898
Anal. Chem. 1982, 5 4 , 898-902
Separation of Copper, Cobalt, Nickel, and Manganese from Deep-sea Ferromanganese Nodules by Adsorbing Colloid Flotation Eric Helnen DeCarlo and Harry Zeltlln" Department of Chemistry, University of Hawaii, Honolulu, Ha wail 96822
Oulntus Fernando Department of Chemistry, University of Arizona, Tucson, Arkona 8572 1
A method for the simultaneous removal of copper, cobalt, manganese, and nickel as Cu2+, Co2+, Mn2+, and Ni2+, respectively, from acid-dlgested deep-sea ferromanganese nodule solutlons Is described. The catlonic specles are concentrated by adsorblng colloid flotation (ACF) uslng the catlonlc surfactant laurylamlne hydrochlorlde and an In situ generated iron( I I I ) hydroxlde collector. Recovery of the elements is specles and treatment dependent. Quantitative recovery of Cu is attalned above pH 6.0 while quantitative recovery of Co, Ni, and Mn can only be achieved at pH 9.0. Flotatlon of the dedred species Is achleved in less than 5 min by udng an air flow of 17 & 2 mUmln. The collected floc and foam are dissolved in acid and subsequently diluted for analysis by atomlc absorptlon spectrophotometry (AAS).
The separation and recovery of economically valuable metals from deep-sea ferromanganese nodules have become subjects of great interest. The nodules which are located at the bottom of the world's oceans constitute a major and strategic national resource and reservoir of cobalt, copper, and nickel. The deposits of greatest economic interest are located in the equatorial North Pacific Ocean. These deposits are unique in that their enrichment in the transition metals copper, nickel, and cobalt of about 3% dry weight exceeds many of the presently exploited continental reserves (1). Studies on the recovery of these metals have been focused on extractive hydrometallurgical processes which include leaching with acids, ammonia, or aqueous solutions containing reducing agents such as SO2 (2-8), pyrometallurgical methods which employ high-temperature smelting and chlorination (9, IO), solvent extraction, and liquid ion exchange (11,12). Within the next decade, the large-scale recovery of metals from the deep-sea nodules will be undertaken and an emerging industry that is concerned exclusively with ocean mining should become a reality. This scenario, however, may be delayed and is complicated by the fact that a number of grave socioeconomic, legal, and environmental problem remain as yet unresolved. A well-developed physical separation technique which has been used for many years on a large scale is ore flotation, and has been applied to terrestrial silicates as well as to sulfide and oxide ores of zinc, copper, and lead (13). Flotation technology, however, had not yet been employed with the deep-sea ferromanganese nodules. The extremely overall small particle size (20-100 A) of the mineral phases encountered in nodules has been considered to be the primary reason for the lack of application of ore flotations which are generally useful in the 100 pm particle size range. Previous work initiated in our laboratory had demonstrated the feasibility of using adsorbing colloid flotation (ACF) for the separation of a variety of anionic and cationic species from seawater as well as other solutions of high ionic strength (14-19). A recent 0003-2700/82/0354-0898$01.25/0
report has demonstrated the feasibility of the simultaneous separation of several anionic species from solutions resembling acid-digested nodules (19). This work had been performed in hope of applying the method to the removal of potentially toxic species such as AsO:-, Se032-,Sb043-,and Ge(OH)30from deep-sea nodules using a positively charged ferric hydroxide collector generated from in situ iron, an anionic surfactant (sodium lauryl sulfate), and air. However, preliminary attempts at separating these anionic species directly from acid-digested nodules were not successful as the colloidal ferric hydroxide collector generated upon pH adjustment was found to bear a negative surface charge (Le., collector was above its point of zero charge or pzc). This was attributed to the presence of a variety of other potential-determining species present in the highly complex acid digested solutions. Our experience with ACF suggested the feasibility of applying this technique to the deep-sea nodules for the recovery of copper, cobalt, and nickel. Accordingly it seemed appropriate that a negatively charged colloidal ferric hydroxide should be used for collecting cationic species such as Cu, Co, and Ni present in the nodules. This paper describes the simultaneous separation on a laboratory bench scale of Cu(II), Co(II), and Ni(I1) from the bulk of aluminosilicates and trace anionic species present in acid digested deep-sea ferromanganese nodules in a 0.5 M H3B03matrix using ACF. In situ negatively charged colloidal iron and manganese hydroxides have been successfully employed in conjunction with the cationic surfactant laurylamine hydrochloride to float Cu(II), Co(II), and Ni(I1) in several minutes by bubbling air through the solution. The enriched foam layer is removed manually and subsequently dissolved in aqua regia. Analysis of the resulting solution for evaluation of the recovery of the metals of interest is then performed by atomic absorption spectrophotometry (AAS).
