Estimation of oxyhydroxide specific surface area using the amounts of OH groups adsorbed

Article abstract of DOI:10.1023/A:1013026300138

A procedure for pH-metric determination of the limiting adsorption of OH groups (A_OH) by Fe^III, Zr^IV, Cr^III, and In^III oxyhydroxide hydrogels from 0.1 and 1.0 M solutions of NaCl is described. Data on the molecular area occupied by a single OH group on the hydrogel surface (S_OH) and the S_spec values, which were calculated from A_OH and S_OH. are presented. The S_spec value does not depend on the pH of hydrogel precipitation: the true S_spec value can be determined only from sorption of the OH groups at the actual point of zero charge of the hydrogel. The A_OH values for hydrogels were found to change only slightly during aging of hydrogels in electrolyte solutions.

Full text of DOI:10.1023/A:1013026300138

1582
Russian Chemical Bulletin, International Edition, Vol. 50, No. 9, pp. 1582—1588, September, 2001
Estimation of oxyhydroxide specific surface area
using the amounts of OH groups adsorbed
S. I. Pechenyuk,« S. I. Matveenko, and V. V. Semushin
I. V. Tananaev Institute of Chemistry and Technology of Rare Elements and Mineral Raw Materials,
Kola Research Center of the Russian Academy of Sciences,
26a ul. Fersmana, 184200 Apatity, Murmansk Region, Russian Federation.
Fax: +7 (815 5) 57 6425. E-mail: root@chemy.kolasc.net.ru
A procedure for pH-metric determination of the limiting adsorption of OH groups (AOH
)
by FeIII, ZrIV, CrIII, and InIII oxyhydroxide hydrogels from 0.1 and 1.0 M solutions of NaCl
is described. Data on the molecular area occupied by a single OH group on the hydrogel
surface (SOH) and the Sspec values, which were calculated from AOH and SOH, are presented.
The Sspec value does not depend on the pH of hydrogel precipitation; the true Sspec value can
be determined only from sorption of the OH groups at the actual point of zero charge of the
hydrogel. The AOH values for hydrogels were found to change only slightly during aging of
hydrogels in electrolyte solutions.
Key words: hydrogel, oxyhydroxide, limiting adsorption, hydroxy group, specific surface
area, zero-charge point, aging, ionic medium.
Estimation of the specific surface area (Sspec) of
oxyhydroxides with electrolyte solutions, especially so-
lutions of alkali. In order to use the OH adsorption
values to estimate Sspec, it is necessary to study first the
pattern of the adsorption isotherm, i.e., the dependence
of the equilibrium (for crystalline oxides) or pseudo-
equilibrium (for gels) adsorption of OH groups on their
equilibrium concentration in solution. In addition, the
molecular area for a OH group on the oxyhydroxide
surface should be determined.
Published data6—17 concerning the molecular area of
an OH group are contradictory; surface concentrations
ranging from 1 to 15 OH groups per 1 nm2 have been
reported for various oxyhydroxides.
amorphous hydrogels of metal oxyhydroxides is a rela-
tively difficult task. Most of the known methods of
determination of Sspec are based on the adsorption of
various substances from solutions or from the gas phase.
Prior to measuring the adsorption of gases and vapors,
samples are pre-treated to remove sorbed water,1 and
this can considerably decrease the Sspec value. The
results obtained by traditional methods of measuring
Sspec in the analysis of amorphous metal oxyhydroxides
depend on the way of treatment of the colloid suspen-
sion and the particular procedure.2 Measurements of
adsorption from solutions give more reliable data on
Sspec in these systems. Drying of the sample prior to
BET measurements of Sspec causes degreadation of the
fine structure existing in the aqueous medium.3 The
estimation of Sspec from the size of gel particles1 is faced
with experimental difficulties and is not sufficiently
precise because it does not take into account the inter-
nal surface of particles.
In this work, we determined the limiting adsorption
values of OH groups for samples of FeIII, CrIII, ZrIV
,
and InIII oxyhydroxide hydrogels prepared by precipita-
tion at different pH (pHpr) and washed from the mother
liquor and also for samples of calcined oxides with
known Sspec, namely, γ−Al2O3, Cr2O3, Y2O3, Sm2O3,
and In2O3. The results were used to calculate the mo-
lecular area of a single OH group on the hydrogel
surface.
