GB2076545A - Surfactant-sensitive Membrane Electrode and Method of Determining Surfactant Concentrations - Google Patents

Abstract

An ionic surfactant, ion-sensitive membrane electrode is characterised in that the membrane has been formed from a solution of a complex of a water-insoluble copolyelectrolyte and the surfactant to which the electrode is sensitive; the copolyelectrolyte comprising units having an ionic group and units of such structure and in such proportion that the resulting copolymer has a glass transition temperature (Tg) below 20 DEG C. An anionic surfactant- sensitive electrode is described based on poly(ethyl acrylate)10-co-(3-benzyl- 1-vinylimidazolium chloride)1.

Description

SPECIFICATION Surfactant-sensitive Membrane Electrode and Method of Determining Surfactant Concentrations This invention relates to surfactant-sensitive membrane electrodes and to methods of determining surfactant concentrations therewith.

It is known that when a suitable ion-exchange membrane separates two solutions having different concentrations of the same electrolyte, e.g. an ionic surfactant, an electrical potential difference will exist between them. This is often called the membrane potential and is related to the relative concentrations of electrolyte in the two solutions. By employing an electrode containing a surfactant solution of known strength separated from a surfactant solution to be tested by an ionexchange membrane and measuring the potential between it and the surfactant solution, a determination of the concentration of the solution under test may be made.

Cutler, Meares and Hall, in J. Electroanal. Chem. 85(1977)145-161, describe a heavily plasticized polyvinyl chloride membrane in which the PVC has anionic or cationic groups chemically attached thereto. In making the membrane, a solution containing a mixture of low molecular weight PVC containing the ionic groups, higher molecular weight PVC and plasticizer is prepared and cast to form a film. Membrane electrodes made therefrom are said to have limited life due to the leaching out of plasticiser. They are also said to become insensitive after 6 hours of continuous immersion; this can be reversed by washing and drying.

The present invention provides surfactant-sensitive membrane electrodes in which the membrane is formed from a solution of a single copolymer without the need for any further additions.

Such membranes are simple to prepare, long lived and will not age due to loss of plasticizer.

According to the present invention there is provided a surfactant ion-sensitive membrane electrode characterised in that the membrane has been formed from a solution of a complex of a water-insoluble copolyelectrolyte and the surfactant to which the electrode is sensitive, the copolyelectrolyte comprising units having an ionic group and units of such structure and in such proportion that the copolymer has a glass transition temperature (Tg) below 200C.

The monomers employed to render the copolyelectrolyte rubbery, i.e. having a Tg less than 200C, may, for example, be olefins (1 -butene, 5-cyclohexyl-1 -pentene, 1 -decene, 2-methylpropene, 1,2butadiene), acrylates (butyl acrylate, ethyl acrylate, methyl acrylate, butyl methacrylate, ethyl methacrylate), styrenes (4-decyl styrene, 4-dodecyl styrene, 4-ethyl styrene, 4-hexyl styrene) or vinyl ethers (vinyl butyl ether, vinyl ethyl ether, vinyl methyl ether).

The copolyelectrolyte may comprise 597% by weight, preferably 8096% by weight, of nonpolar polymer units which may be derived from one or more monomers. Additional comonomers which may be employed in addition to those listed above include styrene, vinylpyridine, vinyl acetate, vinyl chloride and alkyl acrylates and methacrylates. Crosslinkable monomers may also be employed e.g.

divinyl benzene, chloromethylstyrene or bis-acrylate could be used. Crosslinking can also be achieved by reacting a copolymer containing an imidazole or similar ring with a bis-epoxy compound. It is to be understood that all the above variants may be employed to give the copolyelectrolyte the desired physical properties.

The ionic group may be cationic, e.g. a tertiary nitrogen quaternised with, for example, methyl iodide, ethyl chloride, methyl p-toluene sulphonate or benzyl chloride. Co-monomers containing tertiary N groups which are converted later into the corresponding quaternary salt groups by reacting with a quaternizing agent include vinyl substituted tertiary N-containing heterocyclic compounds, e.g., pyridine, imidazole, quinoline, isoquinoline, pyrimidine, phenanthroline, benzothiazole, purine, pyrazine, acridine or picoline.

