GB2127977A

GB2127977A - Method and sensor for the voltametric determination of oxygen

Info

Publication number: GB2127977A
Application number: GB08324822A
Authority: GB
Inventors: Lubomir Serak; Jaroslav Cap
Original assignee: Czech Academy of Sciences CAS
Current assignee: Czech Academy of Sciences CAS
Priority date: 1982-09-27
Filing date: 1983-09-16
Publication date: 1984-04-18
Also published as: GB2127977B; CS231026B1; GB8324822D0

Abstract

The voltametric determination of oxygen dissolved in liquids or present in gaseous mixtures consists in polarization of a sensor by rectangular potential pulses, each pulse being applied after the concentration of oxygen is restored in a space between a separation membrane (3) and the surface of a working electrode (4). The sensor has a limited space (1) filled with electrolyte (6) formed between membrane (3) and working electrode (4), which has advantageously a cylindric shape, the surface area of the base of the space ranges from one third to ten times the surface area of working electrode (4), and the height ranges from 1 to 500 mu m. The limited space (1) may be formed by means of a hole in a foil (2) inserted between membrane (3) and working electrode (4) or it may be formed by a hole in the insulation body (7) of the working electrode (4) between the membrane (3) and the working electrode (4) (Fig. 2). <IMAGE>

Description

SPECIFICATION Method for the voltametric determination of oxygen and a sensorforthis method of determination The invention pertains to a method for the determination of oxygen by an electrochemical voltametric method, on one hand, and to a sensor, which enables this method of determination, on the other hand.

The voltametric or polarographic determination of oxygen has been known for almost 50 years. The first polarographic determination described V. Vitek; Collection Czech. Chem. Commun. 7,537-547(1935). The polarographic determination with a mercury-drop electrode, which was separated from the measured solution with a permeable membrane, was described by one of the authors of this invention, viz. L. Serak; Z.

Phys. Chem., Leipzig, Sonderheft, 1958, p.84. The determination with a solid working electrode separated from the sample by a layer of electrolyte was described by P. W. Davies and F. Brink; Rev. Sci.

Instrum. 13,524,1942, while the determination with a solid working electrode separated from the measured sample with a membrane was described by L. C. Clark; Trans. Amer. Soc. Art. Int. Organs 2,41-45(1956).

In the papers mentioned above, the voltametric determination of oxygen was gradually improved and, at the present time, it is carried out by a procedure based on the work of L. C. Clark; see e.g. H.P. Kimich, F.

Kreuzer, J. G. Spaan; Biocapt 75,307-312 (1975). In the arrangement of Clark, a sensor consists of a working polarized solid electrode, a reference electrode, an electrolyte and a membrane, which is permeable for oxygen and impermeable for water, ions and large molecules, so that only a thin layer of electrolyte occurs between the membrane and the surface of working electrode. The working electrode is cathodi cally polarized to a constant potential and, at constant temperature, the current passing through the sensor is directly proportional to the amount of oxygen transported to the surface of working electrode and, consequently, to the oxygen concentration in the investigated medium in a close proximity of the membrane.However, excellent properties of the sensor are accompanied with an unfavourable feature, namely with a pronounced dependence of the current signal on temperature (the signal increases as much as by 8% at the temperature increase of 1"C).

This dependence is predominantly determined by a high temperature coefficient ofthe membrane permeabilityto oxygen. The permeability increases exponentially with the increasing temperature. In practical measurements,thetemperature dependence of signal has to be compensated automatically, which fact limits the reproducibility and correctness of measurement at largertemperature differences.

The above mentioned disadvantage is removed by the present method of pulse voltametric determination and the sensor according to the invention, which is based on a periodic polarization of the sensor by rectangular potential pulses, whereas each pulse follows first after the concentration of oxygen, in a limited space filled with electrolyte and formed between the membrane and the surface of working electrode, is restored.

A choice of the length of interval between indivudual pulses, ranging from 1 to 10 s or more, depends on properties of the membrane and on dimensions of the limited space.