Obviously, the most reliable results could be gained
by a method based on adsorption of small-size particles
having a simple composition and a stable structure
whose concentration in solution can be easily, precisely
and quickly determined. In the case of metal oxy-
hydroxide hydrogels, OH groups can serve as this type
of particle.
Experimental
The samples of oxyhydroxide hydrogels were prepared by
previously described procedures.7,18—20 Reagent and analytical
grade initial chemicals were used. Solutions were prepared
using double-distilled water and recrystallized salts. Ferro-,
indio-, chromo-, and zirconogels in amounts of 0.007—0.008
moles each were prepared by alkaline hydrolysis of aqueous
solutions of Fe(ClO4)3 (81.75 g of Fe/L), In(NO3)3 (100 g of
In/L), and CrCl3 (38.93 g of Cr/L), respectively, to pH up to 7,
Previously,4,5 we found that oxyhydroxides of FeIII
,
CrIII, ZrIV, TiIV, InIII, ZnII, AlIII, and other metals are
capable of adsorbing substantial amounts of OH groups
on the surface. This can take place both in the processes
of hydrolytic precipitation of oxyhydroxides from solu-
tions of salts and upon contacting dry oxides or gel-like
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1505—1511, September, 2001.
1066-5285/01/5009-1582 $25.00 ©ꢀ2001 Plenum Publishing Corporation

Oxyhydroxide specific surface area
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 9, September, 2001 1583
Table 1. Example of calculation of the amount of OH groups adsorbed in the check experiment
Vò/mL
Vsusp
/L
[OH]0*
/g-ion L
ðÍ
of filtrate
(aOH eq
/g-ion L
)
VHCl
/mL mL
[OH] eq
/g-ion L
AOH
/g-ion g
–1
–1
–1
–1
–1
–7
0
5
17
30
40
50
0.2
0
4.89
5.82
11.30
12.03
12.10
12.35
<10
<10
0
0
0
<10
0.00206
0.0113
0.0191
0.0261
0
–7
–7
0.205
0.217
0.23
0.24
0.25
0.00506
0.01625
0.02705
0.03457
0.04148
0.00086
0.00242
0.00285
0.00292
0.00303
0.00199
0.01071
0.01259
0.02239
0.0206
0.113
0.191
0.261
Note. Experimental conditions: Zr(OH)4, pHpr = 5.5, 0.1 Ì NaCl.
* The value calculated with allowance for the concentration of the alkali added (0.104 N ).
9, and 11; and by dissolution of 2.19 g of ZrO(NO3)2•H2O in
50 ml of water followed by neutralization with 2 M NaOH up
to pH 5.5, 7, and 10. The precipitation was carried out with
permanent pH-metric monitoring until a specified pHpr (ðH1)
value invariable for ∼5 min was attained in ∼50 ml of the
suspension. The precipitates were separated from the mother
liquor by centrifuging, washed three times with decantation of
water, and centrifuged at n = 2000 rpm for 5 min.
The amounts of OH groups adsorbed were determined by
titration of hydrogel suspensions with aqueous NaOH. The
titration was conducted on a pH-121 pH-meter under Ar in a
quartz half-open cell with an ESL-43-07 glass electrode and a
EVL-1M1 auxiliary electrode immersed in the suspension. Prior
to titration, argon was passed for 1.5 h through the gel suspen-
sion and through a solution of the blank electrolyte (NaCl) in
order to remove the dissolved CO2; then 100 mL of the
suspension was added to 100 mL of the electrolyte solution
prepared in such a way that the concentration of NaCl in the
final volume of the suspension adjusted to a value of 1.0 or
0.1 mol L–1. The pH value of the suspension at this instant was
taken as the initial point of the titration curve. Then, a 0.2 N
solution of NaOH was added in portions, the system being kept
for 5 min after the addition of each portion for the pH to settle.
The same volume of the blank electrolyte solution was titrated
in a similar way. The concentration of adsorbed hydroxyl ions
AOH (g ion g–1) was calculated from the formula
To determine the activity coefficients (γOH) and to check
the readouts of the glass electrode, we determined the true
pseudo-equilibrium* concentrations of the OH groups by titra-
tion of the alkali with an acid.
