Electrode membrane

The electrode membrane of the invention has high ion-selectivity for lithium ions relative to other alkali and alkaline earth metal ions. The membrane is shaped from a polymeric composition comprising a polymeric material, e.g., a polyvinyl chloride resin and poly(1,2-butadiene), a plasticizer and, as the ion carrier, a derivative of 1,10-phenanthroline such as 2,9-dimethyl-1,10-phenanthroline and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline.

BACKGROUND OF THE INVENTION 
The present invention relates to an electrode membrane having selectivity 
for lithium ions. 
It is eagerly desired to develop reliable lithium ion-sensitive electrode 
membranes used, for example, for the determination of lithium ions in 
blood serum in the clinical inspection of patients suffering from manic 
depressive pyschosis. 
Various types of polymeric membranes have been hitherto proposed and 
studies to provide a membrane electrode having selectivity for lithium 
ions. The polymeric membrane of this kind has a basic structure formed of 
a polymeric composition composed of a film-forming polymeric resin, a 
plasticizer, an ion-sensitive substance called an ion carrier and, 
optionally, an alkali metal salt of a hydrophobic anion. For example, a 
lithium ion-selective polymeric electrode membrane can be prepared from a 
polymeric composition comprising a polyvinyl chloride resin, a plasticizer 
such as 2-nitrophenyl octyl ether, an ion carrier such as 
N,N'-diheptyl-N,N'-5,5-tetramethyl-3,7-dioxsanonane diamide and, according 
to need, an alkali metal salt of a hydrophobic anion [see, for example, E. 
Metzger et al., Helv. Chim. Acta, volume 69, page 1821 (1986) and K. 
Kimura et al., J. Chem. Soc. Perkin Trans., II, 1986, page 1945]. 
None of these lithium ion-selective polymeric membranes, however, is quite 
satisfactory in respect of the selectivity for lithium ions relative to 
other alkali and alkaline earth metal ions. 
SUMMARY OF THE INVENTION 
An object of the present invention is therefore to provide a novel 
polymeric electrode membrane having outstandingly high selectivity for 
lithium ions not obtained in any prior art membranes. 
Thus, the present invention provides an electrode membrane having 
selectivity for lithium ions which is shaped from a polymeric composition 
comprising, in admixture: 
(a) a polymeric material; 
(b) a plasticizer; and 
(c) an aromatic compound having a structure of 1,10-phenanthroline 
represented by the general formula 
##STR1## 
in which R.sup.1 and R.sup.2 are each an atom or group selected from the 
class consisting of a hydrogen atom, alkyl groups, aryl groups and aralkyl 
groups and R.sup.3 and R.sup.4 are each a group selected from the class 
consisting of alkyl groups, aryl groups and aralkyl groups.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The polymeric resin, which is the base ingredient as the component (a) in 
the polymeric composition from which the inventive electrode membrane is 
formed, is not particularly limitative and various polymeric materials 
conventionally used in electrode membranes can be used including synthetic 
and natural polymeric materials exemplified by polymers and copolymers of 
ethylenically unsaturated monomers such as polyvinyl chlorides, 
polyethylenes, poly(1,2-butadienes) and the like, polycondensation 
polymers such as polyesters, polyamides and the like, polymers by 
ring-opening polymerization such as epoxy resins and the like, silicone 
rubbers, urushi resin and so on. 
The component (b) in the polymeric composition forming the inventive 
electrode membrane is a plasticizer which is also not particularly 
limitative including conventionally used ones such as dialkyl aryl 
phosphonates, trialkyl phosphates, trialkyl phosphites, dialkyl sebacates, 
dialkyl adipates, dialkyl phthalates, nitrophenyl alkyl ethers, 
2-nitrophenyl aryl ethers and the like. These plasticizers can be used 
either singly or as a combination of two kinds or more according to need. 
The component (c), which is the most characteristic ingredient in the 
polymeric composition forming the inventive electrode membrane, is a 
compound represented by the above given general formula (I), in which the 
symbols R.sup.1, R.sup.2, R.sup.3 and R.sup.4 each have the meaning 
defined above, and serves as an ion carrier. The compound is a derivative 
of 1,10-phenanthroline including various compounds corresponding to the 
kinds and combinations of the groups denoted by R.sup.1, R.sup.2, R.sup.3 
and R.sup.4. Examples of preferable derivatives of 1,10-phenanthroline are 
2,9-dimethyl-1,10-phenanthroline, 
2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, 
2,9-dibutyl-1,10-phenanthroline, 
2,9-dibutyl-4,7-diphenyl-1,10-phenanthroline, 
2,9-dioctyl-1,10-phenanthroline, 2,9-dibenzyl-1,10-phenanthroline, 
2,9-diphenyl-1,10-phenanthroline and the like though not particularly 
limited thereto. It is optional that the polymeric composition is further 
admixed with various known additives including alkali metal salts of a 
hydrophobic anion such as alkali metal salts of tetraphenyl boric acid, 
optionally, substituted on the aromatic nuclei with substituents such as 
halogen atoms. 
The compounding proportion of the above described components to form the 
polymeric composition is such that 100 parts by weight of the polymeric 
material as the component (a) are compounded with 10 to 300 parts by 
weight or, preferably, 200 to 260 parts by weight of the plasticizer as 
the component (b), 0.1 to 15 parts by weight or, preferably, 0.5 to 10 
parts by weight of the ion carrier as the component (c) and, when added, 
0.5 to 15 parts by weight or, preferably, 3 to 5 parts by weight of the 
alkali metal salt of a hydrophobic anion. 