EXPERIMENTAL SECTION Apparatus and Equipment. A Perkin-Elmer Model 2380 atomic absorption spectrophotometer was used for all absorbance measurements. The pH of solutions was determined with either a Beckman Model 72 or a Beckman Expand-0-Matic pH meter calibrated daily against Scientific Products Standard Reference buffers (pH 4.01,7.00, 10.00). All flotations were carried out by using a modification of the cell previously described by Kim and Zeitlin (14). The cell was designed to hold a maximum volume of 200 mL of sample solution. Reagents. All chemicals used were of analytical reagent grade. Aqueous solutions were prepared by using water purified in a Millipore-Model Milli R/Q ion-exchange and reverse osmosis water purifier. The surfactant solutions used in these studies were prepared by dissolving 0.1 g of laurylamine hydrochloride (a cationic surfactant)in 100 mL of ethanol and 1.5 g of Triton X-100 (nonionic surfactant) in 100 mL of ethanol. Concentrated and NH3,and NaOH were dilute solutions of HNO,, HCI, HF, HBBOB, used throughout the studies as required. Atomic absorption standard reference solutions of Fe, Mn, Cu, Co, and Ni (1000 0 1982 Amerlcan Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 6, MAY 1982
899
--
Table I. Characteristics of Deep-sea Ferromanganese Nodules
100 -
nodule
depth, m
latitude
longitude
IIM-1178 VALDIVIA V18-D32 V21-D2
5200
5000 2000 5400
12"N 9"N 14"18'S 34'54"
178"W 148" 30 'W 149"32'W 160'19'W
80 t. w w
s>
kg/mL) were purchased from Anderson Laboratories Inc. Standard mixed ellement solutions for AAS analysis were prepared immediately prior to analysis by appropriate serial dilutions of the individual 1000 kg/mL standards. Analytical Procedure. Deep-sea ferromanganese nodules from various locations in the Pacific Ocean (Table I) were ground and passed through standard U.S.sieves for size fractionation. The various size fractionEi were then stored in sealed and labeled glass jars until needed. Samples used in these studies were taken from the 100-150 mesh size fraction for V18-D32, V21-D2, and HM-1178 nodules (see Table I) and from the 150-200 mesh fraction for Valdivia nodules. Solutions of the individual nodules were prepared by subjecting 0.500 g portions of the ground nodules to a modification of the bomb digestion procedure described by Bemas (20) and diluting to 100 mL with 0.5 M H3B03in volumetric flasks. The boric acid matrix was used to complex any remaining HF after sample dissolution. Five replicate elemental analyses of the nodules were performed on appropriate dilutions of the bomb-digented solutions by AAS. Twenty-milliliter aliquots of the bomb-digested solutions were transferred carefully to 250-mL beakers and the volume adjusted to approximately 160 mL with water. The pH was then adjusted with Concentrated and dilute solutions of NH3 or NaOH to the