Let us consider the check experiment taking Table 1 as an
example. Five portions of the alkali were added to a hydrogel
sample (200 mL) in the same cell under argon. After the
addition of the first portion (5 mL), one-fifth of the suspension
volume (41 mL) was taken and filtered through a fine paper
filter. Then pH of the the filtrate was measured and the filtrate
was titrated with 0.1 N HCl. This gave the equilibrium concen-
tration [OH]eq corresponding to the given titrant volume
VT = 5 mL on the titration curve. It can be seen that at this
point of the curve, all the alkali added was absorbed by the gel.
The next portion of the alkali was added to the remaining 80%
of the suspension, the alkali amount being 80% of that needed
for 200 mL of the suspension ((17 – 5)•0.8 = 9.6 mL).
One-fourth of the remaining suspension was withdrawn
((205 – 41 + 9.6)/4 = 43.4 mL) and treated as described above.
The residual concentration of the OH groups was 0.00206 N.
The amount adsorbed AOH calculated using Eq. (1) was
{(0.01625
– 0.00206)•0.173.6 L}/1.27 g Zr(OH)4•0.8 =
0.002424 g-ion OH/g. The next portion of the alkali was added
to the remaining 60% of the suspension, one-third of the
remaining suspension volume was taken, and so on. Finally, we
got five values of equilibrium concentration [OH]eq corre-
sponding to five points on the pH-metric titration curve. For
convenience and clearness of calculations, the titrant volume
was normalized to 100% of the suspension volume.
AOH = ([OH]0 – [OH]eq)Vsusp
,
(1)
where [OH]0 is the concentration of the OH groups in the
blank solution (g-ion g–1) and [OH]eq is the equilibrium con-
centration of the OH groups in the suspension (g-ion g–1).
The experiments showed that titration was reversible;* hence,
the samples virtually did not change during the titration.
The pH measurements produce the activity values aOH
Based on the aOH and [OH]eq values obtained in the check
experiments, the activity coefficients (γOH) were calculated;
they are given in Table 2. The coefficients were used to
construct a calibration plot suitable for determination of γOH
for any aOH value within the pH range studied. Examples of
calculations for the correction of the pH-metric titration curves
to obtain the adsorption isotherm** are given in Tables 3 and 4.
Figures 1 and 2 show examples of the resulting isotherms of
adsorption of the OH groups by various oxyhydroxide samples.
–
rather than the true OH concentrations; hence, the activity
coefficients of the OH groups should be taken into account.
Besides, for ðH > 12, the addition of new small portions of the
alkali induced very slight changes in the pH, which deteriorates
the accuracy of measurements in this pH region. It is also
necessary to check the readouts of a glass electrode kept in an
alkaline solution for a long period.
*An equilibrium concentration of OH groups is called pseudo-
equilibrium merely because the phases of amorphous hydrogels
are thermodynamically in a nonequilibrium state. Thus, the
system as a whole is in a nonequilibrium state, and the equilib-
rium established between the amorphous gel and an alkali
solution is apparent (i.e., it is a pseudo-equilibrium), although
this state is retained for long periods.
** Titration was conducted at a temperature of 22±4°C; since
the ionic product of water Kw changes insignificantly over this
temperature range, the dependence obtained can be considered
to be an isotherm.
* The reversibility of titration means that upon the back titra-
tion of the alkali by an acid, i.e., upon the addition of acid in
amounts equivalent to the amount of the alkali added during
ditect titration results in the same pH, and the titration curves
for the direct and back titrations thus coincide. This implies
that titration with an alkali does not cause any irreversible
changes in the sample.

1584
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 9, September, 2001
Pechenyuk et al.
