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MENDELEEV COMMUN.,
199
1
Utilization of Polyurethane Foams in Sorption-Photometric Analysis
Stanislava G. Dmitrienko, Olga
A.
Kosyreva, Valentin
K.
Runov and Yuri
A.
Zolotov
Department of Analytical Chemistry, Faculty of Chemistry, M.
I/.
Lomonosov Moscow State University, 119899 Moscow,
USSR
Polyurethane foams may be applied as sorbents for element extraction in sensitive and selective analysis of metal
complexes by diffuse reflectance spectrometry.
Table
1
Distribution coefficients
(D)
of thiocyanate complexes of cobalt and iron(111) between different types of polyurethane foams and aqueous
solutionY
Polymer characteristics Mass of D/cm3
g-
'
(V
25 ml)
polyurethane
Type Monomer Linkage Average pore foam tablet/g Cob Fell"
size/mm
140 Ether Oxyethylene
1
.O
?
0.2 0.04 3.2
x
lo4 3.2
x
lo4
[4H2CH2o-In
75 Ether Oxypropylene 0.6
2
0.2 0.07 500 3600
[--CH2CHMeO--1.
130 Ether [+CH2)2o-C(Ok 1.2
?
0.2 0.075 400 1400
Ester (CH,),C(OH,
2200 Ester [X(O)(CH,)4O--ln 0.09 20 800
"
Polymer tablet dimensions: 16
x
10 mm. pH 2,
cKsc,
0.5 mol dm-3.
'
pH 1.5,
cKscN
2.0 mol dm-3.
Methods of analysis based on the combination of group
sorption preconcentration with subsequent multielement deter-
mination by atomic emission, X-ray fluorescence and other
techniques have been developed intensively.' Sorption methods
are not used so frequently with molecular spectroscopy detec-
tion,
e.g.
absorption spectroscopy, diffuse reflectance, lumin-
escence and photoacoustic
method^.'^
However, the pre-
viously devised methods are characterized by low absolute
limits of detection (10- lo-10-
I'
g) and high selectivity.
Common sorbents are chemically modified silica or organic
polymers with functional gro~ps.~,~
One might suppose that the use of polyurethane foam
sorbents for the extraction of elements from slightly acidic
solutions would be useful for sorption-photometric analysis.'
Polyurethane foams, polymers with ether or ester linkages
(Table 1) are macroporous hydrophobic membrane materials
with weak anion-exchange properties. Therefore they promised
to be very useful in sorption-photometric determination based
upon the extraction of coloured acid complexes of metals
(thiocyanate, cyanide ions) or colourless complexes with sub-
sequent sorbent processing by organic reagents. It also seemed
possible to produce a coloured complex on the sorbent after
modification by a hydrophobic organic photometric reagent.
Finally, the sorption of ion pairs containing hydrophobic
anions or cations could be effective. Thus attempts were made
to utilize polyurethane foams in colorimetric test determi-
nations of metals.'
We now report our investigation of the possibility of combin-
ing metal sorption on polyurethane foams and their subsequent
determination in the sorbent phase by diffuse reflectance spec-
trometry. The polymers used as sorbents are noted in Table 1.
Sorption on polyurethane foam tablets was performed in a
static which supported an even sorbent colour. Diffuse reflec-
tance spectra were produced on a 'Spectroton' c~lorimeter.~
An example of sorption-photometric determination of
metals in the form of their anion complexes is that or iron(111)
and cobalt thiocyanates. These compounds were sorbed on
polyurethane foams, keeping the pH between 0.5-5.0 and
0.5-2.0 and the KSCN concentration at 0.5 and
2
mol dm-3
respectively. The metal distribution ratios increased when ester
polyurethane foams were exchanged for ether-containing ones,
reaching a value of 3.2
x
lo4 cm3 g-' (Table 1). It is interesting
to note that increasing the probe volume leads to an increase in
the metal distribution ratio to lo5 cm3 g-I (solution volume
Fig.
1
Diffuse reflectance spectra of metal thiocyanate complexes
sorbed on polyurethane foams: iron(111)
(a);
cobalt (b); iron(111) and
cobalt (c); polyurethane foam
(d)
c,,/pg (0.05
g
foam)-': (a, c) 5;
cc./pg (0.05 g foam)-': (b, c) 50; cKscN/mol dm-'
(b)
0.5; (a, c,
d)
2;
pH
2
500 ml, polyurethane foam type 140); the recovery factor does
not change at all.