desired value 10.05 pH units. The pH adjustment resulted in the formation of a. colloidal precipitate composed primarily of mixed iron and manganese hydroxides (ratio of Mn to Fe dependent upon solution pH). The sample was quantitatively transferred to the flotation cell, purified air passed through the cell at a flow ratle of 17 i 2 mL/min, and 4 mL of cationic surfactant solution injected by means of a hypodermic syringe. After 1to 2 min the cell was found to have cleared and 2 mL of Triton X-100 was injected in order to stabilize the enriched foam bed. The foam was allowed to collect for 3-4 min and manually scooped into a beaker with a Teflon spatula. A few drops of concentrated HCl and water were used to rinse adhering mixed metal hydroxide from the spatula into the beaker. One milliliter of concentrated HCl and 0.5 mL of concentrated HN03 and additional water were used to break up the foam and dissolve the floc over medium heat. The resulting 20-25 mL of solution were then transferred quantitatively to a 50- or 100-mLvolumetric flask, diluted to the mark with water, and transferred to Nalgene containers for storage. Replicate flotations were performed at 0.5-1.0 pH unit intervals in order to evaluate the reproducibility of recovery of the metals of interest. The solutions (or appropriate dilutions thereof) were analyzed by AAS. The addition of surfactant and/or NaCl to the standards was found to be necessary for the slight signal enhancement observed in the presence of these substances in the samples.
-
60-
ap w
-
1
z
ijj 40-
(1?
; ; 1
20 0-
co
Mn
I NI
1
70
60
50
a0
90
RESULTS AND DISCUSSION Elemental Analysis of Deep-sea Ferromanganese Nodules. The results of elemental analysis of various nodules are shown in Table 11. These represent the average of five replicate determinations performed on samples prepared by -___
Table 11. Elemental Analysis of Some Major and Minor Components in Deep-sea Ferromanganese Nodules Percentage Compositiona nodule HM-1178 VALDIVIR V18-D32 V21-D2 a
Fe 7.61 7.52 16.8 13.9
Air-dry at 110 " C weight basis. ~
.
_
_
_
Mn 20.5 20.6 17.2 15.5
AI ndc 2.44 1.23 2.74
Ti
n.d.c 0.72 1.14
0.94 After 4 h of dehydration at 450 "C.
cu 0.76 0.70 0.13 0.33
co 0.20 0.21 0.45 0.28
n.d., not determined.
Ni 0.85 1.07
0.40 0.54
H,Ob
28.9 27.5 30.5 23.5
900
ANALYTICAL CHEMISTRY, VOL. 54, NO. 6, MAY 1982
1
100
"\ C"
b l >
2 40-
W Lli
20-
0' 50
:
,
I
60
70
a0
1
I
90
PH Relative percentage recovery of metals from V18-D32 nodule floated with laurylamine hydrochloride using NH:, as pH adjustlng reagent: Fe, 0;Cu, 0; Co, 0; Ni, D; Mn, A.
Flgure 2.
I 100 -
80 -
>
Lli L u
>
E
-
60-
8 L
> -
u
-
c
4 40-
Lu c,i
20 -
01 50
60
70
80
90
PH Flgure 3. Relative percentage recovery of metals from VaMivia nodule floated with laurylamine hydrochloride using NaOH as pH adjusting ;Mn, A. reagent: Fe, 0; Cu, 0; Co, 0 ; NI, .