The invention further pertains to the sensor for performing the method according to the invention, wherein a limited space filled with electrolyte is formed between the membrane and the working electrode, which space has advantageously a cylindric form, has a surface area of its base ranging from one third to ten times the surface area ofthe working electrode, and its height ranges from 1 to 500 cam.

This limited space may be created by means of a hole in a foil, which is inserted between the membrane and the working electrode. The limited space can be also formed by a hole in an insulation body of the working electrode between the membrane and the working electrode. It is suitable, if the hole in the insulation body or working electrode is placed in the longitudinal axis of the working electrode.

An information about oxygen concentration is obtained by measuring an instant current in a precisely set moment after the beginning of pulse, when a charging current of double-layer has already quenched and the concentration gradient of oxygen is confined in the limited space between the membrane and the surface of working electrode.

The appended drawings displaytwo examples of the sensor according to the invention in a longitudinal cross-section; Fig. 1 shows the sensor, the limited space of which is created by means of a hole in a foil inserted between the membrane and the working electrode, whereas Fig. 2 shows the sensor with the limited space formed by a hole in the insulation body ofworking electrode between the membrane and the working electrode.

The sensor consists of a jacket of sensor 8 separated by a layer of cement 9from a reference electrode 5 and also from an insulation body 7working electrode, which guides the working electrode 4 in the longitudinal axis of the sensor. Both electrodes are connected by one end to a coaxial cable 10, whereas their other ends project into the space of electrolyte 6. The electrolyte 6is separated from environment by means of a membrane 3,which isfixed to the sensorjacket 8 by means of an fixation ring 11.In the first example of the sensor according to the invention (fig. 1), a foil 2 with a cylindric hole is inserted between the mem brane 3and the jacket 80f sensor in the space of electrolyte 6, which creates a limited space 1 between the working electrode 4 and the membrane 3, whereas in the second example ofthe sensor (Fig. 2), the limited space 1 is formed by a hole in the insulation body 70f the working electrode 4 between the membrane 3and the working electrode 4.

Measurement ofthe instant current in a precisely set momentafterthebeginning of pulse, when the charging current of double-layer has already quen ched and the concentration gradient of oxygen is confined in the limited space 1 between the membrane 3and the surface of working electrode 4, gives data which serveforthedetermination of oxygen concentration.

The invention may be employed at technological conditions forthe determination of oxygen either dissolved in liquids or present in gaseous mixtures.

Claims

1. A method for the voltametric determination of oxygen by means of a sensor, comprising a separation membrane and a working electrode, and an evaluation device of the signal, wherein the sensor is polarized by rectangular potential pulses, whereas each pulse follows first after the oxygen concentration is restored in a space between the sepa ration membrane and the surface of working electrode.

2. A sensorfor performing the method according to Claim 1, comprising a working electrode, an insulation body ofworking electrode, a nonpolarizable reference electrode, an electrolyte, and a separation membrane, wherein a limited space (1) filled with the electrolyte is created between the membrane (3) andtheworking electrode (4), which base hasa surface area ranging from one third to ten times the surface area oftheworking electrode and which heightrangesfrom 1 to 500 m.

3. Asensoraccordingto Claim 2,whereinthe limited space (1) is formed by a hole in afoil (2), which is inserted between the membrane (3) and the working electrode (4).

4. A sensor according to Claim 2, wherein the limited space (1) is formed by a hole in the insulation body (7) or working electrode (4) between the membrane (3) and the working electrode (4).

5. A sensoraccording to Claim 4, wherein a hole in the insulation body (7) of the working electrode (4) is placed in the longitudinal axis ofworking electrode (4).

6. A sensor according to Claims 2 through 5, wherein the limited space (1) has a cylindric shape.

7. A method for the volta metric determination of oxygen by means of a sensor substantially as described and disclosed herein.

8. A sensor for performing the method for the voltametric determination of oxygen by means of a sensor substantially as herein described with referenceto Figure 1 or Figure 2 of the accompanying drawings.