Alternatively the ionic group may be anionic, e.g. an acidic group preferably a carbocyclic or sulphonic acid group. Hence, for example, an acrylic acid, vinyl phenol or vinyl phenyl sulphonic acid or a salt or phenolate thereof may be employed as a comonomer.

The units containing the ionic group may comprise from 95 to 3% by weight of the copolyelectrolyte preferably 20 to 4% by weight.

The membrane may be prepared from the copolyelectrolyte/surfactant complex by casting a film from a solution thereof. Preferred membrane thicknesses range from 0.2 to 0.9 mm. The concentration of the polymeric ionic species in the membrane may vary from 1 0-1 to 10-4M while the concentration of added surfactant in the membrane will be the same or greater.

The complete electrode may comprise, for example, a silver-silver chloride electrode immersed in a potassium chloride solution saturated with silver chloride and containing the species to be determined. The boundary between this solution and the solution to be analysed is the membrane.

The copolyelectrolyte membranes used in the present invention may also be used in the electrode formats described in Research Disclosure publications 16113 (September 1977 pages 29-39), and 1 7638 (December 1978 pages 1 5-16).

As indicated above, the present membrane electrodes are particularly suitable for determining the concentration of ionic surfactants in solution. Such surfactants may be anionic, e.g. alkyl sulphates or alkylaryl sulphonates, in particular n-alkyl sulphates wherein the alkyl group contains 8-10 carbon atoms and alkylnaphthalene sulphonates such as tri-isopropylnaphthalene sulphonates. For this purpose a cationic polyelectrolyte is used for the membrane. Alternatively the surfactant may be cationic, e.g. cetyl trimethylammonium bromide and the polyelectrolyte will have anionic groups.

Potential specific uses for the present surfactant ion sensitive electrode are: (i) determination of free surfactant, by direct potentiometry, in dispersions.

(ii) determination of free surfactant, by direct potentiometry, in water and drain samples.

(iii) determination of total surfactant, by potentiometric titration, in species described in (i) and (ii) above.

The present invention also provides a method for determining the concentraiton of an ionic surfactant in an aqueous solution using an ion-sensitive membrane electrode according to the present invention.

The following Examples are included for a better understanding of the invention.

Example 1 Poly(ethyl acrylate)O-co-(1 -vinylimidazole), A 50 ml flask equipped with a stirrer, water condenser and a N2 inlet was charged with ethyl acrylate (10 g) and 1-vinylimidazole (0.94 g), and azo-bis-isobutyronitrile (AIBN) initiator (0.01 g) was added. The mixture was heated to 700C in a N2 atmosphere and kept at this temperature overnight.

The products of the reaction were dissolved in dioxan and isolated by precipitating the reaction mixture into petroleum ether, and purified further by re-precipitating into petroleum ether from dioxan soln. The precipitate was filtered off and dried at 500C under vacuum to give about 10 g of a yellowish rubbery substance containing (%): C 60.2; H 7.8; N 2.6. Poly(ethyl acrylate)10-co-(1-vinyl-imidazole)1 requires (%): C 60.3; H 7.9; N 2.6.

Similarly, poly(ethyl acrylate)20-co-(1-vinylimidazole), was prepared from ethyl acrylate (20 g), 1vinylimidazole (0.94 g) and AIBN (0.02 g).

Found (%): C 60.0, H 7.9, N 1.3; CtosH,88N2040 requires (%): C 60.2; H 7.9; N 1.3.

Example 2 Poly(ethyl acrylate)tO-co-(3-benzyl-1-vinylimidazolium chloride), - Poly(ethyl acrylate)O-co-(1 -vinylimidazole) (10 g) was dissolved in dioxan (200 ml) and reacted with benzyl chloride (1.3 g) at reflux temperature for 20 hours. The reaction products were isolated and purified by precipitating twice into petroleum ether, and dried at 500C under vacuum. Found (%): C 59.8, H 8.0, Cl 0.9, N 2.3. The fully quaternized product requires (%): C 61.0, H 7.6, Cl 2.9, N 2.3.