solutions. Since recoveries were on the order of -20% less than when using 20-mL aliquots, these studies were not pursued further. From the results obtained several observations can be made: (1) The pzc of the colloidal ferric hydroxide is much lower than would have been expected. (2) The relative recovery of metals increases with decreasing sample size. (3) The recovery of copper increases to quantitative levels at lower pHs than those of cobalt, nickel, and manganese which exhibit increased recovery only above pH
0
50
80 90 PH Figure 4. Relative percentage recovery of metals from V18-D32 nodule floated with laurylamine hydrochloride using NaOH as pH adjusting reagent Fe, 0; Cu, 0; Co, 0; Ni, D; Mn, A. 60
70
7.0. (4) Recovery of the Cu is higher and constant over a wider pH range when using NaOH as a pH adjusting agent rather than NH3. ( 5 ) Recovery of Co, Ni, and Mn increases steadily with pH when using a NaOH as a pH adjusting agent rather than NH3. In view of the exceedingly complex nature of the ferromanganese matrix which contains in the neighborhood of 40 elements present as both major and minor components, it is fortuitous that no serious interferences were encountered during the flotation procedure. From the standpoint of solubility products (KBp)as well as from nodule elemental composition it can be assumed that the precipitate generated upon pH adjustment with ammonia or sodium hydroxide is primarily composed of ferric hydroxide formed between pH 3 and pH 7 and contains increased amounts of manganese above pH 7. Precipitates of iron and manganese hydroxides have been found to have their pzc between pH 6 and 8.5 and pH 4.5 and 7.0, respectively, the exact value largely depending upon factors such as solution composition and history (23, 24). The pzc of colloidal precipitates has also been found to be affected by the presence of other species in solution (S042-, HPOd2-,C20d2-,etc.), as well as by the presence of other metals forming insoluble hydroxides which themselves have low pzc's (25,26). It is not entirely surprising that in a solution as complex as that of an acid-digested deep-sea nodule, the pic of the ferric and manganese hydroxide precipitate is altered drastically. The presence of aluminum (1-3%) and titanium (1-3%) in nodules may partly contribute to the lowering of the ferric hydroxide pzc by the formation of a mixed metal hydroxide system even at low pH. Aluminum hydroxide has a Ksp which is near that of ferric hydroxide as well as a pzc in the range of pH 3.0-7.5, while titanium(IV) oxyhydroxide has a KBP and a pzc near pH 4 (23, 24, 27). As noted before the relative recovery of the metals of interest seems to be related to sample size. The 30 mL aliquot flotations show a lower percentage of metals recovered than do the 20 mL aliquots when using NH, as the pH adjusting reagent. This effect is largely attributed to the influence of the ionic strength of the solution. Wilson and co-workers have
-
-
ANALYTICAL CHEMISTRY, VOL. 54,
repeatedly notedl that an increase in the ionic strength of a solution generally results in a decrease in the efficiency of flotation (25,26). In our studies both the 30-mL and 20-mL samples were diluted to near 160 mL with water before pH adjustment to the desired value. The flotations with the larger aliquots of digested solution thus had a higher ionic strength which could then lead to a decrease in the efficiency of flotation. The data presented in Figures 1-4 indicate that Cu recovery becomes quantitative at lower pH values as compared to Co and Ni. It can rleadily be seen from Figures 1 to 4 that the recovery behavior of Cu follows that of iron. This can be explained in part by the higher pK,, of Cu(OH), and by the Paneth-Fajans-IHahn rule which states that “when two or more types of ions are available for adsorption, and other factors are equal, that ion which forms a compound with the lowest solubility with one of the lattice ions will be adsorbed preferentially” (4’6). Thus at low pH the Cu2+is more likely to adsorb onto the iron hydroxide than either the Co2+or Ni2+ in order to form M(OH), on the surface of the floc. As the pH increases though, the Cu2+and Fe3+recoveries level off while those of Co2+,Ni2+,and Mn2+begin to increase as expected from KBpconsiderations. At this point it is believed that the recovery of the elements of interest is controlled by coprecipitation rather than by adsorption. It is of interest to note that above pH 8.0 the Cu recovery drops off for botlh types of nodules when using ammonia as the pH adjusting reagent. The cobalt and nickel recoveries which began to increase slowly at lower pH values also show a significant drop above pH 8.0. This behavior is consistent with what would be expected if complex formation were to occur. All three of the species Cu2+,Co2+,and Ni2+are known to form ammine complexes. The values of the overall formation constants for the formation of tetraammine complex respectively (28). Thus the formation are 1013.3,W7,and