–1
Table 2. Activity coefficients of the OH groups in the region of
adsorption measurements (1 to 50 mL of alkali added) in
0.1 Ì NaCl
AOH•103/g-ion g
1
4
–
2
[OH ]*
ðÎÍ
aOH
/g-ion L
γÎÍ**
–1
–1
/g-ion L
3
2
1
3
0.001075
0.00206
0.00235
0.00319
0.00527
0.00831
0.00905
0.0113
0.0126
0.0141
0.0169
0.0182
2.95
2.70
2.63
2.51
2.30
2.13
2.10
1.97
2.03
1.95
1.88
1.86
1.90
1.83
1.79
1.76
1.65
1.73
1.67
1.68
1.61
1.63
1.62
0.00112
0.00200
0.00234
0.00309
0.00501
0.00741
0.00794
0.01072
0.00933
0.0112
0.0132
0.0138
0.0126
0.0148
0.0162
0.0174
0.0224
0.0186
0.0214
0.0209
0.0245
0.0234
0.0240
1.0
0.97
0.99
0.97
0.95
0.89
0.88
0.94
0.83
0.79
0.78
0.76
0.66
0.75
0.73
0.70
0.86
0.66
0.68
0.65
0.63
0.59
0.55
0
–1
0.01
0.02
0.03 [OH]eq/g-ion L
Fig. 1. Adsorption isotherms of OH groups (AOH) on the
surface of freshly precipitated CrIII oxyhydroxides with precipi-
tation pH of 7 (1); 8 (2); 9 (3). [OH]eq is the equilibrium
concentration of OH groups in the suspension.
0.0191
0.0196
0.02228
0.02485
0.0261
0.02817
0.0316
0.03217
0.0392
0.03967
0.0432
AOH•103/g-ion g
–1
1
3
2
2
3
* Calculation.
** Experiment.
1
Table 5 presents the limiting adsorption values AOH for hydrogels
freshly precipitated at various pHpr
.
To find the molecular area of an OH group SOH, we
attempted to use calcined crystalline samples of oxides with
constant Sspec values which were assumed to remain unchanged
on contact with a solution. The samples used in these experi-
ments are listed and described in Table 6, which also gives the
amounts of OH groups adsorbed by these samples.
0
–1
0.01
0.02
0.03 [OH]eq/g-ion L
Fig. 2. Adsorption isotherms of OH groups (AOH) on the
surface of freshly precipitated ZrIV oxyhydroxides with precipi-
tation pH of 5.5 (1); 7 (2); 10 (3). [OH]eq is the equilibrium
concentration of OH groups in the suspension.
Table 3. Calculation of the amounts of OH groups adsorbed on the freshly precipitated ferrogel
with pHpr 8 using the data of pH-metric titration of a suspension in 0.1 M NaCl
Vtitr
/mL
[OH]0
/g-ion L
pOHsusp
(aOH eq
/g-ion L
)
γÎÍ
[OH] eq
/g-ion L
AOH
/g-ion g
–1
–1
–1
–8
–1
–8
0
1
3
5
0
7.44
4.85
3.40
2.80
2.50
2.33
2.18
2.06
1.99
1.95
1.90
1.85
1.77
1.74
1.69
1.66
3.63•10
1.41•10
1.0
1.0
1.0
3.63•10
1.41•10
0
–5
–5
0.001075
0.003190
0.005270
0.008310
0.011260
0.01410
0.01690
0.01960
0.02228
0.02485
0.02817
0.03217
0.03600
0.03967
0.04320
0.000213
0.000567
0.000749
0.001050
0.001326
0.001446
0.001497
0.001632
0.001918
0.002014
0.002261
0.002018
0.002309
0.002024
0.002117
0.000398
0.001585
0.003162
0.004677
0.006607
0.00871
0.01023
0.01122
0.01259
0.01412
0.0170
0.000398
0.001617
0.003260
0.004976
0.00734
0.01001
0.01218
0.01368
0.01594
0.01834
0.02358
0.02638
0.03141
0.03473
0.98
0.97
0.94
0.90
0.87
0.84
0.82
0.79
0.77
0.72
0.69
0.65
0.63
8
11
14
17
20
23
26
30
35
40
45
50
0.01820
0.02042
0.02188

Oxyhydroxide specific surface area
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 9, September, 2001 1585
Table 4. Calculation of the amounts of OH groups adsorbed on the freshly precipitated
chromogel with pHpr 8 using the data of pH-metric titration of the suspension in 0.1 M NaCl
Vtitr
/mL
[OH]0
/g-ion L
pOHsusp
(aOH eq
/g-ion L
)
γÎÍ
[OH] eq
/g-ion L
AOH
/g-ion g
–1
–1
–1
–8
–1
–8
0
1
3
5
0
6.89
5.79
3.95
3.00
2.70
2.45
2.25
2.13
2.04
1.95
1.91
1.88
1.78
1.69
1.29•10
1.62•10
1.0
1.0
1.0
1.0
1.29•10
1.62•10
0.000112
0.001
0
–6
–6
0.001075
0.003190
0.005270
0.008310
0.011260
0.01410
0.01690
0.01960
0.02228
0.02485
0.02817
0.03600
0.04320
0.000253
0.000753
0.001055
0.001572
0.001923
0.002059
0.002312
0.002844
0.002310
0.002581
0.003123
0.003487
0.003552
0.000112
0.001
8
0.001995
0.003548
0.005623
0.007413
0.00912
0.01122
0.01230
0.01318
0.0170
0.98
0.96
0.92
0.89
0.85
0.82
0.80
0.78
0.71
0.65
0.002036
0.003696
0.006112
0.008058
0.01099
0.01368
0.01537
0.01690
0.02394
0.03141
11
14
17
20
23
26
30
40
50
0.02042
The Sspec values were measured using the nitrogen
thermodesorption method on a Flowsorb-II 2300 laboratory
electron surface analyzer (for Al and Cr oxides) or using the
low-temperature nitrogen desorption method on a Carlo Erba
Strumentazione Microstructure Lab. Sorptomatic 1900 instru-
ment (for Y, Sm, and In oxides).
