia to b e l o w t h e l i m i t o f d e t e c l i o no f t h e e l e c t r o d e .C a r e must also be used to avoid storing clr opening irmmoniareagentbottles in the same room EXPERI\TENTAL

The ammonium satLlratedclays were prepared ( C h a p m a n . 1 9 6 - 5 )b y a l l o w i n g t h e s t a l t i n g s a m p l e t o s t a n d o v e r n i g h t i n l N a m m o n i u m a c e t a t es o l u t i o n w h i c h h a d b e e n a d j u s t e dt o p H 7 . a f l e l w h i c h the sampleswere leachedwith lN NH, Cl. Excess ammonium siilts were removed by washing w i t h i s o p r o p y l a l c o h o l .a f l e r w h i c h t h e c l a y s w e r e dried and ready for determination ol' CEC. An alternatemethod designedforquantitative recovery o f s m a l l s a m p l e sh a s b e e nd e s c r i b e db y M a c k e n z i e

i l 9 5r ) . F o r t h e m e a s u r e m e not f t h e C E C . . a 5 0 - l - 5 0 m g s a m p l eo f a i r - d r i e d .a m m o n i u m - s a t u r a t ecdl a y w a s w e i g h e d o u t a n d t r a n s f e r r e dt o a l 0 0 m l P y r e x be:iker containing a Teflon-covered stirring bar. 1in. in length. Exactll, -50ml of :rmmonia-tiee. d e i o n i z e d - d i s t i l l ew d a t e r w e r e a d d e d .S t i r r i n g w a s commenced and maintained at a constant level (a n e c e s s a r yc o n d i t i o n )w h i l e m e a s u r i n gb o t h s a m p l e s and standards. A small magnetic stirrer motor. connected to zr PowersturtregLllatorset at a constant value. w:rs used. Air-driven stirrersare not

D e t e r m i n a t i o no f c a t i o n e x c h a n g eu s i n g a m m o n i a e l e c t r o d e

215

recommended.The electrode was immersed in the The CEC of the sample is obtained fi'om the suspension,taking care to prevent entrapment of following eqr-ration: air bubbles under the concave tip. By means of a syringe. i ml of 10 M sodium hydroxide solution 1rr.1{I) was added to the beaker, and a timer was started. This amount of NaOH will quantitatively displace where all accessible NHn* and convert it to NH,]. Electrode readings were taken at }-min intervals until a CEC - the cation exchangecapacity of the sample constant potential was recorded for two or three e x p r e s s e di n m - e q u i v p e r I 0 0 g o f c l a y . a n d ( : c o n c e n t r a t i o no f a m m o n i ai n m o l e s p e r l . readings. Two to five min were usually required before the electrode reached equilibrium. Longer r . : v o l u m eo f w a t e r a d d e d( 5 0 m l i n t h i s c a s e ) times were required for lower ammonia concenr ' : w e i g h to f t h e s a m p l ei n m g trations. To determine the CEC of the clay sample. ,f: a conversion factor, in this case a constant the unknown is compared to two standardsolutions equaltol0 ;. of ammonium chloride of concentrations which With careful technique. this method is capablc bracket the unknown concentration. Standards of of accurate measurement of exchange capacities low concentration (< l0 5M) should be freshly a s l o w a s 0 . 0 1 m - e q u i v / 1 0 0 gw i t h a s a m p l eo f o n l y prepared daily by dilution. with ammonia-free 5 0 0 m g . F o r h i g h e r e x c h i i n g ec a p a c i t i e s .s m a l l e r water, of a 0.1 M pfimary standard solution. Solu- samples are used. The manual for the ammonia tions of l0-1 M are good for I week. The known electrode (Anonymous. 1972\ states that the solutions range from l0 2 to l0-5 M. with most reproducibility of the electrode under standard clays talling within the l0-r]- l0 a M range. conditions is t 0.5mV or + 2 per cent of the The concentrationof ammonia releasedfrom the concentration measured. In order to establish the clay by the sodium hydroxide can be determined p r e c i s i o no b t a i n a b l eo n r e p l i c a t es a m p l e su s i n gt h i s graphically (Fig. l), or can be read directly from the method, four determinationsof CEC were made on previously calibrated scale of the ion-analyzer, e a c h o f t h e f o l l o w i n g s a m p l e s : k a o l i n i t e . S . C . ; which is simply a pH meter with an expansible i l l i t e , I l l . ; i l l i t i c s o i l . I l l ; a n d m o n t m o r i l l o n i t e W . yo. scale.The ion-analyzer is calibrated by bracketing The S.D. found for each m:iteriiil was 0.06. 0.25. 0 ' 7 5 a n d 0 ' 8 3 m - e q u i v /1 0 0 g .r e s p e c t i v e l y .