[email protected], of a copper ammine complex which is highly favored overrides the tendency to form an insoluble hydroxide on the surface of the ferric hydroxide floc. This effect is not as pronounced in nodule V18-D32 as it is in the Valdiuia nodule. The higher Fe content of V18-D32 might in part explain this phenomenon as there would be more localized charged surface sites available for adsorption and coprecipitation of the CU(OH)~ in this case. The relative behavior of Co2+ and Ni2+ in the presence of ammonia can also be explained in terms of complex formation. The cobalt recovery at high pH is higher in both Valdivia and V18-D32 nodules than that of nickel; a fact expected from the greater tendency of Ni2+to form the ammine complex. The observed behavior of Mn2+in these flotation studies indicates that a hydroxy complex of manganese Mn(OH)+ may be present in solution. The formation constant of the Mn(NH3)2+complex is quite low (Kf= 101.3)in comparison with the formation constant of Mn(OH)+. From the preceding comments and observations, it appears that ammine complexes of Cu2+,Co2+, Ni2+, and Mn(OH)+ are not readily floated with a laurylamine HC1 system. The differences in relative recoveries of Cu2+,Co2+,and Ni2+ between Valdiuia and VWD32 nodules may be explained by two factors. The first which was briefly mentioned earlier is due to the presence of larger amounts of iron per given size in the VWD32 nodule thus providing more charged sites for adsorption of the cations and precipitation of M(OH)2on the surface of the floc. The second deals with the concentration effect on the tendency of colloids to adsorb certain ions preferentially (26). In the Valdivia nodules Cu, which according to the Paniath-Fajans-Hahn rule is favored to adsorb preferentially, is also present in higher concentrations than Co, thus again favoring its adsorption. In the V18-D32 nodule Cu is present in much lower concentrations than either Co
NO.6,
MAY 1982
901
or Ni (which are nearly equal in concentration) thus favoring an increase in their adsorption by the so-called concentration effect. During the course of preliminary flotation work it was noticed that the enriched foams were occasionally unstable and in some cases collapsed slowly leaving a scum on the upper sides of the flotation cell. This scum was difficult to collect as it adhered tenaciously to the glass of the cell and occasionally resulted in redistribution of some floc into the underlying mother liquor. In order to facilitate floc collection and ensure quantitative transfer of the floated matter, Wilson suggested the addition of a nonionic surfactant such as Triton X-100 or Tween 20 (29). The addition of a small amount of Triton X-100 to the cell after a large part of the desired material had been floated was found to stabilize the enriched layer. This was achieved in two ways. First, some of the Triton X-100 is mixed with the laurylamine hydrochloride near the bottom of the enriched layer thus creating a region of increased stability less likely to collapse; second, the remainder of the Triton x-100 created a clean layer which pushed and held up the enriched layer. This clean layer acted as a trap for any floc particles which would have otherwise been redistributed when disturbed by manual collection. The addition of nonionic surfactant was thus an aid in terms of both facilitating handling of the enriched foam and ensuring quantitative transfers. It is anticipated that the use of nonionic surfactants would not be necessary in pilot or industrial processes which would most likely be continuous flow rather than batch systems. The continuous overflow of enriched foam in these situations would not be subject to redistribution which can result from mechanical disturbance of the foam bed. The ACF method described has been found to effectively and rapidly remove the elements of interest from acid-digested deep-sea ferromanganese nodules as cations present in the 10-1000 ppm level. The separation could readily be used for preconcentration purposes as the elements of interests are not known to form volatile species with aqua regia and thus permit the floated samples to be further reduced in volume without loss of sample. It is of interest to note that a separation of Fe and Cu from the bulk of the Co, Ni, and Mn could be achieved by a two-step flotation. The first step would involve a flotation at pH 6.5 using NH3 as pH adjusting reagent, followed by a flotation at pH 9 using NaOH as pH adjusting reagent. The second flotation would use Mn2+as the collector in the form of a hydrous oxide. Flotation of Co and Ni with Mn as the collector has been achieved in our laboratory from aqueous leach solutions of sulfated manganese nodules. Since these solutions do not contain a significant amount of iron, manganese is used as the collector. Further work on the flotation of valuable metals from leach liquors is presently under way.