ing adsorption AOH value corresponds to the plateau on
the curve of the specific amount of sorbed hydroxyl ions
vs. pH of the equilibrium solution. This value was used
to calculate Sspec
.
–
It can be seen that the amounts of OH groups
adsorbed at saturation decrease with an increase in pHpr
for all the hydrogels studied. Meanwhile, we found
previously5 that oxyhydroxide hydrogels possess a true
point of surface zero charge, which corresponds to a
definite precipitation pH (pHPZC), does not depend on
electrolyte concentration in the suspension, and is a
constant for a given initial compound and a given ionic
medium. Evidently, hydrogels obtained at pHpr values
lower than that corresponding to the true point of zero
charge (pHPZC) have a positive surface charge, and
some of the OH groups are consumed for neutralizing
this charge. As a result, the AOH and Sspec values
obtained are overestimated. On the contrary, when pHpr
is higher than that of the true point of surface zero
charge (pHPZC), the hydrogel surface is already partly
occupied by sorbed OH groups, and the determined
AOH and Sspec values are underestimated. Thus, the
actual AOH and Sspec values are found when pHpr corre-
sponds to the true pHPZC of hydrogels in the given
electrolyte.
Results and Discussion
The shape of the isotherms is associated with so-
called “high-affinity” or H-type isotherms.21 The limit-
Table 5. Pseudo-equilibrium values of the amounts of OH
groups sorbed on freshly precipitated hydrogels in 0.1 M NaCl
pHpr
Zr
Ñr
Fe
In
5.5
7
0.0033
0.0024
—
—
0.0015
—
—
0.0041
0.0033
0.0025
—
—
0.0024
0.0021
0.0015
—
—
0.00135
—
0.00115
—
8
9
9.5
10
—
—
0.0008
Table 6. The results of determination of SOH for dry oxides
The published data on the sorption capacity of
oxyhydroxides toward OH groups are rather contradic-
tory. Thus it has been reported6 that a ferrogel sample
precipitated from a solution of Fe(NO3)3 by the addi-
tion of aqueous NaOH up to pH 8 had a surface density
of sorption sites Cs of 7–8 sites nm–2. This Figure
involves all the surface sites, which can be occupied
either by OH groups or by protons, depending on the
conditions. Obviously, not all of these sites can be
occupied by OH groups. We found4 that freshly precipi-
tated ferrogels Fe(OH)3 have a pseudo-equilibrium ca-
Oxide
Pretreatment
Sspec
AOH
/ g-ion g
SOH
–1
–1
/m2
g
/Å2
γ-Al2O3 Calcined for 2 h
at 1000 °C
Cr2O3
73.6
0.0002
60
Obtained by decomposi-
tion of (NH4)2Cr2O7
and calcined for 2 h
at 1000 °C
Air-dry,
reactive
–5
3.9
12.6
3.6
1.4•10
2.6•10
5.2•10
1.2•10
46
80
38
45
Y2O3
–5
Sm2O3 Air-dry,
reactive
In2O3
–5
–5
–3
pacity for OH groups of (1.0—1.5)•10 g-ion g–1. If
Air-dry,
reactive
we assume that Sspec for these samples is equal to
g
600—800 m2 –1, the molecular area for an OH group
3.25

1586
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 9, September, 2001
Pechenyuk et al.