The above described polymeric composition can be shaped into a membrane by 
a method of, for example, casting of a solution containing the component 
ingredients. The membrane should preferably has a thickness in the range 
from 0.02 to 1.0 mm. The membrane serves as a liquid-film type 
ion-selective electrode membrane. Namely, the polymeric membrane is used 
as a partition membrane and, when it is contacted with a sample solution 
containing various kinds of ions on one surface and with a reference 
solution containing the cations of a specific kind in a known 
concentration on the other surface, a potential difference is produced 
between the surfaces of the membrane, from which the activity of the 
cations in the sample solution can be determined. 
The procedure for the determination of the activity of cations in a sample 
solution by use of the inventive electrode membrane is described below 
with reference to the FIGURE in the accompanying drawing which is a 
schematic illustration of a typical assembly for the determination of the 
potential difference between the surfaces of the membrane. The sample 
solution A in the vessel 1, which is usually an aqueous solution, contains 
the objective cations in an unknown concentration. A cylindrical vessel 2 
dipped in the sample solution A, of which the bottom wall 3 is formed of 
the inventive polymeric membrane, contains an aqueous reference solution B 
of the objective cations in a known concentration of, usually, 10.sup.-6 N 
to 1N. A first electrode 4 dipped in the reference solution B and a second 
electrode 5 dipped directly in the sample solution A are connected to the 
terminals of a potentiometer 6. In this assembly with the membrane 
electrode 3 contacting the sample solution A and reference solution B on 
both surfaces, a potential difference is produced at the interfaces of the 
membrane and the solutions and inside the membrane and can be determined 
on the potentiometer 6 connected to the electrodes 4 and 5, from which the 
activity of the cations in the sample solution A is calculated. 
In the following, the electrode membrane of the invention is described in 
more detail by way of examples. 
EXAMPLE 1 
A uniform solution of polymeric composition was prepared by dissolving 0.10 
g of a polyvinyl chloride resin, 0.25 g of 2-nitrophenyl octyl ether, 
0.005 g of 2,9-dimethyl-1,10-phenanthroline and 0.003 g of potassium 
tetrakis(4-chlorophenyl) borate in 4 ml of tetrahydrofuran and a polymeric 
film having a thickness of about 0.2 mm and a diameter of 4.2 cm was 
prepared by casting of the solution and evaporation of the solvent. 
An assembly for the determination of the potential difference as 
illustrated in the accompanying drawing was constructed, in which the 
electrode membrane had a diameter of 5 mm as taken from the above prepared 
polymeric film. The reference solution was an aqueous solution of lithium 
chloride in a concentration of 0.01N. Several aqueous solutions were 
prepared to serve as a simulated sample solution including aqueous 
solutions of potassium chloride, sodium chloride and lithium chloride each 
in a concentration of 0.01N, 0.1N or 1.0N and aqueous solutions of calcium 
chloride and magnesium chloride each in a concentration of 0.00002N, 
0.0002N, 0.002N, 0.02N or 0.2N. The test was performed at 25.degree. C. 
The calibration curve in each measurement exhibited sub-Nernstian 
response. 
Calculation was made for the ion-selectivity of the electrode membrane log 
K.sub.Li,j.sup.pot against the changes in the activity of the cations in 
the sample solutions to give the results of: zero for lithium ions; -2.5 
for sodium ions; -3.0 for potassium ions; -2.9 for magnesium ions; and 
-2.9 for calcium ions. These results indicate that the electrode membrane 
of the invention gives a selective response of electromotive force for 
Li.sup.+ ions relative to the cations of the other alkali and alkaline 
earth metals against the changes in the activity thereof under the above 
specified experimental conditions. 
EXAMPLE 2 
The experimental procedure was substantially the same as in Example 1 
excepting the replacement of the 2,9-dimethyl-1,10-phenanthroline with the 
same amount of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline. The values 
of the ion-selectivity of the membrane log K.sub.Li,j.sup.pot against the 
changes in the activity of the cations in the sample solutions calculated 
from the results of the potentiometric measurement were: zero for lithium 
ions; -3.0 for sodium ions; -3.3 for potassium ions; -2.0 for magnesium 
ions; and -3.0 for calcium ions. These results indicate that the electrode 
membrane of the invention gives a selective response of electromotive 
force for Li.sup.+ ions relative to the cations of the other alkali and 
alkaline earth metals against the changes in the activity thereof under 
the experimental conditions. 
EXAMPLE 3 
The experimental procedure was substantially the same as in Example 1 
excepting replacement of the polyvinyl chloride resin with a 
poly(1,2-butadiene). The values of log K.sub.Li,j.sup.pot obtained for 
various cations were: zero for lithium ions; -2.0 for sodium ions; -2.2 
for potassium ions; -1.5 for magnesium ions; and -1.8 for calcium ions. 
EXAMPLE 4 
The experimental procedure was substantially the same as in Example 1 
excepting omission of the potassium tetrakis(4-chlorophenyl) borate in the 
preparation of the membrane. The values of log K.sub.Li,j.sup.pot obtained 
for various cations were: zero for lithium ions; -1.2 for sodium ions; 
-1.6 for potassium ions; -0.9 for magnesium ions; and -1.6 for calcium 
ions.