ACKNOWLEDGMENT The authors thank D. W. Shinn for some helpful suggestions. We wish to express thanks to R. Bleasdell for valuable discussions. The glass blowing skills of W. Cooper are gratefully acknowledged.
LITERATURE CITED Cronan, D. S. In “Marine Manganese Deposits”; Giasby, G. P., Ed.; Amsterdam 1977; p 11-44. Lee, J. H. Ph.D. Thesis, University of Hawaii, Honolulu, HI, 1979, 162 PP. Han, K. N.; Funrstenau, D. W. Proceedings of the 1st Australian Heat Mass Transfer Conference, Section 8, 1973; pp 41-48. Han, K. N. Ph.D. Thesis, University of California, Berkeley, CA, 1971, 212 pp. Brooks, P. T.; Martin, D. A. Rep. Invest .-US., Bur. Mines 1971, No. 7473.
Han, K. N.; Hoover, M.; Fuerstenau. D. W. Int. J . Mlner. Process. 1974, 1 , 215-230. Skarbo, R. R. I J S . Patent 3723095, 1973.
902
Anal. Chem. 1982, 5 4 , 902-907
(8) Brooks, P. T.; Dean, K. C.; Rosenbaum, J. B. Proceedings of the International Mineral Congress, 9th, Czechoslovakla, 1970;pp 329-333. (9) Cardweli, P. H. Proceedings of the Mineral Convention/Envlronment Show, Am-Miner. Congr., Denver, CO, Sept 9-12, 1973. (IO) Hoover, M. Ph.D. Thesis, University of Callfornla, Berkeley, CA. 1972% 254 pp. (11) Van der Zeeuw, A. J. U S . Patent 3701 650, 1972. (12) Immartino, N. R. Chem. Eng. 1974, 81 (25),52-53. (13) Rampacek. C. Inf. C1rc.-US., Bur. Mines 1980, No. 8818. (14) Kim, Y. S.;Zeltlin, H. Sep. Sci. 1972, 7, 1-12. (15) Kim, Y. S.;Zeltlin, H. Sep. Scl. 1971, 6 , 505-513. (16) Chaine, F.; Zeltlin, H. Sep. Sc/. 1974, 9 , 71-77. (17) Matsuzakl, C.; Zeitlin, H. Sep. Sci. 1973, 8, 185-192. (16) Tzeng, J. H.; Zeitlin, H. Anal. Chlm. Acta 1978, 101, 71-77. (19) Decarlo, B. H.; Zeltiin, H.; Fernando, 0. Anal. Chem. 1981, 53, 1 104-1 107. (20) Bernas, 8 . Anal. Chem. 1988, 40, 1682-1666. (21) Burns, R. G.;Burns, V. M. I n “Marine Manganese Deposits”; Glasby, G. P., Ed.; Elsevier: Amsterdam, 1977;pp 185-248. (22) Iskowltz, J. M. MS Thesis, Universky of Hawali, Honolulu, HI, 1979;65 PP.
(23) Parks, G. A. Chem. Rev. 1985, 65, 177-196. (24) Parks, 0. A. Adv. Chem. Ser. 1987, No. 67.
(25) Thackston, E. L.; Wllson, D. J. EPA Report 600/2-80-138; U.S. Environmental Protection Agency: Cincinnati, OH, June 1980. (26) Currin, B. L.;Kennedy, R. M.; Clarke, A. N.; Wllson, D. J. S e p . Sci. Technol. 1979, 14, 669-687. (27) Peters, D. G.; Hayes, J. M.;HeftJe,G. M. “Chemical Separations and Measurements”; W. B. Saunders, Philadelphia, PA, 1974. (28) Meites, L., Ed., “Handbook of Analytical Chemistry”, 1st ed.; MacGraw-Hill: New York, 1963. (29) Wilson, D. J. Department of Chemistry, Vanderbilt University, Nashville, TN, personal communlcation, Sept 1980.