(SOH) would be 66—133 Å2, which corresponds to Cs of
1 or 2 OH groups per nm2. Having measured the values
of pseudo-equilibrium sorption of chlorine-containing
complexes of rhodium and iridium,7 we found the Cs
value for freshly precipitated ferrogels to be 2 or 3 OH
groups per nm2. Other works8,9 give the following Cs
values for goethite: 2.7 OH groups per nm2 and 3 or 4
sorption sites per nm2. The published data for FeIII
oxyhydroxides are also diverse,10—15 namely, 5, 2, 7,
and 15 OH groups per nm2 (for akaganeite). The highest
amount of OH groups sorbed on a neutral γ-Al2O3
surface can be estimated as 2 OH groups per nm2; the
values for the ZrIV oxyhydroxide surface are 8 15 and
516, and that for anatase17 is 1 OH group per nm2. The
data for other oxyhydroxides are scarce. The scatter of
the published data on the sorption capacity of oxy-
hydroxides toward OH groups can be explained by the
fact that the researchers cited used hydrogels with a
nonzero charge of the surface.
all the specific features of the behavior of hydrogels in
the determination of their ðHPZC
.
According to the data obtained, of the four hydrogels
studied, the highest capacity toward OH groups was
shown by chromogel, while the lowest one was noted
for indiogel. Previously, we showed that in the
pHpr range of 7—11, indiogels have the composition
In(OH)3•0.5H2O•0.21NaOH. Sodium hydroxide is
strongly bound to indium hydroxide and is not
washed out by water. In the case of indiogel, AOH
=
–4
–1
9•10 g-ions g corresponds to a molar ratio of 0.15,
hence, one mole of In(OH)3 binds 0.36 OH groups and
3 moles of In(OH)3 bind up to one OH group.
Ferrogels and zirconogels are characterized by very
close AOH values near the ðHPZC point; these hydrogels
also exhibit similar sorption and acid-base properties at
almost equal ionic potentials.5 Accordingly, similar Sspec
values can be expected of the hydrogels. However, the
differences between AOH and, correspondingly, Sspec
values of chromogels and ferrogels are very great over
the whole range of pHpr. This explains why sorption
activities of these oxyhydroxides are sharply different,
despite close acid-base properties and equal ionic po-
tentials of FeIII and CrIII cations.5 Hence, the differ-
ence between the properties of chromogels and ferrogels
is due to different mechanisms of their hydrolytic poly-
merization during alkaline hydrolysis.
Let us consider the Sspec values and the surface
charge of hydrogels in order to analyze data on the
kinetics of heterogeneous hydrolysis.4 Weakly basic
indiogel has a positively charged surface up to pHpr 10,
the surface charge being dependent on the pHpr only
slightly, and the surface area is much lower than that of
other hydrogels studied. These properties can be respon-
sible for the lower sorption activity of indiogel as com-
pared to other hydrogels, this activity being influenced
only slightly by the pHpr.4,5 It is more difficult to
explain why the large surface area of chromogel does
not provide the corresponding increase in the sorption
activity. Taking account of our previous study on the
sorption of negatively charged complex ions,4 we can
suggest that an increase in the sorption activity is coun-
terbalanced by the high concentration of the surface
negative charge on chromogels with pHpr > 8. In addi-
tion, the amphoteric character of CrIII hydroxide im-
plies its higher affinity to OH groups*.
After plotting the dependence of AOH on pHpr (Fig. 3)
using the data of Table 5, we can find the AOH values
corresponding to the noncharged surface:
Zr
Cr
Fe
In
5
ðHPZC
AOH•104/g-ion g
9.4
14
8.4
31
8.1
19.5
10
9
–1
Figure 3 indicates that the dependences of AOH and
4
ðHPZC on the gel pHpr are linear. This proves that the
variation of AOH is governed by the surface charge but
not by the surface area.