Similarly, poly(ethyl acrylate)20-co-(1-vinylimidazole) (20 g) was quaternized with benzyl chloride (1.3 g) to give a product containing (%): C 60.0, H 8.0, Cl 0.7, N 1.3; the fully quaternized copolymer requires (%): C 60.5, H 7.8; Cl 1.6, N 1.3.

Example 3 Preparation and use of Electrode The electrodes were prepared by first casting a suitable membrane. A copolyelectrolyte (0.6 g) was dissolved in 75 cm3 chloroform in a separating funnel and 50 cm3 aliquots of 1000 ppm TIPNS (triisopropylnaphthalene sulphonate) aqueous solution added to a total volume of 200 cm3. After each addition, the funnel was shaken vigorously. The mixture of TIPNS and copolyelectrolyte solution was allowed to stand overnight and the non-aqueous layer run-off. This was evaporated to approximately 10 cm3 and poured into a casting ring. Slow evaporation of solvent was carried out (2-3 days) leaving a suitable membrane for use as an electrode sensing material. A disc was- cut from the membrane and attached to a plastic tube with tetrahydrofuran. After drying the joint, internal filling solution was added (10-3M TIPNS and 10-1M KCI) to the plastic tube and a silver-silver chloride wire inserted as the internal reference element.

An example of a typical response of the electrode prepared using the quaternized copolymer of Example 2 to TIPNS anion is given in the Figure of the accompanying drawing. There is plotted the response of the electrode (in mV) at various TIPNS concentrations (in ppm). A calomel electrode was used as reference. The measurements were carried out at 270C. The results of three separate determinations are given in Table 1.

A membrane electrode of this type has been operating on an intermittent basis in a laboratory for more than six months with no deterioration of response.

Table 1 Caiibration of polymeric surfactant sensitive electrode in aqueous solutions of tri isopropylnaphthalene sulphonate anion at 27 OC.

electrode response {mV) [TIPNS](ppm) 1st run 2ndrun 3rdrun 1000 123.2 121.6 121.2 500 137.6 136.3 136.3 100 177.4 176.5 175.6 50 195.2 194.5 191.2 10 229.3 229.5 227.3 1 248.8 246.4 250.0

Claims (10)

Claims

1. A surfactant ion-sensitive membrane electrode characterised in that the membrane has been formed from a solution of a complex of a water-insoluble copolyelectrolyte and the surfactant to which the electrode is sensitive, the copolyelectrolyte comprising units having an ionic group and units of such structure and in such proportion that the copolymer has a glass transition temperature (Tg) below 200 C.

2. A membrane electrode as claimed in claim 1 in which the copolyelectrolyte comprises 597% by weight non-polar units.

3. A membrane electrode as claimed in claim 1 in which the copolyelectrolyte comprises 8096% by weight non-polar units.

4. A membrane electrode as claimed in any of claims 1-3 in which the copolyelectrolyte further comprises units which are crosslinkable.

5. A membrane electrode as claimed in any of claims 1-4 in which the copolyelectrolyte comprises 204% by weight units containing an ionic group.

6. A membrane electrode as claimed in any of claims 1-5 in which the copolyelectrolyte comprises units containing a quaternary ammonium group.

7. A membrane electrode as claimed in claim 6 in which the nitrogen atom of the quaternary ammonium group forms part of a heterocyclic ring.

8. A membrane electrode as claimed in any of claims 1-5 in which the ionic group is a carboxylic or sulphonic acid group or a salt thereof.

9. A membrane electrode as claimed in any of claims 1-7 in which the copolyelectrolyte is a copolymer comprising units of ethyl acrylate and 3-benzyl-1 -vinyl-imidazolium chloride.

10. A method of determining the concentration of an ionic surfactant in an aqueous solution using a membrane electrode characterised in that the electrode is one according to any of claims 1-9.