cEC- f++

(-5)

RESULTS AND DISCUSSION

The CEC of 6 claysand I soil were determined by the procedure described in this paper and also '= by 4 other common methods of ammonia recovery. The other methods used were: the aeration method of Potter and Snyder as modified by Vanselow (in C h a p m a n . 1 9 6 5 ) ; M a c k e n z i e ' s ( l 9 - 5I ) p r o c e d u r e . a n d b y t w o v a r i a t i o n so f t h e K j e l d a h l m e t h o d . I n t h e u s u a l K j e l d a h l d i g e s t i o n .t h e s a m p l e i s b o i l e d E in 36N sulfirric acid for I hr. then -50 per cent E N a O H i s a d d e d u n t i l t h e s o l r , r t i o ins a l k a l i n e .T n e a m m o n i a i s t h e n d i s t i l l e do f f a n d c o l l e c t e d( i n t n e variation of the method used tbr this study) in an o" ro' rot 0,5 excess of saturated boric acid solution. The borrc A m m o n icoo n c e n l r o t i o nm , o l e/s L . acid fixes the ammonia. However. since boric acid is neutral to methyl red, use of this indicator perF i g . l . G r r r p h L r s e dt o t l e l e r m i n ea m m o n i ac o n c e n t r a t i o n mits direct titration of the ammonia with standardfrom electrode potential. (Adapted with permission of ized HCl. The sampleswere rLrnusing this method. Orion Research lnc. Jiont I trstructionul M anuul .lbr A tnmonia Sp et'iJic I on EI ectrode, p. 6). T h e n f o r c o m p a r i s o n .t h e s a m e K j e l d a h l d i s t i l l a tion procedure was used to separatethe ammonia. the unknown with two standard ammonium chlorbut the ammonia was determinedusingthe ammonia ide solutions. a procedure analogous to the standelectrode instead of by titration. With the other ardization of a pH meter by using two buffer solumethods, the ammonia electrode was used to tions of known pH. determinethe ammonia concentration. !

216

E . B u s r . N s p n c ;a n d C . V . C l e v e N c y T a b l e I . C o m p a r i s o n o f r e s u l t s o f c a t i o n - e x c h a n g ec a p a c i t y d e t e r m i n a t i o n so n - 5c l a y s . I c l a y s o i l a n d I m a c r o - c r y s t a l l i n e v e r m i c u l i t e v , z h i c hw a s g r o u n d t o 200mesh. Numbers are e x p r e s s e di n m - e q u i v / 1 0 0 g o f s a m p l e a n d a r e t h e a v e r a g eo f 4 d e t e r m i n a t i o n s .w i t h t h e e x c e p t i o n o f t h e n u m b e r s i n t h e l a s t r o w , w h e r e l a c k o f s a m p l e p r e v e n t e dm u l t i p l e d e t e r m i n l t i o n s . T h o s e m a r k e d w i t h a n a s t e r i s k i n t h e l a s t r o w i n d i c a t e a s i n g l ed e t e r m i n i i t i o n ,w h i l e t h o s e w i t h t w o n u m b e r s w e r e r u n i n d u p l i c a t e ,t h e t w o n u m b e r sg i v i n g t h e i n d i v i d u a l r e s u l t s Method

Ammonia e l e c t r o d em e t h o d (this paper) M a c k e n z i e( 1 9 - 5l ) d i s t i l l a t i o nm e t h o d ( u s i n ge l e c t r o d e ) Aeration method (Chapman.1965) ( u s i n ge l e c t r o d e ) S t a n d a r dK j e l d a h l d i g e s t i o nm e t h o d ( u s i n ge l e c t r o d e ) S t a n d a r dK j e l d a h l d i g e s t i o nm e t h o d (by titration)

Kaol. S.C.

Kaol. Ca.

Mont. Wyo.

Mont Miss.

28

3.0

86

8l

2.7

3.3

86

8l

2.8

3.3

84

2'8

3.4

3.0':'

3.3r'

Illite Ill.

lllitic Soil Iil.

Vermiculite Transvaal ( g r o u n dt o 200mesh)

l-l

ti-l

a-)

l3

8-5

80

25

t3

ri3

uti

83

a^

l4

88

86.0, 855

80.0. 810

24':'

I 3.3

86.0 86.0

* L a c k o f s a m p l e p r e v e n t e dr u n n i n g m u l t i p l e a n a l y s e s

The results obtained by the five methods are shown in Table l. All methods are in good agreement for the two kaolinites,the two montmorillonites. the illite and the illitic soil. For the vermiculite, the Kjeldahl digestion gave slightly higher results than the other three methods. The discrepancy is though to be causedby fixation of ammonia by the vermiculite, and only the Kjeldahl method. which completely destroys the clay structure, frees all the ammonia. The other methods thus give slightly lower results.

weighing a sample can be found. since suitable portable pH meters are available.