The diffuse reflectance spectra of the sorbed iron(111) and
cobalt thiocyanates are constructed according to the Gurevich-
Kubelk-Munk function coordinates10 [eqn
(I)],
where
E
is the molar absorption coefficient of the sorbate, cis its
concentration,
S
is the coefficient of reflection and
R
is the
diffuse reflectance. The spectra are presented in Fig. 1. The
spectral maxima of the iron(111) and cobalt complexes, 490 and
620 nm respectively, are practically identical with the maxima
of the absorption spectra for the higher thiocyanate complexes
of these metals after their extraction into oxygen-containing
solvents. Values of
F
(at constant metal concentration) are
Mendeleev Commun., 1991, 1(2), 75–77
Table
2
Characteristics of sorption-photometric methods of metal determination
Metal Reagent Limit of Concentration RSD,,, Interference ratios allowing determination
detectionl~g range/pg
Co KSCN 0.2 1-150 0.04 Ca, Mg, Ba, Sr, F-, acetate, tartrate 1
x
lo6; HPOI-, ascor-
bate, citrate 3
x
lo5; C20f- 2
x
lo5; Br-
4
x
10"; Cr"'
3
x
lo4;
I
2
x
10"; Ni 1
x
l(r; Mn" 2000, S203- 1400, Bin', Au"', Ag
1000;
TI'
800, WOI-,
Cu"
(in presence of 0.05 mol dm-3
Na2S203), Pb", Cd", Hg", Fe"' (in presence of 0.5 mol dm-3
NaF), As"' 100; Zn 5; Cu" 1
Fe KSCN 0.01 0.1-20 0.05 Ca, Mg, Ba, Sr, 1
x
lo6; citrate 1
x
10'; acetate
3
X
104; tartrate
1
x
lo4; CrV', C20f- 2000; Br-,
I-,
HPOf- 1000; TI' 200; Mn",
Pb", Cd", WOf
-
,
Au"' 100; Ag 50; Zn 15; Cum (in presence of
0.2 mol dm--' thiourea) 10; Co 5; As"', Hg", Cu" 1
Ni Dimethylglyoxime 0.3 3-100 0.07 Ca, Mg, acetate, tartrate 5000; ascorbate, F-
1000;
500;
HPOf-, S203-, Al"', Cr"'."', Cu", Zn, Cd" (in presence of
5
x
lo3 rnol dm-3 sodium citrate) Fen' (in presence of
0.3 mol dm-3 NaF) 100, Pb" 10; Co 2.5
CrV' Diphenylcarbazide 0.08 1-30 0.07 Cu", Zn 100; Co 10; Ni" 5
CrV' Diphenylcarbazide 0.01 0.155 0.09 Cu", Zn, Ni, Co,
Cd,
Feu' 10
(in presence of
tetraphenylborate)
proportional to the molar absorption coefficients of these
complexes in the extracts." It is thus possible to predict the
sensitivity of sorption-photometric determination of metals in
the form of thiocyanates.
The value of
F
is linearly proportional to metal concentra-
tion in solution, so this was the basis of the determination. The
main features of the methods discussed are summarized in
Table 2. The sensitivities and selectivities of these methods are
greater than those of all other techniques based on metal
thiocyanate extraction. Moreover, the new methods are char-
LL
acterized by a greater concentration range and the linear region
correlates with the initial linear section of the sorption iso-
therms of the complexes investigated.
The potential of the sorption-photometric methods widens
significantly when foams modified with organic reagents are
used. The most effective immobilization of reagents on poly-
urethane foams occurs in the presence of
plasticizer^.^
In
contrast to the known techniques for polyurethane foam
preparation we found that the optimal scheme for sorption-
photometric determination is as follows. Polymer tablets were
first impregnated with plasticizer, excess of the latter was
removed and the plasticized tablets were processed using a
small volume (0.2-0.5 ml) of the reagent in a volatile solvent.
This ~rocedure ensures both effective immobilization of the
reag&t and also its uniform distribution in the sorbent phase.
The possible applications
of
modified polyurethane
foams
Fig.
2
Diffuse reflectance spectra of metal complexes sorbed PO~Y-
urethane foams: nickel dimethylglyoximate (a); chrornium(v1) complex
were
shown by
the
sorption-photometric
determination
of
with diphenylcarbazide in absence (b) and in presence (c) of tetraphe-
nickel with dimethylglyoxime and chromium(~) with diphenyl-
nylborate ion; polyurethane foam
(4.
c,.lpg (0.04 g foam)-': (a, b)
carbazide. As for the thiocyanates, ether foams are the most
20; (,) 10;
CDimethylglyoxime
4.4
x
10-4 mol dm-)
g-
1; cD,p
effective. The conditions for complex formation by nickel and
0.05 mol dm-) g-I;
c,~,,~,,,~,,,,,,
1.2
x
mol dm-3. pH: (a) 8.9;
chromium with reagents in aqueous solution and on the
(b)
3.4;
(c)
1.6 mol dm-) H2S04.
sorbent surface are identical: p~-&10 and 2-4 respectively, the
same is true of the maxima of the absorption spectra and diffuse
reflectance spectra of the substances formed (Fig. 2). Details of
the methods are presented in Table 2. The sensitivities and
selectivities of the sorption-photometric methods are on the
same level as those of known techniques with the above
reagents.
'
The sorption of the cationic chromium(vr) complex with
diphenylcarbazide on polyurethane foam increases according
to the following series when various anions are used:
SO:-
S
NO,
<
MeCO,
<
C10i
<
B(Ph)i
In the case of the tetraphenylborate ion bathochromic and
hyperchromic shifts are observed in the diffuse reflectance
spectrum (Fig. 2). The intense blue colour of the sorbate may
result from charge transfer complex formation in the sorbent
phase; such a complex does not exist in solution. The intensity
of coloration of the polyurethane foams is at a maximum when
sorbing chromium from
1-5
mol dm-3
H2S04.
The limit of
chromium detection in the presence of tetraphenylborate ion
decreases up to 10 times (Table 2).
The ion pair sorption application is shown by the determi-
nation of iron(11)
tris(1,lO-phenanthrolinate).
As for the cati-
onic complexes its sorption increases following the anion series:
The absolute limit of detection of iron is
5
x
pg.
Our investigations reveal not only the potential of polyure-
thane foams for devising highly effective sorption-photometric
methods but also support the possibility of transferring the
Mendeleev Commun., 1991, 1(2), 75–77
MENDELEEV COMMUN.,
199
1
known laws of analytical reactions in solutions to those on a
sorbent surface.
Received in USSR,
14th
December
1990
Received in UK,
4th
January
1991;
Com.
01056898
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Mendeleev Commun., 1991, 1(2), 75–77