Apparently, the Sspec of a hydrogel is dependent on
the nature of initial salt, the swamping electrolyte ions,
the precipitating agent, and the temperature of precipi-
tation but not on the pHpr, while the pHpr influences
only the surface charge. This, apparently, accounts for
–1
AOH•103/g-ion g
4
3
The variation of AOH for hydrogels on aging in
electrolyte solutions are presented in Table 7. It can be
seen that the AOH values for gels aged for 2 h and for 20
h at temperatures near the water boiling point differ
little from each other and from values for freshly pre-
pared samples. This agrees with observations indicating
that the sorption activity22 changes only slightly during
aging under the same conditions. Contrary to the com-
mon opinion that the composition and properties of
1
2
2
3
1
4
5
7
9
pHpr
Fig. 3. Dependence of the limiting adsorption of OH groups
(AOH) by oxyhydroxide hydrogels on the precipitation pH:
(1) chromogels, (2) zirconogels, (3) ferrogels, (4) indiogels.
* The ability of Cr(OH)3 to form anionic hydroxo complexes
–
[Cr(OH)4] or [Cr(OH)6]–3, which is not typical of iron(III)
hydroxide.

Oxyhydroxide specific surface area
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 9, September, 2001 1587
–1
Table 7. Amounts of OH groups adsorbed at limiting sorption
on the surfaces of hydrogels precipitated at pHpr 7 and 8 and
aged by heating in electrolyte solutions
AOH•103/g-ion g
3
2
1
1
Aging
Gel (pHpr)
conditions
Zr
Fe
Cr
In
(7)
(8)
(8)
(7)
2
NaCl, 1 Ì, 2 h, 80 °C
0.0025 0.0012 0.0030 0.00115
Na2SO4, 0.5 Ì, 2 h, 80 °C 0.0024 0.0015 0.0030 0.0011
3
NaCl, 1 M, 20 h, 96 °C
Na2SO4, 0.5 Ì, 20 h, 96 °C 0.0024 0.0017 0.0031 0.0012
0.0020 0.0015 0.0026
—
4
amorphous oxyhydroxides change very rapidly, these
data demonstrate that FeIII, CrIII, InIII, and ZrIV
oxyhydroxides are stable (or metastable) systems, the
surface charge being the only parameter that can be
easily changed.
0
–1
0.01
0.02
0.03 [OH]eq/g-ion L
As noted above, the specific surface area can be
calculated from the limiting sorption values AOH if the
value of molecular area of an OH group on the oxy-
Fig. 4. Adsorption isotherms of OH groups (AOH) on the
surface of the oxyhydroxide hydrogels freshly precipitated from
a 1 M solution of NaCl: (1) chromogel, pHpr 9.5; (2) zirconogel,
pHpr 9.5; (3) ferrogel, pHpr 8; (4) indiogel, pHpr 10.
hydroxide surface is known; we denote this value by SOH
.
The SOH value (in Å2) was calculated from the AOH
values for crystalline samples with known Sspec (see
Table 6) or using published data. The SOH, AOH, and
Sspec values are related to each other by the expression:
strength is the same for all the oxyhydroxides and is about
50 Å2. This is the main assumption we made in calcula-
tions. A Cs value equal to 2 OH groups per nm2 is often
reported for hydrogels. Using the value SOH = 50 Å2
and the above data for hydrogels, we derive the follow-
ing Sspec values:
–1
Sspec = SOHAOHNA/1020 m2
g
,
(2)
where NA is the Avogadro constant and 1 m2 = 1020 Å2.
Thus, from known Sspec, we can get
Zr
Sspec /m2•g E(OH)n 420
Cr
930
Fe
585
I
240
–1
SOH = Sspec•1020/AOHNA.
(3)
The resulting values are at the same level as those
usually reported for amorphous oxyhydroxides. Obvi-
ously, these values are closer to actual Sspec than the
data for surface areas found by the BET method for pre-
dried amorphous samples. For example, the Sspec value
obtained by the BET method for the FeIII oxyhydroxide
precipitated25 from a solution of FeIII nitrate on treat-
ment with ammonia at pH 7.5 and dried over phospho-
It follows from Table 6 that the SOH values for
crystalline dry samples are highly scattered. It is known23
that, on contacting oxides with water or aqueous solu-
tions of electrolytes, gel layers of oxyhydroxides are
formed on their surfaces, although it is unclear to what
extent this affects the Sspec value. It is also known24 that
oxides of rare earth elements (here, Y and Sm)24 rapidly
absorb water molecules from air or aqueous solutions to
give Ln(OH)3 hydroxides. The hydration of the Sm2O3
oxide to give Sm(OH)3 should proceed to a much
higher extent than that of Y2O3.24 It can be suggested
that this induces an at least twofold increase in its
surface area, resulting in SOH ≈ 45—50 Å2. The pseudo-
equilibrium in the hydration of these air-dry oxides at
∼20°C is established over a period of 10 to 15 min4;
hence, under the conditions of our experiments, attain-
ment of such a pseudo-equilibrium is quite likely.