REFERENCES A n o n y m o u s ( 1 9 7 2 ) l t n t r t r < t i o r t u lM u r t t r t t l : A r r t r r t o r t i t t Electnttlt' Model 95-10: Orion Research Inc.. Camb r i d g e .M a s s .2 4 p p . C h a p m a n . H . D . ( 1 9 6 5 ) C a t i o n e x c h a n g ec a p a c i t y . I n Methods o.t' Soil Anulysr.i (Edited by Black, C. A.) P a r t 2 . p p . 8 9 1 - 9 0 1 . N u m b e r 9 i n t h e s e r i e sA . q r o n o r n t : Am. Inst. Agronomy, Madison,Wisconsin. Durst. R. A. (Editor) (1969) Ion Selectite Elettrodes: National Bureau of Standards Publication 314. 452 p p . S u p e r i n t e n d e n to f D o c u m e n t s , W a s h i n g t o n .D . C . H i n c k l e y . D . N . ( 1 9 6 2 ) A n X - r a y s p e c t r o g r a p h im c ethod COr-CLUSIONS f o r m e a s u r i n g b a s e e x c h a n g ec a p a c i t y : A n r c r . M i n e r a The obvious advantages of this method are logist47.993-996. simplicity, speed, accuracy, ability to utilize small K e n n e d y . V . C . a n d B r o w n . T . E . ( 1 9 6 4 ) E x p e r i m e n r s with a sodium-ion electrode as a means of studying sample sizes (50 mg) and freedom from inter-ferencation-exchange rates. Clut's und CIut Mineruls, ces. CEC determinationscan be made as easily as 1 3 ,3 5 I - 3 5 2 . the determination of pH and almost as rapidly. M a c k e n z i e . R . C . ( 1 9 5l ) A m i c r o m e t h o df o r t h e d e t e r mination of cation exchange capacities of clays: J. once the ammonia-saturatedclay has been preC o l l o i d Sci . 6 . 2 19 - 2 2 2 . pared. The results compare extremely well with M a l c o l m . R . L . a n d K e n n e d y .V . C . ( 1 9 6 9 )R a t e o f c a t i o n standard methods. Another attractive feature rs exchange on clay minerals as determined by specific the capability of accurate measurementof extremi o n e l e c t r o d e t e c h n i q u e s : J o i l . l c r . S t t r ' .A n t . I ' n t c . 3 3 ( 2 \ . 2 4 7- 2 5 3 . e l y s m a l l C E C , o n t h e o r d e r o f 0 . 0 1m - e q u i v / 1 0 0 g N I a l c o l m ,R . L . . K e n n e d y .V . C . a n d J e n n e .E . A . ( 1 9 6 9 ) (500 if larger samples mg) are used. The method Determination of cation exchange capacity with the should be easily adaptable for measurement of p o t a s s i u n rs p e c i f i ci o n e l e c t r o d e : I t t t . C l t n ' ( - o n . f .I ' n t t . . ammonium present on clays and soils in the field. Tokyo l.573-583. if a field procedure for drying and accurately M a l c o l m . R . l - . a n d K c n n e d v . V . C . ( 1 9 7 0 ) V a r i a t i o n o f

D c t e r m i n a t i o n o f c a t i o n e x c h a n g eu s i n g a m m o n i a e l e c t r o d e c a t i o n e x c h a n g ec a p a c i t y a n d r a t e w i t h p a r t i c l e s i z e i n s t r e a m s e d i m e n l : ..1W u t e r P o l l u l . C o n t r o l F e d . . P a r t 2 , RL53-R160. M o o d y . G . . 1 . a n d T h o m a s .J . D . R . ( 1 9 71 ) S e l e c t i t eI o n S c r t s l l l l c E l e t r n t t l c . s :M e r r o w P u b l i s h i n gC o . . L o n d o n . l'10pp. M o r t l a n d . M . M . ( 1 9 6 1 ) U s i n g a g l a s s e l e c t r o d ef o r

e s t i m a t i n g e x c h a n g e a b l ep o t a s s i u m i n s o i l s : M i t h i g u t t Agric. Expcrinrcnt Stoliotl Quurterll'Brrll.43 (3), ,+9l-498. P h a m T h i H a n g a n d B r i n d l e y ,G . W . ( 1 9 7 0 ) M e t h y l e n e blue absorption by clay minerals. Determination of s u r f a c e a r e a s a n d c a t i o n e x c h a n g e c a p a c i t i e s .C l r r _ r ' s u n d C l u t ' M i r t e r a l s1 8 , 2 0 3 - 2 1 2 .