RECEIVED for review November 3,1981. Accepted January 25, 1982. “Hydrometallurgical Separation of Metals from Deep-sea Ferromanganese Nodules” (MR/R-4) is a research project sponsored by the University of Hawaii Sea Grant College Progam under Institutional Grant No. NA79AA-D00085, from N O M , Office of Sea Grant, U.S. Department of Commerce. This is Sea Grant publication UNIHI-SEAGRANT-JC-82-04.
Enthalpimetric Study of the Surface Interactions of n -Butylamine with Silica Gel J. Kelth Grlme” The Procter & Gamble Co., Ivorydale Technical Center, Cincinnatl, Ohio 45217
Elizabeth D. Sexton Chemical and Materials Science Division, Denver Research Instltute, Denver, Colorado 80208
The Interactions between n-butyiamlne and sllica gel are Investlgated by thermornetrlc enthalpy tltratlon of the amine in the presence of, but not covalently bonded to, silica gel. The protonatlon of the amine Is used as a crlterion for reactivlty. Analogies are drawn between the behavior of the adsorbed amlne and its silane analogue, 1-aminopropyisiiane. Two types of Interactions are observed: a reversible bonding, presumably hydrogen bonding, and an lrreverslble process which Is not defined. Langmulr isotherm theory Is used to descrlbe the reverslble Interaction.
Silica matrices, primarily silica gel or controlled pore glass, have found widespread use in analytical chemistry for the immobilization of organic functional groups and enzymes. The fundamental principles and applications of this area of technology have been comprehensively reviewed elsewhere (1-5).
Whether the ultimate goal is to bind a metal ion to a silica surface in a preconcentration step or to attach an enzyme for an immobilized enzyme reactor, the initial step is the derivatization of the silica surface in order to incorporate the appropriate functionality. The most common surface modification is amination by reaction with alkylaminosilanes such as 1-aminopropyltrimethoxysilane(APS) and N-(2-aminoethyl) (3-aminopropyl) trimethoxysilane (AEAPS) to produce covalently attached mono- and diamines. Clearly, the reactivity of the immobilized amines will be a critical factor in determining the metal- or enzyme-binding capacity of the modified silica surface. In practice, it cannot
be assumed that the reactivity of an amine covalently bound to silica gel, for example, is the same as its reactivity in homogeneous solution. In a heterogeneous system, a distinction must be made between reactive amines and nonreactive amines, which, for whatever reason, are unavailable for reaction with metal ions, coupling reagents, or enzymes. Techniques such as elemental carbon or nitrogen determination obviously will not differentiate between these two entities. Indirect methods of functional group determination, e.g., back-titrations in which the supernatant solution is sampled, will provide quantitative analytical data regarding the number of reactive functionalities on a silica surface. Indeed, indirect redox titration of bound diol groups has been reported (6);quantitation of bound amines could be achieved in similar fashion by acidimetric indirect titration. The underlying kinetic and/or thermodynaic parameters governing the reactivity of the bound group are, however, concealed by an indirect determination. The use of thermometric enthalpy titrations (TET) for the determination of analytes in solution containing large amounts of insolubles is well documented (7,8). Eatough et al. (9) used a discontinuous titration to study the adsorption of aniline on molecular sieves. It is a logical extension of this technique therefore to apply it to the titration of functional groups in the presence of an insoluble matrix. Motivation for the study contained in this report was provided by the observation that the enthalpogram obtained for the titration of an aminopropylsilylated (APS) silica gel with aqueous perchloric acid exhibited considerable curvature to an extent that precluded extrapolation of a meaningful end point (see Figure 1). Curvature of a titration curve in this fashion can be caused
0003-2700/62/0354-0902$0 1.25/0 0 1982 American Chemical Society