ric anhydride at ∼20°C is ∼300 m2
g
–1. For the
oxyhydroxide precipitated from a solution of FeIII per-
chlorate at pH 7, the pH-metric titration method gives
∼600 m2
g
–1. The Fe(NO3)3/NaOH ferrogel6 with
pHpr = 8 aged in the mother liquor for 4 h and dried
had Sspec of ∼300 according to the BET method, or ∼350
–1
from the of negative adsorption* of Na+, or ∼700 m2 g
from the negative adsorption* of Mg2+
.
Thus, the data we obtained with some assumptions
give reasonable Sspec values.
Apparently, the surfaces of polymeric oxyhydroxides
do not possess a close packing of atoms, typical of
surfaces of nonhydrated crystalline oxides. The ion radii
of the central atoms of the oxyhydroxides studied differ
from each other by a factor of not more than 1.5.5
Therefore, the hydrogel surfaces should be uniform
networks of polymeric chains. A plausible assumption is
that the molecular area of a OH group at a given ionic
The size of the molecular area and the specific
surface area of a hydrogel can depend on the ionic
strength of the swaming electrolyte. In order to verify
this assumption, a number of experiments were carried
out in which sorption isotherms of OH groups were
measured using a 1 M solution of NaCl. The results
* A method of measuring the specific surface area.6

1588
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 9, September, 2001
Pechenyuk et al.
Table 8. Activity coefficients of the OH groups in the region of
adsorption measurements (from 1 to 50 mL of alkali added) in
1 Ì NaCl
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–
[OH ]*
ðÎÍ
aOH
/g-ion L
γÎÍ
–1
–1
/g-ion L
0.00023
0.00053
0.00206
0.00724
0.00724
0.0099
0.01741
0.01788
0.02071
0.0212
0.02447
0.02606
0.02865
0.0287
0.03094
0.03294
0.03565
0.03618
3.89
3.54
2.92
2.32
2.35
2.26
2.01
1.99
1.89
1.98
1.86
1.85
1.79
1.86
1.72
1.75
1.74
1.67
0.000129
0.000288
0.00120
0.00478
0.00447
0.00549
0.00977
0.01023
0.01288
0.01047
0.01380
0.01412
0.01622
0.01380
0.01906
0.01778
0.01820
0.02138
0.56
0.54
0.58
0.66
0.62
0.55
0.56
0.57
0.62
0.49
0.56
0.54
0.57
0.48
0.62
0.54
0.51
0.59
12. K. F. Hayes and J. O. Leckie, J. Colloid Interface Sci.,
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* From the results of titration.
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obtained are presented in Fig. 4. Table 8 gives the results
of calculation of γOH in a 1 M solution of NaCl using
the data of titration and pH measurements by a glass
electrode. It can be seen that the activity coefficient in a
1 M solution, unlike that in 0.1 M solutions, is virtually
constant (0.56±0.023). Due to a higher ionic strength of
the inert electrolyte, the changes in aOH are offset by its
own OH groups. Meanwhile, the AOH values obtained by
titration in a 1 M solution of NaCl virtually coincide
with those obtained for a 0.1 M solution. Therefore, the
molecular area of a OH group does not depend on the
ionic strength of the solution. It may be recommended
to perform the measurements at an ionic strength equal
to 1 using NaCl because in this medium, the γOH values
are constant and the glass electrode is protected from
the action of alkali by the sodium ions present in the
solution at high concentrations.
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2001, À180, 259.
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28, 2125.
This work was financially supported by the Russian
Foundation for Basic Research (Project No. 00-03-
32234).
References
1. S. J. Gregg and K. S. Sing, Adsorption, Surface Area, and
Porosity, Academic Press, London and New York, 1967.
Received February 2, 2001;
in revised form May 17, 2001