R 6 s u m 6 - L ' 6 l e c t r o d e d a m m o n i a q u es e r t d e b a s e i r u n e m 6 t h o d e s i m p l e e t p r 6 c i s ep o u r l a d 6 t e r m i n a t i o n d e l a c a p a c i t e d i c h a n g e d e p e t i t s 6 c h a n t i l l o n s d ' a r g i l e ( e n v i r o n - 5 0m g ) . L a t e c h n i q u e p e r m e t 6 g a l e m e n td e m e s u r e ra v e c p r 6 c i s i o nl e s v a l e u r sd e C E C d e l ' o r d e d e 0 . 0 1 m e q / 1 0 0 g s i d e s 6 c h a n t i l l o n s p l u s i m p o r t a n t s ( e n v i r o n 5 0 0 m g ) s o n t u t i l i s 6 s .C e p r o c 6 d 6 .q u i n 6 c e s s i t el a s a t u r a t i o nd e s s i t e s d ' 6 c h a n g e a v e c l ' a m m o n i u m c o m m e d a n s l e c a s d e s m 6 t h o d e s h a b i t u e l l e s .u t i l i s e l ' 6 l e c t r o d e d a n s l e but dc doser l'ammoniaque lib6r6e par l'argile ammonium lors d'un tlaitement par une base forte. Pour une bentonite du Wyoming, cette techniquea donn6 une CEC de 86 m-equiv./100g avec une d d v i a t i o n s t a n d a r d ( q u a t r e d o s a g e s )d e 0 , 8 3 m - e q u i v . / 1 0 0 g . D e s d o u b l e s e f f e c t u 6 s s u r l e m d m e 6chantillon par la methode Kjeldahl conventionnelleont donn6 comme r6sultats 86,0 et 85,5 me q u i v . /1 0 0g . K u r z r e l ' e r a t - D i e A m m o n i u m e l e k t r o d e d i e n t a l s ( i r u n d l a g e e i n e r e i n f a c h e n ,g e n a u e n M e t h o d e z u r B e s t i m m u n g d e r K a t i o n e n a L r s t a u s c h k a p a z i tkdlte i n e r ( c a 5 0 m g ) T o n p r o b e n . D i e g l e i c h e T e c h n i k i s t a u c h z u r g e n a u e nB e s t i m m u n g v o n A u s t a u s c h k a p a z i t d t s w e r t e n i n d e r C r d B e n o r d n u n gv o n 0 , 0 I m v a l / l 0 0 g g e e i g n e t . w e n n g r c i B e r eP r o b e n m e n g e n( c a 5 0 0 m g ) b e n u t z t w e r d e n . B e i d e m V e r f a h r e n , d a s eine Sattigr.rng d e r A u s t a u s c h p l d t z em i t A m m o n i u m i o n e n w i e b e i d e n i j b l i c h e n M e t h o d e n e r f o r d e r t , w i r d d i e E l e k t r o d e z u r B e s t i m m u n g d e s d u r c h B e h a n d l u n gd e s A m m o n i u m - T o n s m i t s t a r k e n B a s e n f r e i g e s e t z t e n A m m o n i u m s e i n g e s e t z t .F i i r e i n e n W y o m i n g - B e n t o n i t e r g a b d i e M e t h o d e e i n e A u s t a u s c h k a p a z i t t v o n 8 6 m - v a l / 1 0 0 g m i t e i n e r S t a n d a r d a b w e i c h u n g( 4 B e s t i m m u n g e n )v o n 0 , 8 3 m v a U l 0 0 g . P a r a l l e l b e s t i m m u n g e na n d e r g l e i c h e n P r o b e m i t d e r k o n v e n t i o n e l l e n K j e l d a h l - M e t h o d e e r g a b e n8 6 , 0 u n d 8 5 . 5 m - v a l / 1 0 0 g . Pelrolre - Arrrrfia.{Hrrii 3-qeKTpo,lcnyxuT ocHoBaHueMnpocroro r roqHoro MeroAa ofipeaeneHfiq cnoco6nocru raruouoo6sreua se6o:ruuux o6pa:uoa rtuusr (ca 50 nrr). 3rlrlr cnoco6olr rloxso raKxe aKKyparHo BtlMeptrl 3HarIeHHtCEC nopr4xa 0,01 lter